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Question 7: Giant Planet Structure and Evolution

The giant planets comprise 99.5 percent of the mass of the solar system, apart from the Sun, and 96 percent of the solar system’s total angular momentum.¹ They are the most massive remnants of accretion from the Sun’s nebular disk and to have played a substantial role in shaping the overall architecture of the solar system. Each giant planet hosts what is akin to a solar system in miniature, including a substantial, and sometimes complicated, planetary magnetic field and a diverse collection of satellites and rings. Each planet system is unique, a complex byproduct of both common and differing formation and evolutionary processes. Comparative planetology between the hydrogen-rich “gas giants” (Jupiter and Saturn), and the intermediate-size and heavier element-enriched “ice giants” (Uranus and Neptune), is crucial for understanding the processes that govern their present-day interiors, atmospheres, and magnetospheres. Time-resolved, multi-wavelength remote sensing coupled with in situ atmospheric, gravitational, and magnetospheric measurements provide essential tools to reveal the properties of these four worlds. Studies of our giant planets provide the scientific template for understanding a broad class of astrophysical objects. A large fraction of known exoplanets falls in the giant planet size class, and among these, the ice giants appear to be a particularly abundant sub-class in the galaxy whose composition and structure remain poorly understood.

Q7.1 WHAT ARE GIANT PLANETS MADE OF AND HOW CAN THIS BE INFERRED FROM THEIR OBSERVABLE PROPERTIES?

Determining the composition of the giant planets is fundamental for understanding their diversity and distinguishing between the gas giant and ice giant classes. Giant planet atmospheres are comprised primarily of hydrogen and helium, like the Sun itself and the protosolar disk (Question 1, Chapter 4), but their detailed elemental abundances vary owing to different formation conditions, planet masses, and evolutionary paths. Apart from helium and neon (thought to sink deeper into the interiors of Jupiter and Saturn in the form of heavy droplets), most elements observed in giant planets to date are enriched compared with their protosolar values (Question 2, Chapter 5;

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

see Figure 5-2), implying that heavy elements were preferentially delivered during or after the planets’ formation, or that they have been dredged-up from a heavy-element core (Moll et al. 2017). While for Jupiter’s atmosphere the abundance of most major species is known relatively accurately, only sparse remote sensing data are available for Saturn, and even less is available for Uranus and Neptune. Relative to the gas giants, Uranus and Neptune contain more heavy elements with only a modest envelope of hydrogen and helium (Figure 10-1). However, there remain major uncertainties in the ice giant global compositions, and the total mass of heavy elements within them is poorly constrained (Helled et al. 2020).

Q7.1a Are the Helium and Noble Gas Abundances Across the Giant Planets Consistent with Interior and Solar Evolution Models?

Helium and noble gases bear essential information for understanding giant planet origin and evolution. Helium is the second most abundant species after hydrogen in the gaseous envelopes of Jupiter, Saturn, Uranus and Neptune. Although helium contributes substantially to the mean molecular weight of their atmospheres, the determination of its abundance from stellar and radio occultations and thermal emission spectroscopy has proven difficult. The only accurate and reliable determination is from the Galileo probe into Jupiter (Young 2003). In Jupiter and Saturn, helium droplets form in the deep metallic hydrogen envelopes and are transported downward. Measuring He abundance precisely in Saturn is key to constraining Saturn’s evolution and cooling for comparison with Jupiter (Mankovich and Fortney 2020). The shallow envelopes of Uranus and Neptune imply that helium rain does not occur, so that measuring the He abundance in these planets would provide a direct determination of the protosolar He abundance, which is presently only inferred from models of the Sun’s evolution.

Neon, thought to dissolve into helium droplets (Wilson and Militzer 2010), is depleted in Jupiter’s atmosphere compared to its protosolar abundance and a depletion is similarly expected in Saturn’s atmosphere. In Uranus and Neptune, measurement of a low neon abundance would directly constrain models of the early formation of the solar system as well as interior evolution. Measurement of the abundances of heavier inert gases (argon, krypton, and xenon) is key to understanding the mechanisms that led to the formation of giant planets (see Questions 1 and 2) as well as planetary envelope mixing. The Galileo probe found that these gases are enriched by a factor of 2 to 3 in Jupiter compared to their protosolar values (Mahaffy et al. 2000), while abundances in the other giant planets remain unknown. Because noble gases are trapped into solids only at very low temperatures and they have few

chemical interactions, determination of their enrichment compared to that of the major gases is an essential clue to understanding giant planet evolution (Guillot and Gautier 2015). For example, a uniform enrichment in noble gases and major species would indicate an early enrichment with little mixing from the deeper interior. Although it would be desirable to determine helium and noble gases for all the giant planets, having even a second point of comparison from Saturn, Uranus, or Neptune to supplement the Galileo probe measurements in Jupiter would be a tremendous advance.

Q7.1b How Do Bulk Abundances of Major Species and Ice-to-Rock Ratios Compare with Nebular Models?

The gas fractions and ice-to-rock ratios of the giant planets provide fundamental constraints on how these planets formed and evolved, and more broadly, on how different classes of giant planets—for example, gas giant, ice giant, rock giant, super-Earth, and sub-Neptune (Q2.2; Question 12)—are established. The main compositional difference among the giant planets is a higher gas fraction in Jupiter and Saturn compared with Uranus and Neptune. The ice-to-rock ratios in the giant planets are more difficult to constrain, and current data and models are unable to conclusively determine whether Uranus and Neptune are rock-dominated or ice-dominated (Helled et al. 2020). Resolving this question may provide important context for understanding why super-Earths and sub-Neptunes—frequently detected in exoplanet systems—are missing in the solar system. Gravity field data from orbiters and composition data from atmospheric probes and remote sensing, particularly at Uranus and Neptune, would provide the greatest advances in our understanding of giant planet bulk compositions.

