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Question 5: Solid Body Interiors and Surfaces

Planetary surfaces and interiors are tapestries on which the history of each body is woven.¹ This chapter focuses on “solid” bodies, meaning those that are primarily solid and monolithic, thus excluding rubble pile asteroids and comets (at the small end) and gas/ice giant planets (at the large end). Terrestrial planets, icy moons, large asteroids, trans-neptunian objects, and comets all have surfaces and interiors that record their evolution, as discussed below.

The first two questions in this chapter address the fundamentals of planetary interiors: what kinds of internal structures exist (Q5.1) and how they have evolved with time (Q5.2). The next three questions address the evolution of planetary surfaces, and how they have been modified by interior (Q5.3), surficial (Q5.4) and external (Q5.5) processes. Last, the special case of present-day activity is discussed (Q5.6). This is deliberately separated from the other questions because the measurement techniques used to probe active processes (e.g., seismology, radar interferometry) are fundamentally different from investigations of geologic processes that have happened in the past.

Q5.1 HOW DIVERSE ARE THE COMPOSITIONS AND INTERNAL STRUCTURES WITHIN AND AMONG SOLID BODIES?

The fundamental control on the compositions of solid bodies is the range of their constituent building blocks, including the accreted solid matter that condensed from the solar nebular gas, or incorporated presolar grains (solid material that predates the Sun and the solar system). The composition of these solids varied throughout the solar nebula in response to the local condensation temperature and gas composition (Questions 2 and 3, Chapters 5 and 6).

Depending on the timing, energy, and nature of the accretion process, the body can be partly to completely melted, and this can strongly influence the development of internal structure. The amount of short-lived heat-producing radioactive elements incorporated during accretion, which is a function of accretion time, can also strongly influence compositions, chemical characteristics, and thermal evolution of solid body interiors. As such, structure and evolution are largely controlled by initial composition, formation time, and body size.

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

Q5.1a How Much Variability in Composition and Internal Structure Is There Within and Between Solid Bodies, and How Did Such Variability Arise and Evolve?

Many factors influence the initial structure of silicate bodies, including the interior oxidation state. Under reduced conditions, iron metal can become stable, and under very reducing conditions silicon in SiO₂ can be partitioned into the dense metal phase that can segregate to form a core. The presence and abundance of sulfur, oxygen, and hydrogen can also influence core formation processes. Under moderately reduced conditions, sulfur can combine with iron to form a molten FeS core. However, under more reducing conditions, sulfur will strongly partition into the silicate melt and modify subsequent melting and differentiation.

Oxidation state determines how much iron is metal versus in silicate form. Martian meteorites suggest that Mars is highly oxidized, and that the mantle has varying oxidation states. The recent measurement of Mars’s core size by the InSight spacecraft is consistent with an Fe-O-S rich core (Stähler et al. 2021). Spectroscopy from MESSENGER data of Mercury shows little or no oxidized Fe in surface lavas and taken together with its massive core indicates its interior is highly reduced (McCubbin et al. 2012). Venus probably has an Earth-sized core, but we do not know its composition.

Rocky planet mantles can exhibit heterogeneity. Phase transitions or compositional variations (like those arising from magma ocean overturn) can inhibit homogenization by convection. Isotopic and trace element data in martian meteorites demonstrate multiple ancient reservoirs in that planet’s mantle, and the same is probably true on the Moon.

Giant impacts and the addition of material in the waning stages of accretion can also influence the compositions of both rocky and icy bodies (Questions 2 and 3, Chapters 5 and 6). A giant impact on Earth is responsible for shaping the unique Fe-metal-poor composition of the Moon. An early giant impact on Mars is one hypothesis for explaining the global hemispheric dichotomy and likely modified the composition of the planet. A giant impact on Mercury has long remained as one of the theories to explain its large core mass (Q5.1). Giant impacts in the outer solar system may also be responsible for the wide range of bulk compositions of outer solar system icy bodies (e.g., Saturn’s moon Tethys is almost pure ice, while its similarly sized neighbors are a mix of rock and ice).

Icy bodies can have complicated internal structures, consisting of silicate mantles, sometimes with underlying iron cores, which in turn are overlain by an outer layer that contains one or more water ice phases and, in some cases, include a liquid water ocean. Very distant bodies (e.g., Pluto) might also contain substantial fractions of ices normally regarded as volatile (e.g., N₂ and CH₄). Ganymede is the only icy body where a liquid iron core is certain (based on the observation of a core dynamo); conversely, in the case of Callisto it is unclear whether full separation of rock and ice has taken place. The rock–ice ratio can vary significantly, from Europa (rocky) to Tethys (icy) (Figure 8-1). Oxidation state is not as important a factor for icy bodies, but their bulk compositions could potentially involve substantial amounts of carbon. The thermal evolution of icy bodies is controlled by melting of the ice phases and alteration of silicate minerals to clays, resulting in segregation of an ice-rich surface layer and sometimes a subsurface ocean. Melting is driven either by decay of short-lived radionuclides (which are most abundant in icy satellites that formed earliest), or by tidal dissipation (Q5.2a, Question 8, Chapter 11) and is facilitated by the presence of impurities such as salts. Furthermore, the efficient advection of heat by circulating fluids moderates internal temperatures, potentially allowing volatiles to be retained.

In the solid bodies in the solar system, we have directly measured or inferred the nature of internal layering and compositional variations from remotely sensed physical properties, such as static and time-variable gravity, shape, and seismic velocity. Constraints on core mass/size for planets, and for rock/metal or rock/ice relative abundances in bulk bodies, depend mostly on mean density and moment of inertia factors. Refinement of these, as exemplified most recently by measurement of the mean density of Vesta by the Dawn mission, lead to better estimates (Ermakov et al. 2014). The only extraterrestrial bodies for which limited seismic data are available to constrain interior structures are the Moon (from analysis of Apollo seismometer data) and Mars (from preliminary analysis of InSight data). On Mars the lessons learned on the Moon influenced the design of the InSight seismometer, and after one Mars year the mission has measured crustal thickness and upper mantle velocity structure, as well as determine the core radius (Stähler et al. 2021). Seismology on Venus requires either high-temperature electronics or aerial pressure sensors (Brissaud et al. 2021);

technological development for either approach would enable comparably fundamental discoveries to be made in future decades.

Tidally induced heating can also lead to melting and chemical differentiation. This is thought to be important on Io and Europa and also important on Triton during its capture into Neptune’s orbit (Question 8, Chapter 11). Measuring gravitational tides can be used to constrain internal structures, especially the presence of liquid layers (e.g., the outer core of Mars or the ocean of Europa).

The smallest bodies of the solar system—asteroids, comets and trans-neptunian objects (TNOs)—are dominantly porous aggregates of variably sized blocks, dust and in many cases, ice. A population of larger (>100 km diameter) planetesimals in the main asteroid belt, exemplified by the differentiated protoplanet Vesta, comprises a record of the original planetary building blocks. The composition of the constituent material across the small body population ranges from relatively primitive in volatile-rich comets and TNOs to highly processed in small asteroids within the main and near-Earth asteroid belts. Rosetta’s exploration of Jupiter-family comet 67P/Churyumov-Gerasimenko revealed its affinity to interstellar material, including inherited organic material (Grady et al. 2018). Dawn’s mapping of Vesta provided context for laboratory analyses of the howardite-eucrite-diogenite meteorites, revealing the complexities of magmatic evolution on this smallest terrestrial planet (McSween et al. 2013) (Q5.3b). Catastrophic disruption and reaggregation have scrambled the interiors of small bodies in some cases, leading to small-scale compositional diversity as seen in some meteorite breccias, and well as on the surfaces of the bodies. An understanding of the interior composition and structure of larger planetesimals by measuring surface composition, dielectric properties, shape, density, and spin state informs accretionary and dynamical models (Question 2, Chapter 5).

Q5.1b What Kinds of Internal Liquid Layers (e.g., Oceans) or Discrete Regions Occur in Solid Bodies, What Are Their Characteristics, Where Are They, and How Long Do They Persist?

The existence, locations, and compositional variations of liquid layers and discontinuous liquid regions (e.g., molten cores, partly melted magma source regions, brine lakes, subsurface oceans) in solid body interiors and their persistence depend fundamentally on their compositions, the abundance of heat-producing elements, and the influence of internal and external physical processes that can lead to melting.

As for the terrestrial planets, the composition of the core and the inventory of heat-producing radioactive elements combine to determine the longevity of molten metallic cores. Mercury, the Moon, Earth, and Mars all have long-lived molten cores, in some cases consisting of a fluid outer core, and a solid inner core). The state of Venus’s core is largely unknown.

Solid state convection of the planetary mantles can lead to decompression melting and the presence of continuous small extents of melting in planet interiors; in Mercury this may be reflected in the continual change in erupted basalt composition over the 700-million-year history inferred for volcanism on this planet. On Mars, the vast Tharsis rise provides a 4-billion-year record of volcanism, most likely arising from melting owing to a long-lived mantle upwelling. The surface of Venus is covered with voluminous lava flows and displays a uniform crater density; whether this uniformity is indicative of a catastrophic resurfacing event or continuous volcano-tectonic resurfacing is currently unresolved. The variability in volcanic styles and longevity among the terrestrial planets is remarkable and reflects the differences in internal compositional layering and their thermal histories, but in ways that are not well understood. The most volcanically active body with the youngest surface in the solar system is Io, Jupiter’s innermost large satellite, whose interior is tidally heated by the oscillating pull of Jupiter’s gravity. Io may possess a subsurface, silicate magma ocean at the present day.

