Hydrocode simulation of Ganymede and Europa cratering trends - How thick is Europa's crust?

One of the continuing debates of outer Solar System research centers on the thickness of Europa's ice crust, as it affects both the habitability and accessibility of its sub-surface ocean. Here we use hydrocode modeling of the impact process in layered ice and water targets and comparison to Europan cratering trends and Galileo-derived topographic profiles to investigate the crustal thickness. Full or partial penetration of the ice crust by an impactor occurred in simulations in which the ice thickness was less than 14 times the projectile radius. Craters produced in these thin-shell simulations were consistently smaller than for larger ice thicknesses, which will complicate inference of large impactor population sizes. Simulations in which the resultant crater was 3 times the ice layer thickness resulted in summit-pit morphology. This work supports that summit pit craters noted on both rocky and icy bodies, can be created by the presence of a weaker layer at depth. We suggest that floor pits, seen only on ice-rich bodies, require a different formation mechanism to summit pits. Pristine craters formed in a target with high heat flow were shallower than for the same impact into a target of lesser heat flow, suggesting that the 'starting' crater morphology for viscous relaxation, isostatic readjustments and erosion rate studies is different for craters formed in times of different heat flow. We find that the crater depth-diameter trend of Europa can only be recreated when simulating impact into an upper brittle ice layer of 7 km depth, with a corresponding geothermal gradient of 0.025 K/m. As this ice thickness estimate is below similar to 10 km, results from this work suggest that convective overturn of the surface ice may occur, or have occurred, on Europa making the development of indigenous life a possibility. (C) 2013 Elsevier Inc. All rights reserved.