Among the giant planets, C/H is the best-constrained compositional ratio thanks to plentiful spectral signatures of atmospheric methane. This ratio is commonly used as a proxy for the overall heavy-element enrichments of Uranus and Neptune, based on the roughly equal enrichments of C, N, S, P, and heavy noble gases seen at Jupiter (Atreya et al. 2022). However, uncertainties arise because carbon could have been incorporated as part of the nebular gas (as CO), as ices (e.g., in water ice clathrates, or CH₄ and CO₂), or in an additional organic solid phase. It is likely that relative abundances may vary significantly in the giant planets, given their large differences in gas fraction. Compositional measurements are needed for all four giant planets to unravel the numerous nebular processes and sources of primordial materials during giant planet formation (Question 2).

Q7.1c How Are Condensable Species and Disequilibrium Species Distributed and Thus Transported in the Planetary Atmospheres and Interiors?

Apart from noble gases, all species in giant planets either condense or undergo major chemical reactions in the atmosphere or interior. Composition in the observable part of giant planet atmospheres varies with height (Figure 10-2), owing to the interplay between chemical processes (i.e., thermochemistry and cloud chemistry in the troposphere, and photochemistry in the stratosphere) and dynamical transport (e.g., global circulation, diffusive mixing, storms, and vortices). Because giant planets are fluid and mostly convective, it has long been thought that abundance variations are primarily owing to radial changes in pressure and temperature. However, measurements by the Galileo probe and Juno’s microwave radiometer, as well as remote spectroscopy data, provide evidence for latitudinal and longitudinal variations in water and ammonia abundance even in Jupiter’s deep atmosphere at tens of bars (Bolton et al. 2017). The ice giant atmospheres also show compositional contrasts with latitude down to tens of bars, as shown by Earth-based infrared-radio remote sensing (Molter et al. 2021; Tollefson et al. 2021). The implication is that abundance variations are likely to be common, but the main transport mechanisms are unknown. While large-scale circulation shapes the appearances of all giant planets, storms and small-scale processes are known to be significant and may, for example, account for the transport of Jupiter’s intrinsic heat flux. Mapping of abundances of key condensable species, including methane and H₂S, in Uranus and Neptune is needed to understand transport mechanisms in their atmospheres. Disequilibrium species such as CO and PH₃ are also particularly important because their observed abundances provide insights into the composition of the deep atmosphere, at pressures of hundreds of bars (Fouchet et al. 2009; Moses et al. 2020).

Improved measurements of chemically active species help advance the understanding of physical and chemical processes in the stratosphere, troposphere, and deep atmosphere, illuminating similarities and differences between the giant planets and their exoplanet cousins. Chemically active species also touch on a wide range of broader science questions. For example, measurements of the CO mixing ratio (interpreted by realistic models) provide constraints on the O/H ratio in the deeper atmosphere. This provides an important check on abundances calculated from the H₂O mixing ratio directly, because CO is unaffected by water cloud condensation. Condensable species such as ammonia, H₂S, and water trace three-dimensional dynamical flows in the weather layer, and photochemical species are sensitive to external influences (Q7.3b).

Q7.1d How Are Atmospheric and Interior Abundances Related?

Our knowledge of the interior, evolution, and formation of giant planets depends on our ability to link atmospheric and interior abundances. The importance of this link is illustrated by helium in Jupiter and Saturn, in which an atmospheric abundance lower than the protosolar value demonstrates the presence of a phase separation of helium in hydrogen. This separation, and the sinking droplets, profoundly modifies the cooling of these planets (Mankovich and Fortney 2020). The unmixing of other elements is also possible. In Uranus and Neptune, water

may separate from hydrogen (Bailey and Stevenson 2021), which would affect the structure, chemistry, and evolution of these planets. In addition, in giant planets all condensing species are heavier than the atmospheric gas and there is no planetary surface, making it presently unclear how far the condensates sink (Guillot et al. 2020). The implication is that the high-pressure behavior of matter and related transport processes both need to be characterized (Q7.1c, Q7.2b–c, Q7.3a–c). Such issues will become increasingly important as abundances are measured in exoplanetary atmospheres. Given the limited spatial information available for exoplanets, interpretation of such abundances will need to rely on understanding obtained from study of our giant planets for which much more detailed spatial and temporal information can be obtained (Question 12, Chapter 15).

Q7.1e What Are the Isotopic Compositions of the Giant Planets and How Do They Compare with Formation and Interior Evolution Models?

Because the giant planets formed in the presence of the solar nebula, measurements of isotope ratios in their atmospheres are extremely important as points of comparison with other objects in the solar system and with each other. D/H has been measured in many objects in the solar system—including Jupiter, Saturn, Uranus, and Neptune—but most other isotopes have not. The jovian D/H value is used as a proxy for the protosolar value, because deuterium is destroyed in the Sun. Because D/H in Uranus and Neptune is much larger than on Jupiter, but still relatively low compared to cometary values, it can also be used to understand mixing in planetary interiors as well as planetary ice-to-rock ratios (Feuchtgruber et al. 2013). As with D/H, the protosolar ³He/⁴He ratio is based on probe measurements at Jupiter, but there remain unexplained differences between these values and those in meteorites (Mandt et al. 2020). Additionally, heavier element ratios such as ¹²C/¹³C, ¹⁴N/¹⁵N, ¹⁸O/¹⁷O/¹⁶O, ²⁰Ne/²²Ne, or the isotopes and relative abundances of heavier noble gases, can be used to test formation models, infer characteristics of the carriers that delivered these elements to the giant planets, and constrain evaporation processes in the early solar system (Question 1, Chapter 4).

