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Water-Rock Reactions on Non-Planetary Bodies in the Early Solar System

Wednesday, February 22, 2006
Mikhail
Zolotov (Dept. of Geological Sciences, Arizona State University)

Reactions of aqueous fluids with rocks shortly after formation of the solar system affected the oxidation states, mineralogy, organic speciation, ice composition, and surface/atmospheric chemistry of asteroids, icy satellites of giant plants, and possibly Kuiper belt objects. Competitive oxidation and hydration by water affected both inorganic and organic compounds in rocks. High water/rock ratios, elevated temperatures and low pressures favored oxidation. Low temperatures supported hydration. In some icy satellites (Europa, Ganymede) high water content and hydrothermal processes during differentiation may have caused profound oxidation leading to carbonates and sulfates. H2 was produced in all oxidation reactions and separated into the gas phase. Escape of H2 into space promoted oxidation. Low porosity and permeability, filling of pore space with secondary minerals, and sealing of outer zones with ice restricted H2 escape and caused incomplete reduction of minerals formed earlier by oxidation. Interaction of water with organic compounds and carbon grains caused disproportionation of carbon leading to O-bearing organic compounds and CO2, as well as to hydrogenated organic compounds and methane, consistent with observations (e.g., Titan, Triton). Although prolonged heating of bodies caused dehydration and some reduction, water-rock reactions led to net oxidation of primary rocky components, consistent with the elevated oxidation states of metamorphosed chondrites and Io. In most bodies, net oxidation of organic compounds can be attributed to preservation of CO2 and O-bearing organics in solution, ices and carbonates, and escape of H2 formed through oxidation reactions.

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