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Tour of the Solar System: A Transformative Decade of Exploration

The past decade has witnessed an explosive growth in the state of knowledge of planetary science and astrobiology through the invaluable combination of new missions and data, supporting theoretical and modeling research, telescopic observations, and laboratory and experimental advances. In this chapter, the committee discusses some of the most exciting advances from the past decade, organized by destination or destination class as represented by the committee panels. Each section ends by enumerating specific highly impactful discoveries.

MERCURY

Prior to the past decade, the innermost planet had been visited only by spacecraft flybys. That changed in March 2011, when NASA’s Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) spacecraft became the first to orbit Mercury, providing a wealth of new observations of the planet’s interior, surface, exosphere, magnetosphere, and heliospheric environment until the end of the mission in April 2015.¹ MESSENGER’s orbital measurements of Mercury defied premission predictions and transformed our thinking about the formation processes for rocky worlds.

Mercury is unique among the terrestrial planets, starting at its core, which makes up ~70 percent of the planet’s mass, more than any other rocky body in the solar system. Mercury’s silicate-rich crust and mantle are but a thin veneer (combined thickness 420 km) atop the comparatively enormous metallic core (radius 2020 km). Geodetic and magnetic data indicate that the majority of this core is fluid, and geodetic evidence suggests the presence of a small solid inner core. The fluid outer core presently supports an active magnetic field that is about 100 times weaker at Mercury’s surface than Earth’s. Mercury’s magnetic field is predominantly axisymmetric and dipolar, closely aligned with Mercury’s spin axis, although the magnetic equator is offset along the spin axis to the north by ~20 percent of the planet’s radius, for reasons that are not fully understood. MESSENGER also mapped crustal magnetic anomalies across varied geologic terranes on 20 percent of Mercury, with measurements limited to areas of the northern hemisphere where the spacecraft was close to the planet. The recognition that ancient volcanic plains in these areas have significant remanent magnetization implies that Mercury’s core dynamo was active prior to at least 3.7–3.9 Ga, and may have had a field strength similar to present-day Earth.

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¹ A glossary of acronyms and technical terms can be found in Appendix F.

Some of the most notable discoveries of the past decade include measurements of Mercury’s crustal geochemistry: elevated abundances of sulfur and carbon, low abundances of iron and oxygen, and a nearly chondritic chlorine/potassium ratio. These characteristics have important implications for the thermochemical evolution of Mercury and point to a planet that is surprisingly rich in volatiles and that formed under highly reducing conditions. The origin of Mercury’s highly reduced, volatile-rich composition and large core are likely key to understanding its formation (and the formation of other planets and planetesimals at the inner edge of protoplanetary disks). MESSENGER’s geochemical measurements ruled out many theories for the origin of Mercury’s metal-rich composition, and currently two remain: (1) Mercury was once a large planet with a typical metal/silicate ratio expected from formation from chondrites (~30 percent metal, 70 percent silicate), but an early giant impact stripped away a large proportion of silicates that never reaccreted, leaving behind a metal-rich planet, or (2) Mercury accreted its metal/silicate ratio from highly reduced and Fe-rich material in the protoplanetary disk. At present, these two formation models cannot be distinguished from available data, nor can either model seemingly satisfy all observations.

The surface geochemistry measurements from MESSENGER have also led to new ideas about the variety of outcomes from global-scale magma oceans. The terrestrial planets are all thought to have experienced a phase where they were largely or completely molten with subsequent crystallization and silicate-liquid fractionation resulting in the formation of a crust and mantle of varying thickness and composition, depending on their starting conditions. This process is best known on the Moon, and many researchers originally thought that Mercury also possessed a flotation crust of similar composition (plagioclase). However, the low abundance of iron oxide in Mercury’s silicates means that plagioclase would not have been less dense than the melt, and thus could not have risen to the surface of a magma ocean on Mercury. In fact, experimental studies used MESSENGER data to determine that all typical rock-forming minerals are denser than Mercury magmas, except for several atypical low-density mineral candidates, such as graphite and oldhamite (calcium magnesium sulfide). Given that MESSENGER data indicated elevated sulfur and carbon at the surface, both calcium-rich sulfides and graphite have been considered as possible constituents of Mercury’s earliest crust. In the case of graphite, its presence at the surface could also reconcile the seeming contradiction of the planet’s low average surface reflectance with its bulk composition that would otherwise yield bright surface materials. Most of the regions where Mercury’s reflectance is lowest, and thus may have the highest abundance of graphite, are found either in areas with the highest density of large craters, or in the ejecta of some impact craters and basins.

The past decade has also revealed Mercury’s geologic history to have been shaped extensively by volcanism. Its vast smooth plains formed from flood-like eruptions that covered approximately a third of the planet. Furthermore, stratigraphic relationships, compositional similarities, and crater size-frequency distributions also suggest that much of the intercrater plains, the most extensive terrain on Mercury, also formed through volcanic eruptions earlier in Mercury’s history. Regions with the strongest evidence for volcanism are typically higher in reflectance and superpose the more ancient low-reflectance crustal materials, suggesting that their mantle source regions did not contain substantial abundances of carbon or other possible darkening agents. Explosive volcanic deposits, which are thought to deliver materials to the surface that are the most representative of mantle composition, are also among the highest reflectance, with the largest deposit shown to have low abundances of carbon and sulfur relative to the rest of Mercury’s surface. The unexpectedly large number of such explosive eruptions further attests to the high abundance of volatiles native to Mercury, as these pyroclastic eruptions are driven by exsolution of magmatic volatiles.

Vast outpourings of flood basalts may account for a substantial portion of Mercury’s crust, even though Mercury is in a state of global contraction owing to secular cooling. MESSENGER observed a surface dominated by long-wavelength folding and contractional tectonic features including lobate scarps, high-relief ridges, and wrinkle ridges (Figure 2-1). Extensional features are comparatively rare and are found only within volcanic plains located inside impact basins. The most abundant tectonic features on Mercury by far are the lobate scarps, thought to be the surface expression of thrust faults, which can result in cliffs several kilometers high. The global distribution of lobate scarps provide insight into how single-plate planets cool and suggest the buckling of the lithosphere is the result of contraction that has resulted in Mercury’s radius shrinking by up to 7 km over the history recorded by its tectonic features. Although such contractional stresses could deter volcanism, Mercury’s magmas may have been aided in reaching the surface by impact fracturing and thinning of the crust, and the fact that magmas are buoyant at any depth within the mantle and crust. MESSENGER geochemical measurements further suggest that

the volcanic deposits were a product of high degrees of partial melting that were likely possible only early in Mercury’s history before cooling and global contraction took over.

Because of its orbit close to the Sun, Mercury has experienced more intense impact bombardment than anywhere else in the solar system, and impactors strike the surface at an average velocity at least three times that of the Earth–Moon system. However, in contrast to the expectation of a heavily cratered surface, MESSENGER revealed that Mercury has a lower total population of large impact craters (>20 km diameter) and basins (>300 km diameter) than the Moon. This implies that Mercury’s oldest exposed crust is relatively young (<4.1 Ga). Both widespread volcanism and the intense early bombardment of Mercury likely played some role in obscuring older crust. Additionally, the distribution of large impact basins on Mercury is not uniform; more are identified on one hemisphere. This asymmetry in basin distribution may be owing to nonuniform resurfacing by volcanic flows and/or the spin-orbit evolution of Mercury, where the rotational period was originally synchronized with its orbital period, allowing more impact cratering on the western hemisphere than the eastern hemisphere. The impact cratering record also forms the basis for understanding chronology of major landforms on Mercury. These data suggest that most of Mercury’s largest impact basins date to the period ~3.8–4.1 Ga, and that the majority of volcanic plains on Mercury are probably older than ~3.0–3.5 Ga.

While most of Mercury’s geologic activity occurred early in its history, one type of landform unique to Mercury is thought to be extremely young. High-resolution images from the orbital phase of MESSENGER’s mission revealed groups of small (typically less than ~1 km across), irregularly shaped, flat-floored depressions known as hollows (Figure 2-2). These features are found within impact craters, and their extremely crisp morphology suggests their formation is ongoing to this day. Hollows are thought to form owing to sublimation or erosion of a volatile-rich component that is found in Mercury’s crust but is not stable on the surface after it has been exhumed by an impact event. Hollows are thus an additional signifier of Mercury’s surprisingly volatile-rich nature: some of its lithologies may be too volatile-rich to survive the harsh surface environment for long.

One environment on Mercury is, however, well suited to volatile preservation: regions of permanent shadows in impact craters and topographic lows near Mercury’s poles. Deposits of ice were initially suggested to exist near the poles based on Earth-based radar observations, and MESSENGER provided multiple lines of evidence that these deposits are mostly water ice, including hydrogen abundances inferred from neutron spectroscopy, reflectance measurements, and models of the thermal environment (Figure 2-3). Reflectance data and long-exposure images of regions in permanent shadow showed that some deposits have high reflectance, consistent with exposed surface ice, whereas others are lower in reflectance than material anywhere else on the planet. The low-reflectance polar deposits coincide with areas where thermal models indicate that ice is not stable at the surface, and thus they may be covered by an organic-rich lag deposit. The deposits on Mercury appear to be of higher purity and represent a larger total inventory of ice than at the Moon’s poles. The source of the water ice in Mercury’s polar deposits is unknown. The volatiles may have been delivered relatively recently (10–100 Ga) by comets or asteroids, or water ice may come from the interaction of Mercury’s surface with the solar wind or by outgassing of interior volatile species.

Mercury is classically considered an environment inhospitable to life, but the astonishing detection of volatiles such as water ice and complex organic compounds at Mercury’s surface suggest that life’s essential ingredients may be more abundant throughout the solar system than previously imagined. Evidence of past explosive volcanism in the form of extensive pyroclastic deposits indicates that volatiles are also present in the deep interior of Mercury, although the type and extent of volatiles in the interior (e.g., CO₂, H₂O, and SO₂) remain unclear. Intriguingly, the polar regions of Mercury may provide a unique opportunity to investigate chemical reactions that occur between water, organic materials, and other volatiles at extremely cold temperatures, including how prebiotic molecules such as amino acids and nucleobases abiotically form. Although Mercury likely never sustained life, Mercury may indeed hold precious unadulterated information regarding the distribution and construction of life’s building blocks throughout the solar system and inform on the ability of a planetary body to retain volatile-rich material through the catastrophic accretionary events of the early solar system.

Measurements from MESSENGER’s orbital mission also revealed the complex and dynamic ways in which Mercury interacts with the extreme space environment, particularly the interplay between the solar wind, magnetosphere, and core. The magnetosphere was found to be strongly affected by events such as coronal mass ejections, allowing solar wind to intrude deeply within Mercury’s dayside magnetosphere. Intense solar wind driven magnetospheric variations can induce currents that increase magnetic flux levels around the planet. New results suggest the dayside magnetosphere has competing processes: induced currents to fortify the magnetic field that can also undergo fast erosion by enhanced reconnection to allow deeper solar wind entry.

MESSENGER observations demonstrated that plasma contained within the magnetosphere is predominantly solar wind protons that enter the magnetosphere via dayside magnetic field reconnection. The entering plasma subsequently flows to a high latitude “mantle” region and then into the central anti-sunward magnetic tail. The magnetoplasma circulation times (from dayside entry, tail, and back to dayside) are unexpectedly fast, on timescales of a few minutes—much faster than the ~1 hour circulation time in the analogous terrestrial magnetosphere. Some fraction of the exospheric neutral species also becomes ionized and subsequently incorporated into Mercury’s

magnetoplasma circulation. Mercury’s neutral exosphere contains nine known species: hydrogen, helium, sodium, potassium, calcium, magnesium, aluminum, iron, and manganese. The ultraviolet-visible spectrometer onboard MESSENGER discovered emissions from exospheric magnesium and manganese and also confirmed the presence of highly energetic exospheric calcium. The exospheric calcium energy is consistent with a temperature of 70,000 K—much higher than any known process occurring in the analogous lunar exosphere—and hypothesized to be related to CaO or CaS molecular chemistry.

The MESSENGER mission to Mercury represents a milestone in our understanding of the innermost rocky planet of the solar system. Comparisons with the Moon, based on its outward appearance, have proved incorrect. Instead, the formation of Mercury remains enigmatic but resulted in a thin, volatile-rich but reduced, silicate shell around a massive iron-rich core. Subsequent contractional processes, impacts, and volcanism have shaped the surface of this thin silicate outer shell. MESSENGER showed that Mercury’s magnetosphere combined with its geochemical properties make it an important environment for understanding exosphere stability in the solar system. The BepiColombo spacecraft arrives at Mercury in late 2025 to start a 1- to 2-year mission of orbital measurements of the surface composition, geophysics, exosphere and magnetosphere that will substantially augment our knowledge of the planet. After BepiColombo, the next step will be to return to Mercury with a lander. Measurements of the minerals in surface materials will go a long way in helping us understand Mercury’s highly reduced, volatile-rich composition and large core. Similarly, surface measurements of the magnetic field and the exosphere will help us understand the way that Mercury interacts with its space environment.

Key Discoveries from the Past Decade

• Mercury is a volatile-rich world. Contrary to earlier predictions, and despite being closer to the Sun, concentrations of elements that evaporate at moderate temperatures are more abundant on Mercury than on Venus or Earth, and are comparable with those on Mars. Additionally, high abundances of sulfur and low abundances of iron in the silicate mantle of Mercury indicate that the planet formed with less oxygen than the other bodies of the inner solar system, providing insight into the building blocks and formation of terrestrial worlds.
• Mercury’s offset magnetic field and dynamic magnetosphere were revealed by MESSENGER. Mercury’s large metallic core generates an axially dipolar core dynamo magnetic field that is enigmatic owing to its low intensity relative to Earth’s and the offset of the center of the dipole by approximately 20 percent of the planet’s radius to the north. Interaction between Mercury’s magnetic field and the solar wind results in the generation of currents that induce external magnetic fields. With induced external fields that can be as large as the planetary field, the dynamic magnetosphere at Mercury is a unique natural laboratory for exploring magnetospheric physics and exospheres.
• Permanently shadowed regions near Mercury’s poles hold abundant water ice. Many regions on Mercury where water ice is predicted to be thermally stable appear to host such ice deposits either at the surface in a relatively pure, thick layer or beneath organic-rich lag deposits. The origin of these deposits is not yet known, but water ice may have been delivered by comets or asteroids or micrometeoroids, from the solar wind, or outgassing from Mercury’s interior.
• Volcanism played a critical role in shaping Mercury’s surface. Mercury’s oldest surfaces contain high abundances of carbon that may be remnants of a primary graphite flotation crust from an early magma ocean. Much of the surface, however, was subsequently covered by extensive floods of lava of varying composition indicating geochemically diverse terrains on Mercury’s surface that hint at a heterogeneous interior.

Further Reading

Charlier, B., and O. Namur, eds. 2019. “Planet Mercury.” Elements 15(1):9–45. Mineralogical Society of America. https://pubs.geoscienceworld.org/elements/issue/15/1.

Solomon, S.C., L.R. Nittler, and B.J. Anderson, eds. 2018. Mercury: The View After MESSENGER. Cambridge Planetary Science, Cambridge: Cambridge University Press. https://doi.org/10.1017/9781316650684.

THE MOON

A wealth of data from new missions, reinterpretation of old data, and new analyses of Apollo samples and meteorites, have, over the past decade, provided new insight into the evolution of the Moon, Earth, and the early solar system. The Lunar Reconnaissance Orbiter (LRO), in lunar orbit since 2009, continues to return critical data, mapping the lunar surface in unprecedented detail as well as characterizing landing sites for future robotic and human exploration. The Gravity Recovery and Interior Laboratory (GRAIL) mission orbited twin spacecraft in tandem around the Moon in 2012 and mapped the Moon’s gravity at higher resolution than available even for Earth, to reveal the Moon’s interior and near-surface structure. The Lunar Atmosphere Dust Environment Explorer (LADEE) spacecraft orbited the Moon from 2013–2014 to characterize the natural state of the lunar exosphere and dust environment prior to disruption from future landed robotic and human missions. Missions led by other countries, including China (Chang’e 3, 4, and 5) and India (Chandrayaan-2), have also enhanced our understanding of the state of the lunar surface and interior, duration of volcanism, distribution of volatiles, and impact processes over the past decade.

The most widely accepted hypothesis of lunar formation posits that the Moon formed from the debris of a cataclysmic impact of a Mars-size planetary body with the proto-Earth. Although this giant impact scenario is not new, it has been extensively revised in the past decade. The original model of the Moon forming by an impact between the proto-Earth and a Mars-size impactor successfully explained the size of the lunar core, angular momentum of the Earth-Moon system, and mass of the Moon, but not their isotopic similarities. The problem has been that most of the Moon was thought to form from the impactor, rather than being a mixture of impactor and Earth materials, suggesting Earth and the Moon would be different isotopically, unless the impactor happened to have the same isotopic ratios as the proto-Earth. New models propose different impact speeds, impact sizes, or even multiple impact events as scenarios that could result in more mixing between the materials that formed Earth and Moon, and invoke transfer of angular momentum from the Earth–Moon system to the Sun–Earth system. These models can broadly explain the isotopic similarity, but may not satisfy other constraints.

In the past decade, there has also been debate over the formation age of the Moon. Knowing the timing of this event would enable an understanding of when Earth and the Moon attained their present-day configuration and when the last stages of planetary formation occurred in the solar system. Lunar samples from the Apollo missions and from meteorites give a minimum age for the Moon of 4.4 Ga. To determine the true age, two main strategies have been used with different results. The first provides model ages by using measured isotopic compositions of lunar materials and back-calculating when they were equal to an assumed starting value, yielding a lunar formation age of 4.51–4.52 Ga. This method also gives a model age for the most evolved known reservoir in the Moon (known as KREEP for its potassium, rare earth element, and phosphorous content) of about 4.38 Ga, perhaps dating the end of lunar magma ocean crystallization. These ages suggest the Moon formed about 50–60 million years after solar system formation. The second method uses assumptions of how materials were accreted to planets, including the abundances of highly siderophile elements (HSEs), as a constraint for the Moon formation time. The HSEs currently found in the terrestrial mantle would have been delivered after or during the Moon-forming event because any HSEs accreted prior to the impact would have been partitioned to Earth’s core. Using estimates of the impact flux and composition of planetesimals, models of the rate of accretion of materials to Earth tend to predict formation ages for the Moon that are substantially younger than other methods—as much as ~100 million years after the formation of the solar system. Determining the formation time of the Moon is crucial because the impact event defined the initial condition of Earth. Moreover, the Moon-forming impact was likely the last catastrophic event in the inner solar system, and therefore it also defined the end of the accretion phase. In summary, the timing of the Moon’s origin remains an open question.

Further insight into the Moon’s formation and thermochemical evolution has been gained over the past decade from intense study of its inventory of indigenous volatile elements. Whereas all terrestrial planetary bodies are depleted in volatile elements relative to CI chondrites and the Sun’s photosphere, the Moon’s depletion is more extreme and results from a combination of the cataclysmic circumstances under which it is thought to have formed, the geochemical signature of the protoplanet impactor, and/or the process of planetary differentiation. However, while the Moon was once thought to contain less than one part per billion H₂O in its interior, new measurements show that at least some portions of the lunar mantle contain hundreds of parts per million H₂O, and remote detection of hydrated materials in some uplifted crustal materials further attest that the Moon is far from “bone dry.” The discovery that stable isotope compositions of moderately volatile and volatile elements in lunar samples are

fractionated relative to terrestrial values further suggests that the Moon lost a substantial fraction of its volatile elements both during and after it formed. Volatile loss from the lunar interior was likely important for generating transient atmospheres and for distributing some of the volatile species presently on the lunar surface.

Major advances in our understanding of the interior and crustal structure of the Moon and its bulk composition have come from new ultra-high-resolution gravity and topography measurements obtained by GRAIL and LRO, respectively. These data, when combined with Apollo seismic constraints, indicate that the lunar crust has high porosity and a mean thickness of 34–43 km, with an average crustal thickness of ~55 km on the farside and ~30 km on the nearside (Figure 2-4).

Such large-scale crustal asymmetry is further found in nearside/farside differences in porosity, heat-producing elements, and extent of volcanism; the cause of the Moon’s crustal asymmetry remains one of the greatest outstanding mysteries regarding lunar early evolution. The crustal thickness estimates also provide constraints on the bulk composition of the silicate portion of the Moon. In particular, the crustal thickness estimates indicate the Moon has a complement of refractory elements that is a close match to the bulk silicate composition of Earth rather than earlier estimates, based on inferred crustal thickness larger than currently accepted values that indicated the Moon was enriched in refractory elements by up to 50 percent relative to Earth and CI chondrites. GRAIL data, coupled with advances in theoretical models, have also transformed our understanding of how large impact basins (and the associated, mysterious gravity anomalies—“mascons”—that destabilized the orbits of early lunar missions) formed, and how tides shaped the Moon. New insights have been gained into the Moon’s deep interior structure as well, from a combination of data from GRAIL and LRO, modern analyses of Apollo seismic data, and continued laser ranging observations, and indicate that the Moon likely has a solid inner core between 130 and 200 km in radius, possibly overlain by a fluid outer core extending out to ~380 km in radius.

Over the past decade, remanent crustal magnetism studies and laboratory analyses of returned lunar samples have revealed that the ancient Moon had an internally generated magnetic field. Between 4.25 and 3.56 Ga, surface field intensities reached values up to 40–110 μT. This high-field period was followed by a lower intensity field of ~5 μT or less that may have persisted beyond 2 Ga. However, no single proposed dynamo mechanism to date satisfactorily explains both the longevity and intensity of the ancient lunar dynamo inferred from the paleomagnetic record. Ongoing research is further exploring hypotheses such as a basal magma ocean dynamo, the possibility that impact effects could have led to transient amplification of a baseline core dynamo field, or a situation wherein multiple dynamo mechanisms with differing predicted field intensities may have operated at different points in time.