Strategic Research for Q7.1

• Constrain the interior evolution of Saturn, Uranus, and Neptune via in situ sampling of noble gases, elemental, and isotopic abundances, in combination with remote sensing composition observations.
• Determine the bulk compositions of Saturn, Uranus, and Neptune, and the ice-to-rock ratios in Uranus and Neptune, from gravity field and elemental abundance measurements.
• Constrain chemical processes, vertical mixing, and dynamical transport in all four giant planets by simultaneously measuring multiple tracers (e.g., temperature and condensable and disequilibrium species) over varied temporal, vertical, and horizontal scales, from both remote sensing (all giant planets) and in situ measurements at Saturn, Uranus, and Neptune.

Q7.2 WHAT DETERMINES THE STRUCTURE AND DYNAMICS DEEP INSIDE GIANT PLANETS AND HOW DOES IT AFFECT THEIR EVOLUTION?

The deep interiors of the giant planets consist of mixtures of common materials (hydrogen, helium, water, methane, ammonia, and silicates) under a wide range of pressures and temperatures. These materials behave in complex and dynamic ways as they mix, separate, settle, and flow, giving rise to the planets’ intrinsic magnetic fields, as well as asymmetries and variations in the planets’ gravitational fields (Figure 10-3). Jupiter and Saturn have very different gravitational and magnetic fields, despite both being composed primarily of hydrogen and helium, suggestive of distinct histories. The much higher abundances of heavier elements in Uranus and Neptune give rise to distinctive internal structures and dynamics, most dramatically their unusual magnetic fields. While currently available data on the internal structures of the ice giants is very limited, the fact that Uranus emits much less internal heat than Neptune may imply distinct internal structures and histories for these planets as well.

Q7.2a How Does Composition Change with Depth in Giant Planet Interiors?

Recent measurements of Jupiter’s and Saturn’s gravitational fields, as well as observations of waves in Saturn’s rings, have revealed that both planets have dilute cores. Instead of a discrete core (composed primarily of rock and ice) surrounded by a distinct envelope (composed primarily of hydrogen and helium), both planets exhibit a smooth change in composition and density with depth (Wahl et al. 2017; Mankovich and Fuller 2021). This reflects both on how these planets originally formed and to their long-term evolution. For example, the formation and stability of these gradients depend on how well rock and ice mix with metallic hydrogen under high pressures, and their existence can influence how quickly the planet’s interior cools over time (Helled and Stevenson 2017). Further theoretical study, together with improved characterization of material properties under appropriate conditions, are needed to determine which evolutionary histories are consistent with the dilute core structures.

The internal structures of Uranus and Neptune are far less well constrained, and it is not yet certain whether these planets have distinct or diffuse layers in their interiors. At least Uranus’s interior may have been stirred by a primordial giant impact(s) that tilted its rotational axis. High-pressure experiments involving mixtures of materials and pressures relevant to Uranus and Neptune would clarify the conditions under which rock, ice, hydrogen, and helium mix, providing needed information to reconstruct the histories of these planets (Helled and Fortney 2020). Measurements of Uranus’s and Neptune’s gravity and magnetic fields are also needed to make progress on this question, with further improvements also possible from seismology if normal modes are detectable on these planets (Markham and Stevenson 2018).

Q7.2b How Are Elements and Heat Transported from the Deep Interior to the Atmosphere?

With the possible, still unexplained, exception of Uranus, all giant planets emit more heat than they receive from the Sun. This is thought to drive convection and mixing, leading to relatively homogeneous interiors in longitude and latitude. However, recent analyses suggest a more complex picture: the abundances of major chemical species (water in Jupiter, ammonia in Jupiter and Saturn, methane and H₂S in Uranus and Neptune) exhibit significant spatial and temporal variability (Fletcher et al. 2020), indicating that mixing is far from complete and/or is competing with other processes such as precipitation during storms (Guillot et al. 2020). The presence of vortices and waves also indicates that at least part of the deep atmosphere is, on average, stable against convection. This issue is not limited to the so-called “weather layer”—that is, the region characterized by condensation and latent heat release: it also extends to deeper regions, where strong compositional gradients may reflect phase separation (e.g., with helium and hydrogen at Mbar pressures in Jupiter and Saturn), phase transition (e.g., the transition to superionic water in Uranus and Neptune), or a compositional gradient leftover from the formation era.

The question of the transport of heat and elements in giant planets is a major one for the next decade. Solving it will require combining new observations, experiments, and models. The recent revision of Jupiter’s heat flux (Li et al. 2018) shows the need for revisiting thermal balance analyses for all four giant planets, particularly for Uranus and Neptune for which only partial observations are available. Large-scale observations of global circulation, as well as small-scale, high-resolution observations of storm activity, will need to be coupled to the constraints on key abundances and interior structure (Q7.1) to clarify the nature of giant planet internal transport.

Q7.2c What Is the Deep Rotational and Dynamical State of Giant Planets?

Analyses of giant planet gravitational fields have revealed interiors that are more complex and dynamic than previously expected. Recent measurements have constrained how deep the visible winds extend within each planet. While currently available data only place limits on the depths of the winds on Uranus and Neptune (Kaspi et al. 2013), Juno and Cassini data reveal that Jupiter’s winds reach a depth of about 3,000 km (Guillot et al. 2018; Kaspi et al. 2018), while Saturn’s extend to depths of about 8,000 km (Iess et al. 2019; Galanti and Kaspi 2021). Furthermore, the saturnian gravitational field is surprisingly time-variable (Iess et al. 2019; Markham et al. 2020) and contains asymmetries that rotate at about the same rate as its surface winds. Based on ring structures, Saturn is also known to exhibit normal-mode oscillations with a complex excitation spectrum. At Jupiter, zonal flows at depth can advect the magnetic field, leading to detectable secular variations on timescales of tens of years.

Given the spatial complexity of the ice giant dynamos, measurements of Uranus’s and Neptune’s magnetic fields would likely be a powerful tool to constrain their internal dynamics (Soderlund and Stanley 2020).

