Crater Relaxation and Heat Flow in the Saturnian System, and Anticipation of New Horizons Observations at the Pluto-Charon System

Evidence for relaxation of impact crater topography has been observed on many icy satellites, including those of Saturn, and the magnitude of relaxation can be related to past heat flow. We have used stereo- and photoclinometry-derived global digital elevation models (DEMs) of the surfaces of eight Saturnian satellites that we have generated from Cassini data to obtain depth and diameter measurements for more than 500 craters. We have previously performed relaxation simulations to determine what heat flow magnitudes and durations are necessary to achieve the current morphologies of certain relaxed and unrelaxed craters on Rhea and Iapetus (White et al., 2013). Combined with age estimates based on crater counting, we found that Iapetus has not experienced heat flows above radiogenic levels since formation of its basins, but that Rhea appears to have undergone a period of global elevated heat flow reaching 20-30 mW m^-2that caused the relaxation of its largest impact basins.

We have since turned our attention to Dione and Tethys, two satellites that show a more complex history of relaxation across their surfaces than either Rhea or Iapetus, with some areas showing extensive relaxation across a wide range of crater diameters, and other areas showing relatively little relaxation. Such a distribution would indicate a history of strongly differential heat flow across these satellites. New simulations of crater relaxation and associated heat flow on these satellites using crater profiles measured from our DEMs, in conjunction with new crater age estimates for the basins, allow us to map the history of heat flow variation across their surfaces. The results suggest that Tethys and especially Dione have been geologically active in their pasts, and that both may represent geologically less evolved cousins of Enceladus.

We also consider these results in the context of the upcoming New Horizons encounter with Pluto and Charon in 2015. These worlds are comparable to the icy satellites of Saturn, Uranus and Neptune with respect to their size, ice-rock ratio and degree of differentiation, but different in that they do not orbit, and therefore are not gravitationally influenced by, a giant planet. These factors must be considered when interpreting the morphologies and thermal histories of impact craters on Pluto and Charon.