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W.B. McKinnon (Dept. EPSc, Washington Univ, Saint Louis), M.E. Zolensky (NASA JSC, Houston)
Recent work on the origin of Jupiter strongly indicates that, irrespective of model type or details, the Galilean satellites were predominantly 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 (loss of water and volatile organics), and less likely to have been wholesale thermochemical reprocessing of rock+metal compositions (e.g., oxidation of Fe and hydration of silicates). None of the formation models predict or imply accretion of sulfates. Europa's primordial ocean was most likely sulfidic, whether formed promptly during rapid accretion in the presence of non-gas-starved jovian subnebula or bled out over the next few 107 yr as an initially cold interior warmed and ice melted and altered accreted minerals. In the former case, only a portion of Europa's primordial water should have been trapped in the outer layers as hydrated minerals (e.g., serpentine), whereas in the latter most primordial water was likely trapped in the interior. Following accretion and subsequent radiogenic and tidal heating, Europa's primordial ocean would have interacted hydrothermally with the subjacent rock ``core." Zolotov and Shock have hypothesized that sulfides would be converted to sulfates if sufficient hydrogen was lost. Pressure effects and the impermeability of serpentinite imply that extraction of sulfate from thoroughly altered Europa-rock may be extremely inefficient (if indeed Mg sulfate forms at all). Our permissive upper limit sulfate concentration for the evolved ocean is 10 wt% MgSO4 or equivalent. Oxidation of the deep interior of Europa may also occur due to water released by breakdown of hydrated silicates as temperatures further increase. Oxidation of FeS to pyrite and magnetite, as proposed by Lewis for Io, ultimately yields S magma and SO2 gas upon pyrite melting. For a slowly accreted Europa, as in the gas-starved-disk formation scenario, the potential exists for more complete oxidation of FeS, and thus for an even thicker (sub)surface layer of S than seen on Io.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.