The properties of the interior oscillations and asymmetries inside Saturn have been used to constrain aspects of the planet’s internal structure and rotation state (Mankovich et al. 2019; Mankovich and Fuller 2021); more observations and modeling work are needed to ascertain how these deep oscillations and flows are generated and maintained (see also Q7.2d). Deep fluid flows provide opportunities to examine how the fundamental processes that underlie atmospheric dynamics operate under extreme conditions that are otherwise inaccessible. Available data on Uranus and Neptune are very limited (see Figure 10-3), and additional measurements (gravity and magnetic field, planetary shape) are needed to ascertain whether the distinctive compositions of these bodies shape their internal dynamics. Finer details (e.g., direct measurements of differential rotation and detection of interfaces) will require planetary seismology.

Q7.2d How Are the Complex Magnetic Fields of the Giant Planets Generated?

Planetary magnetic fields show remarkable variations that reveal clues to their deep interiors and internal histories. Saturn and Mercury have strikingly axisymmetric fields and slow secular variation. Jupiter and Earth have dipole-dominated fields with ~10-degree tilts, prominent regions of enhanced intensities, and measured secular variation. Uranus and Neptune have multipolar (i.e., nondipole dominated) surface fields with comparable intensities, no clear symmetries along any axis (Soderlund and Stanley 2020), and a true rotation rate that is uncertain (Helled et al. 2020). Secular variations of the magnetic field compatible with some advection by the deep zonal flow have been measured at Jupiter (Moore et al. 2019a). (Unfortunately, these measurements are not possible on Saturn because of the axisymmetry of the field and on Uranus and Neptune because of the singular set of field measurements provided by the Voyager 2 flybys.) These observations lead to fundamental questions about planetary magnetic field generation, including the processes that control field morphology, strength, and temporal evolution, and the aspects of planetary interiors responsible for observed variations across the terrestrial, gas giant, and ice giant planets, as well as within each of these classes. These questions can be answered by determining the detailed configurations of the ice giants’ magnetic fields and their temporal variation, the internal density and composition distribution, whether layers of stable stratification and/or double-diffusive convection exist, the thermodynamic and transport properties of the planets as a function of radius and over time as the planets evolve, the characteristics of zonal winds, meridional circulations, and turbulent convective flows in the deep interior, and the dynamo characteristics of exoplanets to further test hypotheses developed to explain the planetary magnetic fields within the solar system (see also Question 12, Chapter 15).

Q7.2e How Are the Interiors of the Giant Planets Evolving Today?

After formation, the giant planets cool and lose the internal heat acquired during gravitational collapse. However, uncertainties in the current radiated heat flux at each planet, and in equations of state, lead to uncertainties in interior and evolution models (Fortney et al. 2011). For Jupiter, the thermal heat flux value has been revised by 30 percent recently (Li et al. 2018). For Saturn, uncertainty on the atmospheric helium abundance, and therefore on the amplitude of the helium rain process, is too large to constrain models. Last, the extremely limited data from Uranus and Neptune suggest that these two planets are losing heat at very different rates from each other for reasons that are still unclear (e.g., Kurosaki and Ikoma 2017). In addition, discoveries from the past decade have revealed that both Jupiter and Saturn have interiors that may not be fully convective (Leconte and Chabrier 2012; Wahl et al. 2017; Mankovich and Fuller 2021), requiring reassessments of their thermal histories. Mixing in the planetary interior (Vazan et al. 2018) and evolution in the aftermath of a possible giant impact (Liu et al. 2019) are also processes that need to be considered in evolution calculations. The recently discovered rapid orbital migration of some of Saturn’s moons could be owing to ongoing changes in the planet’s internal structure (Q7.5a). Little is currently known about Uranus and Neptune’s interior structure, and ongoing evolution, for comparison with Jupiter and Saturn. Progress on this question requires accurate measurements of heat fluxes, a much better assessment of transport processes in the atmospheres and interiors (Q7.2b), a determination of the deep interior structure (Q7.2a), and, for the case of Saturn, a determination of the helium abundance in its atmosphere.

Strategic Research for Q7.2

• Determine the interior structure and composition of Uranus and Neptune using gravity and magnetic field mapping.
• Search for the locations and extent of discrete layers in the deep interior in all four giant planets, using planet/ring seismology (i.e., the ability to detect planetary seismic waves from perturbations in the motion of ring particles).
• Constrain the rate of heat transport in Jupiter, Saturn, Uranus, and Neptune by measuring thermal balance and vertical temperature profiles.
• Measure vertical mixing in Jupiter, Saturn, Uranus, and Neptune from measurements of deep vortices, storm and wave activity, and disequilibrium species distribution.
• Understand the deep rotation rate and dynamics in Uranus and Neptune from time-resolved gravity and magnetic field mapping, radio occultations, planet/ring seismology measurements, and deep circulation modeling.
• Characterize the intrinsic magnetic fields of Uranus and Neptune through magnetic field mapping and dynamo modeling.
• Constrain the ongoing interior evolution of Saturn, Uranus, and Neptune from helium abundance, thermal balance, satellite tidal evolution, occultations, and gravity and magnetic field measurements.

Q7.3 WHAT GOVERNS THE DIVERSITY OF GIANT PLANET CLIMATES, CIRCULATION, AND METEOROLOGY?

Comparative planetology among the four giant planets affords study of diverse regimes of planetary rotation, size, chemical enrichment, condensation processes, atmospheric mixing, seasonal influences, exterior influences (e.g., auroral and chemical) and interior connections (from metallic hydrogen to watery oceans). Exploration of the giants can reveal how these regimes vary from world to world, setting our terrestrial atmosphere into a broader context (Question 6, Chapter 9), and providing ground truth data for understanding atmospheres on giant and sub-giant exoplanets (Question 12, Chapter 15).

Q7.3a What Processes Maintain Banded Patterns and Unique Polar Regions on Each Giant Planet, How Do They Connect with the Deep Interior, and What Controls Their Variability?