In the outer solar system, except for Io, internal liquid layers are dominated by water and other volatiles rather than silicate melts. The roster of such “ocean worlds” has grown with time (Nimmo and Pappalardo 2016). Europa, Titan, Enceladus, Ganymede, and Callisto all possess subsurface oceans, and other bodies, including Triton, Dione and Pluto, are also candidate ocean worlds. Ceres, the innermost dwarf planet in the solar system, may have had a subsurface ocean in the past and perhaps contains remnants of that ocean today as subsurface brine pockets. Little is known of the compositions, pH, and ages of these oceans, yet those characteristics are key to assessing their habitability (Question 10, Chapter 13). In the case of Enceladus, the composition of its geyser-like plumes can be used to infer the composition of its ocean. However, the age of its ocean remains largely unconstrained. Ocean worlds are often found by combining theoretical modeling with magnetic induction measurements (for Europa, Ganymede, and Callisto), gravity and/or rotation state measurements (for Enceladus and Titan), and other geodetic methods and geological inferences. However, ocean detection and characterization is challenging. In general, the compositions, thicknesses, and dynamics of subsurface oceans on these worlds are unknown. While we know that tidal heating helps to maintain these oceans (Question 8, Chapter 11), the likely presence of an ocean on Pluto and a putative past ocean on Ceres suggest that such heating is not required. Additional insulating layers such as gas hydrates (clathrates) within these worlds may have helped maintain these oceans. Nevertheless, the ages and long-term evolution of the oceans are largely unconstrained (Q5.2c).

Q5.1c How Does the Presence of Porosity, Ices, Liquids, or Gases Affect the Physical (e.g., Mechanical, Thermal, Electromagnetic) Properties of the Crust?

The outer layers of solid bodies—the crust and regolith—influence their thermal and mechanical properties. Regolith is the loose, porous, unconsolidated rock and dust on the surface of a planetary body. This layer can stabilize volatile ices within centimeters of the surface by protecting them from diurnal heating and sublimation. The composition of the crust determines its thermal and mechanical properties. For instance, clathrates (ices with structural cages containing gas molecules) within the crust of an icy body lower its thermal conductivity and can keep the interior warm, as is suggested to explain the longevity of brine effusion on Ceres (Castillo-Rogez et al. 2019), and the possible persistence of an ocean on Pluto (Kamata et al. 2019). Clathrates are mechanically strong but unstable at low-pressure and can explosively destruct, which may result in lateral heterogeneity in crustal strength and morphology. The viscosity of ice is sensitive both to the local temperature and to the fraction

of impurities present; thus, the temperature and ice content of the crust of icy bodies will control its mechanical properties. Particularly on small icy or rocky worlds, the low conductivity of the porous near-surface material will keep the interior warm, although high temperatures will close pores by viscous flow. Porous material will also be mechanically weaker than its intact equivalent, and can lead to enhanced scattering, for both seismic and electromagnetic waves. Even small amounts of melt or other liquid can greatly change the mechanical (e.g., viscosity), seismological (e.g., shear-wave speed) and electromagnetic (e.g., conductivity) properties of the crust.

Strategic Research for Q5.1

• Probe the internal structures of the Moon, Mars, and Mercury by establishing a geophysical (seismic/magnetometer) network on the former two bodies and making the first surface seismic/magnetometer measurements on the latter.
• Investigate the properties of subsurface water or magma oceans and melt reservoirs within Europa, Io, Titan, Enceladus, Triton and the uranian moons via electromagnetic sounding (active/passive) or induction, or geodetic measurements from orbiting or landed spacecraft.
• Investigate magmatism, and the effects of interior processes on surface compositions of planetesimals (specifically large asteroids and dwarf planets) via high-resolution imaging, spectroscopy, and topography.
• Determine stable mineral assemblages and the pressure-temperature conditions of melt generation in the interiors of Moon, Mars, Venus, Mercury, Io, and other rocky worlds by carrying out laboratory experiments on returned samples, meteorites, and analog compositions under relevant conditions.

Q5.2 HOW HAVE THE INTERIORS OF SOLID BODIES EVOLVED?

Following their initial differentiation into primordial cores, mantles, ice or rock crusts, and oceans, ongoing mass and energy transport and cycling between surfaces and interiors have led to evolving temperatures and compositions. At the surface, tectonic activity and eruptions are a consequence of internal evolution or external tidal forces. Below the surface, liquid metal cores generate magnetic fields whose histories reflect changes in the temperature and composition of the core and overlying mantle. A great diversity of planetary evolution arises because of different body sizes, different bulk compositions, and the degree of crustal recycling (Figure 8-2). Our ability to reconstruct interior evolution is based on linking sample-based studies, surface observations, and geophysical measurements.

Q5.2a What Mechanisms Have Contributed to Post-Accretion or Post-Differentiation Planetary Cooling and Heating?

The early processes of accretion and differentiation impart a planetary body with an initial heat budget, which can be augmented over time owing to endogenous processes such as radioactive decay and exothermic crystallization, or exogenous processes such as giant impacts and tidal forces. Heat is lost over time via a combination of internal convection, conduction, and either volcanism or cryovolcanism (so-called heat pipe volcanism). The mechanisms through which solid bodies recycle their surfaces have a dominant effect on the composition and thermal evolution of their interior. Plate tectonics, as exemplified by Earth, provides a mechanism for efficient heat loss and direct transport of surface material into the interior. Plate tectonics is largely coupled to mantle convection, wherein cold material buoyantly sinks into the interior, displacing hotter material. Upwelling hot material may also melt by decompression, producing magmas and related volcanism. The reasons why plate tectonics starts and ends remain uncertain. Small rocky worlds such as Mercury, Mars, and the Moon have stagnant lid tectonic surfaces. Venus and ocean worlds such as Europa may also have mechanisms to recycle their surfaces, in the past or possibly at present. Partially mobile surfaces may be possible. On Io, and other planetary bodies earlier in their histories, burial of the surface by volcanism can recycle crusts back into the mantle (Moore et al. 2017). Magmatism and eruptions can also transport heat-producing radiogenic materials to different portions of a solid body, influencing subsequent cooling and volcanism. The various modes of heat transfer in planetary interiors (Figure 8-3)

and their effects on surface mobility give rise to a great diversity of ages of planetary surfaces. The stresses and resurfacing from a convecting interior also create large-scale landforms such as rifts and scarps and contribute to crustal thinning and thickening.

Key to understanding the evolution of planetary interiors is determining how planetary materials deform in response to internal (e.g., radioactivity, primordial heat, and ongoing crystallization) and external (e.g., tides and large impacts) forcing and the feedbacks between that forcing and properties of the interior. Rheology, the relationship between applied stresses and the resulting deformation, affects the ability of interiors to convect and thus to transport heat and mass. Rheology also affects the heating produced by tidal forces, hence the coupling between internal and orbital evolution of satellites (see Question 8, Chapter 11).

The rocky planets and most of the medium-to-large satellites are differentiated and have internal liquid layers. These liquid layers can be responsible for generating magnetic fields and producing fluids that can erupt onto planetary surfaces. Liquid metal cores, partially to totally melted layers in mantles, and subsurface oceans require sufficient heat to melt and remain molten, while crystallization of these layers will produce latent heat. It is unclear whether these internal liquid layers are primordial—formed and maintained by heat from accretion and other heat sources—or if they are formed cyclically or incidentally via episodes of tidal heating or large impacts.

Q5.2b What Processes Control the Production and Evolution of Magnetic Fields?

Within solid planetary bodies, dynamo-generated magnetic fields are typically produced by motion of conductive fluid within a metallic core. Therefore, establishing that a planetary body produced a magnetic field during its history implies that the body underwent differentiation to create a fluid core. Spacecraft-based observations and paleomagnetic studies collectively indicate that Mercury, Earth, the Moon, Mars, Ganymede, and some planetesimals (e.g., Vesta and the pallasite parent body) have generated dynamos at some point in their histories. Dynamo fields are generally sustained by thermal or thermochemical convection driven by heat extraction into the overlying mantle. Thus, dynamo activity is usually controlled by the ability of the mantle to extract heat, so that mantle thermal evolution ultimately dictates whether a dynamo will operate. Dynamo fields may also be produced or affected by mechanical perturbations of cores from large impacts or precession, chemical exchange between cores and mantles, and even convection of sufficiently conductive silicate fluids within basal magma oceans (Lapôtre et al. 2020). In contrast, dynamo activity may be inhibited if, during cooling, core fluids achieve certain combinations of composition, temperature, and pressure that result in top-down or iron snow crystallization regimes that can inhibit convection. Therefore, establishing the field intensity histories and lifetimes of dynamo magnetic fields via paleomagnetism (Tikoo et al. 2017) or crustal magnetization studies (Johnson et al. 2020b), in tandem with gaining information regarding internal structure and composition, can reveal the thermochemical evolution of planetary interiors. Our knowledge of the history of planetary magnetic fields is currently too limited to understand the various ways in which planetary magnetic fields start and end.

