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Question 8: Circumplanetary Systems

Circumplanetary systems—where a system of moons and/or rings orbit a central body—are seen throughout the solar system, and in some cases are akin to mini-solar systems with numerous and varied orbiting bodies (Figure 11-1).¹ A distinguishing feature of these systems is the importance of coupled interactions, which can occur on shorter timescales and have larger effects than on bodies orbiting the Sun, owing to the compactness of circumplanetary orbits, shorter orbital periods, and relatively close moon–moon and moon–ring orbital separations. For example, while tidal dissipation is a general process operating throughout the solar system, in circumplanetary systems it has heightened strength and importance as it can drive substantial and ongoing heating and activity within moons (leading in some cases to active volcanism, subsurface oceans, plumes, and distinct tectonic features), as well as substantial moon orbital migration and associated passage through orbital resonances. Other notable coupled processes include interactions of the planetary magnetic field with a satellite’s intrinsic field and/or its induced field, supply of plasma and neutral content to a planet’s magnetosphere from embedded satellites, and complex dynamics within a planetary magnetosphere that affects the material interacting with embedded moons and rings. Collisional ejecta and sputtered particles that escape a moon typically continue to orbit the planet and may be reaccreted or reimpacted onto neighboring moons and rings, affecting surface compositions and properties. Inner moons can be particularly vulnerable to disruption over their histories, owing to tides and the strong gravitational focusing and high impact speeds set by their host planet’s gravity. Disrupted moons may form rings, and subsequently reaccrete, resetting the moon’s thermal and physical state. Ring systems remain dispersed owing to their proximity to the planet, providing a unique means to study a variety of problems, including the evolution of a self-colliding particulate disk (analogous to early protoplanetary and circumplanetary disks), ring–moon and ring–planet gravitational interactions and resulting wave features, collisionally driven spreading of ring material, and ongoing accumulation into new moons as ring material spreads away from the planet. At the outer edge of some circumplanetary systems are irregular satellites whose complex populations are strongly affected by both the Sun’s and the host planet’s gravity and may record these interactions.

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

Q8.1 HOW DID CIRCUMPLANETARY SYSTEMS FORM AND EVOLVE OVER TIME TO YIELD DIFFERENT PLANETARY SYSTEMS?

The physical, orbital, and compositional characteristics of the present-day circumplanetary material provide clues as to how these systems, and their parent planets, have formed and evolved. For instance, many moons (e.g., Moon, Mars’s moons, Pluto’s moons, and Haumea’s rings and moons) have been attributed to giant impacts, whereas the large moons of the gas giants are believed to have formed directly from the primordial circumplanetary disks, and others may have been captured objects formerly orbiting the Sun (e.g., Triton). The diverse characteristics of the present-day circumplanetary material provide clues as to how these systems have formed and evolved. The regular progression of bulk densities of the Galilean satellites of Jupiter is not duplicated at Saturn or Uranus (see Figure 11-1), indicating some fundamental difference in how these systems formed or evolved. A crucial aspect of moons is that their orbital and thermal evolution are intimately coupled. Furthermore, in some cases the moons have developed subsurface oceans (Q8.1d), which makes them high-priority targets for astrobiology (Questions 9, 10, and 11; Chapters 12, 13, and 14, respectively). Details of the Moon’s formation are covered in Question 3 (Chapter 6).

Q8.1a What Processes Have Led to the Diversity of Bulk Properties of Satellite and Ring Systems?

The regular ring–moon systems of the giant planets (i.e., the rings and moons orbiting in or near the planet’s equatorial plane) probably originated from a disk of gas and solid material that arose as each planet formed. Thus, the bulk composition of these ring–moon systems can constrain the material that flowed into the giant planets during the last phases of their accretion. Meanwhile, comparisons between the compositions of solid bodies and their circumplanetary systems can reveal the conditions and circumstances under which they formed, including whether they were produced by giant impacts.

Recent investigations of the ring–moon systems surrounding Saturn, Uranus, and Mars indicate that the configuration of material orbiting these bodies may have changed substantially over the history of the solar system, with rings accreting into moons and/or moons breaking down into rings at various times (Q8.1c). For example, there is active debate about the age of the Saturn’s rings (e.g., Crida et al. 2019), and what role the hypothesized Uranus-tilting giant impact played in forming or modifying the uranian satellites (e.g., Kegerreis et al. 2018). On one hand, this complicates efforts to reconstruct the initial configuration of the material surrounding these planets. On the other hand, detailed investigations of compositional, geological, and structural variations within and among these systems provide an opportunity to test how fundamental processes such as accretion and fragmentation have operated at different times and under different environmental conditions.

Q8.1b What Determines the Composition and Rock–Ice Ratios of Moons and Rings?

The bulk composition of moons and rings (i.e., the relative abundances of ice, rock, and metal) provides clues to their formation. Bodies built from material excavated in a giant impact are expected to be made of the outer layer of the impacted world—for example, the Moon is presumed to be made in part from Earth’s mantle (Question 3, Chapter 6). Conversely, bodies that formed together with their host planet would have bulk compositions initially set by the protoplanetary and/or circumplanetary disk(s). Last, the composition of a captured body would not be expected to resemble that of its host planet. However, reality is more complicated than this simple analysis. The regular density progression of the Galilean satellites (see Figure 11-1) might suggest a compositional gradient in the protoplanetary disk, or loss of volatiles from the innermost moons. There is no such regular progression at Saturn or Uranus. At Saturn, Tethys, whose low density suggests that it may have formed from debris from an icy mantle produced by an impact or tidal stripping (e.g., Canup 2010), has orbital neighbors Enceladus and Dione, both moderately rocky bodies that seem to have formed from the disk. At both Uranus and Saturn, the ring compositions appear quite different from many of the satellites. Similarly, Phobos and Deimos are spectrally distinct from Mars. Pluto’s small satellites look like impact fragments, but Charon’s bulk composition is similar, but not identical, to Pluto’s outer layers. For trans-neptunian objects, there is an enigmatic trend where smaller bodies are less dense than larger bodies (Brown 2012), but our understanding of their formation and evolution is severely limited by a lack of basic information, including their sizes, masses,

and densities. At present, the combination of factors leading to the various bulk composition patterns observed across the solar system (see Figure 11-1) is not understood.

Q8.1c How Old Are Moons and Rings, and Do They Undergo Cyclic Formation and Destruction?

The mid-size moons of Saturn have received particular attention in the past decade owing to the detailed datasets acquired by the Cassini mission. The moons vary in composition, geological activity, and internal heating with little apparent regard for their distances from Saturn—in contrast to Galilean satellites where geologic activity decreases (and ice content increases) as a function of distance from Jupiter. In order to address these peculiarities, several models have emerged that suggest not all of Saturn’s moons are primordial. In some hypotheses, large impacts led to the destruction of one or more moons, the remains of which accreted to form the current moons (e.g., Asphaug and Reufer 2013), while others suggest the innermost moon or moons grew from material expanding from a massive initial ring, perhaps with rocky “kernels” within Saturn’s rings accreting outer layers of ice (e.g., Canup 2010; Charnoz et al. 2011). Although still an area of active investigation, these different ideas all point to the Saturn system undergoing a much more dynamic evolution than once thought. The life cycle of ring–moon systems may involve several iterations of formation and destruction. Similar hypotheses have been put forward for cyclical formation and destruction of Mars’s satellites, Phobos and Deimos, leading to the hypothesis that Mars may have once had rings (e.g., Hesselbrock and Minton 2017).