As shown in Figure 10-4, all four giants exhibit banded atmospheres, with east-west zonal winds separating domains of different temperatures, aerosols, and chemical composition (Fletcher et al. 2020). Differing conditions within adjacent latitudinal bands influence the prevalence of convection, lightning, and vertical mixing, and on Jupiter these circulations appear to penetrate deeply (at least to the ~hundred bars levels; Ingersoll et al. 2017). The bands are punctuated by large-scale vortices, some of which drift with latitude, particularly on Uranus and Neptune (Hueso and Sánchez-Lavega 2019), while others remain fixed in their respective bands, for example, on Jupiter. Juno and Cassini data have provided important clues on the depths of these bands, but significant questions remain, particularly for Uranus and Neptune for which few data are available (see Q7.2c,d and Figure 10-4). Still uncertain are what determines the wind strength, the energy sources that maintain the zonal flows and thermal gradients, and how these vary with depth (Kaspi et al. 2020), why ice giant banding differs from that seen on the gas giants, and how and why do banded patterns change over quasi-predictable timescales, such as the multi-year cycles of “upheavals” in Jupiter’s belts and zones at cloud level.

Each giant planet appears to display different circulation regimes as a function of latitude, from chemically enriched equatorial plumes and neighboring chemically depleted belts, to finer-scale circulations at the scale of the zonal jets at mid-latitudes. This well-organized banding gives way, at higher latitudes, to more chaotic and turbulent regimes nearer the poles, exhibiting filamentary cloud complexes and organized patterns of cyclones on Jupiter and wave phenomena on Saturn (Adriani et al. 2018). What controls this transition, how it differs between the gas and ice giants, and whether these polar meteorological phenomena are long-lived are key open issues. Unraveling how energy, momentum, and material are transported vertically and horizontally in giant planet atmospheres requires observations to constrain giant planet circulation, from the equator to the poles, the depth and asymmetries in the flows, and the vertical structure of the powerful winds.

Q7.3b How Do Stratospheric Properties Trace Interactions with Internal and External Phenomena?

Planetary stratospheres are transitional domains between the meteorology of the deeper troposphere, and interactions with the external environment. They are sensitive to influences from below, such as rising storm plumes from the troposphere, mixing of gases upwelling from deeper levels, and potentially equator-to-pole gradients of the source material for photochemistry on Uranus and Neptune (Moses et al. 2020). They are also sensitive to influences from above, such as auroral energy deposition and resistive Joule heating in connection with the planetary magnetosphere, and via the influx of exogenic materials from rings, satellites, interplanetary dust, and impacts (cometary and asteroidal). This mix of chemical compounds is then redistributed vertically and horizontally by large-scale and seasonally variable interhemispheric circulation patterns, causing variations in chemical abundances and radiative energy balance from place to place (Hue et al. 2018). Equatorial oscillations of the stratospheric temperatures and winds have been discovered on Jupiter and Saturn and are prone to disruption by meteorological processes (Antuñano et al. 2021)—it is unknown whether Uranus and Neptune exhibit the same phenomenon. Understanding the redistribution of energy is a key challenge for the giant planets, where the middle and upper atmospheres are much warmer than expected from solar heating alone, an imbalance known as the energy crisis. Last, atmospheric chemistry is sensitive to the strength of vertical mixing, such that comparison of Uranus and Neptune, where the strength of mixing differs substantially (Moses et al. 2020), would provide a unique test of how photochemistry operates under very different conditions.

Q7.3c How and Why Do Discrete Meteorological Features (e.g., Storms and Vortices) Evolve?

Large-scale planetary bands are interrupted by, but intricately connected to, meteorological phenomena on smaller scales, including discrete convective clouds and storm plumes, waves in all their guises, and anticyclones and cyclones of various scales (Ingersoll et al. 2004). Some anticyclonic features on Jupiter are long-lived, but vary in size, color, and energetics via interactions with storms and vortices at their edges. Cyclonic features appear to show increased prevalence for moist convection and lightning. On the other hand, some vortices on Uranus and Neptune are short-lived and migrate with latitude, dissipating as they approach the equator (Hueso and Sánchez-Lavega 2019). The mechanisms that control the life cycles of convective storms are poorly understood, including why some (like Saturn’s storms and Jupiter’s bright plumes) appear to show predictable cycles; some of these may be influenced by motions and episodic heat transport deep below the clouds (Li and Ingersoll 2015). The depths of these vortices are unknown, along with what causes the long-term stability or short-term susceptibility of storms. Small-scale eddies and plumes may also be responsible for the maintenance of the zonal jets at larger scales. Lastly, some vortices, like the Great Red Spot, exhibit colorful hazes that appear to be lacking in others, like Neptune’s Great Dark Spot. Understanding how and why meteorological features differ from world to world requires long-term, multi-wavelength datasets to develop a better understanding of how planetary climates influence the types of “weather” that we observe.

Q7.3d What Chemical and Physical Processes Influence the Gas and Aerosol Absorbers That Produce the Diverse Colors and Spectral Properties of the Giant Planets?

The composition of planetary tropospheres and stratospheres is determined by thermochemistry, condensation processes, photochemistry, and the redistribution of material by circulation and meteorological phenomena. The different clouds, hazes, and chemical compositions on each planet reflect differences in these underlying processes, but the basic question of cloud and aerosol composition remains unresolved (e.g., West et al. 2009). We do not yet know how disequilibrium species (see Figure 10-2), which should be sequestered in the deeper troposphere, are transported to the observable weather layer, and how they contribute to observed color. Given the long seasons on these planets, the mechanisms that govern the formation of clouds and hazes as a function of depth, beyond simple thermochemical equilibrium (Guillot et al. 2020), and how the hazes (and their associated colors) change with seasons and auroral influences are not well studied. The nonsolar nitrogen-to-sulfur ratio observed in Uranus and Neptune may represent the bulk abundances of these species, or chemistry in deep water clouds may preferentially sequester ammonia. Additionally, the impact of discrete aerosol layers on the thermal structures of the upper troposphere and stratosphere, and how they differ in each planet, remain unknown. Other dynamical processes, such as precipitation and subsidence surrounding storm plumes, may also lead to dramatic changes in cloud colors. It is particularly important to understand the chemical compositions of major cloud features and dark vortices on the ice giants (Hueso and Sánchez-Lavega 2019), and why they differ substantially from similar features on the gas giants. The complex interplay of dynamics, thermal structure, and composition need to be more thoroughly explored to understand the differences observed at each planet.

