Frontiers in Planetary Rings Science
Shawn M. Brooks, Tracy M. Becker, K. Baillié, H. N. Becker, E. T. Bradley, J. E. Colwell, J. N. Cuzzi, I. de Pater, S. Eckert, M. El Moutamid, S. G. Edgington, P. R. Estrada, M. W. Evans, A. Flandes, R. G. French, Á. García, M. K. Gordon, M. M. Hedman, H.-W. Hsu, R. G. Jerousek, E. A. Marouf, B. K. Meinke, P. D. Nicholson, S. H. Pilorz, M. R. Showalter, L. J. Spilker, H. B. Throop, M. S. Tiscareno
FFrontiers in Planetary Rings Science
Principal Authors:
Name:
Shawn M. Brooks
Institution: Jet Propulsion Laboratory,California Institute of TechnologyPhone: +1 (818) 393-6380Email: [email protected] Name:
Tracy M. Becker
Institution: Southwest Research InstituteEmail: [email protected]
Co-authors:
Kevin Bailli´e Heidi N. Becker E. Todd Bradley Joshua E. Colwell Jeffrey N. Cuzzi Imke de Pater Stephanie Eckert Maryame El Moutamid Scott G. Edgington Paul R. Estrada Michael W. Evans Alberto Flandes Richard G. French ´Angel Garc´ıa Mitchell K. Gordon Matthew M. Hedman H.-W. Sean Hsu Richard G. Jerousek Essam A. Marouf Bonnie K. Meinke Philip D. Nicholson Stuart H. Pilorz Mark R. Showalter Linda J. Spilker Henry B. Throop Matthew S. Tiscareno Co-signers:
Jennifer G. Blank , Richard J. Cartwright Corey J. Cochrane Luke Dones C´ecile C. Ferrari Robert S. French Mark D. Hofstadter Sona Hosseini Kelly E. Miller Edgard G. Rivera-Valent´ın Abigail Rymer Marshall J. Styczinski Lynnae C. Quick PadmaYanamandra-Fisher
CNRS, Institut de M´ecanique C´eleste et de Calcul des Eph´em´erides; Jet Propulsion Laboratory,California Institute of Technology; Univ. of Central Florida; NASA Ames Research Center ; Univ. ofCalifornia, Berkley; Cornell Univ.; SETI Institute; Univ. Nacional Aut´onoma de M´exico; WellesleyCollege; Univ. of Idaho; Univ. of Colorado; Florida Space Institute; San Jose State Univ.; BallAerospace; Arctic Slope Technical Services; Blue Marble Space; Southwest Research Institute; Univ. de Paris; Lunar & Planetary Institute (USRA); Johns Hopkins, APL; Univ. of Washington; NASA Goddard Space Flight Center; Space Science Institute
A portion of this research was carried out at the Jet Propulsion Laboratory, California Institute ofTechnology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004).Copyright: c (cid:13) a r X i v : . [ a s t r o - ph . I M ] A ug xecutive Summary We now know that the outer solar system is host to at least six diverse planetary ringsystems, each of which is a scientifically compelling target with the potential to inform usabout the evolution, history and even the internal structure of the body it adorns.
TheCassini-Huygens mission to Saturn provided a wealth of new information about Saturn’s ringsand continues to motivate new work. Likewise, observations from spacecraft and Earth-basedobservatories have raised many outstanding questions about our solar system’s planetary ringsystems. Evidence for the formation and ongoing evolution of the planets and their environmentscan be found in ring structure, composition, and variations caused by magnetospheres, satellites,and planetary internal modes. Rings are more common than once believed; exciting discoveries ofring systems in unexpected environments, including around small bodies like Centaurs andKBOs, compel us to search for new ones. These diverse ring systems represent a set of distinctlocal laboratories for understanding the physics and dynamics of planetary disks, withapplications reaching beyond our Solar System. We highlight the current status of planetary ringsscience and the open questions before the community to promote continued Earth-based andspacecraft-based investigations into planetary rings. As future spacecraft missions are launchedand more powerful telescopes come online in the decades to come, we urge NASA for continuedsupport of investigations that advance our understanding of planetary rings, through researchand analysis of data from existing facilities, more laboratory work and specific attention to strongrings science goals during future mission selections . We also encourage the active promotion of adiverse, equitable, inclusive and accessible environment in the planetary science community.