The Moon’s dominant form of volcanism involved eruption of voluminous basin-filling mare lavas peaking about 3–3.5 Ga, but samples recently returned from Oceanus Procellarum by the Chang’e 5 mission show that the formation of flood basalts continued until at least 2 Ga. Morphological and compositional diversity is expressed through abundant mafic pyroclastic deposits, less common silicic pyroclastics, and complexes of small domes and cones, and evidence for large-scale shields. Over the past decade, new evidence challenges the notion that all lunar volcanism is ancient. The Lunar Reconnaissance Orbiter Camera (LROC) revealed numerous irregular mare patches, depressions containing patchy rough and smooth mafic deposits (Figure 2-5), thought to be remnants of eruptions younger than 100 My, volcanic materials resurfaced by late degassing, or the product of more ancient eruptions that resulted in materials with atypical physical properties. If these features are confirmed to be related to recent eruptions, the occurrence of such young volcanism implies that the lunar mantle was warmer than previously thought, and/or that heterogeneous or localized concentrations of radioactive elements allowed for small-scale eruptions to continue late into lunar evolution. In addition, silicic volcanism appears to be more prevalent than previously appreciated, raising the question of how substantial volumes of evolved magma could be produced on a single-plate planetary body.

GRAIL and LRO data enabled a new global inventory of magmatic intrusions, suggesting that for every volcanic eruption at the surface, there is an order-of-magnitude more magmatism beneath the surface. Analyses of the Moon’s Bouguer gravity revealed a global population of previously unseen, 100-km-long igneous bodies distributed across the Moon, likely reflecting a period of global expansion and magmatism early in the Moon’s history. On the nearside, even larger dikes and buried rifts were discovered enclosing the Procellarum KREEP Terrane (PKT; a region characterized by low elevations, thin crusts, extensive mare basalts, and abundant heat-producing elements), delineated by a 3.5-ppm-Th contour on the lunar nearside hemisphere. The buried rifts enclosing the PKT may be the result of planetary-scale tectonism and magmatism, and may have been the plumbing for much of the mare basalts that preferentially flooded the lunar nearside terranes. This early period of expansion was succeeded by a period of global contraction as a result of secular cooling. Evidence for this contraction comes from LROC images, which revealed a far greater abundance of contractional tectonic features than previously known, particularly lobate scarps. Similar to those on Mercury, but generally of smaller scale, these features are the surface expression of thrust faults, and their locations suggest that they have been further influenced by stresses from tidal interactions with Earth and from orbital recession away from Earth. Movement along these faults may continue into the present day, and may be the source of the large but rare shallow moonquakes recorded by Apollo seismometers.

The well-preserved state and ancient surface of the Moon has exquisitely preserved the most complete record of the major events that have occurred in our part of the solar system since the formation of the Earth–Moon system.

Although the early record of the accretion and rate of exogeneous delivery of chemical compounds are poorly preserved on Earth, the flux of chemical inventories important to the emergence and subsequent evolution of Earth’s life may be decipherable at the Moon. The Moon’s impact history not only provides a means to understand the linked evolution of the Earth-Moon system since antiquity, but also the chronology of events across the solar system. Indeed, significant advances have been made in the past decade by refined laboratory analysis of samples that have been returned and dated from the Moon’s surface. An example of such a recent advance is an improved age determination for the important nearside impact basin Imbrium to 3.91–3.94 Ga based on U-Pb dating. The history of impacts on the Moon prior to Imbrium remains a subject of substantial uncertainty. Since the 1970s, it has been hypothesized that there was an intensive peak in the impact bombardment of the Moon in the 3.8–4.1 Ga epoch (known as the late heavy bombardment or lunar cataclysm). Modeling and geochemical studies completed over the previous decade have cast doubt on whether an intense impact cataclysm in this epoch is required, in part because the Imbrium impact event may be radically overrepresented in the existing sample collection. At a minimum, the magnitude of the peak in the impact bombardment during this period might be lower than was once thought. GRAIL and LRO substantially clarified the total number of impact basins whose signatures are preserved on the Moon, but the period over which these basins formed remains to be established. Intense interest in the population and ages of lunar basins persists because they have implications beyond the Moon; an intense period of

impact bombardment of the lunar surface would be mirrored across the inner solar system, including on the early Earth. Only the most recent impact events are preserved on Earth, and LRO data combined with the terrestrial evidence suggest that the flux of impactors to the Earth-Moon system may have increased at about 290 Ma, and the breakup of specific asteroid families may be recorded in the Moon’s crater population.

The past decade has seen a major advance in direct observations of the ongoing impact process on the Moon. In particular, repeated meter-scale imaging by LROC enabled detection of ~500 impact craters formed over the first 11 years of the LRO mission (Figure 2-6). These observations provide a direct constraint on the flux of small meteoroids to the Moon. Terrestrial ground-based monitoring of the Moon has also captured impact flashes associated with impacts, and direct observations of the dusty ejecta from small impacts seen by LADEE. Integrating these observations has led to a revised picture of the rate at which the uppermost surface of the Moon is gardened, and emphasized the importance of high frequency, small meteoroid impacts and their ejecta for gradually eroding surface topography and transporting materials vertically and laterally.

Intense interest has also focused attention over the past decade on furthering our knowledge of the abundance, distribution, and origin of the Moon’s near-surface volatiles and understanding their implications for where habitable environments may exist in the solar system. LRO data continues to unravel the mysteries of polar volatiles (Figure 2-7), as neutron absorption measurements provide increased detail regarding the distribution of hydrogen,

and far-ultraviolet and laser reflectometry data are consistent with surface frosts in polar shadowed regions. Whereas previous data were ambiguous as to the form in which hydrogen is present, results from Moon Mineralogy Mapper data confirm for the first time that at least some polar H is present as H₂O. Numerical modeling indicates that volatiles are also thermodynamically stable beneath the surface in areas of temporary sunlight near the lunar poles, and in micro-cold traps at higher latitudes where small shadows persist. New measurements from the Stratospheric Observatory for Infrared Astronomy indicate that H₂O is present in near-surface sunlit regions, likely derived from solar wind H¹ and stored within impact glasses known as agglutinates or possibly between regolith grains. Offsets in the distribution of hydrogen from the poles have been interpreted to suggest that the location of the poles may have changed over time owing to large impacts and/or volcanism that affected the Moon’s obliquity, and thus some portion of the Moon’s polar volatiles are preserved from those ancient times and recorded volatile delivery from various sources such as comets, asteroids, solar wind implantation, and/or volcanic outgassing. Through such interrelated processes, the distribution and nature of polar volatile deposits may be indicative of the geologic and geophysical evolution of the Moon as well as the bombardment history of the inner solar system.

The study of volatiles at the Moon continues into its exosphere. In the past 10 years, a new appreciation of the exospheric hydrogen budget has been gained, where solar wind implanted protons can be backscattered as neutral

hydrogen, energetic protons, or chemically altered in the regolith and released as new H-bearing species, including H₂ and methane. The LADEE mission did not find a substantial and sustained water exosphere. However, transient energetic water plumes were observed to be released from the surface during meteor streams, suggesting that the equatorial surface contains substantial amounts of H-bearing material either as water or OH that can be converted to water during highly energetic impactor events. LADEE also confirmed the presence of argon-40 and helium, found neon in comparable abundances, and discovered regional variations in the sodium and potassium exosphere. Furthermore, a particulate exosphere, produced by micrometeoroid impacts, was found by LADEE to extend many hundreds of kilometers above the surface, although no evidence was found for electrostatic transport of dust at altitudes >1 km.

Over the past decade, numerous lunar space missions, analyses of lunar samples, and modeling approaches have deepened our understanding of the distribution of volatiles, impact history, magnetic field, and geological features not only for the Moon, but for early Earth and planetary objects in general. These observations have also raised new questions that will only be answered by returning to the Moon and making direct in situ measurements and by returning samples to study in laboratories on Earth.

Key Discoveries from the Past Decade

• GRAIL explored the interior structure of the Moon with unprecedented detail, transforming our understanding of rocky worlds. GRAIL produced the highest resolution gravity field measurements of any object (including Earth). These data revealed the Moon’s crust to be far more fractured and porous than expected, exposed previously undiscovered global-scale magmatic-tectonic features, elucidated the formation and evolution of impact basins, and constrained models for both the bulk composition and formation of the Moon. While GRAIL focused on the Moon, these results have had sweeping implications for the understanding the geophysics of planetary bodies across the solar system.
• New analyses of samples from the Moon provide key insights into its formation and physicochemical evolution. Measurements of lunar samples have demonstrated the extent to which volatile elements are depleted in the lunar interior and the isotopic compositions of volatile elements, providing key insights into the process responsible for volatile depletion and at what stage(s) of the Moon’s evolution the depletion occurred. Increasing evidence for the essentially identical isotopic compositions between Earth and Moon for nonvolatile elements has strongly challenged prior Earth–Moon origin models. Additionally, paleomagnetic studies of lunar samples have revealed that the Moon once generated a core dynamo that persisted for over 2 billion years and had surface field strengths that, at times, rivaled that of the current Earth. Last, geochronological studies of lunar samples have elucidated the Moon’s formation age, which ties into our understanding of the timing of giant impacts during planetary accretion.
• The Lunar Reconnaissance Orbiter (LRO) revealed the Moon in unprecedented detail, including ways in which its surface has been altered in recent geologic times. Lunar volcanism was thought to have ended well over 1 billion years ago, but images from LRO revealed irregular patches of basaltic deposits that may have erupted within the past 100 million years. Newly discovered tectonic features indicate their locations may be influenced by tidal interactions with Earth and from orbital recession away from Earth, and movement along these faults likely continues into the present day. Images of craters that have formed after LRO began its mission indicate that impacts affect the surface far from the impact site, and secondary cratering overturns regolith at rates more than 100 times higher than previously thought.
• Water ice lies at the surface within some regions of permanent shadow at the Moon’s poles. New results confirm that at least some polar H is present as H₂O, although its origin and abundance is still not known. Understanding the nature of the Moon’s polar volatiles could provide insight into the origin, timing of delivery, and subsequent evolution of water and volatiles in the inner solar system.

Further Reading

Lunar Exploration Analysis Group. 2017. Advancing Science of the Moon: Report of the Specific Action Team. August 7–8. Houston, TX: Lunar and Planetary Institute. https://www.lpi.usra.edu/leag/reports/ASM-SAT-Report-final.pdf.

Mineralogical Society of America. 2023. “New Views of the Moon 2.” C.R. Neal, B.L. Jolliff, C.K. Shearer, S. Valencia, L. Gaddis, S. Mackwell, and S.J. Lawrence, eds. Reviews in Mineralogy and Geochemistry.

VENUS

A Tale of Two Planets

Viewed from afar—say, a few tens of light-years—the solar system contains two remarkably similar large, rocky worlds. They are close in size: one is only slightly less massive, and slightly smaller, than the other. They are likely the same age, and are presumably made of the same materials in about the same proportions. The smaller of the two orbits the Sun about a third closer than its bigger neighbor.

But actually visit that slightly smaller world and you’ll find a dramatic difference. Whereas Earth has blue skies, liquid water oceans that abound with life, and an oxygen-rich atmosphere, Venus has a global layer of yellow sulfuric acid clouds and suffocates under a thick blanket of carbon dioxide so dense that the pressure at the surface is 90 bars—equivalent to almost a kilometer underwater on Earth. The mean surface temperature at Venus is 740 K, about that of a self-cleaning oven (Figure 2-8). And instead of oceans, the second planet has vast lava plains, towering highlands, and an enormous, equator-spanning rift system.

Since NASA’s Mariner 2 flyby of Venus in 1962—the first successful encounter with any planet, by any nation—the striking differences in surface conditions between Venus and Earth have motivated a major question in planetary science: Why is Earth’s closest sibling not its twin?

Enigmatic Venus

Before the heady days of sustained Mars exploration, Venus attracted substantial attention from the major spacefaring nations of the time, the United States and former Soviet Union. Between the 1960s and 1980s, 35 Venus missions were dispatched (not all successfully reaching their destination). Including those using Venus as a gravity assist during flybys to other destinations, 47 missions visited (or attempted to visit) the second planet since 1961. Since 1991, however, that number has been eight. JAXA’s Akatsuki, in orbit since 2015, is the sole spacecraft

currently operating at Venus. The last dedicated U.S. mission to the planet was Magellan, which launched in 1989 and operated in orbit from 1990 until its decommissioning in 1994.

But during the preparation of this report, the Venus mission landscape changed dramatically. In early June 2021, NASA selected two new Venus missions for its ninth Discovery Program competition—VERITAS (Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy) and DAVINCI (Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging)—and just a few days later the European Space Agency announced the winner of its fifth medium-class mission competition as EnVision, also bound for Venus. In the blink of an eye, the Venus community went from having no missions in play to a fleet. VERITAS, DAVINCI, and EnVision will tackle some of the major outstanding questions that are yet to be answered for the second planet—building on the trove of data returned by those early missions that helped paint a picture of Venus as a tortured planet.

The finding by Mariner 2 of a sweltering surface, and later atmospheric measurements by NASA’s Mariner 5 and Pioneer Venus missions, motivated the development of a “runaway greenhouse” scenario to explain the Venus climate. Under this view, a perhaps-once-temperate planet lost its oceans to the sustained onslaught of radiation from a star that Venus unfortunately happened to orbit just a little too closely. Later missions helped flesh out that picture: a planet with major geological activity within the past billion years that all but erased any earlier record of surface conditions; a planet that, unlike Earth, lacks an internal magnetic field; a planet with a super-rotating atmosphere that at the cloud tops spins sixty times faster than the surface.

But, as is always the case in planetary exploration, we are left with more questions than answers for Venus. Did the planet indeed have a temperate past? If so, what were conditions there like before the climate catastrophe that befell it? Is Venus volcanically and tectonically active today? What are its rocks made of, and what do those compositions tell us about the planet’s formation and chemical evolution? And what lessons might the second planet hold for us regarding the fate that awaits our own, and for our understanding of large rocky worlds orbiting other stars?

A Landscape Both Familiar and Strange

The global cloud layer makes imaging the Venus surface from orbit with conventional cameras extremely difficult. And so Pioneer Venus, Magellan, and the Soviet Venera 15 and 16 missions carried radar instruments, together unveiling Venus as an Earth-sized world quite unlike Earth itself. These missions found no evidence for plate tectonics, the primary way our planet regulates its temperature through geological time via the carbon-silicate cycle. But scientists recognized myriad features from Earth and elsewhere, including huge volcanoes, long lava flows, and an astonishing array of structures attesting to a sustained record of tectonic activity (Figure 2-9). Venus may not have been found to look like Earth, but it clearly had a detailed active geological story all its own.

Indeed, when Magellan acquired almost full global radar coverage of the surface (albeit at a resolution comparable to what the Viking orbiters returned for Mars in the 1970s), one type of feature was conspicuously absent: large impact craters. The planet has fewer than 1,000 craters of any size, and only one greater than 200 km across—a far cry from the hundreds of craters of such size on Mercury, the Moon, or Mars. Our best explanation for this unexpected dearth of large craters? Major volcanic resurfacing that served to bury the large impact craters and basins that were once presumably on the surface. Whether this resurfacing occurred in one catastrophic event or in phases remains unknown, but our best estimate is that the average age of the Venus surface is no more than a few hundred million years.

In addition to the volcanoes and lava flows we recognize across Venus, the planet also hosts an unusual type of landform that is both volcanic and tectonic in nature: the enigmatic corona. Approximately circular features often accompanied by considerable fracturing of the surrounding plains, coronae are sites where upwelling magma from deep in the planet’s interior is thought to impinge on the crust. Some of that material might even return to the interior in a form of local subduction not dissimilar to how tectonic plates are recycled on Earth. Recent computer modeling suggests that at least some of Venus’s ~500 coronae are actively forming, and deforming, today.

Venus offers us other hints of ongoing volcanic activity. ESA’s Venus Express spacecraft, which orbited the planet from 2006 until 2015, found with its infrared spectrometer that some lavas seem to have barely been weathered—consistent with their having erupted geologically recently. That same mission saw short-lived, highly localized increases in surface temperature in regions of the planet interpreted with Magellan data to be where the crust has pulled apart; some of the most concentrated areas of volcanic activity on Earth are in such rift zones. And Pioneer Venus recorded a dramatic reduction in the abundance at very high altitudes of sulfur dioxide over

the 10 years after it made orbit in 1978, as if shortly before the spacecraft arrived a major volcanic eruption had injected a plume rich in that gas high into the atmosphere, which then slowly dissipated.

What of tectonic activity? The surface abounds with fractures, some of which cross the geologically youngest lava plains. Given how widespread apparently well-preserved structures are across Venus (and the planet’s sheer size), it would be a surprise were the planet not tectonically active today. But there once may have been even greater tectonic action: a region high in the north, called Lakshmi Planum, is ringed by mountains that bear more than a passing resemblance to those formed when India collided with Asia. Recent mapping with decades-old Magellan imagery has also found indications that some of the oldest terrains on Venus look like they have been pieced together, and that portions of the planet’s lowlands have recently jostled and moved like pack ice. Together, these observations strongly point to major horizontal motion of the Venus surface at some point since major volcanic burial of the crust began. Both the VERITAS and EnVision orbiter missions will search for evidence of ongoing volcanic and tectonic activity from space and, with an atmospheric entry probe, the DAVINCI mission will look for chemical signatures of volcanism in the atmosphere itself.

Another mystery of Venus is the nature of the planet’s “tesserae,” the oldest and most highly deformed rocks on the planet. Tesserae have no obvious counterpart on any other rocky body except, perhaps, heavily tectonically deformed continental rocks on Earth. Indeed, geophysical data from Magellan suggested that those tesserae that constitute the planet’s highlands correspond to regions of notably thicker crust, drawing parallels between this terrain and correspondingly thick portions of crust on Earth, including continental crust. The formation of continental crust is thought to require the presence of both oceans and plate tectonics—two properties absent on Venus, at least today. Recently, layering has been documented within parts of several tessera exposures; this layering resembles that formed by massive stacks of lava in vast volcanic regions termed large igneous provinces on Earth but is also consistent with sedimentary rocks—which do not form under present climate conditions on Venus.

Indeed, the details of what exactly the surface of Venus is actually made of still remain largely unknown 40 years after the last visit by the Soviet Venera and Vega landers, the only missions to obtain quantitative chemical measurements and photographs of venusian rocks. These landers were technological marvels of their time, but the chemical data they provided were hardly comprehensive, and they made no measurements of mineralogy. Even so, what measurements they did take were enough to show that much of the surface is weathered basalt—consistent with interpretations made with orbital radar data that the expansive plains occupying almost four-fifths of the

planet surface are basaltic lavas. The images returned of those landing sites, at a resolution very much finer than the relatively coarse Magellan radar imagery, also evoke an alien landscape we barely understand (Figure 2-10).

Establishing what materials make up Venus is key to understanding the planet’s formation and evolution: the starting composition of the planet has enormous bearing on the types of rocks we might expect to find on the surface. Modern advances in instrumentation technology hold the promise of incredible breakthroughs for Venus and planetary science by carrying out in situ sampling and analysis of Venus surface materials, both in the planet’s vast volcanic plains and in the tesserae. Just as in situ measurements of rocks by the Spirit, Opportunity, Curiosity, and Perseverance rovers have dramatically expanded our understanding of Mars’s geological character and history, performing such analyses at the Venus surface would rewrite the textbooks about the second planet.

Until we deploy new-generation instruments to the Venus surface, we can take advantage of several “windows” at infrared wavelengths through which an appropriately configured instrument can see all the way to the surface from orbit through the otherwise opaque atmosphere. The Venus Express spacecraft was able to take infrared (IR) measurements of emissivity—the amount of IR radiation emitted by surface materials—of some of the southern hemisphere through one such window. Laboratory work has shown that it is possible to distinguish iron-rich basaltic rocks from iron-poor, silica-rich rocks on the surface, based on their infrared emissivity. Constructing a global map of the distribution of these rock types would represent a major advance in understanding the geological and chemical properties of the planet—which is something both the VERITAS and EnVision missions plan to do. Venus Express found that the largest tessera exposure in the south, Alpha Regio, has a markedly lower emissivity than the surrounding basaltic plains, indicative of it having less iron and more silica than those lavas. There are numerous explanations for this finding, but one intriguing possibility is that Alpha Regio represents material akin to continental crust on Earth—which is marked by relatively low iron and high silica abundances. Excitingly, Alpha Regio is the ultimate destination of DAVINCI’s atmospheric entry probe, which will take detailed, close-up images of this enigmatic landscape as it nears the end of its hour-long plunge through the atmosphere.

Rocks at the Venus surface are chemically weathered by the punishing atmospheric conditions there. Those high-temperature, high-pressure surface conditions are unique among the solar system’s rocky planets and, unsurprisingly, are challenging to replicate on Earth. But some experimental facilities able to simulate these conditions

have come into operation in the past decade, allowing for the study of these weathering processes. Limited by the few constraints we actually have for rocks on the Venus surface, and by even less information about the types of surface-atmospheric interactions that weather rocks there, such studies are nonetheless valuable. For example, laboratory results in the past few years have raised the possibility that mineral grains might weather much faster than had been thought—suggesting that those lightly weathered lava flows seen by Venus Express could be very young indeed. Expanding our capabilities to simulate Venus’s surface conditions in the lab would substantially increase understanding of the geological and atmospheric processes in this relatively unexplored part of the solar system.