Mapping of crustal magnetism not only sheds light on the history of the dynamo (if the nature of magnetizations and their ages can be constrained) but can also be used to understand surface and crustal processes. For example, magnetic anomalies associated with crater interiors and ejecta deposits on the Moon and Mercury may provide information regarding impact cratering dynamics, and the fate of iron sourced from the impactor. On Mars, and potentially Mercury, elongate zones of contrasting crustal magnetization may record large-scale tectonics and crust formation (Johnson et al. 2015). On airless bodies, intense remanent crustal magnetization may affect space weathering and/or regolith sorting (e.g., at lunar swirls; Q5.5a). Crustal magnetization may also record hydrothermal alteration and fluid flow within planetary crusts.

Q5.2c What Are the Chemical and Physical Consequences of Cooling and Solidification on Solid Body Crusts?

The composition and physical characteristics of a planetary body’s initial, primordial crusts set the stage for the remainder of its evolution. Primordial crusts on rocky bodies generally formed from solidification and overturn of magma oceans. Crusts on differentiated icy satellites may have formed from the solidification of water and other volatiles that were initially liquid owing to accretional heating, or from later melting and mobilization of ice within their interiors. As a result, a great diversity of primordial crusts may have formed across the solar system, with compositions ranging from graphite on Mercury (e.g., Peplowski et al. 2016); anorthosite on the Moon; basalt and/or ultramafic magmas on Earth, Mars, Io, and probably Venus; and to ices on outer solar system satellites. These crusts are physically and chemically modified by secondary processes such as (cryo)volcanism,² impacts, weathering, and tectonics.

Magmatism and tectonism are the major internal processes that act to modify primordial crusts through the addition of new material (extrusive or intrusive volcanism), modification of crustal thickness (deformation), or crustal processing (remelting, recycling of material into the interior). The composition of material added to primordial crusts through magmatic and volcanic activity is coupled to the thermal and compositional evolution of the deeper interior (Q5.3b). As planetary objects cool and evolve, their interior compositions will change owing to the progressive extraction of magma at planetary surfaces and freezing of subsurface oceans and cores.

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² While cryovolcanism can be regarded as a subset of volcanism, this report draws a distinction between the two processes for clarity. Thus, (cryo)volcanism should be read as volcanism and/or cryovolcanism.

Studying the composition of materials exposed on planetary surfaces, such as changing compositions of volcanic products erupted on the surfaces of rocky bodies and ocean worlds, thus provides an opportunity to reconstruct internal evolution and ongoing differentiation. For example, remote sensing data, in situ measurements of rocks by Mars rovers and analysis of martian meteorites record changes in composition that have been attributed to progressive cooling of the martian mantle (e.g., Baratoux et al. 2011; Filiberto 2017). Other observable changes in volcanism over time may include the type and quantity of volatiles in magmas, and the degree to which processes such as fractional crystallization occur during magma ascent. In extreme cases, initially basaltic melts can evolve to granitic compositions, as seen in samples and remote sensing data of the Moon (Jolliff et al. 2011), possibly from remelting the crust.

Changes in composition and temperature create stresses and change densities and hence can influence surface tectonics, whether eruptions can bring subsurface materials to the surface, and how surface features will be preserved. Cooling and contraction of the interior of Mercury created some of the largest faults in the solar system and surface compression led to a great reduction in the rate of volcanism (Byrne et al. 2016). Solidification of metallic cores may also enable the eruption of metals onto the surface (Johnson et al. 2020a). Within the ocean worlds, ongoing solidification of subsurface oceans can generate stresses that enable the eruption of water (e.g., Hemingway et al. 2020). The icy satellites preserve a great diversity of tectonic structures that are produced by variable combinations of external tidal forces and internal dynamics within and below their ice shells. Cooling also makes rocks and ice more viscous and hence increases the ability to preserve features on planetary surfaces. Unravelling the processes that create and preserve tectonic structures and their chronology can thus constrain the internal evolution of bodies with solid surfaces.

Combining the various insights offered by the composition of erupted materials, with gravity, topography, radar and electromagnetic mapping, stratigraphy, dated surfaces, and measurements of present-day heat flow, provides opportunities to determine temperature in the past and at present.

Strategic Research for Q5.2

• Determine the timing and flux of volcanism on Venus, Mars, and Mercury using orbital and in situ measurements of crustal composition and mineralogy with accompanying in situ radiometric dating.
• Probe the magmatic history of the Moon by conducting coordinated high-fidelity geochronology, geochemistry, and petrologic analyses either by in situ exploration or by analyzing samples returned by robotic or crewed missions.
• Determine crustal composition, heat production, and origin of crustal dichotomies (if any) on the Moon, Mars, and Venus by in situ geochemical, mineralogical and heat flow measurements by rover(s) or lander(s), by laboratory analyses of returned samples, and by collecting orbital and seismic data.
• Determine the temperature, depth and timing of chemical differentiation, and the compositional and petrologic characteristics of magmas on different bodies by a combination of theoretical and experimental studies on samples or analog compositions.
• Assess the diverse mechanisms that create magnetic fields by measuring the topology and evolution of active geodynamos in Mercury and Ganymede, determining whether Venus had an active geodynamo, and by studying remanent crust magnetization produced by extinct dynamos on the Moon and Mars, via spacecraft measurements or paleomagnetic studies of returned samples.

Q5.3 HOW HAVE SURFACE/NEAR-SURFACE CHARACTERISTICS AND COMPOSITIONS OF SOLID BODIES BEEN MODIFIED BY, AND RECORDED, INTERIOR PROCESSES?

Planetary surfaces, the most external portion of the solid body, are influenced by complex physical and chemical processes within the body’s interior. The evolution of the two regions is therefore linked, and surface features can preserve the history of the interior. The questions that follow highlight multiple ways in which the interior modifies the surface and illustrate the importance of teasing out how different interior processes influence the surface. Constraining a planetary body’s unobservable interior using surface materials is inherently challenging, and many questions about how interior processes influence surfaces remain.

Q5.3a What Internal Processes Control Surface Topography and Produce Tectonic Features?

Surface topography is supported through a combination of dynamic, active processes (such as a plume of hot rising mantle material pushing up on the lithosphere, creating a topographic rise) and the strength of the lithosphere. Lithospheric strength depends on (at least) thickness and composition—for example, ice is generally weaker than silicate rock. Tectonic features are produced by lithospheric or crustal deformation, and their presence requires heterogeneous stresses and strains. Deep understanding of topography, tectonics, and geologic structures requires detailed information of the surface structures and their relative ages, combined with knowledge of a body’s internal properties (e.g., Black and Manga 2016). In particular, it is necessary to know the thickness and the composition of the lithosphere, and whether those parameters have remained constant with time. The state of the underlying mantle can also have important consequences for surface topography and tectonics, and so it is necessary to understand if mantles were warm and deformable (either from tidal heating or radiogenic heat) or essentially frozen—and if so, when that transition happened. Topography and tectonics features can also record tidal and rotational forces (in both orientation and magnitude) and provide key insights to the evolution of a planetary body’s spin and orbit through time. A common question for nearly every solid body is: What is (or was) the duration and magnitude of tectonic activity? The special case of present-day activity is discussed in Q5.6.

The varied activity of solar system bodies gives rise to a diverse array of tectonic features that have yet to be explained. Mariner 10 and MESSENGER revealed that Mercury possesses near-global scarps and folds. The Moon and Mars both have global-scale crustal dichotomies or asymmetries that are still unexplained. Venus’s surface is replete with tectonics, yet it is unclear if they are the result of movement of lithospheric plates (analogous to Earth’s plate tectonics), or some other process. The age and crustal thickness of Venus’s major geologic terranes, the tesserae and the plains, are uncertain. This tectonic menagerie only becomes more varied in the outer solar system. In most cases, the thicknesses of ocean world ice shells are unknown. Plate-tectonic-like motion has been hypothesized for Europa, but still debated. On Enceladus, the formation and evolution of the south polar tiger stripes (the tectonic fractures that source Enceladus’s plume) are unknown. The tectonic history of more distant worlds, like the uranian and neptunian satellites and trans-neptunian objects (including Pluto and Charon) remain elusive.

Q5.3b What Controls (Cryo)Volcanic Eruptions on Bodies with Solid Surfaces, and How Does the Composition of Erupted Materials Vary?

Volcanic eruptions shed light on the internal compositions, temperatures, and degree of processing of subsurface materials on solid bodies without having to directly sample their interiors. Eruptions on planetary bodies are controlled by numerous, interrelated factors, including (but not limited to) amount of internal heating (controls melt generation and degree of melting), how easily melts can erupt (influenced by volatile content, composition, and physical properties of melts), melt composition (influenced by conditions at (cryo)magma source), and crustal thickness, stress state, and composition (influencing melt ascent and eruption style) (e.g., Head and Wilson 2017). The ratio of silicate magmas erupted onto the surface versus those intruded into the crust appears to be highly variable and is known only for Earth with any fidelity. However, typical ratios suggest that only a small fraction of magmas are actually able to erupt on the surface of any planet or moon. The terrestrial ratio of extrusive to intrusive volcanism varies with tectonic setting, is estimated to vary inversely with size of the body (Black and Manga 2016) and is related to the structure and stress state of the crust and the efficiency of mantle degassing. Although much less is known about the eruptibility of cryomagmas on ocean worlds and dwarf planets, they may follow similar trends.