Giant Planet Ring Systems
Saturn : The popularity of Saturn’s rings stems undoubtedly from their stunning appearance,which led to their early discovery. Christiaan Huygens’ realization that rings encircle Saturn [1]was part of a paradigm shift away from the belief in perfectly spherical heavenly bodiesoccupying the concordantly perfect heavens [2]. Saturn’s rings were the subject of repeatedinvestigations in the following centuries. But since visits by Pioneers 10 and 11, Voyagers 1 and 2and most recently, Cassini, our knowledge has grown vastly. Voyager observations laid thegroundwork for Cassini investigations and continue to provide new results [3, 4].Cassini orbited Saturn 294 times during 13+ years, nearly half of a Saturnian year. Theleap in our knowledge of Saturn’s rings from Cassini is so significant and fundamental that itsimply cannot be overstated. Even a basic measure used to characterize rings, optical depth, wasdiscovered to be far more nuanced than our pre-Cassini understanding [5, 6, 7]. We cannotsummarize all of the new knowledge or review all of the new questions. Instead, we discuss a fewsignificant open questions in order to highlight the depth of the current state of knowledge, aswell as the gaps in understanding and avenues for future studies. For a comprehensive summary,see the Rings discipline science section of Volume 1 of the Cassini Mission Final Report [8]. How old are Saturn’s main rings? Are young rings consistent with our understandingof the formation and evolution of the Saturn system as a whole?
Determining the mass ofSaturn’s main rings was a principal measurement goal of Cassini’s proximal orbits, the final 22orbits where Cassini flew between the D ring and Saturn’s cloud tops. The mass of Saturn’s rings( . M Mimas ), derived from gravity measurements [9] and localized surface mass density https://pds-rings.seti.org/cassini/report/Cassini%20Final%20Report%20-%20Volume%201.pdf ∼ − years. A recent origin for a ring system as massive and extensive as Saturn’s holds implicationsfor the rest of the system. Dynamical scenarios consistent with a young ring predict a relativeabundance of impact craters among the inner moons of Saturn generated by planetocentricimpactors [16]. Recent findings suggest an enhancement in the small-crater population [17, 18]that may be consistent with such a scenario and is an active area of research. Further discussionon the determination of the age of Saturn’s rings and its significance can be found in [19]. What does the pollutant (non-ice) material detected in Saturn’s rings indicate aboutthe physical and chemical history of Saturn’s environment?
Cassini provided insight into thecomposition of Saturn’s rings, but details remain stubbornly murky. VIMS near-infraredobservations reveal correlations between the rings’ non-ice fraction and the rings’ structure.Based on these near-IR data [20, 21] and on ultraviolet observations [22, 23], two distinctcontaminants have been identified: a broadband absorber localized in the C ring and CassiniDivision and a broadly-distributed contaminant that absorbs at short visible and UV wavelengths.Cassini RADAR observations suggest silicate concentrations of 6-11% by volume in the mid-Cring, some 4-10% higher than elsewhere in the C ring [24]. Further out, Saturn’s E ring is believedto be fed by cryovolcanic plumes of contaminated water ice emanating from near Enceladus’south pole [25]; these particles likely contribute to the surface contamination of the midsize icymoons [26]. Contamination fraction is a key piece in understanding the rings’ exposure age.Cassini’s proximal orbits also permitted the in situ measurement of Saturnward-drifting materialsourced from the rings. Spatial distribution and compositional information was obtained withCassini’s RPWS [27], CDA [28] and INMS [29] instruments. INMS data conclusively reveal thepresence of CH , CO , CO , N , H O , NH and organics [30]. CDA results include theidentification of silicate material in nanograins [28]. So why is the spectral evidence for any ofthese species inconclusive at best? Additional laboratory work may provide the answer. Figure 1:
Cassini images illustrating medium-scale structurediscovered in Saturn’s rings, including density waves triggeredby satellite resonances (a), unexplained structure in the B ring(b), and the innermost C ring plateau (c).
These features high-light the complex dynamical interaction between ring particlesand external gravitational forces.
Why does Saturn’sinterior have such complexstructure, and what doesthis tell us about giant planetsin general?