Just as for the surface, major questions regarding the Venus interior remain. Presumably Venus, like Earth, has a metallic core, silicate mantle, and a rocky crust (although we do not know even the size of that core: we have to extrapolate our estimates from Earth). Unlike Earth, however, Venus has no strong magnetosphere; Pioneer Venus found that any internally generated magnetic field is at least one hundred thousand times weaker than that of Earth today. And no magnetometer-equipped mission has flown sufficiently close to or landed on the planet to determine if the crust preserves any record of an ancient Venus dynamo. Establishing whether the planet once generated its own magnetic field would help us understand if such a field really is a necessity for a large rocky planet to hold on to its atmosphere, with major implications for our broader understanding of planetary habitability.

A key tool for understanding planetary interiors is seismology, enabled on Earth in no small part because of the large-magnitude quakes here. Whether there are quakes on Venus large enough to probe the interior is an open question. But could we use those putative venusquakes to assess the interior, search for a hypothesized deep molten layer within the planet, or establish whether there is an inner core? Our present capabilities face severe limits to the length of time a seismic station could operate on the infernal surface, but encouraging technological developments promise at least the possibility of long-lived landers operating on Venus, performing not just geophysical investigations but geochemical and atmospheric studies, too. And by taking advantage of just how strongly coupled the thick, lower atmosphere is to the ground—such that seismicity in the crust is transmitted through the Venus air 60 times more effectively than on Earth—it may well be possible to search for seismic signals from balloon-borne instruments, complemented by observations of how the nightside upper atmosphere ripples as it conducts seismic waves.

An Alien Sky … and a Warning?

Among the most striking aspects of Venus is its planet-encompassing layer of sulfuric acid clouds, which reflects 70 percent of incoming sunlight and makes the planet appear so bright in our morning and evening skies. Those clouds are situated in the middle Venus atmosphere, where conditions are relatively hospitable: a balloon-based platform could operate there for weeks or even months. Indeed, the only natural shirt-sleeve environment in the solar system beyond Earth is at about 55 km up in the Venus atmosphere—a fact the Soviet Vega 1 and 2 missions took advantage of in 1985 when deploying two balloons into the middle atmosphere. Those aerial platforms beamed information on temperature, pressure, winds, and aerosols directly to Earth as they traveled a third of the planet’s circumference from night to day (Figure 2-11).

Even so, we still know remarkably little about the Venus atmosphere. We have the basics: the main cloud layer on Venus consists mainly of droplets of sulfuric acid, together with small particles, possibly of sulfur, and an unknown absorber of ultraviolet light that shares some properties with a photosynthetic pigment. Water vapor is now only a minor component in the Venus atmosphere, and horizontal temperature variations are small. The general circulation of the atmosphere is dominated by super-rotation whereby the upper atmosphere rotates much faster than the solid planet itself, wind speed generally decreases with latitude but increases with altitude, and there are major vortices at the poles. At high altitudes, winds blow from the dayside to the nightside.

Yet several of the main features of the Venus atmosphere remain unexplained. How circulation patterns change with altitude and latitude is not clear, as is why the atmosphere super-rotates in the first place. Solar and thermal radiation play a dominant role in many of the processes that define the Venus climate, including the large greenhouse effect resulting from the abundance of carbon dioxide and other atmospheric absorbers. But this effect is despite the lower atmosphere receiving less solar energy than Earth: half of the solar flux received by Venus is absorbed in the cloud top region, with only a few percent reaching the surface (leading to a hazy surface environment a Soviet optics engineer once compared to “a cloudy day in Moscow”). In short, we do not fully understand how and where solar energy is absorbed and redistributed within the planet’s atmosphere.

Surprisingly few details of the chemistry of the Venus atmosphere are known. Key chemical interactions include those between sulfur, nitrogen, hydrogen, and oxygen, driven by solar radiation (and possibly by lightning). Reactions between atmospheric gases and sunlight in the upper atmosphere give way to high-temperature chemical processes in the lower atmosphere, and to poorly understood rock-atmosphere interactions at the surface. But the size distribution, shape, and composition of the majority of the clouds are still undetermined. Other such mysteries abound, including whether organic chemistry in Venus’s atmosphere can produce enough nutrients to support an aerial biosphere—a decades-old focus of speculation based on the relatively clement conditions in the middle atmosphere. This question in particular is all the more pertinent given the tentative, if contested, detection of phosphine in the Venus atmosphere reported in 2020.

The chemical makeup of the atmosphere offers us another critical piece of the Venus puzzle: the geological history of the solid planet itself. The abundances of the heavy noble gases xenon, krypton, and argon (and their isotopes)—measurements of which form a key objective of the DAVINCI atmospheric probe—directly trace to the history of volcanic degassing of the Venus interior, and even the very components from which the planet was built. And, of course, measurements of sulfur dioxide in the atmosphere, and particularly how the abundance of that gas changes with time, would place a firm constraint on estimates of ongoing volcanic activity.

How the upper atmosphere interacts with Venus’s space environment needs to be better described. Early missions, including Mariner 5, the Soviet Venera 9 and 10 orbiters, and Pioneer Venus characterized some of this interaction, establishing the composition of the upper atmosphere and ionosphere, and documenting interactions between the ionosphere and the solar wind (the flowing, magnetized, and ionized tenuous interplanetary medium emanating from the Sun). Another discovery was the magnetization of the Venus upper atmosphere by electrical currents induced by the solar wind—a finding that some magnetic fields are present at the planet, just not any intrinsic to the planet’s interior.

The Venus Express mission found that the planet may be losing more oxygen to space than had been thought. On the other hand, the loss of yet heavier elements was lower than expected, which has implications for the evolution of the overall atmosphere. But we have still to answer such key questions as, say, the way “space weather” associated with coronal mass ejections from the Sun affect how much atmosphere Venus loses to space. Equally, we do not know how deep those magnetic fields induced by the solar wind penetrate into the atmosphere, nor whether the fields can move around the planet. And what of the history of atmospheric loss through time? Because the young Sun might

have been much more efficient at stripping away atmosphere than at present, this question is especially important to our efforts to piece together the ancient history of the Venus climate.

Excitingly, it is this climate history that we have recently begun to rethink. Sophisticated climate simulations have raised the possibility that the Sun was not the main driver of Venus’s present surface and atmospheric conditions. It seems, instead, that either Venus was always wholly inhospitable—with enormous consequences for our understanding of how large rocky planets develop in general—or it hosted a clement climate, oceans, and perhaps even plate tectonics for more than three billion years … until it ran out of luck.

That is because these climate simulations suggest that gigantic volcanic eruptions—of the kind that injected thousands of gigatons of carbon dioxide into the atmosphere in the late Permian on Earth to trigger the largest mass extinction in our planet’s history—may have been responsible for dramatically and abruptly changing the Venus climate. But, whereas one such event on its own might not have dealt the climate a fatal blow, several enormous simultaneous eruptions could have been enough to overwhelm whatever mechanism(s) Venus used to regulate its climate to that point. With a sudden, rapid increase in the volume of carbon dioxide in the atmosphere, perhaps as geologically recent as ~1 Ga, that potent greenhouse gas raised the surface temperature and, ultimately, may have driven off the planet’s putative oceans.

The DAVINCI probe will start to tackle this question, which carries with it a cautionary tale. Huge volcanic eruptions—of the type discussed here, the type that forms what we call on Earth “large igneous provinces”—have taken place throughout the history of our own planet, albeit infrequently. But the factors controlling such eruptions are not well understood for Earth, much less for another planet. It is not clear, therefore, whether Venus was unlucky to have experienced several calamitous events at the same time … or if Earth is lucky that it has not.

The Exoplanet in Our Backyard

Over the past few decades, planetary science has been revolutionized with the discovery of more than 4,000 confirmed extrasolar planets of a variety of sizes and masses extending beyond that of the solar system’s inventory—enabled in large part by the Kepler mission and the Transiting Exoplanet Survey Satellite. Yet many of these exoplanets may be more Venus-like than Earth-like because our current observation techniques are biased toward detecting and observing planets relatively close to their stars. The James Webb Space Telescope (JWST) will soon observe high-priority exoplanet targets, but the vast distances involved mean that even a telescope as powerful as JWST will still acquire sparse and noisy exoplanet data. Here, we can leverage our decades of solar system insights to anchor and validate the models we will need to make sense of our exoplanet observations.

Venus has a particularly critical role to play in guiding our interpretations of exoplanet data, because although Venus-analog worlds will likely be one of the most common classes of planets observed by Webb, they may also be one of the most difficult types of world to interpret correctly if they, too, possess a global cloud layer that blocks the bulk of the atmosphere from view. To understand the chemical signatures of gases and aerosols detected in the atmospheres of those planets, then, we will need to understand the atmospheric composition and chemical cycles on Venus itself—our knowledge of which is, at present, woefully incomplete.

Venus may even have an important role to play in the search for life beyond the solar system. If we establish that Venus once had oceans, then we will need to redefine the traditional boundaries separating potentially habitable worlds from lifeless solely ones on the basis of distance from the host star. Understanding the processes that enabled—or did not enable—early habitability on Venus, and how those conditions were lost, will surely expand—or contract—the regions we regard as suitable locales for habitable worlds as we observe exoplanet systems with diverse architectures and at various ages and evolutionary stages.

And Venus offers us the opportunity to learn how to distinguish certain signs of life—biosignatures—from abiotic false positives on exoplanets. Considerable recent attention has been devoted to understanding false positives for oxygen, a critical biosignature for modern-Earth-like exoplanets, and we could use Venus to better understand mechanisms that form and destroy oxygen abiotically. The possible loss of oceans in an earlier epoch on Venus has been invoked as a mechanism to generate large quantities of oxygen on exoplanets that endure the same fate. In fact, small quantities of abiotic oxygen are generated on Venus today through atmospheric chemical processes, but the details of this process are poorly understood and could lead to dramatic consequences in other chemical contexts. Establishing the processes that remove the oxygen generated by these mechanisms would place valuable bounds on how much we might expect to find abiotically in exoplanet atmospheres. Desiccated, scorched Venus may seem the last place to look for guidance in the search for life beyond Earth, but we would be foolish to ignore the lessons it can teach us.

The Case for Continued Exploration of Venus

Venus is once again back in the spotlight. In the coming years and decades, we will continue to develop the recently emerging view of Venus as a complex, active world, augmented by new spacecraft data, ever more sophisticated climate modeling, and the finding of an increasing number of Venus-size rocky worlds in close proximity to their host stars. And with more and more attention focused on understanding and mitigating human-driven climate change on Earth, Venus’s runaway greenhouse provides a dramatic example of a planet where self-regulating climate feedbacks have failed.

What we have also learned over the past decade in particular is that if we are to fully understand Venus, then we need to study it as a system. No one set of measurements will solve the mystery that is Venus, just as no one mission has answered all our questions of Mars, nor of the Moon, nor any planetary body. But as important as new measurements of the close coupling between Venus’s surface, atmosphere, and space environment are, it is just as important that we have sufficient laboratory data to predict, calibrate, and make sense of such measurements as well.

The data we will acquire from the VERITAS, DAVINCI, and EnVision missions at the end of this decade will fundamentally alter our understanding of the planet as surely as those early Venus missions did, 6 decades ago. Yet even with the discoveries that await those missions, compelling science questions remain—such as those regarding the chemical and physical cycles of the atmosphere, the interactions between that atmosphere and the surface, and the make-up and structure of the planet itself. Studying Venus past and present is critical for comprehending the second planet in its own right; for comparing the divergent evolutionary paths of Venus, Earth, and Mars; for gaining context for similarly sized exoplanets; and simply for understanding the rules that govern Earth-like worlds in general. Learning why Venus took the path it did—why our sibling is not our twin—will tell us whether we are lucky that the sky over our heads is blue, and not yellow.

Key Discoveries from the Past Decade

• Venus is likely a geologically active planet today. Several lines of evidence together suggest that volcanic activity takes place today on Venus, and the planet’s record of tectonic deformation speaks to recent and perhaps even ongoing deformation.
• Models show that the young surface is consistent with Venus being habitable for billions of years. New climate models indicate that Venus could have had modern-Earth-like conditions until as geologically recently as about a billion years ago, before entering a runaway greenhouse driven by several simultaneous, major volcanic eruptions.
• Exoplanet discoveries motivate renewed Venus exploration. The ongoing detection of large, rocky exoplanets close to their host stars, especially those that are amenable to having their atmospheres characterized, increasingly requires that we better understand the atmospheric properties and climate history of the second planet.

Further Reading

Glaze, L.S., C.F. Wilson, L.V. Zasova, M. Nakamura, and S. Limaye. 2018. “Future of Venus Research and Exploration.” Space Science Reviews 214:89. https://doi.org/10.1007/s11214-018-0528-z.

Kane, S.R., G. Arney, D. Crisp, S. Domagal-Goldman, L.S. Glaze, C. Goldblatt, D. Grinspoon, et al. 2019. “Venus as a Laboratory for Exoplanetary Science.” Journal of Geophysical Research: Planets 124:2015–2028. https://doi.org/10.1029/2019JE005939.

Marcq, E., F.P. Mills, C.D. Parkinson, and A.C. Vandaele. 2018. “Composition and Chemistry of the Neutral Atmosphere of Venus.” Space Science Reviews 214:10. https://doi.org/10.1007/s11214-017-0438-5.

Smrekar, S.E., A. Davaille, and C. Sotin. 2018. “Venus Interior Structure and Dynamics.” Space Science Reviews 214:88. https://doi.org/10.1007/s11214-018-0518-1.

Way, M.J., and A.D. Del Genio. 2020. “Venusian Habitable Climate Scenarios: Modeling Venus Through Time and Applications to Slowly Rotating Venus-Like Exoplanets. Journal of Geophysical Research: Planets 125:e2019JE006276. https://doi.org/10.1029/2019JE006276.

MARS

Significant advances in understanding Mars as a system have been made during the previous decade since Vision and Voyages (NRC 2011). These advances resulted from a dedicated and coordinated program of exploration involving in situ laboratories on rovers at carefully selected sites, and measurements of a dynamic surface and atmosphere that link the past and the present, and inform investigation of Mars’s subsurface. Each new measurement—from spacecraft, Earth-based telescopic observations, and laboratory analysis of rocks from Mars that have made their way to Earth as meteorites, combined with detailed analysis and interpretation—has contributed to understanding different aspects of Mars. Vision and Voyages identified three areas in which Mars exploration would provide fundamental new insights:

1. Determine whether life ever arose or existed on Mars.
2. Understand the processes that control weather and climate and the long-term evolution of climate and habitability.
3. Decipher the evolution of the surface and interior and the processes that control them.

These goals were expected to guide exploration for longer than a decade. None of these goals can be addressed fully in isolation from the others; Mars has the complexity of Earth where interior, surface, and atmospheric processes along with solar and impact processes combine to create climate and environments that have changed profoundly over time. Mars’s record of solar-system history is unique and special. Moreover, Mars also provides potential records of prebiotic chemistry and of biosignatures. It is the only rocky planet with an atmosphere where a complete 4-billion-year-plus record of its history sits intact, ready for exploration; even Earth does not have such a detailed record for its entire history. Thus, an independent origin of life on Mars is more testable than life’s origin on Earth because the martian geological record is preserved. Understanding the trajectory of planetary climate and habitability over long timescales—the effects of a brightening Sun, large impacts, early volcanism, and large-scale, long-term climate change—can be traced on Mars in a way that allows comparison with the history of Earth and extrapolation to planets around other stars.

Collectively over the past decade, new observations, measurements, and models have dramatically changed our views of the evolution of the martian surface, interior, atmosphere, and climate, and of the potential for past surface life, or potentially still extant subsurface life. They reveal the history of the planet from 4.5 Ga to the present, a long and complex story of changing climate and the availability of liquid water, and previously unrecognized dramatic changes at the poles even in recent times. These scientific findings have set us up for the coming decade, in which we expect to return to Earth samples from a location chosen for its relevance to the history of water and its potential to have harbored life in the past.

Through Mars’s preservation of a record of its entire history, we can examine the interplay between processes from the deep interior to the upper atmosphere to understand what controls the fate of habitable worlds. Our understanding of Mars feeds into an understanding of terrestrial planets in the solar system and of the possible evolutionary paths of terrestrial planets around other stars (many of which are thought to have undergone extensive loss of their atmospheres and water to space over time analogous to Mars). Here, the committee summarizes some of the major results of the past decade under the same three headings as called out in Vision and Voyages. This summary can only begin to scratch the surface on specific results and identify only some of the interconnections between different components of the Mars environmental system.

Did Life Ever Arise on Mars?

New discoveries in planetary sciences and astrobiology have advanced our understanding of key factors that influence whether Mars was habitable in the distant past or might offer habitable refuges at the present day: the availability of liquid water, organic (prebiotic) compounds, and energy sources. These new findings provide multiple recommended lines of investigation for the coming decade as outlined in An Astrobiology Strategy for the Search for Life in the Universe (NASEM 2019) and represent considerable progress over a decade ago, when spacecraft observations had revealed evidence of past water on Mars but the best-characterized environments appeared too chemically harsh to support life as we know it.

Preserved Organic Compounds

A groundbreaking discovery of the decade relating to the possibility of past life on Mars was the detection of organic matter in the lake sediments of Gale Crater by the Curiosity rover’s SAM instrument (Figure 2-12). The characterization of ringed and straight-chain hydrocarbons and their chlorinated and sulfurized forms, together with the indirect detection of organic matter through chemical analyses of carbon, represents a key milestone in Mars exploration. The refractory macromolecular compounds point to relatively large molecules that are hard to destroy, similar to kerogens in Earth’s geologic record. This is not a discovery of past life; rather, the diversity and composition of these compounds are consistent with large, complex organic molecules. Although the origin of the organic molecules is undetermined, their discovery sets the stage for more detailed characterization and astrobiological investigations to come. The discovery also suggests that organics are an important part of an active martian carbon cycle. Even at the planet’s surface, organic compounds have not been entirely destroyed by radiation or oxidation chemistry, so greater organic abundances might be found beneath the surface.

In parallel with mission exploration, organic compounds in martian meteorites have been analyzed and their structures and isotopic compositions have been characterized in new ways. Laboratory analytical results from examined meteorites (all igneous rocks) conclude that organics trapped in martian meteorites represent a combination

of infall of exogenic carbonaceous materials and abiotic synthesis on Mars during water–rock reactions and magmatic processes. Although meteorite results so far have not found any organic biosignatures, they support the widespread occurrence and preservation of prebiotic compounds on Mars.

Liquid Water Stability and Accessibility

New discoveries offer a way that liquid water can exist on present-day Mars. In general, pure liquid water is not stable at the surface today, owing to average temperatures below its freezing point and low atmospheric pressure; water would either freeze or evaporate quickly. Liquid water in the martian subsurface was reported, albeit debated, for the base of the polar caps using data from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument on the Mars Express spacecraft. On the planet’s surface, liquid water can exist as a transient phase. At the surface, perchlorate minerals have been discovered in soils. Perchlorates are deliquescent, meaning that they can absorb water vapor from the atmosphere and dissolve in it as a liquid; thus, under conditions that occur within the top meter of the surface, films of liquid water can be stable for a large fraction of a sol (a martian day), for certain times of the year over parts of the planet. Temperature data from the Curiosity rover and its measurements of salt abundances showed the possibility of such a mechanism creating habitats in the near surface.

Recurring slope lineae, recognized in 2010, give the appearance of having been formed by water flowing downhill from beneath or on the surface. As small-scale features (meters to tens of meters across), they form and disappear over the course of martian seasons, changing their appearance most rapidly at seasonally warmer temperatures. New analyses and models make it unclear whether these features are related to liquid water or not; they may form from transient water near the surface, from dust avalanching caused by deliquescing salts, or from dry avalanches. If they are related to liquid water, they are telling us something important about water’s availability at or near the surface.

Long-Term Habitable Environments at Gale Crater and Elsewhere on Mars

Exploration of Gale Crater by the Curiosity rover has provided definitive evidence that water flowed and ponded on the surface of Mars billions of years ago. Sedimentary processes that took place over thousands to millions of years indicate that liquid water, although perhaps not necessarily present continuously, was stable and available for considerable periods of time on the planet’s surface. A series of neutral-to-alkaline lakes spanning many square kilometers was followed by even longer-lived episodic groundwaters, occurring hundreds of millions of years later (Figure 2-12). The extensive deposits identified in Gale Crater provide evidence for water, organic carbon, and a chemical source of energy for potential microbial metabolism. The inferred aqueous environment featured fluctuations in the saltiness of water and in redox states. In addition, later overprinting by diagenetic events suggests that habitability could have been sustained in the subsurface for millions of years. This remarkable set of discoveries demonstrates there were long-lived habitats on Mars and that Earth-like life could have inhabited this site.

At the time of the last decadal survey (NRC 2011), data from the Mars Reconnaissance Orbiter’s Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument had documented a diversity of ancient environments with liquid water, including lakes, rivers, and hydrothermal systems. Key discoveries on this front continued into this decade, revealing specific craters with hydrothermal systems associated with lakes, excavations of deeply buried carbonate deposits, widespread hydrous minerals excavated by craters even below the volcanic northern plains, and thick weathering sequences of clay minerals. Collectively, the data indicate that habitable environments were likely widespread across ancient Mars. Only a subset of such habitats has so far been explored. Dozens of studies pinpointed the Jezero crater-Nili Planum system that was subsequently selected for samples to be cached by the Perseverance rover and eventually returned to Earth. This site encompasses lakebed clays and carbonates, a watershed with strata preserving effects of a large basin-forming impact, early volcanic rocks, and mineralized veins of a hydrothermal groundwater system.

Processes and History of Martian Climate

The martian atmosphere has changed on timescales ranging from seconds to seasons to years to billions of years. Determining the evolution of the atmosphere and climate is necessary to understand the time-varying availability of liquid water, its influence on the geology and geochemistry of the surface, and the potential to support life.