Looking outward to the satellite systems of Uranus and Neptune, where data are comparatively sparse, there is further evidence of systems once in upheaval, sculpted by impacts. At Uranus, models suggest that a giant impact titled the planet, destabilizing and possibly destroying its original, primordial satellites. Uranus’s present satellite system thus may be a second-generation of planetary satellites. Uranus’s small moons and rings are darker than the larger moons. The ring particles may consist of heavily processed material which was initially similar to that of the inner moons. At Neptune, the capture of Triton, and its subsequent tidal evolution, likely wiped out much of the original satellite system, the remains of which seem not to have coalesced into a set of mid-sized moons, unlike at Saturn and Uranus. Neptune’s rings, and its ring arcs, consist primarily of fine dust that is sculpted by a variety of dynamical processes, which give clues to how material is aggregating/fragmenting in circumplanetary systems. A major implication is that the evolution of these systems may have been intimately linked to the architecture of the giant planets whose motions governed the timing and extent of material being flung at their developing satellite systems. For both Uranus and Neptune, there are fundamental knowledge gaps in our understanding of the compositions of the small moons and rings.

Q8.1d What Determines the Internal Structure of Moons, and Which Moons Have Subsurface Oceans?

The moons of the solar system have tremendously varied interior structures (Figure 11-2), ranging from largely solid ice balls, to partially molten rocky worlds, and worlds with plausibly habitable subsurface water oceans—so-called ocean worlds. Much of the basic structure of a moon is determined by its initial bulk composition, and the energy imparted from formation, decay of radioactive material, subsequent impacts, and tides. Most large satellites are differentiated, with dense phases (metal and rock) forming a core, overlain by a mantle or crust of low-density phases (rock and ice). However, while this is the norm, it is not the rule. Callisto may be largely undifferentiated, despite having a comparable size and bulk composition to several other differentiated icy worlds, like Ganymede and Titan. Determining the differentiation state of large planetary satellites provides a key window into the earliest epochs of a circumplanetary system (Question 2, Chapter 5).

A handful of icy satellites are heated enough that their outer layers can melt, producing subsurface oceans of liquid water hidden beneath insulating icy shells that are either in contact with the rocky seafloor or high-pressure ices at their base. Ganymede may have several ice polymorphs form with perched oceans between them (Journaux et al. 2020). These ocean worlds may be abodes for life (Questions 9, 10, and 11; Chapters 12, 13, and 14, respectively). Despite their importance, the solar system’s inventory of ocean satellites is uncertain and likely incomplete. There is direct evidence for subsurface oceans within Europa, Ganymede, Callisto, Titan,

and Enceladus, and circumstantial evidence for oceans within several other worlds, including Pluto and Triton (Nimmo and Pappalardo 2016). There is also evidence for a subsurface magma ocean within Io, plausibly formed and maintained by similar processes (Khurana et al. 2011). Subsurface oceans may be persistent and long-lived, or they may go through episodic or cyclic periods of freezing and melting. This is complicated because the thermal evolution of these worlds is likely coupled to their orbital evolution and the gravitational interactions between neighboring satellites and their host planets (Q8.2). For example, the existence, longevity, and evolution of Europa’s ocean is likely tied to the evolution of the orbital resonance between Io, Europa, and Ganymede, their interior structures, and the interior structure of Jupiter (Hussmann and Spohn 2004; de Kleer et al. 2019a). This highlights the need for investigations that span entire circumplanetary systems.

Strategic Research for Q8.1

• Determine the differentiation state, radial interior structure, tidal response, and presence/absence of water and magma oceans and reservoirs within the moons of Jupiter, Saturn, Uranus, and Neptune by measuring their gravity fields, shape, induced magnetic field and plasma environment, and other geophysical quantities.
• Determine the masses, densities, and bulk compositions of Kuiper belt objects and their satellites with surveys for multiple systems and satellites, and characterization of their bulk properties and orbital motion.
• Determine the composition of rings and small moons at Uranus and Neptune to elucidate their origin, evolution, and present-day balance between exogenic and endogenic processes through a combination of geophysical measurements, imaging, and spectroscopic observations, including at spatial resolution sufficient to resolve regional variations and layering.
• Constrain the origin of Phobos and Deimos, including whether they arose from past martian rings, by determining their bulk composition and interior structure via in situ geochemical and geophysical measurements.

Q8.2 HOW DO TIDES AND OTHER ENDOGENIC PROCESSES SHAPE PLANETARY SATELLITES?

A host of processes beneath the surfaces of planetary satellites shape their diverse properties and evolution over time (Figure 11-3). Tides are a prominent such process that are largely responsible for the presence of liquid water oceans and volcanism in the outer solar system, may drive flows in planetary oceans and cores, and fracture the ice shells of ocean worlds. Tectonics, (cryo)volcanism, hydrothermal activity, and magmatism can further shape these worlds and are ultimately the consequence of heat and materials transported from the deep interior outward. Beyond their surface expression, these processes can also have magnetic signatures, both in terms of intrinsic magnetic fields generated by dynamo action and induced magnetic fields owing to interactions between subsurface oceans and the host planet’s magnetosphere. By understanding the diversity of these processes across planetary satellites throughout the solar system, we will learn how they operate on each body as well as collectively. Understanding these processes has broad impact across all of planetary science, including the ability assess the astrobiological potential of planetary bodies and the interplay of oceanography, glaciology, and hydrology with life (Question 9, Chapter 12), the time-dependent supply of liquid water, energy, and nutrients (Question 10, Chapter 13), and how these processes affect biosignature detectability, survivability, and reliability (Question 11, Chapter 14).

Q8.2a How and Where Is Tidal Heat Dissipated Within Circumplanetary Systems?

As a moon orbits its parent world, it is distorted by tides. If the moon’s orbit were perfectly circular and unchanging, its shape would adjust to the tidal forces—producing a tidal bulge. Tidal dissipation in the moon ultimately drives the spin period of the moon to match its orbital period (i.e., becoming “tidally locked”), with the bulge constantly facing its host planet. However, if the moon is on an eccentric orbit, if it spins at a different rate than it orbits, or if the axes of its spin and orbit are not aligned, it may never reach a static shape. Instead, the moon will continuously and repeatedly deform in response to tides, leading to friction and heating within the moon (e.g., Peale et al. 1979). Long-term tidal heating requires maintenance of the orbital eccentricity, which in turn requires tidal dissipation within the planet. The long-term rate of satellite tidal heating depends on how dissipative the planet is, so there is an intimate connection between planetary and satellite evolution. Tidal heating is the main driver of recent geologic activity on outer solar system satellites, including supporting long-lived subsurface water oceans within many satellites and a possible magma ocean within Io, and it may have played a role in shaping other bodies in the early solar system, including the Moon. Despite its importance, there are still many fundamental questions about how tidal heating operates, where the heat is dissipated (be it in the icy shells, water or magma oceans, or deeper rocky layers), whether eccentricity or obliquity tides dominate, how tidal heating affects the surface geology, and how it ultimately escapes to space (Beuthe 2013; de Kleer et al. 2019a).