Cassini has shownhow rings can probe planetaryinteriors, complementing gravityand magnetic field data. Thespiral density and bending wavesused to retrieve information suchas local surface mass densityand viscosity, are typicallyraised by distant satellites. Butoccultation profiles reveal wavesin the C ring raised by internaloscillations and gravitationalirregularities within Saturn itself [10, 31, 32, 12, 33, 34, 35]. Such waves were first identified in Voyager radio occutation data[36]. Combining the analysis of these features with analysis of gravity and magnetic field data2ay yield new information about the structure of Saturn’s interior and provide another measure ofthe planet’s rotation rate. It may well be a viable measurement technique at other ringed bodies.
How do the complex structures in Saturn’s rings form?
Cassini images and occultationsreveal structural features at a wide range of spatial scales, from localized C ring “ghosts” [37] tothe Cassini Division itself. Such structures include non-axisymmetric clumps or densityenhancements in the local distribution of ring particles. Accretionary processes likely play a rolein the formation of some if not all of them. Propellers, potential “missing links” to the progenitorsof the rings, have been observed at multiple locations [38, 39, 40]. Further study of their size andspatial distribution may provide insight into the origins of Saturn’s rings. We still do not fullyunderstand how spokes form. A large number of ring structures remain to be explained.
Jupiter : Jupiter’s dusty ring system remains the only one discovered by spacecraft. Hints of anunseen ring from Pioneer 11 magnetometer data [41] were validated by Voyager 1 with a single,multiply-exposed image of the main ring [42], with Voyager 2 revealing the ring system in detail[43]. The Jovian ring system has been observed by more spacecraft, including Galileo [44],Cassini [45], New Horizons [46] and Juno [47, 48], than any other.The rings of Jupiter are characterized by their low optical depth and the non-gravitationalforces that sculpt them. The main ring extends from . − . R J [44]. Enhanced backscattersuggests a narrow belt of macroscopic particles, the purported dust source of the main ring and thevertically extended halo interior to it, orbiting between Metis and Adrastea [49, 50]. Galileo andKeck images [44, 51] reveal two components to the gossamer rings, whose outer edges coincidewith the orbits of Amalthea and Thebe. Jupiter’s rings are surprisingly dynamic and shaped byinteractions with its magnetospheric environment that we are just beginning to understand. What are the principal mechanisms of radial particle transport and how do they varyacross the jovian ring system?
The Burns et al. [52] model for the Amalthea and Thebegossamer rings appeals to Poynting-Robertson drag to drive the inward migration ofimpact-derived dust. But outward extensions to these rings suggest a more complicated picture[53]. Photometric models of the main ring [54, 55, 45] suggest a relative dearth of particles largerthan ∼ − µ m. Is this the result of particle evolution and dynamics? How main ring particlesmigrate inward to form the halo and their subsequent evolution is equally unclear. Properties thatdetermine halo particles’ charge-to-mass ratio, such as their susceptibility to photocharging andthe local electrostatic potential, dictate their coupling to the magnetosphere and consequentdynamics and are poorly known. Lorentz resonances in the halo likely play a role [56, 57, 58, 59],but alternative particle transport mechanisms have been proposed [60, 61]. The spatial distributionof dust impacts detected by Juno’s Waves instrument [48] recalls the original inner disk [62] thatwas later shown not to exist [50]. Where do these dust measurements fit in the broader picture? What is the origin of the fine structures within Jupiter’s rings? What can they tell usabout the environment at Jupiter?
For such an optically thin ring system, Jupiter’s ringscontain an unexpected amount of fine-scale structure. Researchers have commented on a broadnear arm/far arm asymmetry in Voyager [62, 50] and Galileo [44, 55] data, which is notablyabsent in Cassini [45] and New Horizons [46] data. Vertical corrugations in the main ring imagedby Galileo [44] (note the main ring’s subtle, alternating bright and dark patches in the coverimage) have been attributed to a disturbance somehow induced by the Comet Shoemaker-Levy 9Jupiter impact [53]. This mechanism has been invoked in the more optically thick C and D ringsof Saturn [63, 64]. Clumps in the main ring are another example of fine-scale structure [46].3inally, Showalter et al. [65] note an enhancement of dust just interior to Amalthea and Thebe,suggesting, at least in Amalthea’s case, that ring material is hung up in that satellite’s Lagrangepoints. The short lifetimes of dust at Jupiter imply that these features are either activelymaintained or are ephemeral, caught at just the right time by visiting spacecraft.
Uranus : The dense, narrow Uranian rings are immersed in a sea of micron-sized dust grains,which is strikingly different from Saturn’s massive ring system, Jupiter’s ephemeral rings, or thedusty rings around Neptune. Many open questions regarding their origin and evolution remain.