Q7.3e How Does Moist Convection Shape Atmospheric Structure in Hydrogen-Dominated Atmospheres?

Convective processes are fundamental to understanding the motions of atmospheres and oceans, and the giant planets provide ideal testbeds for understanding the influences of buoyancy, stratification, rotation, and the importance of different condensable species under conditions not found on Earth—namely, a hydrogen-dominated atmosphere in which condensables are heavier than the surrounding environment (Hueso and Sánchez-Lavega 2019). While water-dominated convection can be studied on Jupiter and Saturn, this is largely hidden from remote sensing by overlying ammonia and NH₄SH clouds. Methane-driven convection (and, to a lesser extent, hydrogen sulfide) is more accessible on the ice giants but might be restricted to thin layers. Uranus and Neptune’s high methane abundance implies that they lie in a regime in which moist convection is locally inhibited by the stabilizing effects of molecular weight gradients (Guillot 1995). Convective inhibition in the ice giants may lead

to centuries-long intervals between convective outbursts, a potential explanation for apparent differences in the intrinsic luminosities of Uranus and Neptune (Smith and Gierasch 1995; Li and Ingersoll 2015). Comparing and contrasting convective processes—their vertical velocities, advected chemicals, thermal and chemical gradients, potential for lightning generation, and their relationship to latitudinal bands—between the differing environments will reveal how moist convection works in diverse planetary environments.

Strategic Research for Q7.3

• Determine what processes maintain tropospheric circulation and meteorology of Jupiter, Saturn, Uranus, and Neptune from in situ measurements of the vertical wind field and from multi-wavelength remote sensing measurements of temperatures, composition, and lightning frequency as a function of latitude, longitude, and over many timescales.
• Understand stratospheric coupling and seasonal influences on Uranus and Neptune from multi-wavelength remote sensing of stratospheric temperatures, trace gas composition, and aerosols over long timescales, in connection with ionospheric and magnetospheric variability.
• Constrain storm evolution on Jupiter, Saturn, Uranus, and Neptune with frequent, multi-wavelength remote sensing over multiple timescales from hours to days to years.
• Determine what governs cloud top color and how it ties to transport and chemistry in the atmospheres of Saturn, Uranus, and Neptune from in situ sampling of composition and particle properties, coupled with global imaging in reflected sunlight (at multiple phase angles) and thermal emission.
• Elucidate how convection works on Uranus and Neptune from multi-wavelength remote sensing over many timescales and depths, combined with analytical, radiative transfer, and general circulation modeling.

Q7.4 WHAT PROCESSES LEAD TO THE DRAMATICALLY DIFFERENT OUTCOMES IN THE STRUCTURE, CONTENT, AND DYNAMICS OF THE OUTER PLANETS’ MAGNETOSPHERES AND IONOSPHERES?

The interaction between the strong magnetic fields of outer planets and the solar wind creates fast rotating magnetospheres that serve as natural laboratories for studies of astrophysical plasma processes accessible to in situ measurements from space missions. Our understanding of plasma physics processes such as collisionless shocks, magnetic reconnection, plasma pick-up, field-aligned plasma acceleration, plasma interchange, auroral emissions (visible, ultraviolet, X-ray, and radio), generation of plasma waves, and the formation of radiation belts owes a lot to in situ observations within these giant magnetospheres. A magnetosphere involves a planetary magnetic field, the magnetized flowing solar wind that interacts with it, and a source of magnetospheric plasma (often a moon, but the planet’s atmosphere is also a significant source). Why the jovian magnetosphere is the largest, extremely dense (with >10⁹ kg of plasma), and hot, while the magnetospheres of Uranus and Neptune are near vacuum, is not well understood (Figure 10-5). Most of our current knowledge of the jovian and saturnian magnetosphere comes from orbiting spacecraft (Galileo, Juno, and Cassini) whereas Uranus and Neptune have been visited only once by Voyager 2 flybys and are relatively unexplored. One of the main obstacles to making progress in studies of the magnetospheres is their vast scale and a lack of simultaneous multiple-point measurements to distinguish between temporal and spatial changes.

Q7.4a What Processes Govern the Content and Dynamics of the Giant Planet Magnetospheres?

Sources of plasmas in planetary magnetospheres are outflow from the planetary ionosphere (ions are mostly protons), leakage in from the solar wind (protons and alpha particles), and outgassing of moons with ion species that are source dependent. For example, Enceladus’s water vapor plumes provide O⁺, OH⁺, H₂O⁺. and H₃O⁺ ions; SO₂ from Io’s volcanoes provide S⁺, S²⁺, S³⁺, O⁺, and O²⁺ ions; and Triton’s and Titan’s atmospheres source nitrogen ions (N⁺ and N₂⁺). The relative importance of moon sources and their associated mass loading to the

magnetosphere depends on the volcanic and cryovolcanic activity of moons orbiting within the magnetosphere and the local plasma environment responsible for ionizing outgassed neutrals. The size of the magnetosphere (see Figure 10-5) depends strongly on the strength and spatial configuration of the internal magnetic field generated by the planetary dynamo (Q7.2d). But strength and configuration is also influenced by internal dynamics that heat the plasma (increasing the internal plasma pressure and “inflating” the magnetosphere), as well as by the strength of the solar wind (that decreases with distance from the Sun).