Seasonal Behavior of the Atmosphere

The seasonal and annual behavior of weather in today’s lower atmosphere is dominated by the seasonal cycles of dust, water vapor, and CO₂. Although we have known about dust storms for over a century, improved observations and continuous monitoring over the past decade have resulted in greater understanding of how they begin and evolve, and of annual patterns in when and where they occur. Global and large regional dust storms in 2018 became the most comprehensively studied to date, thanks to concurrent observations made by multiple orbiters and two landed missions. At the same time, advances in atmospheric modeling have led to increased realism in simulated storms. Absent a detailed understanding of today’s dust storms and the complex coupling to water and CO₂ behavior, extrapolation to other epochs and determination of their long-term impact remains difficult.

Dust behavior in the lower atmosphere affects the dynamics and composition of the upper atmosphere via lower-atmosphere heating, expansion, and the modification of vertically propagating waves. For example, the hydrogen abundance in the extended exosphere previously was thought to be relatively constant, owing to the long timescales of its production from water and its thermal escape to space. New observations, however, show an order-of-magnitude variation in hydrogen abundance, occurring in the seasons in which the atmosphere is dustiest. The mechanism for connecting them, involving heating of the lower atmosphere resulting in changes in atmospheric structure and circulation that enhance hydrogen escape, shows the strong coupling between different components of the atmosphere and provides a mechanism by which short-term weather can affect the long-term evolution of water.

The presence and behavior of methane in today’s atmosphere is important as a potential indicator of either ongoing geological or biological activity. In the past decade, Curiosity’s Sample Analysis at Mars (SAM) instrument has identified methane at very low abundances that shows a possible seasonal cycle, suggesting thermally controlled release from the regolith, in addition to occasional much higher methane abundances that are very short-lived. Observations from the ESA-Russian Trace Gas Orbiter (TGO) have not detected methane at all, with upper limits well below the abundances measured by SAM. Although TGO instruments cannot see methane that might be in the lowest scale height of the atmosphere, rapid vertical mixing should prevent methane from staying trapped there for long. For these spacecraft measurements to be consistent with each other thus requires either a very localized source in Gale Crater or a trapping mechanism (via chemical and/or atmospheric dynamical process) that has not yet been identified. Getting to the bottom of the Mars methane mystery and the surface-subsurface processes controlling the methane cycle remains an important goal for future investigations.

Polar and Non-Polar Ice Forcing by Obliquity Oscillations

Mars’s climate is driven partly by the 10,000- to 1,000,000-year variations in the tilt of the planet’s spin axis (axial obliquity), which changes the amount of sunlight received by the poles and the resulting rates of ice loss and accumulation. Estimates based on study of small impact craters identified on the ice show that the north-polar ice deposits, which are many kilometers thick and contain the equivalent of a global layer of water some 20 meters thick, might only have formed around 5 Ma and might be only a recent (and possibly ephemeral) phenomenon. Radar results from the Mars Shallow Radar (SHARAD) instrument on Mars Reconnaissance Orbiter have supported this interpretation based on large-scale growth and retreat cycles for the ice.

Massive deposits of CO₂ ice have been identified within the south-polar cap using radar data. These layers presumably represent atmospheric CO₂ that has condensed to form ice at lower obliquity values and that can be released into the atmosphere at higher values. Estimates of the volume of ice show that its release would more than double the martian atmospheric pressure. If correct, then the observed layers in the polar ice deposits would reflect variations in deposition on much shorter timescales than previously thought; modeling based on the obliquity variations over the past half-million years can successfully replicate some of the observed layer thicknesses. Most importantly, these results demonstrate that the present-day atmospheric pressure may be significantly lower than the average value over the past half-million years or even longer.

A key discovery in the climate history puzzle was the presence of massive mid-latitude deposits of water ice (Figure 2-13). These deposits occur as relatively clean water ice, standing up to hundreds of meters thick and buried by a thin overburden of dust or debris. These deposits recently have been determined to be extensive, based on both

high-resolution imaging (from the High-Resolution Imaging Science Experiment (HiRISE)) and radar measurements (from SHARAD). When these ice deposits were emplaced, whether they represent a one-time deposition or a cyclical deposition and removal associated perhaps with obliquity cycles, and what is the total integrated amount of water they contain are uncertain. However, they certainly represent a source that could produce at least transient liquid water that might be able to support microbial life under some conditions, as well as a potential resource for future human missions to Mars.

Drivers of Climate Change over Billions of Years

Significant progress has been made in the past decade on understanding the mechanism for producing a change from an early, warmer and wetter environment to the cold, dry Mars that we see today. Evidence has long pointed to a very different early martian climate—geological and geochemical data both require the occurrence of liquid water in much greater abundance or for longer periods of time prior to about 3.5 Ga, as compared to the present climate in which liquid water can exist only transiently. Although it has been widely accepted that an early, thicker greenhouse atmosphere contributed to the warming, early models had been unable to produce adequate heating. Recent advances in three-dimensional climate modeling have confirmed that CO₂ and H₂O alone are insufficient to warm early Mars, even with a much thicker early atmosphere. However, two recent approaches have demonstrated the viability of greenhouse models. First, it was recognized that the early water cycle would have been dramatically different when the atmosphere was thicker, owing to the cooling of elevated terrain as occurs on present-day Earth;

ice could deposit at higher altitudes and provide a source for liquid water. Second, recent modeling and laboratory experiments have shown that greenhouse warming from a combination of CO₂ and reducing gases such as hydrogen or methane could have warmed early Mars. Reducing gases could have been produced from volcanism, meteoroid impacts or crustal-alteration, water–rock reaction processes. The occurrence of these gases appears to be consistent with the presence of mineral phases identified by the Curiosity rover that require a reducing environment (although the occurrence of both oxidized and reduced minerals in close proximity complicates the potential interplay between oxidizing and reducing processes). Our growing understanding of Mars’s early atmosphere will have important implications for astrobiology.

What caused the climate to change from one that was at least intermittently warmer to today’s cold and dry environment? Impacts that could cause transient warming, as well as volcanism that could intermittently release reducing gases, were declining from 4.0 to 3.5 Ga. At the same time, the Sun and solar wind were stripping the early, thicker atmosphere to space. Measurements by the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft allow us to understand how stripping of the atmosphere by the Sun operates today, involving breaking apart of H₂O and CO₂ molecules by ultraviolet photons from the Sun and the escape of the individual atoms to space through multiple processes (Figure 2-14). These processes would have been more effective early in Mars’s history when the solar ultraviolet radiation and solar wind were more intense, and we can extrapolate the loss to those earlier times using the derived history of the Sun. The rate of loss of gas to space today is relatively low planetwide; it may have been as much as 10,000 times greater 4 billion years ago.

In addition to loss to space, formation of carbonates within the crust from atmospheric CO₂ and hydration of subsurface minerals by H₂O removed substantial quantities of each. Combined, loss to space, loss of H₂O and CO₂ to the crust, and sequestration in the polar caps can explain the removal of an early thicker atmosphere and accessible water, and thereby explain the transition inferred for climate. The results suggest that loss of both H₂O and CO₂ to space were major factors in the evolution of the atmosphere and climate on early Mars.

Understanding the history of climate and the availability of liquid water requires understanding the complex interplay between processes occurring on all timescales. New discoveries in the past decade have dramatically changed our view of how these processes work, and new and anticipated observations will help us to see how they actually played out through time.

Evolution of the Martian Surface and Interior

Geologic findings in the past decade have significantly altered our understanding of the first billion years of martian history, as well as revealed active surface and subsurface processes on modern-day Mars.

New Discoveries from Very Old Rocks

The ancient igneous crust of Mars is now recognized to be much more compositionally complex than previously realized. The Curiosity rover found volcanic rocks of unusual compositions shed from the rim of Gale Crater, and laboratory analyses of the Northwest Africa (NWA) 7034 meteorite, first described in 2013, extended the occurrence of similar volcanic rocks back into the earliest period of Mars history. The magmas that formed these rocks indicate geochemical evolution in the first billion years of Mars history was distinct from that of more recent times.

Some ancient rock units previously recognized as volcanic have now been reinterpreted as cemented sedimentary rocks. Curiosity found unaltered olivine in a mudstone, indicating minimal chemical weathering in some sedimentary rocks. Mineral sorting and fractionation during transport by water or wind has resulted in significant geochemical variations across Mars that were previously attributed to chemical weathering. Curiosity also identified sedimentary rocks that contain pieces of earlier sedimentary rocks, requiring rock-forming processes to have occurred over long timescales. The occurrence of sedimentary rocks across Mars indicates that the planet possesses a relatively complete temporal record of surface geologic processes, although the incomplete chemical weathering in many locations may be owing to intermittency of waters or cold temperatures when waters were present.

Near the end of its operational life, the Opportunity rover explored Endeavour Crater, a 22-km-diameter impact structure that excavated and redeposited Noachian strata. Prior to this time, Mars rovers (including Opportunity) had only explored younger Hesperian terrains, so this foray provided the first chance to analyze the oldest rocks yet encountered on the martian surface. The lower units of the crater rim stratigraphy represent ejecta from impacts that predate the Endeavour impact, and superposed layers represent breccias (cemented rock fragments) formed during the impact itself. The compositions of the breccias are basaltic, and chemical alteration of the rocks by aqueous fluids produced clay minerals and crosscutting vein minerals that vary with the age of the units. This investigation confirms interpretations from orbital data that the Noachian was a time characterized by basaltic volcanism, large impacts, and intense hydrothermal activity.

Although some Pre-Noachian rocks have been recognized from orbital observations, our understanding of this earliest period of Mars history has been very limited. However, the NWA 7034 meteorite (and paired stones from the same fall) have radiometric ages of 4.4 billion years, placing them deep within this time period (Figure 2-15). These samples are regolith breccias—cemented soils consisting mostly of fragments of basaltic igneous rocks that comprised the early crust. Research on these meteorites in the years since their recovery provides unique insights into the earliest evolution of the planet.

Global differentiation into core, mantle, and crust represents the most profound geologic process that affected the terrestrial planets. Analyses of radiogenic isotopes in NWA 7034 provide time constraints on the rapid differentiation of Mars, as well as rule out a mantle formed by simple extraction of crustal magmas and instead appear to require an early magma ocean. The origin of magnetized rocks in the ancient crust of Mars has been a puzzle since their discovery. NWA 7034 is magnetic and contains an unusual assemblage of iron minerals that formed by hydrothermal alteration, so a similar subsurface alteration process could account for at least some of the magnetized crustal rocks mapped from orbit. Trace elements and oxygen isotopes in mineral grains of the meteorite further confirm that aqueous fluids circulated in the earliest martian crust. Understanding the origin and evolution of the martian atmosphere is critical to interpreting paleoclimates, and stable isotope measurements of atmospheric gases, as done by MAVEN, suggest that a significant portion was lost to space. Xenon isotopes in NWA 7034 indicate hydrodynamic escape of the earliest atmosphere within only a few 100 million years of the planet’s formation.

Insights from InSight

The seismometer on the Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) mission has revealed that Mars is seismically active (Figure 2-16), although the most significant seismic event so far detected is modest by terrestrial standards, at magnitude 4.2. The seismic data have, for the first time, allowed measurement of the thickness of the martian crust, mantle, and core and determined the liquid nature of the core. The low seismic velocity of the crust requires that it is highly fractured and that pore spaces are mostly not filled with ice. InSight’s seismic data allows a determination of the size of the martian core and, combined with Mars’s moment of inertia, the density and composition of the core were constrained, finding a significant proportion of light elements like sulfur or oxygen in addition to iron. InSight’s magnetometer has provided ground truth for orbital measurements by measuring the magnetic field on the planet’s surface for the first time.

A related discovery, newly recognized from decades of orbital data from several spacecraft, is that Mars has a Chandler wobble, an oscillation of the rotation axis that can provide information on temperature and composition of the mantle.

Mars Is an Active World

The more than 13-year record of repeated visible observations by HiRISE on the Mars Reconnaissance Orbiter, supplemented by other orbital cameras, and the more than 20-year thermal infrared observations by THEMIS on Mars Odyssey, reveal much more surface activity than previously recognized, leading to a

new view of surface-atmosphere interactions, especially when combined with the results from Mars Global Surveyor (1997–2006). These activities and interactions include evidence of aeolian transport (including migrating dunes and ripples), frost, gullies, recurring slope lineae, new impact craters and degradation of older craters, sublimation of ice, new insights into dust storms and dust devils, seasonal ice, and growth of unusual geomorphic features within polar ice. These presently active processes have been occurring for millennia, and the long-term observations help constrain their rates.

Measurements from the surface in the past decade have provided more insights into the nature and extent of Mars aeolian activity. Curiosity became the first rover to explore an active dune field on another planet and observed that large ripple bedforms may require a low-density atmosphere, a novel constraint on ancient atmospheric density. Curiosity was also able to characterize daily, as well as seasonal, timescales of aeolian changes. InSight further correlated surface changes with environmental conditions, a key requirement for understanding what drives sand motion and dust lifting.

Mars trembles often, although less vigorously than expected. An average of about one seismic event per day has been detected so far by InSight’s seismic instrument. Most of these events are probably actual marsquakes. Detection of some of the strongest tremors—emanating from the Cerberus Fossae region, where large volumes of water and lava erupted through fissures within the past tens of millions of years, and where boulders appear to have been shaken from hillsides—confirms that Mars is tectonically (and perhaps volcanically) active today. Ongoing seismic activity, by regenerating exposed fracture mineral surfaces and pore space, has implications for potential subsurface habitability and astrobiology.

Collectively, these discoveries highlight Mars as a dynamic habitable world and a key current and future destination in the search for life and understanding the evolution of terrestrial planets.

Key Discoveries from the Past Decade

• Detection of organic matter in the lake sediments of Gale Crater. Curiosity rover data acquired from sedimentary rocks show similar evidence as data from igneous Mars meteorites (exogenic infall of carbonaceous organic matter and chemical reactions on Mars that synthesize or alter organic matter), indicating past indigenous organic chemistry in a martian habitable environment.
• Presence of massive mid-latitude deposits of water ice. Although how and when they formed is not yet known, direct imaging, radar measurements, and crater morphology analyses show large areas in Mars’s northern lowlands where hundreds-meter thick ice slabs occur just meters below the surface, preserving a sizeable reservoir of water and a significant record of martian climate change.
• Possibility of current or recent near-surface liquid water. Enrichments of salt in near-surface rocks and sediments and features that change seasonally with warm temperatures suggest a potential role for small amounts of liquid water in shaping martian geology even in the modern cold, dry climate regime.
• Multiple types of habitable environments were widespread across ancient Mars. Data from orbiters and rovers have revealed evidence for lakes, rivers, playas, groundwater systems, and hydrothermal systems of varying temperatures and water chemistries, preserved in the rock record at thousands of locations, an environmental diversity similar to Earth.
• Kilometer-thick layers of H₂O ice and CO₂ ice in the martian polar caps formed less than 10 million years ago. New radar analyses of polar deposits, coupled with climate models, indicate that much of the ice thickness of Mars’s poles is not billions of years old but rather a product of recent climate change.
• Loss of H₂O and CO₂ to space and sequestration in crustal minerals were major factors in the evolution of the atmosphere and climate. Spacecraft measurements coupled with modeling show that Mars’s climate became drier and colder because of both escape of volatiles to space and their sequestration in minerals in its crust. When compared with volatile evolution on Earth and Venus, this points to the important role of volcanic and tectonic processes in replenishing volatiles and regulating planetary climate and long-term habitability.
• Detection of marsquakes and their use to probe the interior structure of Mars. Data from the InSight lander recorded frequent marsquakes up to magnitude 4 that allowed probing the structure of the martian interior, revealing a thick, fractured crust, mantle structure, and a liquid core that includes a sizeable fraction of light elements.
• Ancient alkali-silica-rich igneous rocks. Data from orbiters and Gale crater have discovered more than just basaltic rocks on Mars. High alkali and silica rocks indicate more differentiated magmas.
• Mars is active today. More dynamic activity occurs on Mars today than was previously known, including migrating sand dunes, recurring slope lineae formation, changing ice landforms, methane release, and many marsquakes centered near Cerberus Fossae. Causes of some of these phenomena, including methane release and marsquakes, remain to be determined.

Further Reading

Ehlmann, B.L., F.S. Anderson, J. Andrews-Hanna, D.C. Catling, P.R. Christensen, B.A. Cohen, C.D. Dressing, et al. 2016. “The Sustainability of Habitability on Terrestrial Planets: Insights, Questions, and Needed Measurements from Mars for Understanding the Evolution of Earth-Like Worlds.” Journal of Geophysical Research: Planets 121(10):1927–1961. https://doi.org/10.1002/2016JE005134.

Jakosky, B.M. 2021. “Atmospheric Loss to Space and the History of Water on Mars.” Annual Reviews of Earth and Planetary Science 49(1):71–93. https://doi.org/10.1146/annurev-earth-062420-052845.

McLennan, S.M., J.P., Grotzinger, J.A. Hurowitz, and N.J. Tosca. 2019. “The Sedimentary Cycle on Early Mars.” Annual Reviews of Earth and Planetary Science 47(1):91–118. https://doi.org/10.1146/annurev-earth-053018-060332.

Udry, A., G.H. Howarth, C.D.K. Herd, J.M.D. Day, T.J. Lapen, and J. Filiberto. 2020. “What Martian Meteorites Reveal About the Interior and Surface of Mars.” Journal of Geophysical Research: Planets 125(12). https://doi.org/10.1029/2020JE006523.

Wordsworth, R. 2016. “The Climate of Early Mars.” Annual Reviews of Earth and Planetary Science 44(1):381–408. https://doi.org/10.1146/annurev-earth-060115-012355.

Wray, J.L. 2021. “Contemporary Liquid Water on Mars?” Annual Reviews of Earth and Planetary Science 49(1):141–171. https://doi.org/10.1146/annurev-earth-072420-071823.

SMALL SOLAR SYSTEM BODIES

Small bodies are rocky and icy worlds that span the solar system. The most commonly known ones are asteroids and comets, and their major reservoirs include the asteroid belt, Trojan populations, trans-neptunian region, and Oort cloud. Small body populations that can potentially strike the planets, such as near-Earth asteroids and comets, are steadily replenished over time by dynamical processes in the major reservoirs. Small bodies also include most meteorite precursors and interplanetary dust particles. For this section, we do not consider objects that have formed equilibrium spherical shapes, roughly >800 km in diameter objects, which are covered in the Ocean Worlds and Dwarf Planets section, or small moons in the outer solar system, which are discussed in the Giant Planet Systems section.

The diversity of small bodies in their sizes, orbits, compositions, and physical natures provides unique scientific opportunities unavailable for larger bodies. First, because many small bodies have undergone minimal processing since formation and had their orbits set in place by early dynamical processes, they are relics of the origin and evolution of the solar system. Second, small bodies are fascinating worlds in their own right, with geologic, geophysical, and geochemical histories that are distinct from those that occur on larger bodies. Third, many have struck the planets over the age of the solar system, yielding beneficial effects, such as the delivery of water and organics, and deleterious ones, such as the destruction of established environments.

The past decade has brought significant scientific discoveries about small bodies that have reshaped our understanding of fundamental planetary processes. Here, the committee highlights several key achievements from the past decade, which have been advanced by complementary discoveries from spacecraft measurements, astronomical observations, theoretical and experimental studies, and laboratory measurements of samples.

In this decade, two NASA Discovery missions will launch to investigate previously unexplored types of solar system small bodies, with Psyche visiting a metal-rich planetesimal in the main belt in 2029 and Lucy encountering the Jupiter Trojan asteroids in 2027. In addition, new space-based telescopic projects, like the James Webb Space Telescope, and Earth-based telescopic facilities, like the Vera C. Rubin Observatory, will begin operations and produce a wealth of new discoveries and data about small solar system bodies.

Early Solar System Formation and Small Body Reservoirs

Computational studies over the past decade have provided new insights into the initial accretion of bodies in the solar system as well as the subsequent structure of the solar system and its small body populations. These dynamical models have used constraints from both meteorite studies and observations of small bodies to develop our latest understanding. A critical step in planet formation is the accretion of planetesimals, defined as asteroid and comet-like bodies. Theoretical studies now suggest that planetesimals are formed by aerodynamical concentration of ~1 cm to 10 cm pebbles in the protoplanetary gas nebula. When the spatial density of pebbles becomes high enough, they gravitationally collapse into 100-km-class bodies, a process referred to as pebble accretion, with some planetesimals turning into binary objects.

The first epoch of giant planet migration may have occurred a few million years after the birth of the Sun, when the protoplanetary gas nebula was still in existence. As the giant planets grew, they traded angular momentum with the gas disk, allowing them to move to new locations. As a consequence, small bodies from the terrestrial planet and giant planet zones were injected into the asteroid belt. The outward migration of Uranus and Neptune then destabilized a massive outer comet disk, leading to new orbits for all of the giant planets and enhanced bombardment rates on all solar system worlds. This event also placed comet-like planetesimals into major reservoirs such as the main belt, Trojan, trans-neptunian region, and Oort cloud populations (Figure 2-17). The giant planet instability may have also ejected one Neptune-size and multiple super-Earth-size worlds from the giant planet zone. The possible capture of an ejected super-Earth in the most distant regions of the solar system at this time may explain the unusual orbits of distant comets and the most distant trans-neptunian objects, but such a body has not been directly observed to date. Last, giant planet migration after the gas disk dissipated may have gravitationally influenced the growth of the terrestrial planets by exciting inner solar system planetesimals. Some of the leftover planetesimals may have played a critical role in the early bombardment of Earth and the terrestrial planets.

