Evaluation of models for the formation of chaotic terrain on Europa

Chaotic terrain on Europa is interpreted to be the result of the breakup of brittle surface materials over a mobile substrate. Two end-member models h ave been proposed for the mobile substrate: a liquid water ocean or solid w arm ductile ice. Direct contact between the brittle surface ice and an unde rlying ocean might be accomplished by melting through Europe's ice shell, b ut plausible sources of thermal energy are insufficient to locally melt thr ough the ice shell. We calculate that a liquid or partially liquid substrat e is probably necessary for tilted blocks to be observed, challenging a sol id-state model. We propose that a model in which chaos areas on Europa form ed over bodies of melt or partial melt within the ice shell may be the best match to observational and thermal constraints. Such bodies of melt within the ice shell may be triggered by warm diapirs of relatively clean ice tha t partially melted an overlying layer of lower melting temperature, such as ice contaminated with salts.