Compositional variation in erupted materials is controlled by several factors. On icy bodies, the degree of connectedness between the subsurface ocean and the surface (e.g., through fractures in the ice shell) can influence the degree of processing of cryomagmas during ascent, possibly resulting in higher concentrations of sulfate, carbonate, or chloride salts in eruptions on larger bodies like Europa and Titan (e.g., Quick and Marsh 2016). Materials that erupted onto the surface of dwarf planet Ceres may be representative of the remnants of a muddy ocean, the gradual freezing of which produced salts whose composition may have changed over time (e.g., Raymond et al. 2020). On rocky bodies, silicate magmas produced by partial melting of the mantle (primary magmas) rarely erupt

directly onto the surface, although they have been suggested to be more common on some planets and planetesimals (e.g., eucrites on Vesta). Instead, on rocky and ocean worlds, (cryo)magma compositions typically evolve during ascent, while in residence in intermediate staging chambers within the crust, and upon eruption. They commonly experience separation of crystals from melt (fractional crystallization) and sometimes contamination by the enclosing rocks (assimilation). A quantified understanding of compositional changes that (cryo)lavas have experienced is necessary if they are to be used to constrain the internal compositions of planets and satellites. As the interior of bodies change, so does their volcanism (see Q5.2c). For example, if no crustal recycling exists, volcanism can provide volatiles with a one-way ticket from the mantle to the surface. Over time, this would cause mantle material to become volatile poor (i.e., to “dry out”), affecting the nature of volcanic eruptions throughout the planet’s history.

Q5.3c How Have Diverse (Cryo)Volcanic Processes Shaped Solid Body Surfaces, Physically and Chemically?

Volcanism is a process that transfers both energy and material from a planetary body’s interior to its surface, and is driven by conditions in a body’s interior, providing a link between volcanic products and internal conditions. Volcanism results in addition of mass to a surface and a reshaping of the landscape; especially voluminous effusions (e.g., flood basalts) may form the bulk of the crust in some locations. Along with volcanic edifices, a myriad of other volcanic landforms is observed throughout the solar system including flows, channels, lava tubes, domes, rilles, calderas, and pyroclastic deposits. In some cases, such as the Tharsis volcanoes on Mars, the edifices are so large they can deform the crust beneath and surrounding them. As discussed in Q5.6b, Io, the most volcanically active world in the solar system, possesses active, 100-kilometer-wide overturning lava lakes, fire fountains, gigantic plumes, and is globally resurfaced by volcanic processes at a rate of roughly 1 centimeter per year. Venus is similarly covered with volcanic landforms, although it is uncertain if Venus is volcanically active today. Solidified lava flows have been observed to cover older topography on bodies including Mars, the Moon, Mercury, Venus, and Io. This process also occurs with smooth cryolava units on ocean worlds such as Europa, Triton, the uranian satellite Ariel, and Pluto’s moon Charon (e.g., Beyer et al. 2019). Pyroclastic deposits have been identified on Mercury, the Moon, and Io, and possibly on Mars and Venus as well, and would serve as a source of sediment and also prevent erosion of the surfaces upon which they are deposited. While we can identify many volcanic features across the solar system, we have nowhere near a complete catalogue of these features. In many cases, we lack high enough resolution imagery of these features to definitively identify them as volcanic in origin. Similarly, understanding emplacement processes is hampered by the difficulty in determining internal flow structures and subtle compositional variations. These issues are particularly problematic on Venus and icy bodies, where an array of unique, likely volcanic landforms have been identified but cannot be fully described, classified, or explained (e.g., Crumpler et al. 1997). The physical expression of volcanism is often controlled by the composition of the lava involved, thus obtaining compositional data of these landforms is also critical.

The addition of fresh (cryo)volcanic material to a planetary surface also alters the surface chemical environment. This has the effect of providing locally distinct chemical regions, or replenishing elements that may have been transported away or taken up by other surface processes. Dustings of fine particles from geyser-like plumes on Triton, Enceladus, and possibly Europa, and brine eruptions on Ceres have produced varying compositions on their surfaces, including local enhancements of nitrogen, to extensive icy mantlings and elevated concentrations of sulfate and/or chloride and carbonate salts, respectively. Much is still to be learned concerning the compositions of flow-like features and airfall deposits on planetary bodies. The majority of volcanic products thus far identified on rocky bodies appear to be basaltic, and on bodies for which we have sufficient compositional data, these basalts are different in composition from the average crust. However, detailed investigations of meteorite samples from Mars and the Moon, as well as exploration by Mars rovers, have revealed local generation of more evolved melt compositions (Q5.2c). This suggests that broad-scale observation of volcanic products may not be sufficient to capture the full story of the chemical modification of planetary surfaces by volcanism.

Q5.3d Where and How Are Surfaces Modified by Hydrothermal/Geothermal Processes?

Materials that have been mineralogically or geochemically altered by hydrothermal processes in the interior can be exposed on the surface, thus modifying its composition. One example is the metamorphic minerals thought to have been formed in hydrothermal systems established below large martian impact craters—systems that can persist for centuries or longer (see also Q4.3d). Such hydrothermally altered rocks in impact ejecta have been analyzed by the Opportunity rover in Endeavour crater on Mars. The occurrence of water ice in permanently shadowed regions of the Moon and Mercury may allow the possibility that hydrothermal systems may have been established below some craters on those bodies as well. Another example is serpentine and clays in carbonaceous chondrite meteorites, formed by early aqueous alteration of ice-bearing asteroid parent bodies heated by rapid decay of short-lived radioisotopes. The emplacement of magmas within planetary crusts containing groundwaters or ices may have also driven hydrothermal reactions. Available observations of Venus are not extensive enough to know whether or not hydrothermal activity may have occurred there, but the ancient tesserae may hold evidence of past fluid-rock interactions. Some geothermal reactions may also involve fluids composed of volatiles that are not water. For example, Mercury and Io may have had fluid-rock interactions involving sulfur, and the Moon shows evidence of rock interaction with H₂ and CO fluids. Although hydrothermal processes may be widespread among the terrestrial planets, our knowledge of them is hampered by limited surface exposures.

Ocean worlds have ongoing geothermal activity. The Cassini spacecraft found silica grains thought to be sourced from erupting plumes on Enceladus, pointing to water–rock interaction at temperatures likely occurring at the seafloor of its subsurface ocean. Melting of carbon- and nitrogen-containing ices could have produced non-aqueous or hybrid fluids in icy worlds. The Dawn mission discovered ammonia-bearing clays and salts formed by extensive aqueous alteration on dwarf planet Ceres. Similar fluid-rock interactions may occur on the ocean floors of Europa, Titan, and other giant planet satellites that are subject to tidal and/or radiogenic heating. As water-rock interactions are only sustainable if fresh rock is available, constraining the timing and duration of water-rock interactions on ocean worlds also provides information on the exposure or production rates of fresh rock surfaces, giving insight into other geological processes. Hydrothermal processes may also be responsible for synthesis of some of the organic compounds on ocean worlds, dwarf planets, and asteroids. Much work remains to be done in deciphering the compositions of subsurface fluids in extraterrestrial bodies and in understanding their origin, chemical evolution, and persistence through time.

Strategic Research for Q5.3

• Identify and classify tectonic and volcanic landforms (both modern and ancient) on rocky bodies (Venus, Mars, Mercury, the Moon, and Io), and provide fundamental constraints on lithospheric properties such as the thickness of the deforming layer via high-resolution topography, gravity and images of the surfaces, and chemical and mineralogical measurements.
• Investigate and classify tectonic and cryovolcanic activity and landforms on icy bodies (Europa, Enceladus, Triton, and Titan) to determine properties such as location of deforming regions and the thickness of deforming layers via global high-resolution imaging and topography, and constraining the ages of cryovolcanic units/structures.
• Probe compositional evolution, surface weathering, and fluid–rock interactions on icy moons and dwarf planets such as Europa, Enceladus, Titan, Triton, Ceres, and the uranian moons using orbiting and landed spacecraft measurements of crustal composition and mineralogy.
• Determine the range of volatile contents and species in planetary melts in igneous samples from Mars, the Moon, and asteroids, to constrain the range and variety in planetary volatile contents, and factors influencing melt generation, composition, and eruptibility using Earth-based laboratory measurements of returned samples and/or meteorites.
• Assess the nature and timing of hydrothermal processes on small bodies and planets via modeling of aqueous alteration, and a combination of high-resolution spectral data and laboratory analyses of samples of asteroids Ryugu and Bennu returned by Huyabusa2 and OSIRIS-REx, from Mars’s moon Phobos from MMX, and from Mars by the Mars Sample Return Program.

Q5.4 HOW HAVE SURFACE CHARACTERISTICS AND COMPOSITIONS OF SOLID BODIES BEEN MODIFIED BY, AND RECORDED, SURFACE PROCESSES AND ATMOSPHERIC INTERACTIONS?