Matching observations with theoretical predictions for tidal hearing is difficult because of a lack of observational constraints of the interior structures of tidally heated worlds, incomplete theoretical models, and the lack of knowledge of how relevant planetary materials (e.g., both within the rock/ice/melt of the satellite, and within the giant planet cores and fluid envelopes) behave under relevant pressure, temperature, and forcing conditions.

Q8.2b What Is the Thermal and Orbital Evolution of Planetary Satellites, and What Is the Role of the Host Planet and Other Satellites in That Evolution?

Under the action of tides, the orbits of satellites in circumplanetary systems change with time. Exactly how a satellite’s orbit changes depend on the internal structure and thermal state of both the satellite and its tide-raising parent body—which determine how and where energy is transferred and dissipated. This results in a complex coupling between satellite orbital evolution and thermal evolution. These coupled processes affect the timing and likelihood of capture into resonances between orbital and/or spin periods, orbital eccentricities and inclinations, and the magnitude and duration of tidal heating—which governs the formation and sustainability of subsurface oceans. Although solid planets and moons cool over time, the coupled evolution of satellites could lead to phases of high heat flows, transient or late oceans, and episodic geologic activity (Ojagankas and Stevenson 1989; Hussmann and Spohn 2004). For example, the Moon’s outward migration owing to tides may have been quite complicated, plausibly leading to periods of high eccentricity, obliquity, and tidal heating, which may leave an imprint on the overall shape of the Moon (its so-called fossil figure—e.g., Gerrick-Bethell et al. 2014; Keane and Matsuyama 2014). How moons respond geophysically to temporal changes in heating and deformation, how changes in their host planets might affect their long-term orbital evolution, and the potential for such dynamic environments to host life are all active areas of research.

Q8.2c What Is the Expression of Tectonic Activity on Planetary Satellites, and Why Is It So Varied Between Circumplanetary Systems?

Tectonic activity on planetary satellites is varied in both style and extent, which can provide insight into satellite evolution, interiors, and the geophysical mechanisms at play in the solar system (Collins et al. 2009). Moons with the potential for tidal activity because of their eccentric orbits tend to have younger surfaces with more diverse geology (e.g., Europa and Enceladus), although they display curious differences in feature types and distributions. For example, Europa possesses “cycloids,” arcuate fractures that can be explained by diurnal tidal stresses (e.g., Rhoden et al. 2021), although similar fractures are not observed on other worlds, and other tectonic structures on Europa remain unexplained. Enceladus possesses a long-lived cryovolcanic plume which is sourced from jets originating from a set of tectonic fractures on the moon’s south pole—the “tiger stripes.” While these faults are the conduits by which Enceladus’s ocean material is ejected to space, we do not know how they form, evolve, or what modulates their behavior (e.g., Běhounková et al. 2017). However, some moons with eccentric orbits display little evidence for tectonism (e.g., Mimas), and some moons with circular orbits display extremely young surfaces and tectonic features similar to their eccentric cousins (e.g., Triton). Enceladus’s long-lived plume system and Europa’s plate-tectonic-like behavior and disrupted “chaos” terrain are unique among explored satellites. Other moons, such as Tethys, Miranda, and Charon, for which tidal deformation likely ceased billions of years ago, display large canyon systems that might be linked to volume expansion during the freezing of ancient oceans, but the processes responsible for the distribution and shapes of these features (e.g., location, orientation) remain enigmatic. For outer solar system satellites, our understanding of tectonic processes is limited by a dearth of data on the response of ice to mechanical stress under relevant conditions and limited investigations into the complex interactions between fractures and other processes, such as cratering. In addition, sources of stress beyond eccentricity or obliquity tidal stresses (e.g., nonsynchronous rotation, physical libration, true polar wander) have been suggested but, for the most part, not confirmed for these moons. Expanding our theoretical models and obtaining additional observational constraints on these processes would help clarify their importance in the evolution and activity of planetary satellites.

The surface manifestations of tectonic stresses are incredibly varied across the circumplanetary systems in the solar system. Many icy moons show evidence of extensional tectonics, such as cycloids, lineaments, and extensional

bands on Europa, fractures on Enceladus, grooved terrain on Ganymede, wispy terrain on Dione, fractures on Triton, and canyons on Tethys, Miranda, Pluto, and Charon. Tectonic activity from shear stresses is less prevalent but is observed on Io, Europa, Ganymede, and Enceladus in the form of strike-slip faults, deformation within bands, and deformation of existing features (e.g., craters). Compressional tectonics is prevalent on many rocky worlds, including lobate scarps and wrinkle ridges on the Moon, and enormous mountains on Io. Compressional tectonics can be more difficult to identify on icy worlds, although compressional bands have been definitively identified on Europa, and mountain ranges on Titan may form from compression. The relationships between local and global stress regimes are not well-understood. Additionally, the level of present-day tectonic activity is uncertain on most worlds. A notable exception is the Moon, where Apollo seismometers and observations of boulder falls suggest that the Moon’s lobate scarps are seismically active today (Senthil Kumar et al. 2019). Theoretical models strongly suggest that many icy worlds are tectonically and seismically active today, including Enceladus—where tides may cause meters of displacement on the tiger stripes every Enceladus orbit (33 hours), and possibly modulate plume output (e.g., Běhounková et al. 2017).

Q8.2d Where Does Liquid Water Exist in the Ice Shells of Ocean Worlds?

We now know that several icy moons in the solar system possess subsurface liquid water oceans (Q8.1d), and there is increasing evidence that liquid exists within their ice shells as well (Cable et al. 2020; Journaux et al. 2020; Schmidt 2020). Enceladus’s plumes have provided proof that icy satellites can develop fracture systems that directly link subsurface liquid water reservoirs with the surface, but the evolution that led to this system is not well understood. In contrast, liquid water may not completely penetrate Europa’s ice shell, but instead collect in melt pockets within the ice shell (i.e., in sills), which may play a key role in the formation of ubiquitous double ridges, disrupted “chaos” terrains, relatively small, distinct pits, uplifts, and smooth dark regions dubbed “spots,” and the purported plumes—although convection of warm ice has also been proposed for most cases. More generally, subsurface water kept liquid by antifreezes (e.g., ammonia) may occur in warmer ice (e.g., upwelling convection cells or sites of localized tidal heating), and brines may be retained in a mushy layer at the base of the ice shell. As oceans freeze, they become pressurized and may fracture the overlying ice shell to allow ocean infiltration. In large ocean worlds (e.g., Ganymede and Titan), liquid water may also exist in high-pressure ice layer(s) at great depth. The extent to which liquid water can move into and out of ice shells has important implications for habitability and the search for life (see Questions 10 and 11, Chapters 13 and 14, respectively). In particular, the habitability of in situ melt pockets depends on the source of liquid water; melt pockets that are not sourced from the ocean are less likely to contain biosignatures. Melting in high-pressure ices may enable silicate–ocean exchange of salts and volatiles, which are necessary for life, but their ability to cycle resources in a way that supports life may be limited.

Q8.2e What Causes Plumes, (Cryo)Volcanism, Hydrothermal Activity, and Magmatism in Planetary Satellites?

