Modeling global-scale mass flows on the Lagrangian satellites of Dione and Tethys

Abstract

We perform geomorphological analysis and modeling of the Lagrangian satellites of Dione and Tethys, based on images obtained by Cassini's Imaging Science Subsystem camera and a digital elevation model covering Helene's Saturn-facing and leading sides. Helene's cratered surface has been heavily modified by mass flows as indicated by pronounced crater rim degradation and downslope-oriented chutes, ridges and mesas, which reach >5 km long and ~ 100–300 m wide, and merge onto broad featureless basins at lower elevations with concave lateral boundaries. Mesas are elevated 20–25 m above adjacent parallel oriented chutes that merge with smooth basin materials, suggesting that they define an earlier mass flow surface that has undergone dissection by renewed mass flows. Similar landforms are observed on Calypso. Telesto also exhibits concave seams in the center of its smooth plains, which are interpreted as sites where opposing mass flows converge. We employ landform evolution modeling in order to investigate how these features might form via mass wasting. We test two hypothetical scenarios of the mass flow mechanics. The first scenario postulates shallow diffusive flows, and involves cessation of the mass flow followed by a quiescent period, during which a thin, indurated surface layer forms that armors the loose regolith underneath. Upon resumption of flow, the downslope motion of avalanching materials erodes the hardened surface to create chutes and high standing mesas. A disadvantage of this scenario is the slow formation of an encrusted surface layer. The second scenario assumes deep flows with a Bingham rheology, and we find that development of chutes and mesas appears to be an intrinsic consequence of progressive, differential cessation of Bingham flows as surface relief declines. We conclude that Bingham flow modeling more successfully replicates the observed morphology of the Lagrangian satellites. The preferential location of long mass flows on the leading side of Helene suggests the accumulation of exogenous granular ice as a major source of the mobile material. Estimated rates of deposition derived from the current E-ring system are very low, suggesting alternative sources such as cryovolcanic or impact-generated debris from Dione or Tethys, or a catastrophic recent event in the inner satellite system of Saturn.