What processes define the structure — including the locations, widths, eccentricities,and inclinations — of the Uranian rings?
The small Uranian satellites likely play a significantrole in forming, sourcing, and sculpting the ring system, though none of these interactiveprocesses are well understood. The confined, narrow structure of the rings suggests unseenshepherding satellites, though more dynamically complex confinement mechanisms are requiredto explain Saturn’s narrow F ring [66]. Currently, there is no definitive explanation for the spacingbetween the narrow rings, though it is known that these dense rings oscillate on orbital timescalesagainst the backdrop of a much more slowly evolving (over several decades) dusty ring system. Ahypothesized, self-sustaining mechanism for narrow rings has not been confirmed in detail;refinements would likely apply to other narrow ring systems, such as those of Chariklo [67]. Thesatellite Mab [68] is embedded in, and is presumably the direct source for, the dusty µ ring.Further study could elucidate the moon’s environment, including its meteoritic bombardment rate. What is the origin of the Uranian ring system and how has it evolved?
What little isknown about the composition of Uranus’ larger moons is intriguing: though H O rich, CO iceappears preferentially on their trailing hemispheres [69], while the near-IR spectra of the ringsand smaller satellites remain featureless [70]. This suggests that their composition may dependupon their present environment as well as their origins. If the rings are remnants of the originalsource material for the planet, their composition may be indicative of the formation and migrationhistory of Uranus, especially when compared with the composition of Neptune’s rings. What can we learn about Uranus from ring studies?
Satellites and rings capture criticaldetails about the magnetosphere and interior of their host planet; observations of radiolyticprocessing on the satellites and rings, as well as searches for ring rain along magnetic field linesas seen at Saturn [71, 29] would provide insight into the tilted magnetosphere. Oscillations withinthe planet that trigger waves in the rings can be used to probe Uranus’ interior [72, 21]. Notably,ring precession rates are still the best constraints on Uranus’ J and J gravity harmonics [73]. Neptune : The Neptunian ring system consists primarily of micron-sized dust, which has arelatively short lifetime and must be replenished constantly. Denser arcs of material embeddedwithin the outer Adams ring have been observed to change in brightness, drift in position, andeven completely vanish [74]. Smaller scale changes and compositional detail about the ringsystem are difficult to assess due to the low optical depth of the rings and Neptune’s distance.
What is the origin of Neptune’s rings system?
The rings and small moons may beremnants of the original Neptune system that was disrupted during the capture of Triton, andtherefore may provide constraints on the inventory of material at Neptune’s initial orbital distancefrom the Sun [75, 76]. Better compositional measurements of the rings and satellites coulddetermine if they share a common origin. Comparisons with Triton’s composition wouldelucidate differences in the primordial material of the Kuiper Belt versus the region in whichNeptune formed. Finally, better constraints on the particle size distribution of the rings would be4ndicative of the collisional age of the rings.
How are the Neptunian rings and arcs sustained?
The arcs’ stability and confinementare still areas of active research, with solar radiation forces and inelastic particle collisionschallenging their maintenance through resonances or co-orbital moonlets ([77, 78, 79, 74]).Resonances may be used to explain the location of the LeVerrier ring, though whether thatresonance is with an undiscovered moon or driven by structure within the planet is unknown. Theshort timescale changes that have been observed from Earth [74] suggest active evolution of therings and arcs, perhaps similar to what is observed at Saturn’s F ring [80].Intriguingly, the radial layout of the rings and moons at Neptune are the reverse of that atUranus. The innermost moons of Neptune are effectively as close to the planet as the Uranianrings, so understanding how Neptune’s inner moons hold themselves together could explain whythese ring systems are so different. Further, unlike the Saturnian ring satellites, most of Neptune’ssmall satellites reside inside synchronous orbit. This means that the moons are moving inwarduntil they become tidally disrupted, with unique consequences for the structure, development, andpossible recycling of the Neptunian rings and moons. The capture of Triton likely had asignificant dynamical effect on the Neptune system, most evident by the highly eccentric satelliteNereid. Investigating how the rings responded to this disruption event would provide uniqueinsights into the evolution of planetary disks facing large-scale disturbances.
New & Old Ring Discoveries
An entirely new regime of rings science was uncovered within the last decade when discrete ringswere discovered around the Centaur Chariklo [81], the Kuiper Belt Object Haumea [82] andpossibly around the Centaur Chiron [83].
