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

Abstract

a b s t r a c tEvidence 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.). 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 >100 km 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 mW m À2 , 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 this heating, and if so may provide an important anchor point for Saturn system chronology. None of these heating mechanisms are viable for Iapetus in its isolated position far from Saturn, and as such it has remained dynamically inert since formation, confirming conclusions based on thermal modeling of Iapetus' interior. Rapid and complete relaxation and subsequent erosion by bombardment of a 'first generation' of large basins on Rhea is regarded as an explanation for the lower counts of large basins on Rhea relative to Iapetus, and the overall lower amplitude of topography on Rhea compared to Iapetus.

Rhea gravity field and interior modeling from Cassini data analysis

“…Support for degree-2 topography, however, may be more limited, especially if the degree-2 shape and gravity field were established early, when Rhea's heat flux was still high (White et al, 2013). In that case, it may be more appropriate to assume that the J 2 (=-C 20 ) and C 22 surface topography is relaxed and therefore conforms closely to the geoid (equipotential surface at reference radius R), given by…”

Fault geometries on Uranus’ satellite Miranda: Implications for internal structure and heat flow

“…In the jovian system, Europa displays near-ubiquitous ridges and troughs that may have resulted from tensional stresses associated with nonsynchronous rotation and/or diurnal tides (Nimmo and Manga, 2002;Ruiz and Tejero, 2003;Tobie et al, 2003;Showman and Han, 2004;Ruiz, 2005), diapirism (Croft, 1987), and/or dike intrusions (White et al, 2013). Ganymede exhibits regions of ridged and grooved terrain that have been interpreted to be extensional in nature and in some cases show evidence for strike-slip motion (Lucchita, 1980;Murchie and Head, 1988;Pappalardo et al, 2004;Pappalardo and Collins, 2005) and transtension Pappalardo et al, 1998;Deremer and Pappalardo, 2003).…”

Shallow normal fault slopes on Saturnian icy satellites

“…These values are T s = 87 K for Tethys [ Hanel et al ., ], and the average trailing hemisphere temperatures of 88 K for Rhea [ Cruikshank et al ., ], and 90 K for Dione [ Cruikshank et al ., ] (Table ). Estimates for F range from 18 to 30 mW m −2 for Tethys [ Giese et al ., ; Chen and Nimmo , ], 15 to 30 mW m −2 for Rhea [ Nimmo et al ., ; White et al ., ], and 24 to 90 mW m −2 for Dione [ Hammond et al ., ; Phillips , ] (Table ).…”