The surfaces of solid bodies record evidence of atmospheric interaction, and thereby provide information about how the atmosphere may have evolved through time (see also Question 6, Chapter 9). Constraining the nature of the interaction between atmospheres and solid surfaces requires detailed morphologic, stratigraphic, and compositional information of the solid surface, and how these characteristics may be expressed in different locations and preserve different times in the past. Generation and transport of sediment, regolith formation, composition (including the products of chemical weathering) and distribution, and how the regolith composition changes with time and place all provide vital details about how the atmosphere shapes a planetary surface. Given that “the present is the key to the past,” precise information about the current atmosphere (including composition, temperature, and pressure), and how it varies laterally, vertically, and temporally (on diurnal, seasonal, and longer timescales), is essential for improving our understanding of atmosphere-surface interactions.

Q5.4a Where and How Have Fluvial Processes Sculpted Landscapes?

Landscapes shaped by liquids inherit physical, chemical, and mineralogical properties reflecting the climatic, fluid, and geologic conditions at their formation. Investigation of these fluvial landscapes and subsequently formed sedimentary materials can reveal the history of atmospheric conditions, timing of habitable conditions, and the impact of distinct planetary conditions on fluvial processes. Such landscapes and preserved sedimentary rock records are clearly observed on Mars, where multiple orbital and landed missions have confirmed the presence of liquid water in the past, which formed rivers, deltas, lakes, and possibly oceans (McLennan et al. 2019). However, it is unclear if the current landscapes as observed from orbit form a complete record of Mars history, or if surface-based observations would reveal a much longer history of fluvial activity. Possible records of past surface fluid flow may also exist on Venus and Titan. Venus was not previously considered a candidate for habitable surface conditions owing to its extreme surface conditions, but work in the past decade has suggested that Venus’s deviation from Earth-like conditions may have been geologically recent, and some rocks on the venusian surface might record fluvial activity from more clement past surface conditions (Way et al. 2016). Titan has modern fluvial activity, but with organic fluids and a water-ice crust, it is unclear whether a sedimentary record of past fluvial activity is preserved. Some features on the dwarf planets Ceres and Pluto have been ascribed to fluvial activity, although this is more uncertain. For all of these worlds, it remains unclear how planetary hydrologic cycles redistributed, modified, and concentrated materials in the past, at what time, and for how long.

Q5.4b Where and How Have Glacial Processes Sculpted Landscapes?

Glaciers sculpt landscapes through strong erosive processes at the base of massive ice piles and through evaporation, melting, and sublimation of ice deposits. Glacial processes have affected landscapes on terrestrial and icy bodies from Earth to Pluto and possibly beyond. On terrestrial bodies, wet-based glacial activity causes substantial erosion and smoothing of surface topography, as well as smoothing of impact features, and is recognizable long after the activity stops. On Mars, as on Earth, the strong topographic effects of glacial activity are frequently used as a constraint for paleoclimate models. However, the distribution of identified glacial features on the surface of Mars does not match the expected distribution based on current paleoclimate models, so improved understanding of the past extent of glacial surface modification may improve understanding of the martian paleoclimate. Further insight into Mars’s present and paleoclimate would be obtained by constraining the current amounts and spatial distribution of near-surface and subsurface ice. A record of past glaciation is also contained in the polar caps on Mars as observed by radar, but we have yet to understand the chronostratigraphy of Mars’s polar layered deposits and the nature of the atmospheric-surface interactions preserved in these layers.

Glacial activity on dwarf planets such as Pluto, and on outer-planet moons, generally involves exotic ices rather than water ice. Nitrogen ice glaciers are flowing today on Pluto, filling a vast solid nitrogen ice sheet within an impact basin, and geomorphic evidence exists for more extensive glacial activity in the geologic past. Similar or

related glacial activity may be occurring on Triton and on other dwarf planets in the Kuiper belt. These different ices and mixtures of ices create glaciers with distinct physical properties compared to Earth, so further study of such glacial processes is important for understanding and further investigating surface processes and their records on bodies farther out in the solar system. The record of glaciation on icy bodies and dwarf planets remains uncertain, as does the exact composition of the glaciers, and the processes controlling their accumulation, movement, and destruction.

Q5.4c How Has Regolith Generation and Subsequent Gravitational or Aeolian Transport of Material Driven Landscape Evolution?

Regolith, or unconsolidated surface material, is found on nearly all solid solar system surfaces regardless of size, density, solar distance, or age. The generation of such regolith during impact cratering, physical or chemical weathering, precipitation, and/or chemical sintering frequently involves interactions between surfaces, planetary volatiles, atmospheres, and/or the surrounding space environment, and is discussed in more detail in Q5.5b. Further transport of such materials downslope by mass wasting or through aeolian transport has shaped landscapes with bedforms recording ancient processes.

Bodies with even thin, transient atmospheres also contain a record of aeolian transport processes that may record past climates, atmospheres, and atmospheric densities. For example, large aeolian ripple bedforms, which form under atmospheres less dense than Earth’s current atmosphere, are recorded in both modern sediments and sedimentary rocks on Mars (Lapôtre et al. 2016) and have been putatively identified on comet 67P/Churyumov-Gerasimenko (Thomas et al. 2015). Other preserved landscape-forming aeolian bedforms, such as transverse aeolian ridges on Mars, Venus’s dune fields, extensive fields of longitudinal dunes on Titan, and putative aeolian features on Pluto and Triton are not yet well-understood. Investigation of such aeolian landscapes and those preserved in sedimentary records on Mars and potentially Titan or Venus would help expand our understanding of aeolian systems on Earth to other planetary conditions and improve understanding of aeolian physics more broadly.

Mass wasting—the movement of unconsolidated materials downslope under the force of gravity—depends on composition, preexisting weaknesses, and other factors. All planetary bodies have gravity and at least some topographic relief, if only from impact craters, so mass wasting may be one of the most common surficial processes. Small impacts themselves can give rise to diffusive down-slope motion of material, or trigger landslides. Liquid has prompted mass wasting on Mars and perhaps on Titan. The flanks of volcanoes are also subject to slope failure, and mass wasting is a natural part of volcano evolution. The origin of young martian gullies, mass wasting at a much smaller scale, remains controversial (McEwen et al. 2011). Mass wasting also occurs on scarps on small bodies, such as asteroid Vesta (Krohn et al. 2014).

Q5.4d What Are the Signatures of Chemical Weathering/Alteration, and How/Why Have Surface Mineralogies Varied over Time?

Chemical weathering and alteration of surface materials represent the main way atmospheres and hydrospheres interact chemically with the solid surfaces of planetary bodies and produce secondary minerals from reactions between minerals and species in atmospheric gases or fluids. The specific secondary minerals formed are determined by the initial mineralogy of the surface, chemical species in the atmosphere or fluid, concentration of gas/fluid species, and the pressure and temperature conditions of the near-surface environment. Therefore, chemical weathering is highly dependent on the climate of planetary bodies and its chemical signatures can vary between planets, as well as over time on a single planet. This fact makes preserved weathering products useful tracers of past climate conditions.

Signatures of weathering are contained in both mineralogy and chemistry of weathering products. For example, orbital mineralogical data has been used to identify changing weathering products on Mars over geologic time, by identifying clay minerals in ancient layers, sulfates in progressively younger material, then anhydrous iron oxides in modern regions (Bibring et al. 2006). Alteration at the surface and within the crust may have largely drawn down the planet’s surface water inventory (Scheller et al. 2021). However, it remains unclear how extensive this

weathering is on Mars, and what it can reveal about the presence (or absence) of surface water or groundwater, and the compounds dissolved into that water through Mars’s history. Venus may once have had a more clement surface environment, potentially hosting liquid water for substantial amounts of time (Way et al. 2016). In such a scenario, weathering would likely have occurred there and would be starkly different than the current weathering style occurring under hot, high-pressure, dry conditions. Similarly, weathering processes involving dissolution of soluble organic material by methane rain have been proposed for Titan, but there is currently not enough data to document this process or use it to unravel the history of surface–atmosphere interactions on Titan, including climate change (Neish et al. 2015). Aqueous alteration, which could be considered a form of chemical weathering, has occurred on asteroids or comets, and likely on (or within) icy bodies as well.

Strategic Research for Q5.4

• Search for evidence of weathering or the presence of ancient water on Venus by characterizing the chemical compositions and mineralogy of surface rocks paired with high-resolution imaging and topography using global and local scale measurements.
• Map and measure the geologic, chemical, and mineralogical characteristics of Mars’s Noachian stratigraphic record to correlate local and regional sedimentary depositional episodes and provide insight into the range and diversity of environments and their relative timing via in situ measurements from a long-distance rover or airborne vehicle.
• Characterize the paleoenvironment, weathering, habitability, geochemistry, petrology, geochronology, and geologic history of returned samples from an ancient martian sedimentary sequence, as well as regolith and any igneous rocks, via laboratory analyses of samples returned from Mars.
• Investigate the fundamental processes that govern hydrologic cycles by investigating Titan’s hydrologic cycle via global high-resolution imaging and topographic and mineralogical data.
• Constrain the history of glaciation and erosion on icy bodies via experimental studies of the material properties (e.g., strength, density, volatilities, and rheology) of relevant planetary ices, planetary ice-regolith mixtures, and ice clathrates at relevant temperatures and pressures.
• Constrain weathering rates and regolith formation, including physical, chemical, and mineralogical changes to surface materials during weathering under conditions relevant for Venus, Mars, and Titan using experimental studies of abrasion, weathering reactions, and kinetics.

