Enceladus and the Icy Moons of Saturn (2016) 3019.pdf

SYSTEMATICS OF THE INNER SMALL SATELLITES OF SATURN. P. C. Thomas1, M. S. Tiscareno2, P. Helfenstein1 1Cornell University, 2SETI Institute

Introduction The libration satellites have deep coverings subject More than a decade of encounters of the Cassini to long-range downslope motion. In at least the case of spacecraft with the small satellites of Saturn shows ob- Helene, the covering is being eroded and is not in an jects having distinct morphologies occupying distinct equilibrium with local deposition. The source and rea- dynamical settings[1]. The prime scientific challenge son for apparent discrete nature of the deposition is not in the study of these objects is determining how these yet clear. morphologies restrict the evolutionary development of Photometry and colors objects in the different dynamical niches and thus help The small satellite albedos increase and colors be- in constraining the broader history of the Saturn sys- come bluer [3] progressively from Atlas to the Trojans tem. showing the influence of E-ring particles that originate The distinguishing features include: Satellites in from Enceladus [4]. The differences are likely caused the main ring system have equatorial ridges [2]. The by water ice with minimal contaminants that are dis- coorbitals are by far the most massive, have the highest tinct from those in the darker components of regoliths (and nearly identical) mean densities, and have the of outer satellites such as Iapetus, Phoebe or Hyperion most asteroid-like appearances. The three resolved [3,5]. However, surface resolved albedo and color var- satellites in diffuse rings or ring arcs have the lowest iegations also exist in relation to local geology, such inferred mean densities and appear to be equilibrium as downslope transport [6]. ellipsoids, unique among small solar system objects. Dynamics and possible ages Libration zone satellites have deep coverings of debris Consensus on the dynamical history of the small subject to long distance downslope motion. and mid-size satellites of Saturn is currently in Distinguishing forms flux. Longtime suspicions that Saturn’s rings are only In interpretation of an object’s history, the presence ~100 Myr old have recently intensified as the rings ap- of a surface with evidence of different phases or pro- pear less massive than previously thought [7] and be- cesses is important, as is the presence of a surface indi- cause the pollution of the rings’ pristine water ice by cating an equilibrium of processes without evidence of interplanetary dust is faster than previously thought discrete events. The equatorial ridges of ring-related [8]. Also, the orbital evolution of moon orbits due to satellites, Pan, Daphnis, and Atlas suggest at least a Saturn’s internal friction may be faster than previously two-stage history, the latter involving addition of ring thought [9]; this result has led some to conclude that materials at low latitudes. Prometheus, the inner F- the moons also cannot be more than 100 Myr old ring shepherd, shows evidence of stripping (by im- [10]. Formation from young massive rings, rather than pact?) of an outer mantle of material from about half from the protosatellite disk, was previously discussed the satellite’s area exposing a core of perhaps 2/3 the for the small moons [11] and should be revisited in satellite’s volume. The putative exposed core is heav- light of these new results. ily cratered, so any exfoliation occurred relatively Hyperion early in the satellite’s history. Janus and Epimetheus Hyperion is distinguished from the inner small satel- have the most asteroid-like appearances among the lites by its size, sponge-like morphology, but not its small inner satellites, namely surfaces accumulating mean density. Its dynamical and surface history is craters with local debris accumulation, basically show- likely very distinct from that of the inner small objects. ing one style of surface history. References: [1]Thomas, P.C., et al. (2013) Icarus 226, The most remarkable surfaces are the smooth ellip- 999-1019. [2] Charnoz, S. et al. (2007) Science 318, soids of the ring-arc related moons, Aegaeon, 1622. [3] Buratti et al. (2010) Icarus 206, 524–536. [4] Methone, and Pallene. Methone and Pallene are suffi- Verbiscer et al. (2007) Science 315, 815, [5] Fi- ciently well resolved to show they have equilibrium lacchione et al. (2010) Icarus 206, 507–523, [6] Mor- shapes consistent with mean densities of 310 and 250 rison et al. (2010) BAAS 42, p.955. [7] Hedman, -3 kgm , respectively. Aegaeon is consistent with a M.M., Nicholson, P.D. (2016) ArXiv e-prints -3 mean density of 540 kgm [1]. These objects are ap- arXiv:1601.07955. [8] Kempf, S., et al. (2015) EPSC parently shaped by surface processes and materials that 2015, EPSC2015-411 10. [9] Lainey, V., et al. (2015) are effectively fluid on modest geological time scales. ArXiv e-prints, October 2015. [10] Cuk, M., Dones, This unique arrangement, occurring within ring arc L., Nesvorny, D. (2016) The Astrophysical Journal materials, suggests recently active interchange of mate- 820, 97. [11] Charnoz, S., Salmon, J., Crida, A. (2010) rials, but limits on their ages and fluxes are not well es- Nature 465, 752-754. tablished.