Active volcanic or cryovolcanic eruptions (where melted or vaporized ices are extruded; Geissler 2015) occur on Io, Enceladus, Triton, and possibly Europa. The Moon is not volcanically active today, but possesses a rich volcanic history as revealed by both orbital datasets and returned samples—most recently with the samples collected by the Chang’e-5 mission which reinforce the concept of long-lived lunar magmatism (Che et al. 2021). Beyond the Moon, past volcanic and cryovolcanic activity is hard to identify with certainty, although many satellites—including Ganymede, Titan, Ariel, and Charon—appear to have experienced (cryo)volcanic activity in their past. But some satellites, like Callisto and Mimas, have apparently never experienced such activity—or such activity happened early enough (billions of years ago) that it was erased by subsequent impacts. This variability in behavior is not simply a function of distance from the planet, orbital eccentricity, or body size, and thus represents a major puzzle. Tides are presumably the dominant heat source for recent volcanic activity, and they are known to modulate the plumes on Enceladus (Hedman et al. 2013) and possibly Io (de Kleer et al. 2019b). However, geologically recent (<1 Ga) volcanic and cryovolcanic constructs on worlds where tidal heating is either

insignificant (e.g., Ahuna Mons on Ceres), or ceased long ago (e.g., Wright and Piccard Montes on Pluto, irregular mare patches on the Moon [see Figure 2-5]), suggest that tidal heating is not always required to power volcanic activity on planetary satellites. For large and rock-rich bodies, such as Europa, the contribution from radiogenic heating alone may be sufficient to drive volcanism.

On Earth and the Moon, intrusive magmatism (where the magma never reaches the surface) dominates the volcanic history. On the Moon, the ratio between the volume of intruded magmas to erupted lavas may be as high as ~50:1 (Head and Wilson 1992). However, the extent and role of intrusive magmatism within other planetary satellites, and in the rocky mantles of ocean worlds, is harder to conclusively identify. Io-like volcanic activity on Europa’s rocky seafloor cannot be ruled out; it would lead to vigorous hydrothermal activity and could alter ocean chemistry. Silica particles in Enceladus’s plume may be evidence for high-temperature water–rock interactions (Sekine et al. 2015), but evidence for water–rock interactions within other worlds is lacking. Understanding intrusive magmatism on tidally heated worlds is a key factor in determining the heating, chemical inventory, and habitability of ocean worlds.

Q8.2f How Are Heat and Material Transported Through—and Ultimately Out of—Satellite Interiors?

Fundamental characteristics, such as the presence or absence of a magnetic dynamo and the survival of a subsurface ocean, depend on how heat is transferred from a body’s interior to its surface (Soderlund et al. 2020). In smaller bodies (e.g., asteroids), heat tends to be transferred by conduction, possibly affected by porosity (which disappears at high enough temperatures). In larger bodies (e.g., planets and some large satellites), conduction can be augmented by convection (where warm solid rock or ice rises and transports heat closer to the surface—as in Earth) or “heat pipe” volcanism (where melt rises to the surface, transporting heat with it—as in Io). For most planetary bodies, the balance between conduction, convection, and heat pipe volcanism is not well known.

Large icy bodies like Callisto or Titan may experience convection in their outer ice shells, but beneath a thick, rigid, cold, conductive lid. Conversely, Europa’s ice shell may experience “mobile lid” tectonics (i.e., similar to Earth’s plate tectonics), which would increase the rate of heat transfer, while at Enceladus the ice shell appears static, but cryovolcanism enhances heat transfer. Ocean circulation further controls how heat and material are exchanged between rocky seafloors and the overlying ice shells, depending on the composition and temperature structure of the ocean. If the rocky core is porous (as at Enceladus), its heat may be transported convectively by hydrothermal circulation. Io is certainly in the heat-pipe regime, as was the early Moon (and plausibly other early terrestrial worlds), but the details of this process are largely unknown (Moore et al. 2017).

Q8.2g Which Planetary Satellites and Dwarf Planets Have or Had an Intrinsic Magnetic Field, and What Processes Govern These Dynamos?

Ganymede is the only known satellite with an intrinsic magnetic field at present (Kivelson et al. 1996). The Moon records the presence of an ancient dynamo through remanent magnetization of crustal rocks (Wieczorek et al. 2017). Paleomagnetic studies of meteorites further demonstrate that small bodies, such as the asteroid Vesta, may also have had core dynamos early in their histories (Scheinberg et al. 2017). It is not yet known whether dynamos—past or present—await discovery in the satellites and dwarf planets that have not yet been explored in detail. An active dynamo within moons orbiting within a planetary magnetosphere is expected to be detectable from magnetometer measurements during a single low-altitude flyby, and distinguishable from crustal or induced fields. The rare exception to this being a dipolar field observed at a moon that contains a similar phase but lower magnitude than that anticipated from the magnetic-wave-driven induction response. Weak dynamo fields on dwarf planets or satellites residing in the solar wind may require several encounters or orbits to definitively characterize as sourced from a dynamo or from induction, while mapping of crustal fields would require much greater spatial coverage. Disentanglement of these fields and any induced magnetic fields driven by an ocean could be achieved by comparing observations with models of the structure, amplitude, and temporal variability of the magnetic field.

Beyond knowing what bodies have dynamos, it is also important to characterize the magnetic fields. Ganymede’s intrinsic magnetic field is known to be dipole-dominated, but its detailed configuration and evolution over time have not yet been determined (Journaux et al. 2020). Conversely, few constraints exist on the Moon’s ancient dynamo field morphology. Samples returned by the Apollo missions revealed that it was anomalously strong, and a weaker dynamo may have been active until relatively recently (Wieczorek et al. 2017). For both bodies, the mechanisms of magnetic field generation—and their evolution over time—are not well understood. For ocean worlds such as Ganymede, there may further be interactions between the intrinsic core field and the oceanic induced field together with the magnetosphere of the host planet. By understanding these processes in planetary satellites, we will not only learn how intrinsic magnetic fields operate in these bodies specifically, but also facilitate tests of dynamo theory more broadly through comparative planetology.