Q5.5 HOW HAVE SURFACE CHARACTERISTICS AND COMPOSITIONS OF SOLID BODIES BEEN MODIFIED BY, AND RECORDED, EXTERNAL PROCESSES?

Planetary surfaces often retain records of the external environments in which planetary bodies formed and evolved. Deciphering these records, however, requires detailed knowledge of the processes by which the environment and surface interact. Space weathering influences the physical and chemical properties of planetary bodies, and how they reflect and absorb light. Impacts and volatile sublimation/redistribution significantly modify the surfaces of bodies throughout the solar system. Impacts disrupt and redistribute materials on planetary surfaces, excavate materials from depth and introduce exogenic materials. Volatile activity shapes the surfaces of smaller bodies, drives many tenuous atmospheres, and volatiles trapped on airless bodies provide crucial historic records.

Q5.5a How Do Space Weathering Processes Modify Surface Characteristics and Compositions?

The surfaces of airless bodies are bombarded by micrometeoroids, the solar wind, magnetospheric plasma, solar energetic particles, and galactic cosmic rays (e.g., Pieters and Noble 2016). These processes impart profound changes to the physical properties, chemistry, and spectral properties of airless bodies, and are collectively known as space weathering. Because of the complex interactions of space weathering agents operating in concert, and the difficulty of reproducing the environment, processes, and relevant timescales in laboratory simulations, space weathering is only partly understood.

Charged particles (from the solar wind, and/or the relevant planetary magnetosphere for planetary satellites) collide with individual regolith grains and lose energy as they are implanted into grains, breaking bonds. With increasing radiation exposure, regolith grain surfaces accumulate damage and lose their periodic crystalline structure (become increasingly amorphous). Some fraction of these charged particles also causes sputtering, where atoms are ejected from regolith grains and are either lost to the exosphere or deposited onto nearby grains. Micrometeoroid impacts also result in the production of impact glass (for rocky surfaces) and vapor, which can be deposited onto the surfaces of regolith grains.

For silicate targets, these processes form opaque nanoscale particles (e.g., metallic iron, troilite) within the glass coatings on grains, which strongly affect light absorption and reflection. Micrometeoroid impacts further produce glass-welded agglomerations of opaque-rich soil grains called agglutinates. Because charged particle and micrometeoroid bombardment are both thought to result in similar physical and chemical changes (apart from agglutinates), it is not known whether one process dominates, their relative importance in altering the regolith, or if there are substantial differences when both processes operate together. Further, the relative roles of more-energetic but infrequent particles—solar energetic particles (SEPs) and galactic cosmic rays (GCRs)—are not known. SEPs and GCRs may cause dielectric breakdown (“sparking”) that would also result in the production of glass and possibly nanoscale opaques (Jordan et al. 2015); the contribution of dielectric breakdown to space weathering is unknown.

For icy surfaces such as Europa, radiation from magnetospheric corotating plasma converts crystalline ice into amorphous ice, forms radiolytic volatiles (e.g., H₂O₂, O₂) and non-ice compounds (e.g., acid hydrates), and alters other materials (e.g., changing the colors of salts); these processes are thought to produce a gradual brightening of features with increasing age. These processes, however, compete with impact gardening and thermal processing, and can be confused with micrometeoroid-delivered materials. For most planetary satellites, the plasma effects are usually strongest on the trailing hemispheres of synchronous moons (as charged particles caught in the planet’s rapidly rotating magnetosphere “run-into” the trailing hemisphere), whereas impact gardening is dominant on the leading hemispheres (as the moon “sweeps up” orbiting debris in the planet’s orbit). In some circumstances, such as the inner midsize moons of Saturn, such effects can be reversed, with enhanced MeV electron bombardment on the leading hemispheres of Mimas and Tethys, and enhanced E-ring particle bombardment on Mimas’s trailing hemisphere (Schenk et al. 2011). Deciphering these data allows constraints to be placed on the radiation-exposure ages and compositions of surface-feature but requires improved modeling and laboratory data.

These chemical and physical effects of space weathering are of great interest in part because they profoundly affect surface appearance. Newly exposed and comminuted regolith, such as is observed in young impact crater rays, is often substantially brighter than the space-weathered surroundings at optical wavelengths on airless bodies. Significant space weathering effects have been observed across wavelengths, from the ultraviolet to the thermal infrared. These spectral changes are thought to be largely owing to the accumulation of nanoscale opaque particles in grain coatings and agglutinates that are highly efficient light absorbers. Thus, as space weathering proceeds, the composition of a surface can be increasingly masked. The degree to which spectra have been changed can be a powerful tool to understand the duration of surface exposure and date surfaces, although the weathering rates are poorly known and depend on surface composition and space/magnetospheric environment.

Mature regolith samples from the Moon and asteroid Itokawa demonstrate that space weathering products depend on initial composition. However, comparison of space weathering processes across the solar system, or even across a single complex planetary body, needs to also consider environmental differences. For example, the degree to which micrometeoroid impacts produce melt and vapor depends on impact velocity, which varies with heliocentric distance. Solar wind fluence is highest closest to the Sun, but GCRs play a larger role in the outer solar system. The effective solar wind flux scales with latitude, and thus for bodies with minimal axial tilt, space weathering may vary with latitude. Magnetic fields can shield the surface from charged particles to some degree but may also help to transfer species between nearby bodies (e.g., from Earth to the Moon or between the inner Galilean moons). However, the fluxes and energies of species that reach and thus weather the surface and their dependence on magnetospheric conditions are not well known.

Developing a general understanding of how different space weathering processes modify the surfaces of planetary bodies and how those vary with local conditions will help to determine rates of geologic processes, understand duration of surface exposure of materials, understand surface composition, and provide a valuable framework that can be used to better interpret remote sensing observations across the solar system.

Q5.5b How Have Impacts Affected Surface and Near-Surface Properties?

Impacts are one of the most ubiquitous geologic processes, affecting nearly every solar system body and in many cases dominating their landscapes. Impacts modify the compositions of planetary surfaces by redistributing target material and delivering exogenic material. Impacts modify crustal structure through both melting and fracturing of the target material (Melosh 1989), and some models suggest impacts may also induce volcanism. Impact processes are considered in detail in Question 4 (Chapter 7).

Nearly all remote sensing data and returned samples are of regolith (with the notable exception being martian igneous meteorites); thus, it is critical to understanding the formation, structure, and evolution of regolith in order to interpret these data and samples. Impacts play a critical role in the formation of regolith, particularly on airless bodies, via ejecta/catastrophic disruption and micrometeoroid impacts (Q5.6c). The rate at which impacts create regolith relative to other processes such as thermal fatigue, however, remains unknown, particularly on smaller and/or icy bodies. While we know impacts mix or “garden” regolith, the rates and depths of this mixing are not well known but have major implications for radiation processing of near-surface material (e.g., on Europa). Our knowledge of how the regolith varies vertically is poor, even for the Moon, and yet understanding this third dimension is essential for constraining lunar surface evolution, the origin of samples, and for any successful lunar human engineering endeavors.

Regolith can also preserve ancient layers of ejecta that both record the local history of impact events and the ancient space environment (variations in solar/GCR activity; Q5.5a). Impacts also excavate material from depth, providing a window into the composition of the lower crust and the mantle or ocean below; the correlation between crater size and source depth of surface material, however, is poorly constrained, as is the degree to which crater ejecta is deposited versus local material is exhumed with increasing distance from the primary crater. Impacts deliver exogenic material to planetary surfaces. For neighboring bodies, like Phobos and Deimos, or satellite systems in the outer solar system, material can be exchanged through so-called sesquinary impacts, where ejecta from an impact on one satellite can orbit the primary and either reimpact the original satellite or another satellite. Improved modeling of these processes allows observed surface compositions to provide insight into both the materials comprising the original planetary crust, from the surface to the mantle/ocean beneath, and those materials delivered to the surface.

In addition to redistributing material, the energy imparted by impacts and the resulting shockwave modify the existing target material through melting and fracturing. Major outstanding questions exist, however, about the details of these processes. The distribution of melted material is debated (e.g., the melt deposit antipodal to, and possibly originating from, the lunar crater Tycho), and the fate of melted water-ice “bedrock” on icy worlds (e.g., Titan) is not well constrained. Fundamental questions remain regarding the composition (and homogeneity thereof) impact melt composition and if cooling occurs slowly enough to allow for differentiation.

Q5.5c Where and How Do Volatile Deposition, Sublimation, Transport, Redeposition, and Loss Take Place, Now and in the Past?

