Other
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.p53a..07z&link_type=abstract
American Geophysical Union, Fall Meeting 2005, abstract #P53A-07
Other
1034 Hydrothermal Systems (0450, 3017, 3616, 4832, 8135, 8424), 1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 6008 Composition (1060), 6218 Jovian Satellites, 6281 Titan
Scientific paper
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. Water condensed as ice in the solar nebula, was incorporated into the composition of these bodies together with rocky components represented by extremely reduced and anhydrous nebular condensates (e.g., Fe-rich metal, forsterite, low-Ca pyroxene, troilite, Ca-Mg-Al oxides, phosphides), presolar grains (SiC, graphite, diamond, Al-, Mg-, Ti-oxides) and organic compounds and polymers. Radioactive decay of short-lived radionuclides on small bodies, and accretionary heat and decay of long-lived radionuclides on large bodies provided energy to melt ice. On smaller bodies, low gravity precluded separation of water from rocks and restricted fluid dynamics. On larger bodies, water was separated from descending rocks, limiting the duration of water-rock reactions. Competitive oxidation and hydration by water affected both inorganic and organic compounds in rocks. Oxidation of minerals led to formation of ferrous silicates, magnetite, pyrrhotite, Ni sulfides, Ni-rich metal alloys, chromite, phosphates, carbonates and sulfates. Hydration caused formation of phyllosilicates (serpentine, chlorites, smectite clays, amphiboles, and micas), hydroxides, and hydrated sulfides and salts. 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 even sulfates. Since water was the only early oxidizing agent, the elevated oxidation state of Io implies its early aqueous history. Hydrogen was produced in all oxidation reactions and preferentially separated into the gas phase. Escape of H2 into space promoted oxidation, consistent with the high oxidation state of highly porous CI and CM carbonaceous chondrites. Low porosity and permeability, filling of pore space with secondary minerals, and sealing of outer zones with ice restricted H2 escape, retarded some oxidation reactions and caused incomplete reduction of minerals formed earlier by oxidation. For example, magnetite was partially converted to ferrous silicates. The limited oxidation and signs of reduction in some meteorite types (unequilibrated ordinary and CV3 carbonaceous chondrites) is consistent with low amounts of incorporated water ice and low porosity of parent asteroids. 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. 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. In small bodies, organic matter was only partially altered. In some icy satellites, prolonged reactions and elevated pressures favored formation of light hydrocarbons and methane, consistent with observations (e.g., Titan, Triton). 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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