Strategic Research for Q8.2

• Characterize the spin states and orbital and rotational evolution of planetary satellites (including at Jupiter, Saturn, Uranus, and Neptune) with spacecraft and radar observations, and long temporal baseline astrometry.
• Characterize the current orbital evolution of planetary satellite systems across the solar system (including Earth, Jupiter, Saturn, Uranus, and Neptune), and determine if they are in thermal equilibrium, by measuring how the satellite orbits are currently evolving, how their host planet responds to satellite tides (including phase lag), and by measuring satellite heat flows.
• Assess the past and present geologic activity of the uranian satellites, Triton, and large Centaurs and trans-neptunian objects with moon or ring systems—including their cratering record, tectonic, and cryovolcanic activity—and understand how and why they differ from satellites in other systems by imaging their surfaces with resolution, coverage, and spectral range that is at least comparable to those of the Galilean satellites and mid-sized moons of Saturn.
• Determine the differentiation state, radial interior structure, tidal response, and presence/absence of water and magma oceans and reservoirs within the Galilean, saturnian, and uranian moons, as well as Triton, by measuring their gravity fields, shapes, induced magnetic fields and plasma environments, and other geophysical quantities.
• Determine if/how tides have shaped the crustal structure, tectonics, and (cryo)volcanism of the large/mid-sized saturnian (e.g., Enceladus, Titan) and Galilean (Io, Europa, Ganymede, Callisto) satellites by characterizing the three-dimensional structure of their crusts through topography, gravity, ice-penetrating radar, and other geophysical methods.
• Determine if/how tides have shaped the crustal structure of the Moon, by characterizing the three-dimensional structure of its crust through seismology, electromagnetic sounding, heat flow, and other geophysical methods.
• Characterize the thermophysical processes of the icy shells of the large/mid-sized satellites of Jupiter and Saturn (e.g., Europa, Enceladus, and Titan), and determine their present-day activity via in situ geophysical analyses, including seismology, electromagnetic sounding, heat flow measurements, and other methods.
• Characterize tectonic and eruptive processes on Io, Europa, Enceladus, and other active bodies, and assess their relationship to tides, crustal structure, and interior processes with high-resolution imaging over a range of illumination conditions, tidal stress conditions, and observational cadences, and long-term monitoring of activity at various temporal/spatial scales and wavelengths.
• Characterize volcanic and magmatic processes on the Moon and assess their relationship to tides, crustal structure, and interior processes via in situ geochemical and geophysical analyses (including seismology, electromagnetic sounding, and heat flow measurements), and/or returned samples from key volcanic and magmatic sites across the Moon.
• Determine the size and state of the Moon’s solid inner core through seismology measurements, electromagnetic sounding, and other geophysical investigations.

• Determine the rheological behavior of icy crusts, including determining how and when they fracture with laboratory and analogue studies of ice failure and rheological behavior at the conditions relevant to icy satellites.
• Determine how, why, and when tectonic and (cryo)volcanic surface features form on rocky and icy satellites through modeling of fractures, fracture systems, fracture-water interactions, and (cryo)magmatic processes in ice and rock shells.
• Determine the nature and origin of Ganymede’s intrinsic magnetic field through magnetic field mapping over time, improved knowledge of its interior structure, and dynamo modeling.
• Understand how subsurface water and magma oceans are affected by convection, orbital forcing, magnetism, and salinity, and how those processes modify heat fluxes and circulation patterns with fluid dynamical models.

Q8.3 WHAT EXOGENIC PROCESSES MODIFY THE SURFACES OF BODIES IN CIRCUMPLANETARY SYSTEMS?

Satellite surface landforms and composition provide a record of the processes that shaped each of them, key to understanding their formation and evolution. Some of this record is owing to exogenic processes, such as radiation, impacts, and the exchange of material between bodies within circumplanetary systems. It is important to understand how these processes affect what we see today on each satellite, as exogenic processes can be superposed on intrinsic and endogenic processes—obscuring more primitive surface composition and morphologies, leading to misinterpretation.

Q8.3a How Do Weathering and Atmospheric Loss Affect the Environment and Objects in Circumplanetary Systems?

Weathering is the general alteration of surface materials owing to prolonged exposure to both small impacts and energetic radiation. Sputtering is a subset of this, covering the ejection of surface particles at the molecular level upon radiation exposure, which can be the source of tenuous satellite atmospheres, like those on the Moon, Europa, and Enceladus (Question 6, Chapter 9).

Radiation-induced weathering is observed on nearly all planetary bodies without a substantial atmosphere (e.g., so-called airless bodies: Mercury, the Moon, asteroids, and many icy satellites) all show evidence of such alteration. But the details of this process depend on surface composition, and the object’s location in the solar system. For the inner solar system, exposed minerals containing iron or sulfur darken with prolonged exposure to radiation, as confirmed by study of returned samples. In the outer solar system, radiation-induced weathering is largely controlled by the desaturation of organics, which reddens surface coloration. More information about space weathering can be found in Question 5 (Chapter 8). Radiation also affects ice grain size and can brighten or darken the surface of icy satellites.

In circumplanetary systems, the loss of material from one body can act as a source to neighboring bodies. This can be the result of atmospheric loss owing to radiative processes, as seen from Earth to Moon, and Pluto to Charon. Sputtering of material off the surface can also contribute to “polluting” the surrounding environment. In addition, endogenic activity such as plumes or volcanism can launch material into space (e.g., Enceladus and Io), creating diffuse rings at Jupiter and Saturn, and contaminating other neighboring surfaces. Saturn’s rings also act as a volatile source to Saturn itself; volatile “ring rain” was observed by Cassini (see Q8.4c), a loss mechanism that may suggest the rings are young. Some radiation and mass exchange processes may be in steady state, or if not, can be used as chronometers to constrain event histories.

Q8.3b What Is the Role of Circumplanetary and Heliocentric Impacts in Creating the Observed Structures on Moons and Rings?

Impact craters are the most common landform on planetary surfaces, seen on nearly every major satellite surface (Question 4, Chapter 7). As older surfaces have been exposed to more impacts, the cratering record can be used as a chronometer for dating surfaces (Figure 11-4). This is one of the primary tools used to constrain both absolute and relative histories of planetary and satellite surfaces. However, calibration of the cratering record

to absolute age is not uniform across the solar system and is complicated by secondary impacts (craters formed from the impact of suborbital ejecta from a single impact) and sesquinary impacts (craters formed from the impact of ejecta that initially escaped the target body, orbited its host body, and then reimpacted the target body, or another body in the circumplanetary system). The inner and outer solar system have different size frequency distributions (SFD) of impactors, as the source population is asteroidal for the inner solar system, and cometary for the outer solar system. There is a larger uncertainty in the SFD for the outer solar system. Detailed observations of uranian satellites would provide useful new cratering data to help constrain outer solar system chronology, as some surfaces appear to be quite old, perhaps primordial. Future missions could determine whether unseen terrains on the satellites of the ice giants are consistent with known processes, features, and statistics.

Planetary rings can also be shaped by interplanetary impacts. At least four such individual events were seen during the Cassini tour. Disruption from these events was very localized, but models show that interplanetary impactors sculpt some aspects of ring structure, for example, the ramps observed near Saturn’s ring edges. Interplanetary impacts are likely the major source of small dust particles in the rings, because collisions within the ring are too gentle to generate such dust.

Q8.3c What Is the Fate of Ejecta from Impacts onto Moons and Rings?

The fate of ejecta from interplanetary impacts in circumplanetary systems encompasses a wide range of possible outcomes, depending on ejecta velocity relative to the escape velocity of the impacted moon (Question 4, Chapter 7). If moving much slower than escape velocity, ejecta will remain localized to the impact region of the target body. If close to escape velocity, ejecta will cover the impacted moon more broadly. Above escape velocity, ejecta will orbit around the primary planet, ultimately dispersing into a diffuse, dusty ring owing to the range of initial individual debris velocities. Examples of such dusty rings include those coincident with Phoebe at Saturn, and likely Mab at Uranus. Phobos and Deimos may have exchanged surface material through a similar process. Such dust will slowly spiral inward, likely coating the surfaces of other satellites (Q8.3d).

