Zusammenfassung
Recent models for the origin of Jupiter indicate that the Galilean
satellites were mostly derived from largely unprocessed solar nebula
solids and planetesimals. In the jovian subnebula the solids that
built Europa were first heated and then cooled, but the major effect
was most likely partial or total devolatilization, and less likely
to have been wholesale thermo-chemical reprocessing of rock + metal
compositions (e.g., oxidation of Fe and hydration of silicates).
Ocean formation and substantial alteration of interior rock by accreted
water and ice would occur during and after accretion, but none of
the formation models predicts or implies accretion of sulfates. Europa's
primordial ocean was most likely sulfidic. After accretion and later
radiogenic and tidal heating, the primordial ocean would have interacted
hydrothermally with subjacent rock. It has been hypothesized that
sulfides could be converted to sulfates if sufficient hydrogen was
lost to space, but pressure effects and the impermeability of serpentinite
imply that extraction of sulfate from thoroughly altered Europa-rock
would have been inefficient (if indeed Mg sulfates formed at all).
Permissive physical limits on the extent of alteration limit the
sulfate concentration of Europa's evolved ocean to 10% by weight
MgSO4 or equivalent. Later oxidation of the deep interior of Europa
may have also occurred because of water released by the breakdown
of hydrated silicates, ultimately yielding S magma and/or SO2 gas.
Geological and astrobiological implications are considered.
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