A key factor in our ability to constrain planetesimal and planet formation models is through direct laboratory analysis of extraterrestrial materials. Dramatic advances in spatial resolution and sensitivity for structural, elemental, and isotopic measurements have occurred over the past decade (Figure 2-18). Additionally, we have many new asteroid and comet fragments to explore in the form of more than 25,000 new meteorites discovered since 2011, captured interplanetary dust particles, and ultra-carbonaceous micrometeorites collected from Antarctic ice. When combined with the continued analysis of returned samples from the Stardust and Hayabusa missions, they collectively provide us with a set of diverse planetesimal samples that formed from different compositional reservoirs in a range of solar system regions and eras, and insight into what might have been the prebiotic chemistry of Earth—a key question for astrobiology.

Characterization of newly discovered ungrouped meteorites and some carbonaceous chondrite groups have provided one of our first windows into planetesimals that formed in the giant planet and trans-neptunian regions. Analysis of these samples in state-of-the-art laboratory facilities have revealed the complexity of the history of water and organic chemistry over the age of the solar system, including mixing of materials accreted to comets and asteroids. Measurements of hydrogen and nitrogen isotopic compositions of planetary materials have been crucial for understanding the extent of radial mixing in the solar nebula. Isotopic measurements of meteorites have also enabled new insights into compositional reservoirs in the early solar system. Advances in mass spectrometry and sample preparation have enabled precise measurements of many new isotopic systems (e.g., Mo, Ru, W, and Ti). This has suggested that meteorites (and planetesimals) are potentially derived from two distinct nebular reservoirs: one in the inner and another in the outer solar system, with their separation hypothesized to have been caused by the formation of Jupiter.

Outer Solar System Small Bodies

Major advances in our knowledge of outer solar system planetesimals have been driven primarily by observational surveys. Nearly two-thirds of the known trans-neptunian objects were discovered in the past decade, a striking statistic that illustrates just how new our knowledge is of this population. Some trans-neptunian objects

discovered have orbits that travel beyond 1,000 astronomical units, more than 30 times farther from the Sun than Neptune. Gravitational interactions with Neptune cause some objects to evolve onto crossing orbits with the giant planets, producing the Centaur population, transitional objects between Jupiter and Neptune on their path to becoming Jupiter-family comets.

The orbits of the most distant trans-neptunian objects (TNOs) appear to have an orbital alignment. This alignment has been suggested to potentially be owing to the presence of an as-yet undiscovered giant planet, probably several times more massive than Earth (Figure 2-19). It is hypothesized that this planet would be on an elliptical

orbit more than 10 times farther away than Neptune. although observational searches have begun, surveys in the next decade may bring additional evidence to support or refute this current hypothesis.

One highlight of small body exploration this decade was the exploration of the TNO 486958 Arrokoth. In 2014, the Hubble Space Telescope discovered Arrokoth, an optimal target for the second flyby of the New Horizons mission in the dynamically cold classical TNO population. Cold classical TNOs are thought to be undisturbed by giant planet migration and formed in situ, and so they should represent pristine planetesimals whose nature can be used to probe the earliest epochs in the primordial trans-neptunian region. From ground-based stellar occultation campaigns, however, all that could be determined was that Arrokoth had a complex shape about 36 km long.

On January 1, 2019, the New Horizons spacecraft flew by Arrokoth, giving the world the first images of a distant TNO (Figure 2-20). The images showed that Arrokoth is a contact binary with two flattened lobes attached by a bright narrow neck. This shape is consistent with theoretical work on planetesimal formation, where the gravitational collapse of a cloud of pebbles can lead to such a two-lobed structure. Arrokoth’s surface is red with methanol ice and organic material, both of which tell us the kinds of constituents that formed in the far reaches of the solar nebula. Arrokoth is also lightly cratered, which may indicate that few <1–2 km objects exist in the trans-neptunian region. Arrokoth appears to be a representative cold classical object in both its color and its slow spin, and thus much can be gleaned about planetesimal and planet formation from this intriguing body.

Ground-based observations have also led to the notable discovery of rings around some small bodies. In 2014, Chariklo, a 250-km body, was the first Centaur shown to have rings, detected through stellar occultations.

Since then, rings have been discovered around the dwarf planet Haumea, and the Centaur Chiron is suspected to have rings as well.

This decade also brought the unprecedented exploration of a Jupiter-family comet with the European Space Agency’s Rosetta mission. In 2014, Rosetta became the first spacecraft to orbit a comet, conducting 2 years of detailed observations of the comet 67P/Churyumov-Gerasimenko and deploying the Philae lander to the comet’s surface. In particular, Rosetta monitored the comet’s evolution during its closest approach to the Sun and beyond, providing new insights into the geologic features, surface properties, active outburst processes, and interior structure of the comet. Measurements by Rosetta nearly doubled the inventory of coma organics detected, including the confirmation of the amino acid glycine. This mission provided fundamental insight into the dynamic complexity of cometary comae and nuclei, ground-truth for past missions, and context for both future missions and telescopic observations.

Observational studies of comets revealed new insights into the D/H ratio, which has now been measured in over a dozen objects with remote sensing and in situ techniques, yielding a range of D/H values from equivalent to Earth’s ocean to up to a factor of three higher. No consistency has been observed in periodic versus long-period comets; this variability leaves the question of cometary delivery of Earth’s ocean water and/or organics still unclear. Remote sensing has advanced to routinely measure the volatile distribution in fainter cometary comae with facilities such as the Atacama Large Millimeter Array, where asymmetric outgassing, distributed and extended source species, and variability have been found. Observational studies have expanded the molecular inventory of complex organics detected in comets, providing new understanding into the materials available during the formation of the terrestrial planets.

Main Asteroid Belt Planetesimals and the Moons of Mars

A highlight of the past decade was the first spacecraft exploration of the asteroid Vesta, the second-largest body in the main belt (Figure 2-21). The Dawn spacecraft’s 14-month orbital investigation confirmed the affinity of Vesta’s mineralogy and elemental composition to the Howardite-Eucrite-Diogenite (HED) meteorite clan. It also provided constraints on Vesta’s internal structure that confirmed its igneous-differentiated nature.

The presence of two large overlapping impact basins in the southern hemisphere of Vesta (Veneneia and Rheasilvia), and the global trough systems (fossae) tied to stresses of those impacts indicate that while Vesta experienced significant impact-induced stress from these titanic blasts, it remained intact. Despite the deep excavation within those basins, no olivine was detected, defying expectations that this mineral, common in

planetary mantles, would be exposed. The missing olivine is likely sequestered in the deep mantle, whereas the upper mantle is dominated by orthopyroxene-rich diogenite. The ~1 billion-year-old crater-based age for the Rheasilvia impact basin is consistent with it being the source of Vesta’s dynamical asteroid family and the HED meteorites.

Geochemical modeling based on Dawn constraints concluded that Vesta formed within 1.5 Ma of the first condensates in the solar system. It was likely made of volatile-depleted material with a bulk composition that was ~3/4 H-type ordinary chondrite and ~1/4 carbonaceous chondrite. Unexpectedly, dark, hydrated material covers a large portion of Vesta’s surface and is thought to be remnants of carbon-rich low-velocity impactors, possibly including the impactor that created the Veneneia basin. Pitted terrains within young impact craters, as well as curvilinear gully systems, point to hydrated minerals or possibly even ice buried in the subsurface that has been excavated and mobilized by impacts. In addition to hydrated material delivered by the impact of carbonaceous asteroids, ice mixed into near-surface materials might be delivered by ice-rich comet-like bodies that strike Vesta at low impact angles. These significant discoveries at Vesta support models of volatile delivery to many different asteroids by carbon- and possibly ice-rich impactors.

In addition to Dawn’s spacecraft encounter, telescopic observations and meteorite studies have continued to expand our understanding of the main belt and associated near-Earth asteroids and comets. Recently, the Spectro-Polarimetric High-Contrast Exoplanet Research (SPHERE) instrument on European Southern Observatory’s Very Large Telescope (VLT) has been used to observe large (>100 km diameter) main belt planetesimals. Detailed shape models and density estimates of dozens of objects have been derived, leading to the recognition that 434-km-diameter asteroid 10 Hygiea appears to have retained significant volatiles. Telescopic observations have characterized main-belt comets and active asteroids—small bodies that have asteroid-like orbits but show comet-like visual characteristics—which blur the distinction between comets and asteroids in the small body population and attest to the volatile content of outer main belt objects.

A major advance has come from observations by the Wide-Field Infrared Survey Explorer (WISE), which characterized the sizes and albedos of hundreds of thousands of inner solar system bodies. This has enabled new theoretical studies of asteroid families, clusters of asteroid fragments produced by catastrophic collisions. Using theoretical models, these data can be used to probe the nature of asteroid disruption events, when they took place, and whether they produced surges of impactors capable of striking the terrestrial planets. WISE data has also been used to produce sophisticated models of the near-Earth object population, an important resource because the vast majority of hazardous objects have yet to be discovered.

From meteorite studies, the timescales of water-rock alteration in ice-rich planetesimals, as shown by Mn-Cr age dating of secondary minerals such as carbonates and fayalite, have been crucial for revealing the overall framework of early solar system chronology. Now, instead of dates for primary aqueous alteration within parent objects spanning 10 million years, they are constrained to an interval of 1 million years to 4 million years following solar system formation. Meanwhile, advances in the sensitivity and spatial resolution of magnetometers have enabled the first magnetic measurements of many meteorite groups and identification of an asteroid dynamo and the nebular magnetic field. Such investigations continue to expand our understanding of the formation and evolution of inner solar system planetesimals.

The two moons of Mars, Phobos and Deimos, are irregularly shaped small bodies that resemble asteroids in their shapes, low densities, and spectral characteristics. However, the explanation that the moons are asteroids captured into orbit about Mars has always had dynamical challenges to explain the origin of the moons, given both moons have near-circular, near-equatorial orbits, in contrast to highly elliptical and inclined orbits expected from capture models. In the past decade, there have been multiple new studies investigating an alternate origin for the martian moons—formation from a giant impact into Mars. Giant impacts have been recognized as important events across the solar system, including with the formation of Earth’s Moon, and the impact modeled to form the martian moons is envisioned to have been smaller than the event that formed the Moon. Dynamical and geochemical models developed in the past decade have made testable predictions to distinguish between the competing hypotheses for the martian moons. Other studies have focused on explaining the different spectral units and large systems of grooves on Phobos to gain insights into the moons’ origins. JAXA’s Martian Moons Exploration (MMX) mission,

planned for launch in 2024 with NASA as a contributing partner agency, is positioned to test these theories for the origin of the martian moons, through spacecraft measurements of the two moons as well as bringing samples of Phobos to Earth in 2029.

Exploration of Near-Earth Asteroids

Near-Earth asteroids are fragments of main belt asteroids that can approach or possibly strike Earth. They offer unique investigation opportunities owing to their proximity and accessibility. Because the near-Earth population originated elsewhere in the solar system before these objects entered Earth approaching orbits, studying near-Earth objects (NEOs) provides insights into the diversity of asteroids and comets, the properties and evolution of small planetesimals, and the timing and nature of terrestrial planet impacts over time.

Over the past decade, ground- and space-based surveys have discovered more than 17,000 NEOs, while ground-based radar observatories have further characterized more than 700 of these. Direct imaging of NEOs through radar and adaptive optics facilities has highlighted the population’s spectrum of sizes and shapes. Radar facilities, such as that at Arecibo Observatory in Puerto Rico and the Goldstone Solar System Radar in California, provided data to generate three-dimensional models of small bodies without requiring a spacecraft encounter. In fact, the radar-derived shape model of asteroid Bennu was utilized by the OSIRIS-REx mission for planning prior to arrival, and comparison with the subsequent high-resolution spacecraft-derived shape model showed excellent agreement.

The simultaneous and complementary spacecraft exploration of two NEOs in 2017–2020 by JAXA’s Hayabusa2 and NASA’s OSIRIS-REx missions generated surprising results. Each of these missions rendezvoused with sub-kilometer diameter carbonaceous near-Earth asteroids, Ryugu and Bennu, respectively. Initial spacecraft observations of both asteroids revealed them to have similar spinning top-like shapes with low-albedo surfaces covered with large boulders and a surprising paucity of large smooth areas, in contrast to prearrival predictions made based on ground-based observational data.

Detailed reconnaissance confirmed the presence of hydrated surface materials on both bodies. Ryugu’s material appears to resemble thermally metamorphosed or shocked carbonaceous meteorites, whereas Bennu’s is similar to aqueously altered carbonaceous meteorites. Each spacecraft studied and interacted with its respective target to characterize the surface and collect samples. Hayabusa2 deployed two rovers and a lander to the surface prior to its first touchdown, then deployed a small crater-forming impactor which excavated subsurface material for collection prior to the second touchdown. The impact experiment produced a crater dominated by gravity in a surface of cohesionless materials. During proximity operations, OSIRIS-REx discovered that small particles were being ejected from multiple locations on Bennu; fortunately, this activity posed no danger to the spacecraft but it indicated that Bennu is shedding material, but not from cometary-like outgassing events.

Both missions shared the goal to deliver samples to Earth for coordinated, integrated analysis. Analysis of the returned samples will provide ground truth comparisons with Earth-based and spacecraft observations of the nature of asteroids and with our meteorite collections. It will also provide key constraints into the entire history of both asteroids, from their preserved presolar grains components through to pre- and post-accretion environments and geologic activity, to surface processes and the overall dynamical evolution of each asteroid—advances to be made in the coming decade. Hayabusa2 successfully returned to Earth with approximately 5.4 grams of sample on December 6, 2020. OSIRIS-REx touched down successfully on Bennu to obtain a sample (Figure 2-22) and will return with at least several hundred grams to Earth in September 2023. The upcoming analysis of the asteroid samples delivered to Earth by Hayabusa2 and OSIRIS-REx will bring new discoveries of primitive building block materials.

Our understanding of the lifecycles of small, rubble pile asteroids and comets has seen great advances in the past decade. The decade started with the discovery and analysis of several active asteroids, small bodies that shed material in a variety of contexts (e.g., ice sublimation, collisions, landslides driven by rotational processes). These findings motivated more detailed and precise models of the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect, a thermal torque produced by the absorption and reemission of sunlight, which can cause some small

bodies to reach disruptive rotation rates. Rotational spin-up from YORP is on par with collisions in influencing the geologic evolution of small gravitational aggregates like Bennu or Ryugu.

Additionally, the active asteroids motivated theoretical studies focused on the mechanical properties and behavior of small rubble pile asteroids. This led to the identification of weak molecular forces between components as an important additional mechanical force that shapes the evolution of these bodies. Constraints on the strength of such forces have been estimated by astronomical observations of active asteroids, buttressed by in situ measurements from the Hayabusa2 and OSIRIS-REx missions. These two missions have further expanded our insight into rubble pile bodies by providing our first direct estimate of the internal mass distribution within the asteroid Bennu, and have placed strong constraints on past spin rates and global failure mechanisms for Ryugu. To further understanding of the processes shaping small rubble pile asteroids, NASA’s SIMPLEx-class Janus

mission is, at the time of writing, scheduled to launch with Psyche and send twin smallsat to explore two binary asteroid systems in 2026.

Knowledge gained about near-Earth asteroids also has implications beyond the scientific exploration of the solar system. For example, the orbits of both Bennu and Ryugu are Earth-crossing, classifying them as potentially hazardous asteroids to Earth. By characterizing the shapes, sizes, and physical properties of NEOs, we glean insights into the planetary defense mitigation efforts that might be the most effective against such an object in the future. Another critical aspect of planetary defense is to identify near-Earth objects and accurately predict their future orbital pathways. A more in-depth discussion of planetary defense activities is provided in the Planetary Defense chapter.

NEOs in accessible orbits are also targets for human exploration and resource utilization. In the past decade, the NASA Near-Earth Object Human Space Flight Accessible Targets Study (NHATS) identified at least ten objects offering round-trip voyages of less than a year and requiring a DV (i.e., velocity change) of less than 6 km/s. For reference, this is less energy than a one-way trip to the lunar surface. At Ryugu and Bennu, Hayabusa2 and OSIRIS-REx characterized the low-gravity asteroid landscapes similar to those human explorers may encounter. Space-suited astronauts in neutral buoyancy simulations prepared for exploration and sampling operations at near-Earth asteroids and the martian moons to realistically simulate operations and surface sampling techniques. Such skills will be needed when astronauts or robots visit a resource-rich NEOs in the future, possibly to extract water from hydrated clay minerals, such as those detected on Bennu by OSIRIS-REx, for use as propellant. The upcoming analysis of returned samples will further characterize this material, telling us whether water can be readily extracted. From a cost-benefit perspective, it may be possible that NEO water resources are more plentiful and easier to access and utilize than those in the permanently shadowed regions on the Moon.

Connections Between Impacts and Astrobiology

Asteroid impacts have had a profound influence on both the habitability of Earth and the evolution of life. For example, 66 Ma, a 10 km body hit in what is now Mexico’s Yucatan peninsula and triggered a mass extinction event that ended the reign of the dinosaurs. The heavily cratered Moon is testament that other impact-related catastrophes took place in Earth’s deep past, but well-preserved impact effects in the terrestrial geologic record are rare. This raises the question of how else impacts have influenced our biosphere.

One intriguing example comes from the disruption of the L-chondrite parent body in the asteroid belt ~470 Ma. This event sent an enormous shower of small particles and sub-km impactors to Earth, with the flux of fine-grained extraterrestrial material increasing by three to four orders of magnitude. This dust cooled Earth prior to the start of the Ordovician ice age, while the numerous impactors stressed many abodes of life. Together, this bombardment potentially explains the timing and nature of the Great Ordovician Biodiversification Event, a ~30 million year period that produced nearly modern levels of marine invertebrate biodiversity.

Impact craters can also be used to tell us about tremendous changes in the history of Earth. For example, most craters on Earth are found on stable regions called cratons and are younger than 650 million years old. Their near total absence from older terrains takes place at the same time as the so-called Great Unconformity, a large gap in Earth’s stratigraphic record. Both these missing records appear linked to heavy erosion that took place in an era known as Snowball Earth, at a time when Earth was cloaked in global ice. Erosion linked to these ice sheets may have removed 3–5 kilometers of worldwide crust and sent it to the ocean floor. The history of life and our biosphere may be determined by such events.

The onset of the Snowball Earth era ~700–800 Ma is poorly understood, but it is fascinating to find that it is close in time to another major impact shower occurring ~800 Ma. This event was identified using lunar impact spherules returned by the Apollo astronauts and the calculated ages of numerous large lunar craters. Like the 470-million-year shower, the ~800-million-year event is marked by a number of changes in the history of life: the return of anoxic conditions to the deep ocean for the first time since ~1.8 Ga, an abrupt decrease in carbon isotopes (d¹³C) in Australia’s Bitter Springs formation, and major changes in the abundance, diversity, and environmental distribution of marine eukaryotes. It is plausible that the onset of glaciation at this time was brought about by an event similar in character to the 470-million-year event.

Critical connections between life and impacts can also be indirect. For example, large impacts appear to enhance the output of existing volcanic plumes via seismic shaking. This was first identified for the Chicxulub impact 66 Ma, with the erupted volumes of both mid-oceanic ridges and large igneous provinces suddenly increasing at that time. Going back to the Archean era more than 2.5 Ga, impact spherule beds tell us that tremendous impacts took place, with some projectiles being several tens of km in diameter or more. Models show that these events can instigate volcanic plumes from the core-mantle boundary, with eventual consequences for Earth’s surface and atmosphere. The largest events may even help to initiate plate tectonics.

Last, impacts likely played a crucial role in the development of life on Earth and possibly Mars as well. For example, water and organics were probably delivered to Earth and Mars during the planet formation era by volatile-rich projectiles that originated beyond the orbit of Jupiter. In addition, early bombardment of these worlds may have also helped produce periods of surface habitability by both exposing and distributing subsurface materials. The question is whether life had enough time to emerge in a sustained manner within these intervals. Early Earth studies allow us to explore a new planet in the solar system that is very different from the current Earth. The search for connections between impacts and astrobiology has only just begun.

Small Bodies from Beyond the Solar System

One of the major discoveries in the past decade was the identification of the first interstellar object detected passing through the solar system. Interstellar object 1I/‘Oumuamua, estimated to be between 100–1,000 meters long, was discovered in 2017 by the Pan-STARRS survey. Aptly named ‘Oumuamua, meaning “a messenger from afar arriving first” in the Hawaiian language, this first discovery was followed only 2 years later by the discovery of the second interstellar object, 2I/Borisov, roughly half a kilometer in size. The interstellar origins of these objects were determined by their hyperbolic trajectories through the solar system, thus indicating that they are not bound by the Sun’s gravity.

While the discovery of two interstellar objects is a historic achievement of this decade, it is also noteworthy how different the two objects are from each other. 1I/‘Oumuamua had no measurable activity, and appeared more asteroidal, while 2I/Borisov exhibited comet-like behavior, with its nucleus surrounded by a coma and an extensive tail. However, while cometary in appearance, 2I/Borisov was observed to have an extreme CO abundance with respect to H₂O, unlike most comets from the solar system. These two apparitions, although fleeting, provided the ability to observe the chemistry and physical conditions in small bodies from other planetary systems. Upcoming sky surveys in the next decade, such as planned for the Vera C. Rubin Observatory, are anticipated to find many more interstellar objects that will transit the solar system and continue this new field of discovery.

Although the rapid traverse of interstellar objects through the solar system has given us our first tantalizing glimpses into small bodies from extrasolar worlds, the microscopic traces of many such worlds in the form of interstellar and presolar grains is, and has been, our most accessible source of extrasolar materials for prolonged investigation. This decade, the collector tray from the Stardust mission that was devoted to gathering interstellar grains was investigated in state-of-the-art analytical facilities. The preliminary analysis identified seven pristine interstellar dust grains that exhibited a diversity of crystal structures and elemental composition. The analysis of isotopically anomalous SiC grains isolated from a meteorite provided the first successful dating of interstellar grains, which showed a range of ages that inform models for the origin of these particles beyond the solar system. As analytical techniques continue to advance, new measurements of interstellar materials contained in meteorites and other extraterrestrial samples will continue to provide constraints into the evolution of the rocky components of other solar systems.