Ejecta owing to collisional “grinding” of captured, irregular satellites is likely a primary source of dust in giant planet systems, acting as the major source of surface deposition on outer, regular satellites—for example, Phoebe dust on Iapetus. Internal collisions in planetary rings will also create a population of dust that can be

readily ionized, enabling it to then be carried along magnetic field lines into the atmosphere, as seen on Saturn. Deeper searches at Uranus and Neptune may show new dusty rings. In the inner solar system, Phobos and Deimos may be the result of reaccretion of material from a major Mars impact, or they may be captured asteroids. How does the likely ongoing exchange of surface dust affect our ability to discriminate between these origin theories?

Q8.3d What Causes the Global-Scale Asymmetries Observed on Moons in Circumplanetary Systems?

On tidally locked satellites, the surface can be geographically divided between the nearside (i.e., the planet-facing hemisphere) and the farside, or alternatively, the leading hemisphere (the hemisphere facing the direction of travel along the orbit) and trailing hemisphere. Leading/trailing hemispherical albedo and color asymmetries are evident on most tidally locked satellites in the outer solar system. Examples include dark trailing hemispheres on Io and Europa, reddened trailing hemispheres on several saturnian satellites, and the archetype of this feature, Iapetus, with a dark leading and bright trailing hemisphere. The processes that lead to such leading-trailing dichotomies are generally understood, arising from exogenic processes. Dust sourced from a larger orbital distance tends to migrate inward, preferentially depositing on the leading hemisphere of any satellite that encounters it (e.g., dust from Phoebe coating Iapetus, E-ring dust coating the inner saturnian satellites). Trailing hemispheres of outer planet satellites tidally locked outside of synchronous orbit are exposed to an enhanced flux of energetic particles, as the planet’s magnetosphere is rotating faster than the orbital velocities of the satellites. Despite this understanding, the extent and thickness of dust coverage on most satellite surfaces are poorly constrained. Quantified measurements of this coverage can help to infer the source of this material, and when the deposition occurred.

Not all global-scale asymmetries are well-understood, and many may arise from endogenic processes. The Moon’s nearside-farside asymmetry—the nearside has a thinner crust and more extrusive volcanism—has remained unexplained since its discovery at the dawn of the space age. Hypotheses range from asymmetric thermal evolution owing to Earthshine, to asymmetric convection of the mantle, asymmetric crystallization of the magma ocean, tidal processes, and giant impacts including South Pole–Aitken or even larger. Enceladus also exhibits a pronounced global asymmetry: with most present-day activity concentrated near to the south pole (Hemingway et al. 2018). Io also exhibits an unusual leading-trailing asymmetry in volcanic output; there are more, smaller volcanoes on the leading hemisphere, and fewer, larger volcanoes on the trailing hemisphere (although the total volcanic output is comparable; de Kleer and de Pater 2016). The cause of these asymmetries (endogenic or exogenic), their relationship to the circumplanetary environment, and how deep into the body they extend, remain unclear.

Strategic Research for Q8.3

• Determine the shape, structure, and composition of Uranus’s and Neptune’s rings and small moons to elucidate their origin, evolution, and present-day balance between exogenic and endogenic processes through a combination of geophysical measurements, imaging, and spectroscopic observations, including at high spatial resolution sufficient to resolve regional variations and layering.
• Determine the history of weathering and impact processes shaping planetary rings and satellites at Jupiter, Saturn, Uranus, and Neptune from remote sensing and direct measurement of ejected dust and gas.
• Determine the locations and distributions of source bodies within dusty rings around Uranus and Neptune, and how fine particles are generated, lost, and transported throughout these systems with a combination of high-resolution imaging and measurements of the mass flux and composition flowing into and out of the rings.
• Test the hypotheses for the origin of planetary asymmetries, including leading-trailing asymmetries on planetary satellites around Jupiter and Saturn, the nearside–farside asymmetry on the Moon, and elsewhere, by characterizing their magnetospheric and dust environment, and by geological, geochemical, and geophysical investigations of the dichotomies themselves.
• Determine how radiation affects materials in the atmospheres and on the surfaces of planetary satellites and characterize the associated chemical pathways and yields with laboratory studies of relevant materials, at relevant conditions.

Q8.4 HOW DO PLANETARY MAGNETOSPHERES INTERACT WITH SATELLITES AND RINGS, AND VICE VERSA?

The co-rotating charged particles of outer planet magnetospheres interact with the surfaces and atmospheres of the moons and rings embedded in the magnetospheres and erode them. The sputtered volatile molecules and dust particles are ionized by solar extreme ultraviolet light and electron impact processes and get picked up by the magnetospheric plasma. Three of the giant planet magnetospheres (Jupiter, Saturn, and Neptune) derive most of their plasma from their satellites (Io, Enceladus, and Triton, respectively). Much of the kinetic and thermal energy of the magnetospheric plasma in the outer planet magnetospheres is derived from the rotations of their parent bodies. How planetary ionospheres lose their angular momentum to the outflowing plasma is a topic of great interest in both understanding the dynamics and energetics of the current day magnetospheres and the angular momentum budget histories of the giant planets.

Studying the magnetospheric interaction with orbiting moons and rings can elucidate the mechanisms behind the evolutions of their surfaces and provide insight into the gain and loss of plasma by the magnetospheres and the sculpting of their radiation belts. Magnetospheric interaction is the dominant mechanism by which material is exchanged between satellites. The rotating magnetic fields of giant planets and their magnetospheres also provide cyclical sounding signals that can be used to probe the interiors of the satellites for electrically conductive liquid water and magma oceans.

Q8.4a How Do Magnetospheric Magnetic Fields, Neutral Atoms and Molecules, Plasma, and Dust Populations Interact with Moons and Rings?

The magnetospheres of the giant planets provide a time-variable magnetic environment at the orbiting moons. The periods and amplitudes of these magnetic waves depend on the rotation of the planet, the intensity and tilt of the planet’s intrinsic magnetic field, magnetospheric morphology, and the orbital geometry of the moons (e.g., eccentricity and inclination of the orbits). These magnetic waves and their harmonics can generate induced dipole magnetic fields in moons harboring global subsurface water or magma oceans. Careful analysis of these induction fields can characterize the oceans in terms of their depths, thicknesses, conductivities, and potentially, fluid circulations (Vance et al. 2021).

The interaction of the magnetospheric plasma and fields with orbiting moons affect the local structure and strength of the magnetic fields at the moons through several contributions from the planetary magnetic field, the plasma dynamic interaction of the magnetosphere with the moon and its ionosphere, the induced magnetic moment from the magnetic waves driving electrical currents in subsurface oceans, the induced magnetic fields sourced from oceanic fluid motion, and any magnetic fields intrinsic to the moon (as with Ganymede). The magnetospheric plasma density, plasma velocity, and magnetic field strength local to the moons determine the nature of the interaction with the moon. These parameters define the local Alfvén speed, and the local flow of the magnetized plasma determines if the interaction is sub-, trans-, or super-Alfvénic. The stability and organization of the upstream flow at the moons may be predictable (as in the cases of the Galilean moons) or may be highly variable (as is the case with Titan) depending on where in the magnetosphere the moons reside. Depending on the energy spectra of the incident ions and electrons and the local magnetic field geometry, some fraction of the magnetospheric plasma may access the exosphere and the surfaces of the moons. This process of plasma precipitation can lead to local aurorae, ionization of the atmosphere and formation of localized ionospheric populations, and implantation, sputtering, and radiolytic processing of the surface. This same process can structure the planet’s radiation belts (Figure 11-5, top panel). Moreover, these processes can vary over geologic timescales if the host planet’s intrinsic magnetic field changes in intensity and/or direction. Such changes may be reflected in the surface characteristics of the moons and rings.