Magnetospheric dynamics depend on the coupling between the solar wind, the planetary ionosphere, and the magnetosphere which comprises the planetary magnetic field and plasma sources described above. Earth-like magnetospheric convection is expressed by coupling between the solar wind and the planet’s ionosphere and driven by the process of magnetic reconnection at the dayside magnetopause. The characteristic solar wind Alfvén speed decreases with distance from the Sun, and, for a constant solar wind speed, the Alfvén Mach number increases. This results in the boundary between the solar wind and giant planet magnetospheres (particularly if inflated with hot plasma) being frequently dominated by a viscous coupling (e.g., Kelvin-Helmholtz instability) across the magnetopause boundary layer rather than by global magnetic reconnection (Masters 2018).

For the rapidly rotating gas giants, the coupling between the ionosphere (that is collisionally coupled to the spinning gas planet) and the magnetosphere produces rotation-dominated plasma flows. The energy of the plasma in the jovian and saturnian magnetospheres is derived mostly from the rotation of the planet with some

contributions coming from the solar wind imposed electric field (Khurana et al. 2004; Kivelson and Bagenal 2007; Gombosi et al. 2009). Both Jupiter’s and Saturn’s magnetospheres are also marked by large plasma acceleration events connected to magnetotail reconnection and plasma interchange (Gombosi et al. 2009; Bagenal 2013; Vogt et al. 2014). The excited aurorae in the waveband spanning radio to X-ray spectrum are both a window into the workings of these magnetospheres and one of the main loss processes of the energy of the confined plasma. A combination of in situ plasma and magnetic field observations with simultaneous or near simultaneous remote observations of the aurora would help to determine the nature of magnetospheric energy transport and quantify the competing processes governing magnetospheric dynamics and structure in the outer solar system.

Q7.4b What Is Responsible for the Differences Between the Magnetospheres of the Gas Giants and Ice Giants?

The active moons Io and Enceladus dominate the magnetospheres of Jupiter and Saturn, respectively, filling their large magnetospheres with plasma disks that co-rotate with the planet. The strong magnetic field of Jupiter harnesses the spin momentum of the planet and heats the plasma as it moves out (to >20 keV in the middle magnetosphere), inflating the magnetosphere to 100 times the radius of the planet on the sunward side and stretching past the orbit of Saturn downstream in the solar wind. The weaker Enceladus source coupled to the magnetic field of Saturn produces a similarly rotation-dominated magnetosphere that is smaller in extent. The plasma is colder (temperatures below 1 keV), and the magnetosphere is dominated by neutral material sourced from Enceladus’s plumes.

The highly tilted and off-centered dipole moments of Uranus and Neptune generate asymmetric magnetospheres and twisted magnetotails which were only fleetingly explored by Voyager 2 flybys. Because of this geometry, their magnetospheric interaction with the solar wind varies greatly with rotation and with season (Paty et al. 2020). Their magnetospheric plasma composition is dominated by protons (along with N⁺ ions in the case of Neptune sourced from Triton), suggesting that solar wind coupling to their ionospheres produces Earth-like global convection. However, the process whereby a tenuous solar wind couples to these asymmetric magnetospheres, and the dynamic roles of their sparsely observed satellites in terms of sculpting the radiation belts and sourcing plasma to the magnetosphere, remain unclear. One of the major unresolved questions in planetary magnetospheres is how the extremely low plasma density in the uranian magnetosphere maintains electron radiation belts similar in intensity to Earth’s. These differences illuminate relative roles of the strengths of planetary fields, plasma sources (including their composition), corotation breakdown from finite effective conductivities of the ionospheres, and plasma processes such as interchange and reconnection operating in these magnetospheres.

Q7.4c How Is Energy Redistributed with Latitude and Altitude Within Giant Planet Ionospheres/Thermospheres, and What Is Responsible for Their High (and Variable) Temperatures?

Giant planet thermospheres are influenced by both external processes (e.g., solar heating and auroral heating) and by internal processes (e.g., waves and circulation patterns in the middle atmosphere), such that tracing the redistribution of energy can reveal insights into thermospheric and ionospheric circulation. All four giants exhibit thermospheric temperatures far warmer than can be explained by solar heating alone (García Muñoz et al. 2017), hinting that dynamical energy redistribution may be a significant contributor, and evidence of rapid thermospheric flows associated with auroral ovals has been observed on Jupiter and Saturn. How energy is redistributed latitudinally through the upper atmosphere, from the auroral domains to the lower latitudes, and whether tropospheric meteorology influences thermospheric temperatures as a function of latitude, is not well constrained. Nor is the mechanism responsible for the slow cooling trend of Uranus’s ionosphere over multiple decades: does its weak atmospheric mixing and low homopause, coupled with the extreme axial tilt, produce a uniquely variable ionosphere? Further exploration of the ice giant systems is needed to compare the redistribution of energy in their ionospheres/thermospheres with those of Jupiter and Saturn.

Q7.4d How Do External Inputs and Local Ion Chemistry Produce the Complex Variability Observed in Ionospheres?

Although overall composition should be similar in the ionospheres of the giant planets, the insolation, seasonal forcing, magnetic field configuration, and ring influxes are very different. At Saturn, variation in H₃⁺ emission and electron density may relate to variable influxes of ring material (including neutral nanograins near the equator and charged grains at mid-latitudes), to gravity waves breaking in the lower thermosphere, and/or to other extreme ionization or transport processes. At Uranus, nonseasonal thermospheric temperatures vary on timescales greater than a solar cycle. But these temperatures are derived from H₃⁺, implying that seasonal change and spatial and temporal variability in this ion could influence derivations of temperature (Moore et al. 2019b). For Neptune, H₃⁺ has not yet been detected, and models of the ionosphere predict H₃⁺ concentrations greater than the current upper limit. This may suggest that the thermospheric temperature and/or methane homopause level differ from what was observed by Voyager, or that an influx of external material, perhaps from Triton/Neptune’s rings, has depleted ionospheric H₃⁺ densities. While many other ionic species are predicted to be present at the ice giants, none have yet been detected. Radio occultations, ultraviolet/infrared spectral limb scans, and in situ mass spectral measurements from deep-dive orbiter passes would provide crucial new data for understanding ionospheric variability.