Key Discoveries from the Past Decade

• Distinct chemical and physical reservoirs were produced during the evolution of the early solar system and the formation of the giant planet. These momentous events distributed small bodies across the solar system; chemical signatures measured in meteorites and remote observations of volatiles point to the extent of mixing between the reservoirs and the compositions of the building blocks available to the terrestrial planets.
• Outer solar system small bodies display a wide diversity of properties that are distinct from their inner solar system counterparts. The unprecedented number of outer solar system small bodies discovered

• in the past decade and the first spacecraft exploration of a primitive trans-neptunian object suggest distinctive accretion and evolutionary processes, with still much to discover about this population we have only begun to explore.
• Inner solar system planetesimals provide new insights into planetary evolutionary processes. The first orbital expedition to the main asteroid belt documented Vesta’s very early formation, battering by two giant impacts, and preservation of hydrated materials delivered to its surface by impactors. Meteoritic and telescopic studies investigated processes ranging from early solar system magnetic dynamos to current-day outburst events from active asteroids, and new models for the origin of the martian moons by a giant impact were developed, emphasizing the wide array of processes that affect the evolution of small bodies.
• The physical properties of near-Earth asteroids indicate that they are complex assemblages that are actively evolving. Exploration of two near-Earth asteroids revealed rugged surfaces dominated by boulders and provided key ground-truth for models of the shapes, structures, strength, and lifecycle of the near-Earth asteroid population.
• The first interstellar objects passing through the solar system were identified, The two interstellar objects discovered were strikingly different from each other. Along with analytical measurements of interstellar grains, these objects provide insights into origin environments unlike those of the solar system.

Further Reading

Bockelée-Morvan, D., U. Calmonte, S. Charnley, J. Duprat, C. Engrand, A. Gicquel, M. Hässig, et al. 2015. “Cometary Isotopic Measurements.” Space Science Reviews 197:47–83. https://doi.org/10.1007/s11214-015-0156-9.

Fitzsimmons, A., C. Snodgrass, B. Rozitis, B. Yang, M. Hyland, T. Seccull, M.T. Bannister, W.C. Fraser, R. Jedicke, and P. Lacerda. 2018. “Spectroscopy and Thermal Modelling of the First Interstellar Object 1I/2017 U1 ‘Oumuamua.” Nature Astronomy 2:133–137. https://doi.org/10.1038/s41550-017-0361-4.

Grundy, W.M., M.K. Bird, D.T. Britt, J.C. Cook, D.P. Cruikshank, C.J.A. Howett, S. Krijt, et al. 2020. “Color, Composition, and Thermal Environment of Kuiper Belt Object (486958) Arrokoth.” Science 367(6481). https://doi.org/10.1126/science.aay3705.

Jewitt, D. 2012. “The Active Asteroids.” Astronomical Journal 143(3):66. https://doi.org/10.1088/0004-6256/143/3/66.

Lauretta, D.S., D.N. DellaGiustina, C.A. Bennett, D.R. Golish, K.J. Becker, S.S. Balram-Knutson, O.S. Barnouin, et al. 2019. “The Unexpected Surface of Asteroid (101955) Bennu.” Nature 568:55–60. https://doi.org/10.1038/s41586-019-1033-6.

Rosenblatt, P., S. Charnoz, K.M. Dunseath, M. Terao-Dunseath, A. Trinh, R. Hyodo, H. Genda, and S. Toupin. 2016. “Accretion of Phobos and Deimos in an Extended Debris Disk Stirred by Transient Moons.” Nature Geoscience 9:581–583. https://doi.org/10.1038/ngeo2742.

Russell, C.T., and C.A. Raymond. 2015. “The Dawn Mission to Vesta and Ceres.” Pp. 419–432 in Asteroids IV, P. Michel, F. DeMeo, and W.F. Bottke, eds. Tucson: University of Arizona Press. https://uapress.arizona.edu/book/asteroids-iv.

Spencer, J.R., S.A. Stern, J.M. Moore, H.A. Weaver, K.N. Singer, C.B. Olkin, A.J. Verbiscer, et al. 2020. “The Geology and Geophysics of Kuiper Belt Object (486958) Arrokoth.” Science 367(6481):eaay3999. https://doi.org/10.1126/science.aay3999.

Taylor, M.G.G.T., C. Alexander, N. Altobelli, M. Fulle, M. Fulchignoni, E. Grün, and P. Weissman. 2015. “Rosetta Begins Its Comet Tale.” Science 347:387. https://doi.org/10.1126/science.aaa4542.

Thomas, N., B.J.R. Davidsson, L. Jorda, E. Kührt, R. Marschall, C. Snodgrass, and R. Rodrigo. 2020. “Editorial to the Topical Collection: Comets: Post 67P/Churyumov-Garasimenko Perspectives.” Space Science Reviews 216:107. https://doi.org/10.1007/s11214-020-00727-1.

Vokrouhlický D., W.F. Bottke, S.R. Chesley, D.J. Scheeres, and T.S. Statler. 2015. “The Yarkovsky and YORP Effects.” Pp. 509–531 in Asteroids IV, P. Michel, F. DeMeo, and W.F. Bottke, eds. Tucson: University of Arizona Press. https://uapress.arizona.edu/book/asteroids-iv.

Wadhwa, M., T.J. McCoy, and D.L. Schrader. 2020. “Advances in Cosmochemistry Enabled by Antarctic Meteorites.” Annual Review of Earth and Planetary Sciences 48:233–258. https://doi.org/10.1146/annurev-earth-082719-055815.

Watanabe, S., M. Hirabayashi, N. Hirata, N.A. Hirata, R. Noguchi, Y. Shimaki, H. Ikeda, et al. 2019. “Hayabusa2 Arrives at the Carbonaceous Asteroid 162173 Ryugu—A Spinning Top-Shaped Rubble Pile.” Science 364(6437):268–272. https://doi.org/10.1126/science.aav8032.

GIANT PLANET SYSTEMS

Overall Architecture

The giant planets—Jupiter, Saturn, Uranus, and Neptune—dominate the solar system, containing more than 99 percent of the mass and angular momentum outside of the Sun. Their formation, and likely orbital migration, dictated the structure of the rest of the solar system, controlling terrestrial planet growth rates, locations, and much of their volatile inventories, as well as the ultimate distribution of all small body populations in the solar system. Their ever-changing atmospheres provide windows to the deeper hidden interiors, which lock away the secrets of the origin of the solar system. Each giant planet harbors a rich and diverse system of satellites and rings, embedded within enormous and complex magnetospheres (Figure 2-23). The tidal interactions between many icy satellites

and their host planets provide provides a third energy source in addition to energy of accretion and radiogenic heating, helping to maintain subsurface oceans which may harbor life. To understand whether the solar system architecture is typical or unique requires comparing a deep understanding of the formation and evolution of the giant planet systems, while surveying the size, number, and composition of the more than 2,000 giant exoplanets discovered to date.

The outer planets are often divided into two groups: (1) the gas giants, Jupiter and Saturn, which are H-He-dominated; and (2) the “ice giants,” Uranus and Neptune, which possess H-He atmospheres containing ~10–20 percent of their total mass and a larger proportion of ices. There are many similarities among the giant planets: all appear to have interiors dominated by a large, dense core; deep, dynamic hydrogen/helium atmospheres; multiple satellites, some of which suffer additional heating by tidal dissipation to have subsurface oceans; multiple rings that interact with small moons; and substantial magnetospheres. The Galileo, Cassini, and Juno missions have shown that the interior structures of gas giants, Jupiter and Saturn, are far more interesting and complex than expected, and have advanced our understanding of their atmospheres, magnetospheres, satellite, and ring systems.

However, even within each sub-group there are clear differences among the planets. Saturn is not simply a smaller version of Jupiter and there are key differences between Uranus and Neptune. Each planet is unique and has different key questions associated with it. These differences give clues to their origin and unique histories, provide key insight into how giant planets are built, and can untangle the dynamic history of our early solar system.

Atmospheres

Giant planet atmospheres are natural planetary-scale laboratories for studying the interplay between dynamics, meteorology, chemistry, and cloud formation, and represent the transition region between the external magnetosphere and the hidden deep interiors. These atmospheres are in a constant state of motion and change, transporting energy and material from place to place in response to long seasonal cycles and meteorological phenomena. Their deeper atmospheres are characterized by horizontal bands of clouds, composition, and temperature, organized by east-west winds and punctuated by spectacular vortices and storms. Their cloud-free stratospheres are warmed by sunlight absorbed by methane gas. The stratospheres interact with the external planetary environments. Material from “ring rain,” micrometeoroids, and even large impacts deposits water and other external material into the stratosphere. At the upper edge of the stratosphere, which blends into the thermosphere and ionosphere, energy is also deposited in the form of charged particles streaming in from the magnetospheres, some of which generates delicate auroral patterns. Circulation of the stratospheres and thermospheres then redistributes this energy with latitude.

Clouds condense in the atmospheres of the giant planets. The troposphere is where most of the solar heating occurs, but in all the giant planets except Uranus, more heat comes from the deeper interior than from the Sun. Heat is transported within this “weather layer” by mixing—on small scales by storms and on large scales by atmospheric circulation, both of which vary substantially with time. The composition of clouds in the giant planets is diverse. In addition to clouds formed from water, the giant planets host clouds of ammonia ice and ammonium hydrosulfide, joined by hydrogen sulfide and methane clouds in the even colder atmospheres of Uranus and Neptune. Molecules such as carbon monoxide and phosphine are mixed upward from deeper levels where high-temperature thermochemistry dominates, exposing them to sunlight to help provide the breadth of colors in the visible clouds.

Hubble and Keck observations over many years have shown the appearance of more clouds on Neptune than seen by Voyager, with changes occurring on short timescales. Additional Great Dark Spots akin to that seen by Voyager have been seen by Hubble recently to appear and then go away on a multiyear timescale. Uranus’s variability is even more striking, challenging the idea that the atmosphere is driven entirely by seasonal effects. When Uranus approached equinox in 2007, after a long period in which no changes were seen by Hubble, an outbreak of clouds all over its surface occurred. After that outbreak, no additional outbursts were expected, but in fact many large bright cloud outbreaks have since been observed.

Giant planet atmospheres are mostly hydrogen and helium, with other common elements like carbon included as hydrogenated molecules in small amounts, and a general trend of more enriched elemental abundances in the ice giants than the gas giants. The noble gases (helium, neon, argon, krypton, and xenon) are largely unaffected by atmospheric chemistry. Measurement of their elemental and isotopic abundances provide key pieces of the puzzle of how the solar system formed as different models predict a different pattern of abundances.

In the stratosphere, methane is broken apart by solar ultraviolet and energetic particle radiation to recombine as more complex hydrocarbons. Water coming in from external sources can also participate in stratospheric chemistry, and chemical signatures of external inputs such as cometary impacts can persist for centuries. The complex molecules are then redistributed by stratospheric circulation, sediment downward to contaminate the deeper clouds, and can sometimes condense to form thin haze layers.

Studies of the weather, climate, and atmospheric circulation of the giant planets provide many parallels to processes operating in the atmospheres from Earth to exoplanets. Owing to the fast rotation of these planets and the absence of a solid surface, winds blow in mainly the east-west direction. The fastest winds are near the equator, where Jupiter and Saturn have super-rotating (eastward) jets and Uranus and Neptune have retrograde (westward) jets. Gigantic convective storms on Jupiter and Saturn are known for lightning activity and strong updrafts that allow mixing of cloud and precipitation particles of different types (e.g., water and ammonia), and show episodic and nonseasonal behavior which might someday allow us to predict giant planet weather. Earth-size vortices spinning in both cyclonic directions (like hurricanes) and anticyclonic directions have been observed on Jupiter and Saturn. On Uranus and Neptune, only anticyclonic vortices have been detected, with similar properties but shorter lifetimes than Jupiter’s Great Red Spot.

The planetary banding on all four giants may demarcate circulations similar to the Hadley and Ferrel cells in Earth’s atmosphere, but although this has been studied in depth at Jupiter and Saturn, the applicability to the ice giants remains a mystery. The polar regions differ from world to world, with long-lived clusters of cyclonic vortices on Jupiter, a hexagon on Saturn, a seasonal polar hood on Uranus, and a hot and chemically depleted vortex at Neptune’s summertime pole. Understanding how the dynamics, clouds, and composition vary from world to world, particularly via exploration of ice giants, will provide powerful new insights into the formation processes and environmental processes shaping giant planets in all their guises.

Impacts have been observed many times in Jupiter’s atmosphere, and streaks from small impacts appear on Saturn’s rings in Cassini high resolution images. Whether Uranus and Neptune are impacted by objects from the Kuiper belt is an open question. The observed overabundance of CO and HCN in Uranus and Neptune relative to what chemists predict for these hydrogen-rich atmospheres suggest that cometary and asteroid impacts might be responsible.

Interiors, Including Deep Structure, Circulation, and Heat Balance

The bulk composition and internal structure of the outer planets are still poorly constrained, but knowledge of them is critically important to understanding the formation and evolution histories of the planets. Additionally, various physical and chemical processes and their interplay govern the interiors; dynamics and magnetic fields, dynamo generation and composition, dynamics and rotational contribution to the density distribution, as well as processes like convection, core erosion, and immiscibility. In addition, the deep interiors of the giant planets serve as natural laboratories for materials at high pressures and temperatures, as a result, constraining the interiors of the outer planets is also of interest to the high-pressure physics community. Last, the planets in the solar system serve as prototypes for exoplanetary science. A better understanding of Jupiter, Saturn, Uranus, and Neptune will improve our understanding of distant giant planets elsewhere in the Milky Way galaxy. Therefore, understanding the deep structure and global composition of giant planets is a key objective in planetary science.

In the past decade, the Cassini-Huygens (Figure 2-24) and Juno (Figure 2-25) missions around Jupiter and Saturn, respectively, have led to many exciting and important discoveries. With extremely accurate measurements of the planets’ gravity fields, the depth of Jupiter and Saturn’s zonal flows has been determined and found to be

3,000 km and 9,000 km for Jupiter and Saturn, respectively. The flows decay where the electrical conductivity is sufficiently high to drag the flow into nearly uniform rotation at greater depth. With the knowledge of the dynamical contribution to the gravity field, the interior structure can be further constrained. For Jupiter, models matching the Juno measurements indicate that the envelope is not well-mixed and that the core is diluted (i.e., has an extended region of enriched elements heavier than hydrogen and helium in the deep interior). For Saturn, the detection of oscillations in its rings by the Cassini spacecraft has demonstrated that part of its interior is stably stratified, with indications that the core is diluted.

The bulk compositions of Uranus and Neptune are poorly constrained, and accurate measurements of their gravity fields are essential for their characterization. Such measurements can also be used to constrain the depth of the winds on the ice giants. To understand the nature of Uranus and Neptune better, improved measurements of their magnetic fields and thermal fluxes are required. There is a clear need to link the magnetic fields and the deep interiors. A better determination of the internal structure and the variability of the planetary magnetic fields can further constrain the composition and internal structure via the required conditions to sustain a dynamo and the role of ohmic dissipation in decaying the winds. Knowledge of the fluxes and heat transport mechanisms within the outer planets can reveal information not only on their evolution and structure, but also on the link between the atmosphere and deep interior. The existence and nature of the magnetic fields provide important observational constraints on the present-day interior structure and dynamics of the outer planets. Dynamo generation is thought to require large-scale motions in a medium that is electrically conducting, and rapid (at least moderate) rotation. In Jupiter and Saturn, the conducting material is metallic hydrogen in a region where heat is transported by convection. Since metallic hydrogen is associated with a relatively deep region in the planets, the resulting magnetic field is nearly dipolar. Recent Juno observations explored the morphology of Jupiter’s magnetic field. When viewed at the dynamo surface, Jupiter’s magnetic field is characterized by an intense isolated magnetic spot near the equator with negative flux, an intense and relatively narrow band of positive flux at ~45 degrees latitude in the northern hemisphere, and relatively smooth magnetic field in the southern hemisphere. The north-south dichotomy in Jupiter’s magnetic field morphology could be a result of Jupiter’s dilute core, which either limits the dynamo action to the upper layer of Jupiter or creates spatially separated active dynamos within the planet. Saturn’s intrinsic magnetic field is unusually weak, with surface field strength ranging from 20 to 50 μT. Surprisingly, Saturn’s magnetic field seems to be symmetric, to within the accuracy of the data, with respect to the spin-axis. Both the weak strength and the extreme spin axisymmetry of Saturn’s magnetic field might be linked to helium rain that could create a stably stratified layer atop the deep dynamo. However, this is only a speculation, and this topic is still being investigated.

The magnetic fields of Uranus and Neptune are poorly understood, and the available measurements are very limited. One possibility is that the magnetic field in these planets is generated by an exotic form of water called “super ionic.” This is motivated by models that predict that Uranus and Neptune consist of mostly water in their deep interiors. However, other elements could also lead to high electrical conductivity to generate a dynamo, and therefore the link between the composition and the magnetic field in these planets remains unknown. The location at which the magnetic field is generated in Uranus and Neptune (where the material is electrically conducting and the region is convective) is expected to be in an outer shell (i.e., closer to the “surface”). This could explain the multi-polar nature of the magnetic field of Uranus and Neptune. Understanding the magnetic fields of the giant planets is not only crucial for putting constraints on the composition, but also on the heat transport mechanisms within the planets and the interplay between rotation, interior, and dynamics.

Magnetospheres

Jupiter’s magnetosphere—the sphere of influence of its magnetic field—is the largest planetary structure in the solar system, 10 times the volume of the Sun and stretching out past the orbit of Saturn. When we compare the magnetospheres of planets, however, it is usual to compare with the size of the planet (e.g., radius of planet,

R_P) and use the distance from the planet’s center to the subsolar boundary (basically the smallest scale) for comparison. At Jupiter, Saturn, Uranus, and Neptune, the magnetospheres are 63–92 R_J, 22–27 R_S, 18 R_U, and 24 R_N, respectively. The range in sizes at Jupiter and Saturn shows the variability owing to changes in the pressure of the solar wind. At Uranus and Neptune, the single Voyager 2 flyby of each did not provide enough data. As the solar wind spreads out with distance, the pressure at farther planets correspondingly decreases, but the major cause of these ranges in size between these magnetospheres comes from the different strengths of the magnetic fields generated by their internal magnetic dynamos. In the case of Jupiter, a major additional factor is the internal pressure of hot ionized gases—plasma—trapped in the magnetic field that further inflate the magnetosphere by about a factor of two.

But size is not everything. The characteristics of the magnetospheres of Uranus and Neptune are radically different from those of Jupiter and Saturn (Figure 2-26) owing to a combination of two factors: the geometry of the internally generated magnetic fields and the sources of plasma. At Jupiter and Saturn, beyond a couple radii away from the planet, the strong internal fields (generated in large volumes of metallic hydrogen) are approximated by a simple dipole—like a bar magnet—with a small tilt (10 degrees for Jupiter, 0 degree for Saturn) from the planet’s spin axis. At Uranus and Neptune, the field is much more complicated (probably owing to dynamos generated in a shell of water), with not just a large (50–60 degrees) tilt from the spin axis, but also a highly irregular and nondipolar form. This means that over the Uranus and Neptune spin periods (17 and 16 hours, respectively) the geometry of the planetary field relative to the solar wind, and the magnetic field embedded therein, change dramatically. Consequently, the magnetospheres of Uranus and Neptune are thought to be highly dynamic, and any plasma sources are quickly flushed out of the system.

The magnetospheres of Jupiter and Saturn are dominated by plasma sources from their geologically active moons—Io and Enceladus, respectively. The interaction of the surrounding plasma with Io’s atmosphere causes tons of atmospheric gases (mostly SO₂ and dissociation products O, S) to escape from the moon every second. This cloud of neutral material along Io’s orbit is quickly ionized to produce 260–1,400 kg/s of plasma (dominated by ions of O and S, plus electrons) that forms a torus of plasma that is coupled to Jupiter via the magnetic field and co-rotates around the planet with Jupiter’s 10-hour spin period. As this Io-genic plasma moves out into the vast magnetosphere, it becomes heated by processes that are not well determined (to tens to hundreds keV). Ultimately, the plasma is lost mostly via ejection of blobs down the magnetotail, but also through acceleration into Jupiter’s atmosphere where it excites intense auroral emissions. Similar processes occur at Saturn with the plasma source (~250 kg/s) being water group ions from ionization of material spewed out by Enceladus’s plumes. But lower energy electrons at Saturn mean that the neutrals survive longer and spread out in the system. The resulting ratio of neutral- to ionized-particles is 1:100 at Jupiter and 100:1 at Saturn.

The Voyager 2 flybys of Uranus and Neptune showed weak (<1 kg/s) plasma sources of mostly protons that could originate in the planets’ ionospheres, the solar wind, or icy moons. Nevertheless, Uranus showed a remarkably intense radiation belt. There are glimpses of auroral emissions but the physical processes driving these intriguingly complex magnetospheres are undetermined.

Moons

The four giant planet systems each have distinctly different systems of orbiting moons, sometimes thought of as miniature solar systems. Like the incredible variety of exoplanetary systems, the diversity of our neighboring moon systems allows us to explore the diversity and complexity of planet formation and subsequent dynamical processes that modify or disrupt them, right in our “celestial backyard.” Satellite systems can also preserve the nongaseous composition of the planets and solar nebula in the region they formed. The satellite systems of the giant planets reveal four very different outcomes of planet formation processes and the subsequent dynamical evolution that can modify or disrupt them, as recorded in their surface features and crater populations. As a result, the thermal and geologic processes that have shaped and sculpted the satellites of these four systems are also distinct.