The magnetospheric environment surrounding dense rings determines whether vapor and small charged particles released from the rings remain trapped in the vicinity of the rings or are transported away from the rings into the planet. The environment therefore strongly affects how quickly massive rings (like those of Saturn) evolve compositionally and erode over time. Cassini measurements revealed that large amounts of particles and vapor are flowing between the rings and the planet, but only directly sampled a small part of the

rings’ environment, leaving open many questions about the structure and magnitude of this flow, especially given that the plasma environment surrounding the rings almost certainly changes with seasons and local time. More detailed information about the rings’ magnetospheric environment is needed to properly estimate the current evolution rates of these dense rings.

The dust-sized particles that form more tenuous rings like Saturn’s E-ring or Uranus’s μ-ring are strongly perturbed by nongravitational forces, so the plasma environment strongly affects both the particle size distribution

and morphology of these rings. Similarly, both the prevalence and evolution of the dusty spokes over Saturn’s B-ring are clearly influenced by seasonal changes in the magnetospheric environment. These diverse systems provide opportunities to better understand the sources, sinks and transport rates of small particles in various contexts, ultimately informing how dust coats icy moons (Q8.3d). Furthermore, these insights can shed light on how other dusty systems operate, including the complex debris disks recently seen around distant stars.

Q8.4b How Do Moons and Rings Contribute to the Neutral, Plasma, and Dust Populations Surrounding Their Host Planets and Influence the Dynamics of the Magnetosphere?

Moons can contribute to the dust, neutral, and plasma environment in the magnetosphere in direct and indirect ways. Volcanic moons like Io, which reside in high-radiation environments, can provide more than 1,000 kg/s of gas that is rapidly ionized and creates a plasma torus in the inner magnetosphere (Figure 11-5 bottom panel; Bagenal and Dols 2020; de Pater et al. 2021). In contrast, the plumes of Enceladus provide ~500 kg/s of water vapor and ice grains to Saturn’s orbital environment, sourcing the E-ring and an extended neutral cloud that slowly ionized and provides a distributed cold source of plasma to the magnetosphere. Europa contributes sputtered neutral atoms and molecules to a comparably less extensive but stable and observable neutral cloud at Jupiter. At both Jupiter and Saturn, the location and extent of the neutral and plasma sources shape radial mass transport in the magnetosphere, determining the edge of its inner portion that co-rotates with the planet, and, at least in part, the structure and intensity of the planetary aurora.

The inward diffusion of plasma in a magnetosphere leads to the formation of energetic (mega-electron volt, MeV) ion and electron radiation belts. The rings and moons of the jovian and saturnian system sculpt these radiation belts by absorbing these particles. An additional source of energetic protons very close to the planet is the cosmic ray albedo neutron decay process that creates MeV protons from fast neutrons generated from the bombardment of atmospheres of the planets and their rings by cosmic rays. Electric fields related to wave processes, convection of plasma (bursty flows), and temporal variations of the ambient magnetic field are required to energize and facilitate inward convection of charged particles, however the details of these processes are poorly understood. Plasma waves also lead to the losses of these particles into the atmosphere of the planet as they scatter the stably trapped bound belt populations into trajectories that can enter the atmosphere.

Q8.4c How Does “Ring Rain” Affect Planetary Atmospheres and the Circumplanetary Environment?

Observations from both ground-based telescopes and the Cassini spacecraft have revealed that the rings, and material ejected from Enceladus, have profound effects on the environment around Saturn. Most dramatically, as much as 10,000 kilograms of ions, molecules, and small dust grains appear to be flowing from the rings into the planet every second. Some material flows into the mid-latitude ionosphere along magnetic field lines, some sediments downward over the equator. This flow affects not only the long-term evolution of the rings, but also the composition of the upper atmosphere by introducing large amounts of water and organics into regions where these molecules would otherwise be rare. Oxygenated materials may contribute to photochemistry, haze production, and the radiative balance of Saturn’s middle and upper atmosphere. This exogenic contamination of Saturn’s atmosphere may even complicate attempts to determine Saturn’s bulk composition remotely, a key measurement for understanding planetary origins. The discovery of Saturn’s substantial planet–ring connection begs the question of whether a similar connection exists on the other giant planets, particularly for the substantial rings of Uranus and Neptune.

Material flow between the rings and planet also affects the configuration, composition, and plasma density of the inner magnetosphere, producing a highly interconnected system. The complexity of this system is demonstrated by the perturbations to the magnetic fields measured when Cassini flew between the rings and the planet, as well as the clear influence of magnetospheric asymmetries on multiple dust populations around the main rings. Detailed examinations of this region promise new insights into material transport and electromagnetic connections within planetary magnetospheres.

Strategic Research for Q8.4

• Quantify material sources, sinks, and mass transport between Jupiter’s magnetosphere and moons via in situ magnetic field and plasma measurements.
• Characterize the magnetospheric interactions between Uranus and Neptune’s atmosphere, moons, and rings with observations of planetary aurorae, measurements of satellite and ring exospheres and ionospheres, in situ measurements of the global distribution of the plasma composition, density, velocity and temperature, neutral density and composition, and charged dust in both the orbital plane of the moons and rings and at high inclinations.
• Characterize how Saturn’s rings, neutral clouds, and charged dust are coupled with Saturn’s magnetosphere and atmosphere with high-resolution, high-cadence observations of Saturn’s rings, including imaging, composition, magnetic field, and plasma measurements.
• Develop a more complete framework for integrating and assimilating sparse plasma and magnetic field observations and determining the interior structures of planetary satellites (including recovery of ocean depth, thickness, salinity, and dynamics), and ionospheric/magnetospheric processes (including plasma interactions, ion-neutral, mass-momentum transport, field-aligned current, and induced/intrinsic magnetic fields) with advanced analytical and numerical approaches.

Q8.5 HOW DO RINGS EVOLVE AND COALESCE INTO MOONS?

The solar system contains numerous collections of many small objects in orbit around larger bodies in the form of either small moons or planetary rings (Tiscareno and Murray 2018). Recent work has shown that both individual objects and the overall distribution of material within these systems have complex and dynamic histories. At the individual particle level, objects can either accrete into larger bodies or erode away, while on larger scales material can either be transported or confined by a wide variety of internal processes and external forces. The two most important processes involved in these phenomena are collisions and mutual gravitational attraction, both of which have complex and context-dependent outcomes. Similarly, it is not yet clear how fast particle populations spread and disperse under different situations. Hence major uncertainties and controversies remain about the conditions needed for colliding particles to accrete into larger bodies or to break apart into smaller fragments, and about the current evolution rates and even the overall history of these systems of rings and small moons. Improving our understanding would also clarify the formation and early evolution of planetary systems (including the solar system), because it too is/was shaped by particle accretion, transport, and erosion. For example, protoplanets in the protoplanetary disk resemble moonlets and other objects forming now in planetary rings. These embedded objects perturb the disk to create gaps and edges and can trigger accretion. The disk reacts back on the embedded objects to cause them to migrate.

