A model of Europa's crustal structure: Recent Galileo results and implications for an ocean

Recent results from the Galileo Near-Infrared Mapping Spectrometer (NIMS) e xperiment suggest that hydrated magnesium sulfate salts may be present on t he surface of Europa. We use this interpretation and a model for the chemic al budget of Europa's outer water-rich layer to determine (1) how Europa's profile atop the mantle may have evolved by cooling and fractional crystall ization from an initial liquid state and (2) the potential effects of this evolution on the surface geology. This model leads to predictions that the lower part is composed of layers of heavily hydrated magnesium sulfates, wh ich effectively isolate any ocean from direct physical contact with the sil icate mantle. These lower layers are overlain by an ocean which has an icy solid upper layer present at the surface; this ocean may have persisted thr ough time. Eutectic conditions are predicted to lead to thickening of the o verlying ice shell and freezing out of the ocean, although tidal heating ma y maintain the liquid layer. Mechanisms such as solid-state convection can further serve to thicken, thin, and/or mix the ice shell. Thermal diapirism could be induced by convection, and compositional diapirism could be drive n by density instabilities within the layers. In this model, early global c ompression would have given way to later global expansion.