Thomas, P., Helfenstein, P., Burns, J. (2013). "Inner small satellites of Saturn: Their varied surfaces tell dynamic tales" American Astronomical Society, DPS Meeting #45 #406.07.


Abstract
According to images from the Cassini Imaging Science Subsystem (ISS), the surface forms and overall shapes of Saturn's inner small satellites occur in groups that populate different orbital niches. Co-orbitals Janus and Epimetheus are the most lunar-like of the small satellites; ring moons Atlas, Pan, and Daphnis have latitude-dependent morphology likely related to how ring material is supplied (Charnoz et al., 2007). The shepherding moon Prometheus may show a stripped mantle/core structure. Arc/ring embedded moons are small, smooth ellipsoids, unique among well-imaged small solar system objects. The Trojan satellites (Calypso, Telesto, Helene) have deep coverings showing multi-step histories of deposition and erosion, and include branching networks of downslope transport.

We report the quantitative characteristics of these bodies' shapes, mean properties, and surface characteristics. The differences may arise from the amounts of loose material available to cover the surfaces. Modeling of ejecta sources from large icy satellites in addition to interactions with ring particles may be required to explain all the variation among these small, icy bodies.

The semi-global drainage patterns on the Trojans are especially enigmatic. Why is there nothing comparable on other small satellites? The tapered albedo markings on the Trojans suggest process-specific surface properties. Cassini ISS UV3/IR3 color ratios show that, for Helene, erosion and downslope motion result in a surface that is bluer in color; or a less active surface remains/becomes redder. Sustained exogenic processes such as E-ring particle impacts and charged-particle bombardment compete with geological processes, but on the Trojans, both leave strong signatures. The different amounts of interconnected surfaces on the small satellites range from the cratered landscapes of Janus and Epimetheus, through the semi-global drainage patterns of the Trojans, to complete smoothing of the arc/ring embedded objects. Further work on the systematics of cratering and ejecta motion in the inner Saturn system may elucidate why the "drainage pattern" result is so unusual.