Q8.5a What Determines the Distribution of Particle Properties Within Rings and Other Systems of Small Bodies?

Most parts of dense planetary rings are evolving over timescales that are long compared to their orbital periods, and so in these regions the distributions of particle sizes, densities, relative velocities, and rotation states can be approximated as a nearly steady-state system where smaller particles are being assembled into larger bodies at roughly the same rate as larger bodies are being broken apart (Cuzzi et al. 2009; Colwell et al. 2018). Important aspects of collisional outcomes among these bodies can therefore be determined by quantifying variations and trends in the particle property distributions across the rings. For example, correlations between the typical particle size and the mean relative velocity can constrain the critical impact speeds where aggregation transitions to fragmentation. On the other hand, more dynamic regions of the rings where parameters like density or vertical extent change rapidly owing to external forces can reveal how changes in one aspect of the dynamical environment can affect other particle properties. Furthermore, the magnitude of the rings’ response to the driving forces can reveal nonlinear processes or instabilities relevant to the formation of larger structures. Meanwhile, the properties of

more tenuous ring systems dominated by fine debris released from larger bodies can provide information about dust production, transport, and loss in different environments (Hedman et al. 2018).

Q8.5b How Are Ring Particles Assembled into Moons and More Transient Aggregates?

Cassini observations of a variety of structures within the rings (Figure 11-6) revealed that Saturn’s rings contain a population of exceptionally large particles (up to several hundred meters in diameter, Spahn et al. 2018) that can be regarded as transitional between typical ring particles (which range between millimeters and a few meters in size) and small moons (which are several kilometers across). These objects may be analogous to growing objects in protoplanetary disks. At the same time, other observations show that different parts of these rings contain a wide variety of transient agglomerations of particles that may form quickly and last for only a few orbit periods. Such agglomerations can explain the overall brightness of the ring in different viewing geometries (Schmidt et al. 2009; Salo et al. 2018; Tiscareno et al. 2019). The formation of transient aggregates and more permanent larger bodies are likely related processes, and so we may be able to better understand how particles accrete into larger bodies if we can determine under what conditions different types of transient aggregates form, as well as the sizes, lifetimes, and ultimate fates of the larger objects.

Q8.5c What Is the Life Cycle of Planetary Rings?

Recent measurements of the total mass of Saturn’s rings and the mass flux between those rings and the planet indicate that the composition and mass of Saturn’s rings may change substantially on timescales of hundreds of millions of years, which is much shorter than the age of the solar system (Waite et al. 2018; Crida et al. 2019; Iess et al. 2019; O’Donoghue et al. 2019). The histories of rings and moons seem to be closely intertwined. Theoretical studies have shown that several of Saturn’s and Uranus’s moons (including the innermost mid-sized moons Mimas and Miranda) may have spawned from the rings well after the solar system formed (Charnoz et al. 2018; Hesselbrock and Minton 2019; Neveu and Rhoden 2019). Even Mars’s moon Phobos may be the latest incarnation of material that has cycled between rings and a moon multiple times over the planet’s history (Hesselbrock and Minton 2017). The origin of Mars’s moons—be it from capture of passing asteroids or accretion from a disk of material in Mars orbit—is still uncertain (e.g., Ramsley and Head 2021). Material in the rings can be perturbed and confined by nearby shepherding moons and mean-motion resonances with more distant bodies to produce arcs, gaps and other features that evolve over timescales of years to decades. Hence, it may be possible to use the structure, composition, and shape of ring-generated moons to ascertain how large objects grow from rings and, based on how quickly the structure and composition of the rings change over time, to constrain the timescales relevant to the evolution of these systems.

Q8.5d Which Worlds in the Solar System and Beyond Have (or Had) Rings?

The past decade revealed that giant planets are not the only objects in the solar system with rings. The recent discovery of rings around small bodies in the outer solar system, along with evidence that several of Saturn’s moons might have had rings in the past, has greatly expanded the known environments where rings can occur (Ortiz et al. 2017; Sicardy et al. 2018). However, relatively few small bodies have rings, and it is not yet clear what conditions favor the existence of rings around a given body. Additionally, some worlds may have once possessed rings that were subsequently lost, like Mars (whose moons Phobos and Deimos may have originated from a ring), or Iapetus (whose prominent equatorial ridge is hypothesized to be a collapsed ring). Measuring the properties of these rings would help us better understand the conditions and/or materials that favor ring formation. This information will not only help us to better understand the solar system, but also clarify which planets outside the solar system are likely to have rings.

Strategic Research for Q8.5

• Determine the prevalence of rings and satellites around small bodies, including Centaurs and trans-neptunian objects, to understand how circumplanetary systems form and evolve with high-resolution telescopic observations and occultation campaigns, in situ exploration, and other methods.
• Determine the composition of Uranus and Neptune’s rings and small moons to elucidate their origin, evolution, and present-day balance between exogenic processes and endogenic processes through a combination of geophysical measurements, imaging, and spectroscopic observations, including at high spatial resolution sufficient to resolve regional variations and layering.
• Determine the locations and distributions of source bodies within Uranus’s and Neptune’s dusty rings, and how fine particles are generated, lost, and transported throughout the uranian and neptunian systems with a combination of high-resolution imaging and measurements of the mass flux and composition flowing into and out of the rings.
• Elucidate the origin of Jupiter, Saturn, Uranus and Neptune’s small regular satellites and ring-moons, and their relationship and interactions with their rings, by measuring their composition and structure.
• Characterize the present-day evolution of Jupiter, Saturn, Uranus, and Neptune’s rings by measuring the mass flux and composition flowing into and out of the rings.
• Observe how particles in dense rings around Saturn, Uranus, and other objects aggregate into larger bodies and fragment into smaller bodies by observing dense rings with sufficient spatial resolution and imaging cadence to resolve the individual ring particles and aggregates and their evolution over various dynamical timescales (~minutes to observe individual collisions, ~years to observe their orbital evolution).

• Constrain the origin of Phobos and Deimos, including whether they arose from past martian rings, by determining their bulk composition and interior structure by in situ geochemical and geophysical measurements.
• Determine whether equatorial ridges on worlds like Iapetus, Pan, and Atlas are produced by the deposition of ancient rings or by other processes, with a combination of high-resolution remote sensing observations of equatorial ridges, and theoretical models for ring collapse and other competing hypotheses.
• Determine the long-term evolution of planetary rings with high-resolution imaging, stellar occultation techniques, and computer simulations of their orbital evolution and dynamics.
• Quantify the evolution of ring structures (including arcs, gaps, and edges) over timescales of years to decades using stellar occultations and high-resolution images.
• Determine the collisional properties of ice as relevant to understanding impact processes on icy satellites, and the aggregation/disruption of ring particles with laboratory studies of ice under outer solar system conditions.

SUPPORTIVE ACTIVITIES FOR QUESTION 8

• Determine how planetary materials with relevant compositions and melt fractions behave under the temperatures, pressures, and forcing conditions relevant to understanding processes within circumplanetary bodies—including tidal dissipation, convection, melting and melt transport, and magnetic induction, with laboratory and ab initio studies of planetary materials.

REFERENCES