Jupiter has a primordial system of large moons and an extensive population of smaller captured bodies. Jupiter’s four large Galilean moons reveal the fundamental importance of gravitational tides in generating internal heat, producing the giant volcanoes on Io and melting ice on Europa and Ganymede. At Saturn, we see a suite of smaller, ice-rich moons of uncertain age that likely experienced tremendous dynamical upheaval (some of which are still active today), one large ice-rock world, and a number of smaller outer captured moons. Saturn’s lone large moon, Titan, has a dense nitrogen-methane atmosphere, with wind-blown dunes and organic precipitation producing Earth-like river channel systems, lakes and seas, suggesting a world akin to an early Earth in hibernation in the colder regions of the outer solar system.

Uranus and Neptune present even more system diversity, evident even in brief flybys by Voyager 2 in 1986 and 1989. At Uranus, a giant impact into the planet itself may have disrupted the initial satellites, leading to the solar system’s only example of a second-generation satellite system. The uranian moons are similar in size to the smaller ice-rich moons of Saturn, which range from ancient, battered relics of satellite formation to currently active ocean worlds such as Enceladus. Several moons, such as Saturn’s Dione and Tethys, and Uranus’s Ariel and Miranda, have been extensively fractured and resurfaced by water- and ammonia-rich lavas, betraying internal sources of heat from gravitational tides and radioactive decay. At Neptune, the capture of Triton (a likely Kuiper belt object) led to the total disruption and/or ejection of the original satellite system. Triton itself is likely an ocean world, with an active surface, complex geology of volcanic resurfacing and crustal convection, and a thin nitrogen atmosphere producing surface frosts and ices that migrate during Triton’s long deep seasons. Triton appears to be a near twin of Pluto but has experienced a very different dynamic and geologic history.

Astrobiological Potentials of Giant Planet Moons

Three key ingredients required to support life on Earth are liquid water, source(s) of energy (oxidants and reductants), and core biological elements (C, H, N, O, P, and S or CHNOPS). Strong geophysical evidence exists for subsurface water oceans in the jovian satellites Europa, Ganymede and Callisto and the saturnian satellites Enceladus and Titan. Evidence from surface geology that can be interpreted as owing to a subsurface ocean also exists for Saturn’s satellite Dione and Neptune’s satellite Triton. Two mid-size moons of Uranus, Titania and Oberon, are large enough to be capable of harboring subsurface oceans, especially if their H₂O layers contain sufficient amounts of ammonia or other antifreeze. A third, Ariel, might have been tidally heated to the point of creating a water ocean in its interior.

Whether an icy satellite would develop and maintain an ocean depends on the amount of initial primordial heat, the rate of internal heating from radiogenic and tidal sources, the rate at which the heat escapes the satellite and the freezing point of the liquid (determined by antifreeze concentrations). The largest of the giant planet satellites such as Ganymede, Callisto, and Titan likely have sufficient remnant primordial heat and current radiogenic heating to maintain internal oceans whereas the primary source of heating in Enceladus is very likely tidal heating. Enceladus also show strong evidence of communication between the ocean and the rocky seafloor, while for Europa little is as yet known about communication between its ocean and the surface. Heating of the ocean and communication between it and the moon’s surface are important respectively, for habitability and the search for life. The same source of energy that keeps oceans viable over geological time would also support chemical disequilibrium in the ocean through leaching of new materials from the mantle that can be exploited by life forms. In terms of key biological elements, both oceans of Europa and Enceladus contain salts, while the latter also contains carbon- and nitrogen-bearing molecules. The compositions of potential water oceans within the uranian moons and Triton are unknown.

Water plumes linked to the subsurface oceans have been observed on Enceladus, and tentatively on Europa, and provide the most attractive targets for in situ detection of biosignatures through plume flybys. The detection and interpretation of the biosignatures in ocean-derived materials is aided by the fact that, as highlighted in An Astrobiology Strategy for the Search for Life in the Universe (NASEM 2019), “slow” life that is barely able to survive in an austere environment is easier to detect because its environmental noise level is low. The direct detection of biosignatures by a lander on the surface of Europa is challenging because of the harsh surface radiation environment but the surface of Enceladus would preserve biogenic evidence over a long time and is conducive to landed astrobiological science. Expanded understanding of habitability of chemosynthetic subsurface environments, brine stability, and adaptations of life to saline fluids have widespread implications for the search for life on Enceladus, Europa, and other ocean worlds, as discussed in the next section.

Rings

Each of the giant planets is surrounded by a distinctive ring system composed of many small particles orbiting the planet. Jupiter has the most tenuous ring system, which is composed primarily of fine dust grains that were probably knocked off the planet’s small inner moons. By contrast, Saturn has the most elaborate ring system, with many different components ranging from extremely tenuous rings of dust-size particles (including one generated by Enceladus’s cryovolcanic activity) to the much denser and more massive main rings composed primarily of particles millimeters to meters across. Uranus also has a ring system that includes both dense and dusty components, but its rings also contain a surprisingly large number of exceptionally narrow structures, as well as an unusually blue dusty ring associated with a small moon. Last, Neptune’s ring system contains multiple dusty features, along with a set of dense but incomplete ring arcs that have been slowly changing over time.

These rings provide important information about the dynamics and history of their host systems. For example, the extensive data returned by the Cassini mission enabled Saturn’s rings to be used as a seismometer

that records the oscillations and asymmetries in the planet’s gravitational field, thereby providing new insights into Saturn’s internal structure and rotation. More dramatically, a variety of measurements of both the mass of Saturn’s main rings and the mass fluxes between the rings and planet made around the end of the Cassini mission suggest that the rings may be only about 100 million years old. While the age of Saturn’s ring system is still being debated, the possibility that the rings are young, together with the surprisingly rapid tidal evolution of Saturn’s moons has led to a reevaluation of the Saturn system’s history. At the same time, theoretical investigations of the material around Uranus and Neptune have revealed that some of the solid material around these planets may have cycled back and forth between rings and moons multiple times. The differences between Uranus’s and Neptune’s ring-moon systems could therefore indicate that they are in different phases of this cycle.

The rings around the giant planets also provide insights into the physical processes that operate astrophysical disks, including the protoplanetary disks that gave rise to planetary systems like our own. For example, the distribution of particle sizes and structures within dense rings depend on fundamental disk processes like particle aggregation and fragmentation. Furthermore, the discovery of embedded objects within Saturn’s rings has enabled direct observations of orbital migration arising from interactions with surrounding disk material. At the same time, the discovery of multiple new ring systems around small bodies orbiting among and beyond the giant planets suggests that rings can be found in a broader range of contexts than previously appreciated.

Connection to Exoplanets

The first planet detected around a normal star like the Sun was a giant planet like Jupiter, but in a very close orbit around its parent star. Such were the easiest exoplanets to detect, but with the advent of the Kepler and TESS satellites using the transit technique, we now know that these are not the most abundant planets. Uranus and Neptune mass objects (20 times the mass of Earth) are more plentiful, and bodies of 10 Earth masses and below (the sub-Neptunes and super Earths) perhaps even more so. However, planets in orbits like those of Jupiter and Saturn around the Sun remain notoriously difficult to detect. Those in closer orbits are being studied by Hubble, and soon James Webb Space Telescope, to understand what their atmospheres are made of. This makes comparison with the detailed studies of the solar system’s giant planets especially valuable. Comparing the abundances of elements like carbon and oxygen in the atmospheres of giant exoplanets with those in their parent stars gives us clues to how they formed, so that we have a window into whether the way our own giant planets formed is typical of that of their sister planets elsewhere in the Milky Way galaxy. Atmospheric processes (cloud formation and spatial variability) in giant planets allow us to understand clouds in exoplanets and the sources of variable light curves.

Summary

Our knowledge of each of the giant planet systems was enabled by a suite of missions to these bodies, and both Earth orbiting and ground-based telescopes. In situ mission elements, such as the Galileo probe, provided key ground truth for the missions that came both before and after, especially by obtaining measurements not possible via remote sensing. In the past decade, Juno has provided deep insights into our remaining Jupiter knowledge gaps, such as its interior structure and composition, and will continue to provide science throughout the Jupiter system in its extended operations phase. Cassini, as a comprehensive, system-encompassing mission enabled a deeper understanding of both the individual bodies and of the key interactions within the Saturn system. The continued analysis of rich multi-instrument Cassini data informs our understanding of the interplay between the rings, satellites, planetary atmosphere, interior, and magnetosphere and their coupled evolution. While advances in understanding Uranus and Neptune following the Voyager flybys came from Earth-based observing and modeling, they are ready for orbital and in situ exploration and spectacular new discoveries in the upcoming decade.

Key Discoveries from the Past Decade

• Jupiter and Saturn have dilute cores, not the small well-defined cores assumed by models. Gravity data from Cassini’s final orbits at Saturn, and Juno’s high inclination orbits at Jupiter have revealed that the deep interior structure of both planets is not sharply defined as most models had assumed. Rather, they likely have extended envelopes enriched with heavy elements. New models of giant planet formation and evolution are needed to explain this, along with similar data for Uranus and Neptune to understand if this is common.
• Belt-zone structure of Jupiter and Saturn goes deep, yet Jupiter has polar cyclones very different from the Saturn polar hexagon. Findings from Juno show that Jupiter likely has deep winds, as does Saturn based on Cassini results. Lab and numerical studies show that circumpolar jets can form vortex streets or polygons depending on the vertical structure and depth of the wind field. However, as they both have deep structure, Juno’s observations that Jupiter’s poles have cyclones, while Saturn’s does not, is surprising and may be owing to subtle differences in the local environment.
• Saturn’s ring-moon system has changed dramatically over time and the change is ongoing, unlike Jupiter’s. Cassini’s long exploration of Saturn’s rings revealed new ringlets, changes in dust content, vertical features, waves, and dynamical interactions with its many moons. Some of the ring moons show evidence of accretion in the form of equatorial ridges, such as on Daphnis, Pan, and Atlas.
• Magnetic spots on Jupiter (like sunspots); Saturn’s magnetic field is surprisingly symmetric. Juno’s mapping of Jupiter’s magnetic field found a patch of intense strength near the equator and strong secular variation. In contrast, Cassini’s mapping of Saturn’s field showed strong axisymmetry, but with the magnetic equator shifted northward from the planetary equator. However, the many latitudinal variations in Saturn’s magnetic field require a complex internal dynamo.
• Changing ammonia abundance with depth and latitude on Jupiter suggests violent storms and large ammonia “mushballs” bringing ammonia deep into the atmosphere below the cloud base. Jupiter was thought to have a well-mixed troposphere below the clouds, but Juno data revealed that ammonia varies with latitude and with depth; it is only well mixed in a narrow low latitude band. Combined with observations of lightning, it is theorized that ammonia-rich hailstones in thunderstorms carry the ammonia deeper into that atmosphere than was expected.
• Changeable Uranus and Neptune: big outburst on Uranus in 2014; brightening of seasonal polar hood on Uranus; Neptune dark spots are frequent and short-lived (years). Increased seasonal storms were expected around Uranus equinox in 2007, however, much larger bright cloud outbreaks were observed since then. This challenged ideas about solar insolation and convection on Uranus. Additionally, new dark spots were discovered on Neptune in 2015 and 2018, studies of the full lifecycle of large anticyclone formation. New storms occur every few years and last for 3–5 years.
• Titan’s polar seas are deep and mostly methane. Owing to the extreme surface temperature, ~95 K, and the abundance of atmospheric hydrocarbons, Titan’s seas are predominantly composed of methane and ethane. The largest of the seas was mapped by Cassini’s RADAR system and found to have a depth of ~160 m.
• Evidence of ongoing sizable impacts on Uranus and Neptune, and Saturn’s rings. The observed overabundance of CO and HCN in Uranus and Neptune, above thermochemical equilibrium levels, suggests an ongoing, and external, source; cometary and asteroid impacts are likely responsible. Additionally, Cassini’s high spatial resolution images of Saturn’s ring show streaks from small impacts.

Further Reading

Guillot, T., D.J. Stevenson, S.K. Atreya, S.J. Bolton, and H.N. Becker. 2020. “Storms and the Depletion of Ammonia in Jupiter: I. Microphysics of ‘Mushballs.’” Journal of Geophysical Research: Planets 125:e2020JE006403. https://doi.org/10.1029/2020JE006403.

Morales-Juberías, R., K.M. Sayanagi, T.E. Dowling, and A.P. Ingersoll. 2011. “Emergence of Polar-Jet Polygons from Jet Instabilities in a Saturn Model.” Icarus 211:1284–1293. https://doi.org/10.1016/j.icarus.2010.11.006.

Müller, S., R., Helled, and A. Cumming. 2020. “The Challenge of Forming a Fuzzy Core in Jupiter.” Astronomy & Astrophysics 638:A121. https://doi.org/10.1051/0004-6361/201937376.

Spilker, L. 2019. “Cassini-Huygens’ Exploration of the Saturn System: 13 Years of Discovery.” Science 364(6445):1046–1051. https://doi.org/10.1126/science.aat3760.

OCEAN WORLDS AND DWARF PLANETS

In addition to Earth, we have identified more than 20 worlds throughout the solar system that may once have had or currently support large liquid water oceans. These so-called ocean worlds include several icy moons of Jupiter and Saturn, which harbor confirmed modern oceans, as well as icy moons of Saturn, Uranus, and Neptune and several dwarf planets, including Pluto and Ceres, in which candidate oceans may exist (Figure 2-27). As identified in the high-level recommendations of An Astrobiology Strategy for the Search for Life in the Universe (NASEM 2019), exploration of these ocean worlds presents an opportunity to find extant life beyond Earth and may provide natural laboratories from which we can study the prebiotic processes that led to the emergence of life on Earth, as well as chronicle the development and sustainability of habitable environments across the solar system. Over the past decade, the study of ocean worlds has also spurred the development of a new cross-cutting field: comparative oceanography. Oceans, much like planetary atmospheres, now extend to worlds beyond Earth. Studying them will lead to a better understanding of how Earth’s oceans and cryosphere work and, hence, how best to protect them.

Starting with the initial discovery of hydrothermal vent ecosystems near the Galapagos Islands coinciding with Voyager 2’s first images of Jupiter’s moon Europa, exploration of ocean worlds has complemented advances in terrestrial ocean science that are redefining our understanding of how terrestrial life may have emerged. For example, definitive evidence for de novo abiotic synthesis of organic compounds at deep-sea hydrothermal

vents and continental subsurface settings on Earth informs investigations of hydrothermal activity with similar characteristics within Saturn’s moon Enceladus (see below for details). These parallels, along with the broad scientific questions associated with ocean world exploration, have attracted a diverse cohort of scientists that includes those not traditionally involved in solar system exploration. Terrestrial oceanographers, cryosphere scientists, and microbiologists are working alongside planetary scientists and engineers to define the next generation of ocean world exploration missions. The ocean worlds themselves are also extremely diverse, with significant differences in ice shell thickness, geologic history, and surface characteristics that provide a plethora of environments to explore.

In the past decade, the response to discoveries made in ocean world exploration have not only excited the scientific community but have also garnered significant public interest. The New Horizons encounter with Pluto, for example, was mentioned on the front pages of 450 newspapers around the world; on the day of the flyby itself, the mission earned 1.7 billion mentions across 21 social media platforms and reached 144 million people through Facebook alone. Cassini’s Grand Finale similarly reached 1.7 billion mentions in social media and was the topic of 791 major media articles. The Jet Propulsion Laboratory won an Emmy award for Outstanding Original Interactive Program for its coverage of the Grand Finale.

The next decade of solar system exploration will be even more exciting and could be the decade in which life beyond Earth is detected (see the Future of Ocean World and Dwarf Planet Exploration section below). The discoveries of the past decade have identified ocean worlds that could be habitable today, and advances in technology have provided the tools to search for evidence of life within these environments. In the following subsections, the committee describes major discoveries made in the past decade and list outstanding questions in planetary sciences and astrobiology associated with ocean moons of the giant planets and dwarf planets found in both the inner and outer solar system. This section ends with a discussion of the future of ocean world and dwarf planet exploration, including major discoveries that could occur over the course of the next decade.

Ocean Moons

Among the confirmed ocean worlds are the jovian satellites Europa and Ganymede, as well as the saturnian satellites Enceladus and Titan. Candidate ocean worlds, where ocean presence is not confirmed but available evidence can best be interpreted as owing to an ocean, include Jupiter’s satellite Callisto, Saturn’s satellite Dione, and Neptune’s satellite Triton. Smaller natural satellites of Saturn—including Mimas, Tethys, Rhea, and Iapetus, as well as the natural satellites of Uranus (Miranda, Ariel, Umbriel, Titania, and Oberon)—are ocean worlds candidates whose composition and proposed heat budget are consistent with the possible presence of an ocean, but no strong evidence has been found to date in their limited observations. These ocean moons represent a diverse set of targets that express a wide variability in ocean depth and seafloor pressures, geologic history, accessibility (e.g., plumes of liquid water erupting from the surface), overlying ice layer thickness, presence or absence of high-pressure ice layers, presence or absence of atmospheres, and surface environments (e.g., Europa’s irradiated icy surface versus Titan’s hydrocarbon-based sedimentary surface). Of these bodies, the oceans of Europa, Enceladus, and Titan are the best documented and have emerged as key targets in the search for an independent emergence of life and the study of prebiotic and possibly biotic processes across the solar system.

While Voyagers 1 and 2 collectively investigated all the giant planet systems between 1979 and 1989, the acquired data from these flybys were insufficient to determine the presence or absence of subsurface oceans. The subsurface oceans of Europa, Ganymede, and possibly Callisto were discovered via magnetic induction by the Galileo spacecraft, which orbited Jupiter from 1995 to 2003. The oceans of Enceladus and Titan were identified and characterized by the Cassini spacecraft, which orbited Saturn from 2004 to 2017. Following on from the discoveries from Galileo, the Europa Clipper spacecraft will perform multiple flybys of Europa in the early 2030s (launching in the 2024). Dragonfly, selected as NASA’s fourth New Frontiers mission, will explore the equatorial region of Saturn’s moon Titan with a rotorcraft drone in the mid- to late 2030s (scheduled to launch in 2027). ESA’s L-class Jupiter Icy Moons Explorer (JUICE) mission, currently slated to launch in the early to 2023, will fly by Europa and Callisto before entering orbit around Ganymede in the 2030s. Collectively, these upcoming missions are designed to characterize the habitability of these confirmed ocean moons. Follow-on missions, which are actively being designed and proposed today, will address the question of whether they are in fact inhabited (see below).

Europa

Prior to the previous decadal survey, Visions and Voyages (NRC 2011), the Voyager and Galileo missions revealed that Europa has a global liquid water ocean that is sandwiched between a dynamic ice shell and a silicate core (Figure 2-28a). The outer surface of water ice was shown to be crisscrossed with long linear features and marked by “chaos” terrain where the surface ice has been disrupted, broken, and in some cases, rotated and frozen into new positions. The presence of non-ice components was determined spectrally and suggests that salt-rich material originating in the ocean may have been emplaced on the surface, with these substances subsequently processed by Jupiter’s strong radiation. Much less is known about the interior, although the ocean is expected to be saline and has the potential for active material exchanges at the ice-ocean and ocean-silicate mantle boundaries.

Our understanding of such exchange processes represents a major development over the past 10 years and is significant because the energy needed to power life is potentially sustained through dynamic water-rock interactions on the seafloor coupled with radiolytically produced oxidants created on the surface and cycled into the ocean below (Figure 2-28b). Apparent detection of subduction on Europa has been used to argue that this satellite is the second known body in the solar system to exhibit plate tectonics-like behavior. If confirmed by future exploration, subduction would provide a mechanism by which surface materials may be transported downward through the ice shell. Conversely, evidence for plumes of water that sporadically erupt from the surface have been detected, tentatively, by multiple, independent observations (Figure 2-28d,e). Perhaps relatedly, pockets of liquid water, from which the plume material may be sourced, may be perched within the ice shell above the subsurface ocean. Ground-based observations show that chaos terrains are associated with high abundances of NaCl, or table salt, suggesting the expression of saline liquids (Figure 2-28c). If NaCl is the dominant salt in Europa’s ocean (as on Earth) then it, too, may have been subject to extensive high-temperature seafloor water–rock interactions.

In the coming decade, the Europa Clipper mission will begin its exploration of the Jupiter system, conducting dozens of close flybys of Europa (closest approach of each encounter is 35–100 km), with the goal of assessing the moon’s habitability and addressing questions such as: How does physical and chemical oceanography affect Europa’s past and present state (e.g., ocean thickness and geochemical exchange between the ocean, ice shell, and seafloor)? How does Europa evolve, both internally and on its surface? Tectonism, subduction, and convection are just a few of the wide-scale processes for which Europa offers a second type example, helping to inform not just how Europa works, but also how these processes work on Earth. Do the plumes originate in Europa’s ocean, or from water pockets in the icy shell? Beyond plume eruptions, what other modes of surface–interior exchange are most common on Europa? Perhaps the grandest question one can ask, however, is: Does Europa harbor life? This question will likely be the objective of the next mission, beyond Europa Clipper, which may involve landing on the surface and/or directly sampling Europa materials to search for evidence of life. If Europa does harbor life, what regulates its habitability and what biochemistry does it utilize? If Europa does not harbor life, but a habitable ocean exists, what are the limits on the emergence of life itself?

Enceladus

Thanks largely to the Cassini mission, the past decade has revealed Saturn’s geologically active moon Enceladus to be a habitable world that contains significant liquid water, energy to sustain metabolism, and conditions favorable for the assembly of complex organic molecules. Much of this evidence has been supplied via analysis of material from a “cryovolcanic” plume of icy particles and gas that erupts from fractures in Enceladus’s South Polar Terrain (Figure 2-29).