How do rings form and evolve around small bodies, and how do they compare withthe discrete rings of the outer planets?
Markedly distinct from scenarios invoked for ringformation around the outer planets, the existence of rings around small bodies suggests a new setof ring-forming mechanisms, including outburst activity or micro- or macro-impacts onto theprimary body in addition to satellite breakups. Studying the structure and composition of therings provide clues to their origin. Of paramount interest is how water-ice-rich rings could begenerated around Chariklo, though the central body lacks any H O spectral features [84]. Thisparadox could have strong implications for how the rings formed, including the presence ofsubsurface ice if the rings were created from impacts or activity from the Centaur.
How stable are rings, and can they constrain the dynamical evolution of small bodies?
Centaurs are presumably primordial objects, possibly originating from the Kuiper Belt, that havebeen perturbed into planet-crossing orbits with finite lifetimes of ∼ years. The rings’ structureand the potential presence of smaller satellites to generate and/or gravitationally sculpt the ringsmay constrain the evolution of the system, guiding interpretations of its age. The age and stabilityof the rings have implications for previous dynamical interactions with the outer planets as theCentaurs’ orbits evolved, and may be used to trace their movement through the Solar System. Did other Solar System objects once host rings?
Intriguing numerical simulationssuggest that Phobos and Deimos may have coalesced from an ancient ring that dissipated throughdeposition of debris onto Mars in a cyclical ring-moon formation process [85]. Observationalevidence of a potentially collapsed ring exists in the form of an equatorial ridge on Saturn’s moonIapetus [86]. A more complete understanding of the evolution of ring systems may be gleanedthrough the establishment of the end-state of ancient rings on various planetary bodies [87].5 n what type of environment could a ringed exoplanet have evolved?
With moresophisticated instrumentation, rings will be detected around exoplanets. Applying our knowledgeof the diverse ring systems in our own Solar System to any structural or compositionalmeasurements of exo-rings will result in substantial advances in understanding the origin andevolution of ringed exoworlds and their host environments.
Key Recommendations for NASA
We strongly urge NASA to continue to maintain a robust Research & Analysis program tosupport planetary rings science. • The Cassini mission resulted in a deep trove of data of Saturn’s rings. Continued analysis,including cross-instrument comparative science, is still needed. We urge continued supportfor the CDAP program, as well other data analysis programs (e.g., NFDAP) that cansupport rings science, well into the next decade. Finally, we recommend continued supportfor the SSO to conduct observations from Earth-based facilities. • Observational data of rings can only be interpreted in the context of ground-truth data. Weurge an increase in funding for laboratory studies that make relevant compositionalmeasurements, conduct ring particle interaction experiments, and theoretical modelingprojects through R&A programs like SSW.
We strongly endorse NASA support for Earth-based rings science observations. • Stellar occultation campaigns are the leading technique for discovering new ring systems[88, 89, 81, 82], enable detailed measurements of ring structure and particle sizedistributions [90], and provide hazard mitigation support for NASA missions ([91]). Weurge support for new and existing facilities with the necessary time resolution for stellaroccultation measurements, as well as campaign efforts using smaller, portable telescopesthat can involve collaborations between experts and non-experts (e.g., for Arrokoth [92]). • Facilities covering broad ranges of wavelengths, from ground-based radar to space-basedUV capabilities, are needed for the continued assessment of ring composition.High-resolution images enable deep searches for new rings and long-term monitoring forring variations that answer some of the outstanding dynamical questions presented above,through, for example, a dedicated Solar System space-based telescope [93].
We urge NASA to prioritize strong rings science goals when evaluating mission proposals tothe outer solar system. • Planetary ring systems are being increasingly appreciated for what they tell us about theirenvironment, the origins of their systems and the interior structure of the bodies they circle,as well as their relevance to circumstellar disks beyond our Solar System. Missions lackingrings science goals forfeit insights into the broader scientific pictures of these systems. • Spacecraft provide unique platforms from which many of the open questions above can beanswered. The value of in situ measurements to rings science has been definitively shown,most recently by Cassini and Juno. Remote sensing observations benefit from geometriesthat cannot be obtained from Earth. New technologies will permit previously unattainablemeasurement goals. For example, Hinson et al. [94] describe a novel radio occultationexperiment with New Horizons with sensitivity sufficient to penetrate even Saturn’s B ring.6 eferences [1] Huygens, C., 1659. In
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