Comparison of the Cratering Records of Ceres and Rhea

Abstract:

Comparing the cratering records of dwarf planet Ceres and the Saturnian satellite Rhea, offers a great opportunity in comparative planetology to fill the gaps of understanding of the cratering history of the two bodies. Both bodies show strong indications for a water-ice rich crust. For Ceres, the amount of ice in the crust is indeterminate. Early Dawn imaging data shows complex craters on Ceres which are smaller than those on the basaltic asteroid Vesta. The smallest complex craters on Ceres are similar in size (~10-15 km) to those on Rhea, which might indicate a rather high water-ice content in Ceres’ crust. The surface gravity on both bodies is almost equal, differing by only ~4%. Thus, regardless of their absolute values many variables required to relate projectile and crater size should be very similar on both bodies (surface gravity, strength to gravity transition, simple to complex transition, target density). The remaining variables such as projectile density and impact velocity are comparatively well known for Ceres but still in discussion for the Saturnian satellites. If the crater size-frequency distributions for craters >5 km from Rhea and Ceres are plotted together and are corrected for different projectile flux and exposure time, both records plot nearly on top of each other. This could indicate a common projectile population that impacted both bodies at nearly the same velocity. However, if the impacting projectile populations are very different, the impact velocity would have to compensate for such differences. Different ice temperatures may also play some role. Reducing the degrees of freedom increases the chance of understanding the projectile source and dynamics in the Saturnian system. We acknowledge the support of the Dawn and Cassini Instrument, Operations, and Science Teams. This work is supported by the German Space Agency (DLR), grants 50OW1101, 50OH1102 and 50OH0305.