The plume’s source within Enceladus is a global subsurface ocean, as established by Cassini in 2014 from analyses of Enceladus’s gravity, topography, and wobble as it rotates. The ocean is deduced to be about 40 km (25 mi) deep, or 10 times deeper than Earth’s oceans, although the reduced gravity leads to seafloor pressures that are equivalent to depths of ~1,000 m on Earth. The chemical compositions of icy particles and gas in the plume provide a “free” sample for analysis that can reveal the nature of the underlying ocean, and its potential to sustain life. Tiny silica grains emanating from the plume point to ongoing high-temperature water–rock reactions made possible by tidal heating below the ocean floor. Sodium and potassium salts, an alkaline pH, and a low-density rocky core provide further evidence for water–rock interactions at depth. Importantly, hydrogen gas, methane, and carbon dioxide were also found in the plume—similar to newly discovered hydrothermal systems and subsurface fluids on Earth. Terran systems have recently been demonstrated to sustain the spontaneous synthesis of simple organic compounds, even in the absence of life, and to support some of Earth’s most primitive forms of microbial metabolism. If Enceladus’s ocean and hydrothermal activity are sufficiently long-lived (current estimates for the age of Enceladus range from 100 million to 4.5 billion years), life could potentially have gained a foothold there.

Cassini showed that the Enceladus plume also contains a diversity of organic molecules spanning a range of sizes and containing, in addition to carbon, hydrogen, and oxygen, the bio-essential element nitrogen. Owing to high-speed collisions between plume particles and the Cassini spacecraft that break down organic matter during collection, however, what were detected are likely to be fragments of even larger molecules. These molecules may have been produced by living organisms, or by abiotic processes; ascertaining their nature and source is a major unanswered question that awaits future exploration.

In general terms, we now know that Enceladus is habitable. But is it inhabited? Answering this civilization-scale question requires a new mission. Modeling from Cassini data indicates that the plume feeds Saturn’s E-ring and, thus, is likely to be long-lived. This is important because it would ensure that future missions to Enceladus can investigate subsurface processes by accessing expelled ocean material without the need to dig or drill into or beneath the ice-shell in the decade(s) ahead.

Titan

Titan is unique among the ocean moons: it has two ocean realms (Figure 2-30). The first, like its ocean world siblings, is a liquid water ocean that lies beneath a water-ice shell. The second consists of liquid hydrocarbon (natural gas) lakes and seas that sit on a surface, shrouded by a dense, hazy atmosphere of nitrogen and methane. A decade ago, we knew that this atmosphere drove Titan to be a dynamic world shaped by Earth-like processes and populated by familiar features including dunes, rivers, lakes, and other earthly landscapes. Within this atmosphere, chemical reactions produce a plethora of organic compounds, some as large as terrestrial proteins. Methane rains onto and carves channels into Titan’s organic- and water-ice-rich landscape, eventually pooling in lakes and seas. The presence of large equatorial dunes demonstrated that, in addition to rain, wind further modifies and transports this organic material. Modeling showed that impacts may support ephemeral liquid-water environments while geologic evidence hinted at cryovolcanism. Laboratory work had shown that if Titan’s organics were to mix with this melt water, amino acids (some of the building blocks of life) would be produced within days.

In the past decade, research has confirmed Titan’s global subsurface water ocean. While high-pressure ice may line the ocean floor (see Figure 2-30), such a layer does not prevent exchange (although it does slow) between the ocean and rocky core (a key to its habitability). Titan’s icy crust is thought to be convecting, and the temperature and pressure at the top of the convection zone are similar to those within terrestrial deep glacial ice, which hosts diverse microbial life between ice grains. The chemical complexity of Titan’s atmospheric constituents is now known to increase in polar winter and with decreasing altitude. Our current list of ~20 atmospheric compounds represent only the tip of the organic factory iceberg. The depth (10023001 m) and major components (methane and ethane) of the hydrocarbon seas are now known and theoretical investigations are exploring if they might support spontaneous assembly of cell membrane-like structures and other elements that could enable a non-water-based alien biology.

These discoveries make Titan a key target for exploration, and many mysteries remain. The processes that create complex species in Titan’s atmosphere are not well understood and operate without, presumably, biological

catalysts like those responsible for large molecules on Earth. Titan’s exploration thus offers fundamental insights into the chemistry that may have preceded and facilitated the rise of biochemistry on early Earth. The Dragonfly mission, which will explore Titan’s equatorial regions in the mid-2030s, will be essential to this effort by providing our first detailed understanding of the surface composition. New isotopic measurements of noble gases and methane would resolve key questions concerning the ocean composition, the evolution of the interior and atmosphere, and the formation of Titan, including the age of Titan’s atmosphere and how it mysteriously remains methane-rich. Determining if Titan’s ocean is interacting with its rocky core would provide a key constraint on the habitability of large ocean worlds both within and beyond the solar system. Global high-resolution imaging and topography would allow us to use Titan’s surface and climate system as a natural laboratory; for instance, to study how planetary-scale hydrologic cycles control the physical and chemical evolution of a landscape in an environment akin to, but less complex than, Earth’s. Similarly, detailed observations of the composition, physical conditions, and seasonal evolution of Titan’s polar lakes and seas would allow us to constrain their role in the hydrologic cycle and climate, and perhaps even their potential habitability.

Other Bodies

The identification of ocean worlds as a new class of planetary bodies brings a new perspective on the diversity of worlds across the solar system. Reexamination of pre-Cassini mission era data from the Galileo and Voyager 2 spacecraft has led to new discoveries at Jupiter’s large moons Ganymede and Callisto, the uranian moons, and Neptune’s largest moon Triton. Results from Galileo demonstrated the likelihood of oceans beneath the surfaces of Ganymede and Callisto, but it was not until the discovery of a habitable ocean at Enceladus during the Cassini mission that moons of the uranian system and at Neptune became widely recognized as potential ocean worlds.

Ganymede and Callisto

Initial studies of Ganymede’s ocean suggested that a layer of high-pressure ice, likely sandwiched between the rocky core and subsurface ocean, would significantly limit water–rock interaction, and hence limit the ocean’s habitability potential. Recent modeling work, however, suggests that not only is exchange through a high-pressure ice layer possible, but that Ganymede’s interior may instead consist of alternating layers of high-pressure ices and salty oceans that would permit direct water-rock interactions. Since Galileo, observations with the Hubble Space Telescope have further supported the presence of an ocean by documenting the dynamics of Ganymede’s aurora. Magnetic induction measurements made by Galileo also indicated that Callisto may have a subsurface ocean, but recent analysis has suggested that some or even all of the induction signal could have instead come from the ionosphere, as opposed to a salty subsurface ocean. At Ganymede, these induction measurements showed that it is the only known satellite with an active magnetic field generated in its metallic core. The Juno mission will study Ganymede’s magnetosphere and its interaction with the jovian magnetosphere. In the early to mid-2030s, ESA’s JUICE mission will reassess the presence or absence of an ocean within Callisto during multiple close flybys before entering orbit around Ganymede to determine the latter’s interior structure and dynamics, map its surface, investigate its tenuous atmosphere, and characterize its intrinsic magnetic field.

Other Saturnian Moons

In addition to Titan and Enceladus, Saturn hosts five moons of intermediate or comparable size: Mimas, Tethys, Dione, Rhea, and Iapetus. In the past decade, the Cassini mission did not detect ongoing geological activity at these moons, although it found indications on Tethys and Dione of past activity (e.g., fractures) and past warm interiors. It also revealed that the interiors of Mimas and Dione comprise a rocky core and ice shell, with possible evidence for an ocean in between arising from their wobble (for Mimas) or gravity and shape (for Dione); however, ocean-free interpretations remain at least as likely. The interiors of Tethys and Iapetus are unconstrained, but likely contain little rock because their densities are similar to that of water ice. In the past decade, the moons’ ages have become debated. Their surfaces, covered with impact craters, seem to be billions of years old. While the moons’ rapid orbital expansion could indicate recent formation (e.g., from a collision disrupting an older moon), this expansion may instead be a consequence of how tidal energy is dissipated inside Saturn, permitting the moons to be as old as the solar system. The moons’ ages thus remain uncertain, which bears on their potential to harbor oceans and on their habitability.

Uranian Moons

Despite a dearth of data, most dating back to Voyager 2, the uranian moons Miranda, Ariel, Umbriel, Titania, and Oberon are considered credible ocean world possibilities for four reasons. First, some moons show possible evidence for cryovolcanism, perhaps sourced from an ocean. Second, some moons show evidence for ammonia, a powerful antifreeze able to sustain liquid water down to 173 K. Third, both Miranda and Ariel are tectonically deformed, with indications of high heat flows at the time of deformation. Last, all five moons are either larger than the confirmed ocean world Enceladus or comparable in size. While only Titania and Oberon would be large enough for oceans to persist solely through radioactive heating, it is likely that all these satellites were heated by tides in the past.

Triton

Triton is a Kuiper belt object that was captured into Neptune’s orbit. Voyager images showed a young surface with relatively few craters and discovered active plumes that stand out among the icy satellites of the outer solar system and put Triton in a class with Io, Europa, Enceladus, and Titan—moons with geological processes active today. While error bars on the absolute crater model ages for Triton are large and the origin of the plumes remains unknown, they do tell the story of a young, dynamic surface heavily modified by internal geologic processes. Theoretical modeling of Triton’s orbital evolution after capture and tides associated with its tilt (obliquity) suggest that tidal energy could maintain a subsurface ocean to the present-day, adding Triton to the family of candidate ocean words. Triton’s unique surface features suggest cryovolcanic processes, which offer the prospect of studying surface–ocean exchange. Earth-based observations show the presence of H₂O and CO₂, which are presumed to form the surface bedrock. Ethane has also been tentatively detected, and the volatile ices N₂, CO, and CH₄ are also present. Recent comparisons to the Enceladus plume suggest that Triton’s plumes could be driven by internal processes rather than solar heating. Near-surface mixtures of ammonia and water can freeze and leave ammonia-rich ice near Triton’s surface whose antifreeze properties may help facilitate cryovolcanism. These observations show that Triton is both exotic and complex, with an array of unique surface features that, when explored in detail, could indicate ongoing exchange between the surface and a subsurface ocean.

Dwarf Planets

Because the current definition of a dwarf planet includes a requirement on its shape, the number of objects known to be dwarf planets is quite small. The large asteroid Ceres is the only dwarf planet in the inner solar system. The Kuiper belt contains Pluto, the archetypal example of a dwarf planet, and Haumea and Makemake, comparable in size to Pluto’s moon Charon. Further out is Eris, the same size as Pluto. Various other candidate dwarf planets exist in the trans-neptunian region.

So far, only Ceres and Pluto have been investigated by spacecraft. Ceres was orbited by the Dawn spacecraft from 2015 to 2018, and New Horizons flew by Pluto in 2015. As a result, these two dwarf planets have been studied intensively, and both are now also regarded as candidate ocean worlds (see below). Other dwarf planets of similar size are credible ocean world possibilities, but there is currently no observational evidence to support this.

Ceres

A decade ago, Ceres—a dwarf planet and the largest object in the main asteroid belt—was but a blurry disk in images from Hubble and other large telescopes. Over the past decade, its exploration by NASA’s Dawn mission has revealed it as a potentially once-habitable candidate ocean world and the most water-rich body in the inner solar system after Earth. Ceres has had sufficient water and radioactive heat to potentially host a deep ocean throughout most of its history, leading to a layered interior with the opportunity for extensive water–rock interaction.

The Dawn mission also revealed recent and even ongoing geological activity on Ceres (Figure 2-31), the presence of liquid water below its ice-rich crust, organic matter (locally) and carbon (globally), and the presence of an exosphere and volatile transport. Material, perhaps erupted from a gradually freezing subsurface layer of salty water and mud, can be found as “bright spots” or “faculae” at multiple locations These observations demonstrate that Ceres has harbored liquid water that has driven, and may still be driving, geologic activity, contains organic compounds, and once experienced water-rock interactions akin to those occurring in hydrothermal environments on Earth.

Ceres shares many similarities with the confirmed ocean worlds Europa and Enceladus, including composition and geologically recent extrusions of salt-rich fluids onto its surface. Unlike these moons, however, Ceres lacks tidal heating so water will have frozen more rapidly. Furthermore, Ceres may represent a surviving member of the protoplanets that transported water and organic material into the inner solar system, including Earth. This makes Ceres a prime target for studying the emergence, evolution, and longevity of ocean world habitability (including Earth).

Despite the success of Dawn, many unanswered questions remain. Is Ceres currently habitable? If not, was it habitable in the past? If so, how long ago was it habitable and how long did habitable conditions last?

How extensive are the liquid reservoirs within Ceres today? What does Ceres reveal about the habitability, over time, of ice-rich bodies and of their ability to generate habitable environments? What processes have driven recent geological activity? Did Ceres form at its current location, further out among the giant planets, or did it migrate inward from the outer reaches of the solar system? What does Ceres’s origin reveal about the potential for planets to migrate on a large-scale? Answering these questions will require follow-on missions to Ceres, including eventual sample return.

Pluto

At the time of the past decadal survey (NRC 2011), almost nothing was known about Pluto. Its mass and approximate radius were known, as was the existence of one large moon (Charon) and at least two small ones. Its density indicated it was made up of ice and rock. But it was still a point of light in a telescope, more the province of astronomers than geologists. In 2015, NASA’s New Horizons spacecraft flew through the Pluto system, revealing an unexpectedly complex and dynamic world (Figure 2-32).

Pluto has active glaciers of nitrogen ice, which carve their way down to fill a plain called Sputnik Planitia. Sputnik Planitia’s solid nitrogen appears to be convecting like a pot of oatmeal on the stove, resulting in a very young surface age. In the slow course of Pluto’s seasons (248-year orbital period), this nitrogen is thought to vaporize and freeze back on the mountaintops, in a manner analogous to Earth’s hydrological cycle. Other materials, like methane, also appear to freeze out only on the peaks of Pluto’s highlands.

Pluto’s atmosphere is made mostly of nitrogen and methane. New Horizons discovered prominent atmospheric haze layers (see Figure 2-32), made of small particles created by irradiation of the gas molecules. These particles eventually fall out to create the dark red material seen on the surface. The hazes themselves may make Pluto’s atmosphere much colder than was expected.

In addition to impact craters, Pluto’s surface is scarred by large fractures, some of which appear to be quite young. These fractures suggest that the crust is expanding, as would occur if a subsurface ocean has been slowly freezing. They thus also suggest an interior in which the ice and rock have completely separated. Two large, mysterious mountains might be cryovolcanoes built from piled-up, now refrozen water magma, and there are hints of cryovolcanism at some of the young fractures. Enigmatic branching channels might be evidence for ancient glacial activity. Other terrain displays thin blades of hardened snow or ice. Such penitentes are found on Earth, but Pluto’s penitentes are 500 times taller.

Theoretical arguments showed that Pluto could retain a subsurface ocean beneath its ice shell. More subtly, Sputnik Planitia’s location opposite to Charon could be explained if a subsurface ocean was present. Although not definitive, these arguments are sufficient to classify Pluto as a candidate ocean world. Despite its achievements, New Horizons imaged only half of Pluto and Charon at useful resolutions; the other halves remain terra incognita.

Other Bodies

Many Pluto-scale worlds orbit the Sun beyond Neptune, including all known dwarf planets except Ceres (see Figure 2-32). What little is known about these distant worlds, and the unexpected complexity discovered up close at Pluto, makes them intriguing targets for exploration. Telescopic observations indicate a striking diversity in their basic properties including the existence and relative size of moon(s) or rings, density, brightness, color, and surface composition. Many have densities similar to Pluto, implying a roughly half-rock, half-ice makeup. This significant rock content implies that the decay of radioactive isotopes contained in the rock can heat these bodies, drive geological activity, and, perhaps, generate and maintain subsurface liquid water oceans.

The largest dwarf planets contain ices composed of methane or nitrogen that, at these worlds’ frigid surfaces, can condense, vaporize, and flow, enabling processes analogous to those that have sculpted the wide variety of landscapes observed on Pluto. Orcus and possibly Quaoar’s surfaces contain ammonia; this potent antifreeze may enable subsurface liquid water, both in the past and perhaps even today. Sedna and Gonggong’s red surfaces suggest the presence of complex organic molecules, while the bright surfaces of Eris, Haumea, and Makemake appear refreshed, potentially by ongoing glacial processes as on Pluto. Haumea is unique, with an elongated football shape owing to its rapid rotation (4 hours), a nearby family of small, similarly icy objects on related orbits, and rings. All of these features indicate a past collision that ejected fragments of Haumea and spun it up. Haumea’s rings, in particular, are key to understanding how rings can form and evolve around solid bodies. As was the case at Pluto, the diversity of compositions and shapes found among the dwarf planets hints at a trove of unexpected discoveries that await exploration. Such exploration, as pioneered by New Horizons, is essential to understand how these worlds formed and evolved, and whether they (as well as other yet-to-be-discovered dwarf planets) represent habitable oases in the outermost reaches of the solar system.

Future of Ocean World and Dwarf Planet Exploration

The initial ocean world discoveries made by the Voyager, Galileo, and Cassini missions motivated Vision and Voyages to recommend a Europa mission to confirm the presence of a subsurface ocean and take the first steps in understanding the potential of the outer solar system as an abode for life. This recommendation is being implemented as the Europa Clipper mission, which will enter orbit around Jupiter in the early 2030s. Europa Clipper will explore the jovian system alongside ESA’s JUICE, which will perform flybys of Europa and Callisto before entering orbit around Ganymede in the mid-2030s. Vision and Voyages also recommended a Uranus orbiter and probe, which would provide the first detailed investigation of an ice giant system, and, in the event of an optimistic budget environment, an Enceladus orbiter mission. While these last two missions have yet to be implemented, the discoveries made over the past decade have done nothing but strengthen the recommendations put forth by Vision and Voyages. Around midway through the past decade, discoveries made during Cassini’s second extended mission led to the inclusion of ocean worlds (Enceladus and/or Titan) as a mission theme to the New Frontiers 4 target list. This addition resulted in the selection of Dragonfly, which will explore Titan’s equatorial terrains using a drone quadcopter in the mid-2030s.

Over the next decade, we will move toward the next stage in the astrobiological investigation of the outer solar system and continue to investigate the physical and chemical processes that shape the ocean worlds. This will include developing technology and designing missions that will directly search for evidence of life at Europa and/or Enceladus. Exploration of the ice giant systems, Uranus and/or Neptune, remains a priority and would provide an opportunity to confirm the presence or absence of subsurface oceans on the larger uranian satellites and Triton. A Titan orbiter would provide critical context to extend Dragonfly’s regional exploration to a global scale, while a lake probe would provide in situ exploration of an environment inaccessible to Dragonfly and directly assess the potential habitability of the liquid hydrocarbon seas. Returning to Ceres, perhaps even to bring a sample back to Earth, would permit investigation of both its habitability, as well as the generation and evolution of habitable environments within ice-rich bodies in general. As highlighted in An Astrobiology Strategy for the Search for Life in the Universe (NASEM 2019), the ocean worlds represent a diverse and rich set of targets for exploration over the next several decades. Enough, in fact, to sustain a multi-decade program of ocean world exploration, although much can still be accomplished through directed flagships and competed missions within the Discovery and New Frontiers programs.

Key Discoveries from the Past Decade

• Intermittent plumes on Europa. Although Europa has a young surface, space telescope observations suggesting intermittent plumes of water vapor jetting into space were a complete surprise. Any such plumes would allow us to directly sample the subsurface ocean and will be a major focus for Europa Clipper.
• A habitable ocean at Enceladus. At the time of the last decadal survey, Enceladus was known to harbor subsurface water, but its distribution was unknown. Geodetic observations confirmed that the liquid layer was global in extent (i.e., an ocean) and in direct contact with the rock beneath, making it a potentially habitable environment.
• Titan is a world of two oceans. The existence on Titan’s surface of methane seas and lakes was known at the last decadal survey, but subsequent geodetic observations also pointed strongly to a subsurface, liquid water ocean, sandwiched between two layers of ice. The combination of surface seas and a subsurface ocean makes Titan unique.
• Brine deposits on Ceres. An unexpected result of the Dawn mission was the detection of surface brine deposits, in some cases associated with recent cryovolcanism. This discovery suggests that water–rock interactions persisted throughout much of Ceres’s history, increasing its astrobiological potential.
• Pluto’s unexpected surface diversity. Pluto hosts solid nitrogen glaciers, channels apparently carved by liquid, an actively convecting ice sheet that is unique in the solar system and several possible cryovolcanoes. Almost none of these features were expected; Pluto demonstrates that even very distant objects can be geologically active and potentially host subsurface oceans.

Further Reading

Hand, K.P., C. Sotin, A.G. Hayes, A. Coustenis. 2020. “On the Habitability and Future Exploration of Ocean Worlds.” Space Science Reviews 216:95. https://doi.org/10.1007/s11214-020-00713-7.

Hayes, A.G., R.D. Lorenz, and J.I. Lunine. 2018. “A Post-Cassini View of Titan’s Methane-Based Hydrologic Cycle.” Nature Geoscience 11:306–313. https://doi.org/10.1038/s41561-018-0103-y.

Lunine, J.I. 2017. “Ocean Worlds Exploration.” Acta Astronautica 131:123–130. https://doi.org/10.1016/j.actaastro.2016.11.017.

Nimmo, F., and R.T. Pappalardo. 2016. “Ocean Worlds in the Outer Solar System.” Journal of Geophysical Research: Planets 121:1378–1399. https://doi.org/10.1002/2016JE005081.

Stern, S.A., W.M. Grundy, W.B. McKinnon, H.A. Weaver, and L.A. Young. 2018. “The Pluto System after New Horizons.” Annual Review of Astronomy and Astrophysics 56(1):357–392. https://doi.org/10.1146/annurev-astro-081817-051935.

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