Impact basin relaxation on Rhea and Iapetus and relation to past heat flow

• Published in: Icarus (New York, N.Y. 1962) N.Y. 1962), 2013-04, Vol.223 (2), p.699-709
• Main Authors: White, Oliver L., Schenk, Paul M., Dombard, Andrew J.
• Format: Article
• Language: English
• Subjects: Cratering; Iapetus; Saturn, Satellites; Thermal histories
• ISSN: 0019-1035; 1090-2643
• DOI: 10.1016/j.icarus.2013.01.013

► Impact basins >100 in diameter are unrelaxed on Iapetus, but are relaxed on Rhea. ► Iapetus has only experienced radiogenic heat flow since formation. ► Rhea has experienced an elevated heat flow of tens of mWm−2 since formation. ► Rapid despinning and tidal resonances may have caused Rhea’s elevated heat flow. ► Rhea may have displayed a first generation of large basins that are now erased. 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 (e.g. Moore, J.M., Schenk, P.M., Bruesch, L.S., Asphaug, E., McKinnon, W.B. [2004]. Icarus 171, 421–443; Dombard, A.J., McKinnon, W.B. [2006]. J. Geophys. Res. 111, E01001. http://dx.doi.org/10.1029/2005JE002445). We use new global digital elevation models of the surfaces of Rhea and Iapetus generated from Cassini data to obtain crater depth/diameter data for both satellites and topographic profiles of large basins on each. In addition to the factor of three lower amplitude of global topography on Rhea compared to Iapetus, we show that basins on Iapetus >100km in diameter show little relaxation compared to similar sized basins on Rhea. Because of the similar gravities of Rhea and Iapetus, we show that Iapetus basin morphologies can be used to represent the initial, unrelaxed morphologies of the Rhea basins, and we use topographic profiles taken across selected basins to model heat flow on both satellites. We find that Iapetus has only experienced radiogenic heat flow since formation, whereas Rhea must have experienced heat flow reaching a few tens of mWm−2, although this heat flow need only be sustained for as little as several million years in order to achieve the observed relaxation magnitudes. Rhea experienced a different thermal history from Iapetus, which we consider to be primarily related to their different formation mechanisms and locations within the saturnian system. A recent model for the formation of Saturn’s mid-sized icy satellites interior to and including Rhea (Charnoz, S. et al. [2011]. Icarus 216, 535–550) describes how Rhea’s orbit would have expanded outwards after its accretion from a giant primordial ring, which would have instigated early heating through rapid despinning and tidal interaction with Saturn and other satellites. Rhea’s basins would therefore be required to have formed within the first few tens of Myr of Rhea’s formation in order to relax due of thi