inclusion of various amounts of impurities — e.g., is a salty ice-block crust more susceptible than pure ice to tidal flexure and heating? Would such a system preserve an interior "ocean" to the present day, or are the disturbed terrains only frozen-in remnants from a tectonically active period earlier in the satellite's history?

Geochemical Processes That Influence the Composition of a Potential Ocean

Compounds can reach oceans on Europa in essentially two ways: melting or overturning of the overlying and surrounding ice, or reaction of water with the underlying rock. Besides water and salts (or other compounds brought out from the interior during cryovolcanism), ice melting and overturning would deliver the other volatile constituents of the ice (such as NH3, CO2, and organic compounds) to the oceans, along with their alteration products resulting from sputtering processes or photochemistry. In addition, the ice would also contain embedded ions responsible for the sputtering processes (largely S+, O+, and H+ from Io and/or Jupiter), as well as dust, meteorites, and other exogenous material. Water-rock reactions will contribute soluble compounds to the solution and are likely to strongly influence the pH, oxidation state, and concentrations of the major ions. Depending on the temperature and oxidation state of the water-rock system, there may be a potential for hydrothermal organic synthesis.

Sputtering experiments on ice-salt mixtures that could be appropriate to the study of the surface of Europa are lacking. Nevertheless, there are speculations that sputtering of sodium and magnesium sulfates in ice could produce compounds like NaOH and H2SO4, either of which could affect the pH if the ice were to melt. Redox conditions in the melt would be affected if potential sputtering products like SO2, MgS, or O2 were released to the solution. In addition, sputtering of organic compounds in ice from exogenous or endogenous sources may lead to ejection of small molecules like CO or HCN, and to cross-linking reactions in more refractory residues. Sputtering of hydrocarbons can lead to the production of H2, CH4, larger molecules, and a carbonized residue,28 and experiments on other organic compounds have yielded CO, CO2, HCN, HCO, other fragments, larger molecules, and carbonized residue.29,30,31 The consequences of hydration reactions in the molten equivalents of these ices are unexplored.

Laboratory simulations of water-rock reactions appropriate to conditions on Europa are lacking. The suggestion that water on Europa would be a sulfate-rich solution if the composition of the rock were like that of carbonaceous chondrites may be corroborated once better resolution of NIMS spectra is achieved.32,33 These measurements also suggest the presence of natron or other carbonate minerals in the ice of Europa. Experiments are needed that test the production of salt solutions during heating, dehydration, and decarbonation of model compositions for the interior of Europa.

The presence of sulfur as sulfate may require that the water-rock reactions occur at relatively low temperatures. As temperature increases, water-rock reactions should be capable of reduction of the sulfate. The range of temperatures at which sulfate reduction can occur depends largely on the composition of the rock, and to a lesser extent on the water-to-rock ratio. For example, calculations of water-rock reactions using the Murchison meteorite's bulk composition show that sulfate reduction is possible above about 100° C at low water-to-rock ratios.34 However, the rates of sulfate reduction may be extremely slow at low temperatures even if they are strongly favored by thermodynamics. Although many hydrothermal sulfur redox experiments have been conducted in terrestrial systems, experiments designed to represent processes that can occur in icy satellites are lacking.

THEORETICAL STUDIES

Thermal History of Europa

Previous studies of Europa's thermal history argue convincingly that enough heat is generated by radioactive decay in the rocks inside Europa that separation of rock and metal from water must have occurred early in the evolution of the satellite. There is also enough energy from radiogenic heating for early differentiation of a metal core from the silicate-metal mixture. The present structure of metal core, rock mantle, and water outer shell must



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