Can redistribution of material by sputtering explain the hemispheric dichotomy of europa?

Abstract

Sputtering and redeposition make up a significant geologic process on Europa and may contribute to the observed color differences between the leading and trailing hemispheres. Sputtering is particularly efficient on Europa as an erosive process, due to the high rate of ion bombardment from the Io plasma torus, together with Europa’s easily sputtered icy surface. We estimate the global average sputtering erosion rate on Europa at 14.7 mm Myr −1. However, 42% to 86% of sputtered water molecules survive to redeposit onto the surface again. Due to gravitational escape and removal by electron-impact ionization, the number of redepositing particles cannot overcome sputtering erosion, and the global average result of sputtering is net erosion. However, neither sputtering nor redeposition is globally uniform, and differences in the global distributions of the two processes can result locally in net deposition. We propose that Europa’s hemispheric color dichotomy might be explained by net deposition on the leading hemisphere, which may obscure the non-ice signature by covering it with a thin water frost. To test this hypothesis, we have created a simulated model of the sputtering erosion/redeposition process on Europa. Our objectives are to determine the conditions under which net deposition occurs on the leading hemisphere and to evaluate the effects on this process of Jupiter’s gravity, of Europa’s rotation, and of the loss of water molecules to the jovian magnetosphere. We have followed the trajectories of hundreds of thousands of simulated sputtered water molecules in a Monte Carlo process, evolving their orbits under the gravity of both Europa and Jupiter. We have performed this model multiple times, in order to explore the effects of different assumptions of the global distribution of impacting ions, as well as of the sputtered particle ejection velocity. Based upon our results, we conclude that net deposition occurs under a wide range of conditions, making sputtering erosion and redeposition a plausible explanation for the hemispheric color dichotomy of Europa.