Volatiles exist and interact with regolith under a wide range of chemical, geological and gravitational conditions, from Mercury to TNOs. Many airless bodies cold trap substantial volatile deposits (including water ice) in permanently shadowed regions (PSRs). PSRs, like those on the Moon and Mercury, may retain a record of volatile transport across the solar system and planetary dynamics, although this is still uncertain and depends on their unknown age. Landscapes and climates of entire worlds (e.g., Mars, Callisto, Pluto, Triton, comets, possibly the dwarf planet Eris), are controlled, in some cases almost exclusively, by volatile interactions (Mangold 2011). On Ganymede, water frost redistribution is modulated by the background magnetic field, resulting in bright polar caps

(Khurana et al. 2007). However, the interaction timescales are generally unknown. For example, it is unclear how TNOs are modified into the Centaur and Jupiter family comet populations—which have different characteristics (e.g., colors)—as they migrate closer to the Sun.

Volatiles trapped on terrestrial bodies (e.g., the Moon, Mercury, asteroids) may retain rich records. For example, lunar PSR deposits may preserve the volatile history of the Earth–Moon system, including the delivery of organics to Earth. The abundances of (moderately volatile) alkali elements on Mercury may reflect magmatic abundances or imply thermal redistribution of material. Fundamental questions remain, however, regarding the transport, retention, physical and chemical alteration, and loss processes operating on such deposits over seasonal, diurnal, and precessional timescales (see also Question 6, Chapter 9). The relative contributions of impacts, volcanism, and solar wind to volatile inventories are poorly constrained. There is a lack of strong constraints on relevant rates, including diffusion, low-temperature chemical reactions, clathrate formation/retention (in thicker deposits), and loss mechanisms. There is little information regarding the chemical, physical, or mechanical/structural evolution of these ices, the distribution of volatiles beyond the poles, and the variability of these characteristics with solar distance (e.g., between Mercury and the Moon).

In the outer solar system, volatiles are not trace species, but rather, substantial components of planetary bodies. The surface materials and activity (e.g., glaciers) observed on these bodies, in particular, provide critical clues to understand the process(es) that affect how these bodies incorporate and retain volatiles and refractory materials (Q5.4b). Investigating the most distant objects in the solar system (e.g., TNOs, Centaurs, interstellar objects, and dynamically new comets) provides insight into the most volatile elements. Studying Jupiter family comets, the most accessible comets, provides insight into less-volatile elements (such as water, semi-volatile organics, and refractory materials) that can inform the dust-to-ice ratio of cometary materials, and test if comets delivered Earth’s volatiles and organics. Large advances are possible by exploring these yet unknown bodies (e.g., Centaurs and TNOs) and by investigating the chemical/physical/mechanical alterations of volatile and nonvolatile materials in situ or within a returned sample from a primitive small body.

Surface-volatile sublimation is known to evolve landscapes and climates, from low-volatility ices like water in the inner solar system to high-volatility ices like CO₂ at Mars and Callisto, and CH₄, CO, and N₂ at Triton, Pluto and beyond. The relative importance of sublimation, however, is often poorly constrained due both to limits in observational data and difficulties replicating relevant temperatures and pressures in laboratories, thereby limiting knowledge of relative rates and material properties. It is unknown, for example, if Triton’s plumes are driven by cryovolcanism (Q5.6b) or surface/near-surface volatile sublimation, if such plumes exist elsewhere, and where not (e.g., Pluto), why not; how seasonal sublimation of Mars’s polar caps influence its climate; how volatiles are transported on icy satellites/dwarf planets, and how this influence their climates; how dominant a geologic process sublimation is on icy satellites/dwarf planets; and how far into the Kuiper belt sublimation-driven activity exists.

Strategic Research for Q5.5

• Determine the origin, time of delivery, vertical and lateral distribution, and current cycling of cold-trapped lunar volatiles via in situ analyses of isotopes (e.g., deuterium/hydrogen), sulfur, organics, abundance and distribution of volatiles, and local exospheric measurements.
• Investigate the effects of sublimation, space weathering and interior processes on the surfaces of ice-dominated worlds (including icy satellites, active asteroids, and comets) via high-resolution imaging, spectroscopy, and topography.
• Investigate the role of space weathering processes on airless rocky bodies using high-resolution imaging and spectroscopy of planetary surfaces coupled with laboratory studies of representative/analog materials and laboratory analyses of returned samples.
• Determine how small bodies (asteroids and comets) incorporate and retain volatiles via analyses of returned samples or in situ analyses of volatile and nonvolatile materials.
• Assess processes producing lunar regolith heterogeneity by measuring the thickness variations, and vertical and lateral compositional variability of the lunar regolith, using geophysical profiling, high-resolution multi-spectral imaging, and petrologic analyses of in situ or returned samples.

Q5.6 WHAT DRIVES ACTIVE PROCESSES OCCURRING IN THE INTERIORS AND ON THE SURFACES OF SOLID BODIES?

Activity is present on the surfaces and in the interiors of both rocky and icy bodies throughout the solar system. Active processes (volcanism, impact cratering, tectonic, fluvial, and aeolian activity) manifest differently, but all reveal critical information about the evolution of planetary bodies. The driving mechanisms behind geologic activity might be endogenic, driven by the body’s internal heat (Q5.2a), or exogenic, controlled by energy derived from the Sun (Q5.5c), or by the influx of impactors (Q5.4c, Q5.5b). Through a more thorough understanding of the process controlling the activity, the better the rate of change, frequency or duration of the activity can be constrained, such as the location of volcanism on Venus, the occurrence of mass wasting events, or the duration of eruptive activity on Enceladus.

Q5.6a Where and How Are Convection and/or Crustal Recycling Taking Place?

Earth is the archetype of one style of planetary crustal recycling via plate tectonics, although plate tectonics has been hypothesized for some other worlds (Q5.2a). Crustal recycling has been proposed for at least two other planetary bodies, Venus and Europa. In both cases, though, the form of crustal recycling differs. On Venus, plume-induced subduction has been proposed to occur at coronae. Regardless of whether subduction is occurring, coronae are identified as the manifestation of plume-lithosphere interactions that is potentially ongoing across Venus. Crustal recycling on Europa appears more like Earth’s plate tectonics, with obvious spreading zones and putative subduction zones. The extent of these recycling zones on Venus and Europa is not yet known. A style of crustal recycling that involves remelting or removal of the lower crust has been proposed for both Venus and Io, although the details and tectonic implications are uncertain There is debate about whether Mars experienced “sea-floor spreading” style plate tectonics early in its history, perhaps recorded by Mars’s remanent magnetic field.

Other proposed regions of convection include the Sputnik Planitia basin on Pluto, where a cellular floor structure is proposed to form from localized convection in a kilometers-thick surface nitrogen ice sheet, and a similar process may occur in Triton’s so-called cantaloupe terrain in the geologic past. Convection and crustal recycling on the icy bodies is poorly understood. Tidal forces acting on Io and the ocean worlds (Europa, Enceladus, Titan, Triton, Pluto, and Charon, in particular) may affect the circulation patterns within the oceans and mantles of these bodies (Question 8, Chapter 11), but the physical characteristics of this forced convection remain mysterious.

In the absence of crustal recycling, any (cryo)volcanic activity will tend to “dry out” planetary interiors such that they become depleted in volatile compounds (such as H₂O and CO₂). Volatiles within the mantle and crust contribute to volcanic and tectonic activity; a loss of volatiles over time, for example, would reduce the likelihood of explosive volcanic eruptions (Q5.3b).

Q5.6b Where and How Are Active Melt Generation, Outgassing and Plume/(Cryo)Volcanic Activity Taking Place, and What Melt and Gas Compositions Are Produced?

Beyond Earth, there are very few places where active outgassing or (cryo)volcanic activity has been directly observed. Jupiter’s volcanic moon Io is a testament to the power of tidal forces in driving interior heating and the resulting eruptive activity. At Saturn, diminutive Enceladus (~500 km diameter) is another prime example of the work of tidal heating: the plume of water vapor, ice particles and other compounds discovered by the Cassini mission attests to a body of liquid water beneath the ice shell. Possible plumes of water vapor have also been reported at Europa, another ocean world. Dark, nitrogen-driven plumes were observed by Voyager 2 at Neptune’s moon Triton, although it is unclear whether these are internally driven or whether solar heating of sub-surface nitrogen ice is responsible. Quiescent outgassing and outbursts have been observed at active primordial bodies beyond 5 AU (Centaurs and dynamically new comets), while so-called “main belt comets” and even small bodies such as Bennu exhibit activity, but the driving processes, beyond simple solar heating, are poorly understood. In the inner solar system, the combination of primordial heat and radiogenic activity (in different proportions) would have driven endogenic and eruptive activity for at least some portion of each

planet’s history. Despite abundant evidence for the work of volcanism in shaping the surfaces of the terrestrial planets and our own Moon, only Io is demonstrably volcanically active, although Venus has provided us with tantalizing hints of present-day volcanic eruptions. However, just because we may not have “caught them in the act,” it does not necessarily mean that other bodies (such as Mars) do not harbor interior melt and are not capable of eruptions today or in the future. On Earth, we define a volcano as active based on whether it has erupted within the past 10,000 years. Whether we can observe eruptive activity on other bodies, should it be occurring, is a matter of when, how often, and how we look for it.