Strategic Research for Q7.4

• Determine the dominant processes governing the magnetospheres of Uranus and Neptune via plasma, particle, and magnetic field observations with simultaneous or near simultaneous remote observations of the aurora.
• Constrain the structure, dynamics, and temporal evolution of the magnetospheres of Uranus and Neptune with long-term magnetic field measurements.
• Investigate the evolving composition of the ionospheres of Uranus and Neptune from remote sensing, magnetospheric plasma, and in situ ion/neutral composition measurements.
• Determine the variability and thermal structure of the thermospheres and ionospheres of Uranus and Neptune from radio occultations and infrared and ultraviolet spectral limb scans, distributed in latitude and time.

Q7.5 HOW ARE GIANT PLANETS INFLUENCED BY, AND HOW DO THEY INTERACT WITH, THEIR ENVIRONMENT?

Giant planet systems have many components: the planet and its rings, satellites, and magnetosphere. Each element interacts with the others (Question 8, Chapter 11), but also with external influences from the Sun and cosmic rays. Inputs into the giant planets (both energetic and mass/compositional) affect upper atmospheric composition, which then precipitates to lower levels over time; this input needs to be understood to relate current compositional measurements of the giant planets to current and primordial bulk abundances. The planets themselves also lose energy to the satellites and rings through tidal interactions, which can further elucidate the history of their internal structure.

Q7.5a How Is Angular Momentum Lost, and Tides Dissipated, from the Giant Planets?

Recent observations reveal that all of Saturn’s moons are migrating outward at surprisingly fast rates (Lainey et al. 2017, 2020), which means that angular momentum is being transferred from Saturn to its moons much more efficiently than previously expected. The observed orbital evolution rates suggest that they are driven by ongoing changes in the planet’s internal structure (Fuller et al. 2016; Lainey et al. 2020). It is not yet clear what is happening inside Saturn to produce these changes, nor whether similar processes are operating within the other giant planets. Planned missions to Ganymede and Europa can be used to refine their orbital evolution and constrain Jupiter’s tidal dissipation. Clarifying the orbital history of the giant planet moons via precise astrometry and long-term monitoring promises to provide more insights into the recent history and evolution of the giant planets.

Q7.5b How Is Atmospheric Composition Influenced by Ring Rain, Large Impacts, and Micrometeoroids?

Measurements made by Earth-based telescopes and the Cassini spacecraft reveal a large flux of material into Saturn’s atmosphere from the rings, which may provide a total water flux to Saturn of 10,000 kg/s (Moore et al. 2015). While no other planet is surrounded by as massive a ring system as Saturn, some fraction of circumplanetary debris surrounding all the giant planets can be transported along magnetic field lines into the planet. Additionally, exogenic material is continuously delivered to the planets via impacts. Larger impacts (e.g., Shoemaker-Levy 9 at Jupiter) are governed by physics similar to terrestrial airbursts, but with very different chemistry. New observations taken soon after large impacts in giant planet atmospheres are needed to test these models across a range of impact geometries and impactor properties. Observations and models of stratospheric chemical signatures in Uranus and Neptune have begun to elucidate the balance between influx from micrometeoroids versus major cometary impacts within the past several centuries (Cavalié et al. 2014; Moreno et al. 2017). Monitoring of smaller impacts on Jupiter (but not yet on the other giant planets) currently provides the only constraints on the small end of the impactor size distribution, a parameter important for establishing surface ages near the planets.

Q7.5c How Does Seasonally Variable Solar Insolation Influence Middle Atmosphere Chemistry and Haze Production?

Despite their large heliocentric distances of 10 AU to 30 AU, the giant planets beyond Jupiter show significant seasonal influence from the Sun. Saturn and Neptune have obliquities like that of Earth, and Uranus’s axis is tilted by an extreme 98° from the ecliptic, leading to hemispheres bathed in sunlight for long portions of an orbital period. Indeed, changes in Uranus’s polar hazes have been observed over the uranian year, and there is some evidence for seasonal change in Neptune’s brightness and banding, as well. Thus, very low solar irradiation can still cause photochemical change over time, modulated by the atmospheric circulation, and these processes, and the chemical/dynamics pathways involved, are not yet well understood.

Strategic Research for Q7.5

• Determine the role of tidal dissipation in angular momentum transfer at Jupiter, Uranus, and Neptune from measurements of satellite orbital migration.
• Quantify the diverse external influences on the atmospheric chemistry and dynamics of Jupiter, Saturn, Uranus, and Neptune via time-series imaging and spectral measurements of the effects of impacting objects from micrometeoroids to comets.
• Constrain the influence of seasonal solar insolation on Uranus and Neptune’s atmospheric chemistry and hazes from measurements of temperature, haze optical depth, and gas abundances over long time periods.

SUPPORTIVE ACTIVITIES FOR QUESTION 7

• Laboratory measurements and numerical simulations of opacities under outer planet conditions (both at high pressures and for long columns at low pressures/temperatures), high-pressure equations of state, chemical reaction rates, and transport properties (i.e., viscosity, thermal and electrical conductivity, and diffusion coefficients).
• Numerical and analytical models of dynamic processes in the atmosphere, interior, and magnetosphere of giant planets, coupled to an investment in high-performance computing.
• Continued data analysis from ongoing and past missions, along with acquisition and archiving of planetary datasets from ground-based facilities to enable maximum science return, as well as generate high level science products to inform the development of future missions.
• Long-term monitoring of atmospheric dynamics, waves and oscillations, auroras, and impacts, ideally by a space-based telescope.

REFERENCES