In the inner solar system, melts are mostly silicate in nature and dominated by mafic compositions. Magmatic volatiles include primarily H₂O, CO₂, SO₂, based on terrestrial experience, but could also include more reduced species like CO, H₂S or other compounds on other bodies (Q5.1a). Io also exhibits dominantly silicate volcanism, inferred to be of mafic to ultramafic composition, with sulfur or SO₂ the primary volatile species. Excluding Io, solid bodies at distances from Ceres to Pluto and beyond generally contain ice as a major constituent. Some of these bodies (Europa, Ganymede, Callisto, Enceladus, and Titan) are thought to possess a subsurface ocean, based on multiple lines of evidence (Q5.1b). Subsurface oceans on other bodies, including Ceres, Pluto, and Triton, are supported by theoretical modeling or a single observation type. Cryomagmas might derive directly from the oceans or result from localized melting within the ice shell. In either case, cryomagma compositions are likely to be dominantly water, possibly containing salts, ammonia, or minor amounts of other constituents. Candidate volatiles driving explosive volcanism include water vapor, CO₂, N₂, SO₂, ammonia, and methane, based on cosmogenic abundance or detections in Enceladus’s south polar plumes. Water sublimation drives comet and active asteroid outgassing at distances <5 AU, while activity at primordial Centaurs and dynamically new long-period comets may be driven by annealing and crystallization of amorphous ice and/or sublimation of volatiles such as CO.

The processes of volcanism and cryovolcanism are direct manifestations of the heat sources (e.g., radiogenic and tidal) within a planetary body. Understanding the nature of volcanism through time, and where it is active today, provides information about interior evolution and compositional variability across the solar system. Active cryovolcanism, and the ability to sample plume constituents, has the potential to yield profound discoveries about the habitability (and the potential inhabitants) of icy satellites (Question 10). Sampling the gases given off by primordial active bodies reveals the nature of unprocessed primordial material, the driving mechanisms of the activity, and provides context for understanding the record retained in short-period Jupiter family comets and active asteroids.

Q5.6c Where and With What Intensity Are Tectonic Processes and Deformation Currently Occurring?

Tectonic processes are likely to be active on many planetary bodies in the solar system, from small icy moons to the terrestrial planets, and can shed light on the strength of planetary crusts and the ease with which material may be exchanged between their surfaces and interiors. For icy bodies in particular, tectonic processes can illuminate the evolution of their oceans. Fractures may transport materials from subsurface oceans, or other extensive fluid pockets within these worlds, to their surfaces. For example, the observed brightness variations of Enceladus’s plumes appear to be related to motion of south polar faults over a tidal cycle (although the exact link between plume output and tectonism is unclear). Diurnal tides may also be responsible for the active formation of cycloidal ridges on Europa and double ridges on Triton. Europa’s cycloidal ridges may form over a matter of days and require an extensive, near-surface liquid layer. As such, they may be indicative of both current tectonic activity and a global subsurface ocean on Europa. Likewise, double ridges are amongst the most youthful features on Triton’s surface. Their formation in regions of the surface that have experienced enhanced heating and possibly convective overturn suggests that their presence also requires the existence of a near-surface ductile layer, possibly an internal ocean. Io is currently experiencing both volcanic and tectonic deformation on a massive scale. If a magma ocean is present on Io, tectonic deformation could be caused by diurnal motion as its lithosphere deforms atop this liquid layer. Steadily increasing tidal stresses are plausibly responsible for the grooves on Phobos.

All large planetary bodies in the inner solar system are currently understood to be experiencing tectonic deformation. The scale of deformation detected by remote instruments varies from small-scale fractures to large-scale gravity perturbations. For example, on the Moon and Mercury small-scale thrust faults indicate that these bodies are tectonically active today with the fault movement caused by continued cooling and shrinking of these bodies; on the Moon, the Apollo seismic network provided direct evidence of present-day tectonic activity. A combination of analog studies, models, and data observations of larger-scale tectonic landforms on Venus, known as coronae (hundreds of km across), also suggest more recent activity. Active crustal recycling is inferred from the observed tectonic landforms on Venus (Q5.6a).

Understanding of planetary interior processes has been greatly enhanced by in situ data collection, particularly seismic data. The InSight lander is detecting hundreds of marsquakes, some caused by atmospheric phenomenon and others are of tectonic origin. A few large enough to locate are associated with young faults and fissures in the Cerberus Fossae region of Mars (Giardini et al. 2020). These larger marsquakes are of similar magnitude to intraplate tectonic activity on Earth. Having data from several bodies allows for comparative studies that facilitate greater understanding of the formation and state of planetary interiors. Currently available data indicate that Earth is the most seismically active body measured, followed by Mars and then the Moon (Banerdt et al. 2020) (Figure 8-4). The dearth of in situ seismological investigations on Venus, ocean worlds, and dwarf planets, and in many cases, the lack of global, high-resolution imagery, limits our ability to tie tectonic activity and deformation to processes occurring in their interiors.

Q5.6d Where and How Are Active Sedimentary and Regolith Processes Occurring?

The physical and chemical processes controlling the generation and movement of planetary regolith materials vary across the solar system. On airless bodies, thermal stresses and impact-related processes dominate regolith formation (see Q5.5b, and Question 4, Chapter 7). For instance, micrometeoroids pulverize solid materials and chemically

weather regolith particles through formation of nanophase iron, also known as space weathering (Q5.5a). At a larger scale, impacts continue to actively mix the regolith. Images of new craters on the Moon show that even decameterscale impacts affect the surface up to thousands of meters away from the impact event, and secondary cratering is gardening the uppermost regolith at rates >100 times faster than previously thought (Speyerer et al. 2016).

Regolith formation may be intimately linked to dynamic processes occurring on dwarf planets and ocean worlds and can shed light on their internal evolution. Regolith formation occurs on icy bodies by various processes; for instance, when unconsolidated, icy material is emplaced on their surfaces as a result of impact cratering and, in the case of Enceladus, plume eruptions. Because active regolith formation may erase craters and mute surface topography on ice-rich worlds, constraining the composition and thickness of these unconsolidated layers, as well as the rate at which these layers form, allow us to place improved constraints on the evolution of dwarf planet and satellite surfaces. Furthermore, because thin regolith layers (if rocky) act to shield ice-rich layers from sublimation on bodies like Ceres and Callisto and may serve as insulating layers, investigating the properties of planetary regolith can inform thermal models for icy bodies. The presence and composition of regolith on planetary surfaces may also shed light on planetary evolution. For example, compositional analysis of regolith on Ceres revealed that it underwent extensive aqueous alteration, a significant amount of ice-rock fractionation, and that its non-ice regolith has a composition similar to primitive meteorites (CI/CM chondrites) (Prettyman et al. 2017).

Investigations of active sedimentary processes can tell us much about how the climate and surface conditions of planets and satellites have changed over time. Mass wasting of particulate matter down high-slope regions occurs on planetary bodies with and without atmospheres (e.g., Mercury, Moon, Mars, icy satellites). Recent mass movement of material is observed to occur owing to the loss of volatiles from the subsurface. For example, imagery of mass-wasting events along the edges of the martian polar ice caps have been captured. Recurring slope lineae have been observed for many years on Mars, but their exact formation mechanism is still debated. The same is true for movement of materials on comet 67P/Churyumov-Gerasimenko. These mass wasting processes reveal important information about the rate of volatile loss on planetary surfaces.

The presence of an atmosphere facilitates the lofting of fine-grained particles into near-surface planetary atmospheres, creating a suite of ever-changing depositional and erosional landforms. High-resolution imagery shows the movement of dune fields on Mars, indicating large-scale movement of sand-size particles. Venera lander data, both panoramas and light flux data, suggest atmospheric transport of particulates by near-surface winds, although whether these winds were typical or modified by the landing event are unclear. Titan’s extensive, mid-latitude dune fields are a testament to the dry climatic conditions in the mid-latitudes and suggest that, like fluvial deposition, aeolian deposition is still ongoing. These landforms and their temporal changes alert us to the most recent weather patterns of these planetary bodies. Comparison of active landforms with inactive dune fields, for example, reveals how recent weather or climate patterns have changed and can reveal the processes that led to the observed changes.

Strategic Research for Q5.6

• Investigate the potential for active volcanism and deformation, and where and how crustal recycling is happening, on Venus with synthetic aperture radar, infrared, ultraviolet, or repeat-pass interferometry measurements of the venusian surface and atmosphere.
• Constrain the rate of active surface changes on Mars related to dune migration, mass movements, sedimentation, or ice sublimation using either long-term, repeat-pass, high-resolution altimetry or imaging.
• Characterize present-day plate mobility and recycling on Europa, Titan, and Enceladus by visible imaging at regional scale, global, high-resolution gravity and topography, and/or repeat-pass interferometry.
• Characterize individual eruptions and determine the rate of volcanic activity on Io using repeat high-resolution imagery and/or spectroscopy.
• Characterize the style and intensity of active tectonism occurring on rocky or icy worlds, through seismic and other geophysical measurements.

SUPPORTIVE ACTIVITIES FOR QUESTION 5

• Measure optical constants of the range of materials expected in the solar system under relevant pressure and temperature conditions (from Venus to airless icy satellites) to serve as the basis by which we constrain the compositions of planetary surfaces from remote sensing data.
• Continued meteorite collection activities in the Antarctic and associated curation.

REFERENCES