Accretion, core formation, H and C evolution of the Earth and Mars |
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| Authors: | Kiyoshi Kuramoto |
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| Institution: | Center for Climate System Research, University of Tokyo, Meguro-ku, Tokyo 153, Japan |
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| Abstract: | Recent understandings of planetary accretion have suggested that accumulation of a small number of large planetesimals dominates intermediate to final growth stages of the terrestrial planets, with impact velocity high enough to induce extensive melting of the planetesimal and target materials, resulting in formation of a large molten region in which gravitational segregation of silicate and metal, that is, core formation proceeds. In case of homogeneous accretion, volatiles contained in each planetesimal are likely subjected to partitioning among gas, silicate melt, and molten metallic iron at significantly high temperatures and pressures in such a massive molten region. Each phase would subsequently form the proto-atmosphere, -mantle, or-core, respectively. Such chemical reprocessing of H and C associated with core formation, which is followed by both degassing from mantle and atmospheric escape, may result in a diverse range of H2O/CO2 in planetary surface environments, which mainly depends on the H and C content relative to metallic iron in planetary building stones. This may explain inferred difference in volatile distribution between the Earth's (relatively H2O-rich, CO2-poor) and the martian (H2O-poor, CO2-rich) surface environments. Such volatile redistribution may be systematically described by using the retentivity of H2O, ξ, defined as follows: ξ = 1 ? (CO]0 + 2CH4]0 + 2C(gr)]0)/H2O]0, where i]0 represents mol number of species i partitioned into non-metallic phases, that is, gas and silicate melt in impact-induced molten region. When ξ > 0.5, relatively H2O-rich and CO2-poor surface environment may eventually evolve, although a small portion of H2O partitioned into the NON=metallic phases are possibly consumed by subsequent chemical reactions with reduced C-species with producing CO2 and H2. When ξ< 0.5, on the contrary, H2O consumption by the above reactions and selective loss of H2 to space may result in relative H2O-depleted and CO2-rich surface environment. Given the building stone composition by the two-component model by Ringwood (1977) and Wänke (1981), ξ is found to decrease with increasing the mixing fraction of the volatile-rich component: ξ > 0.5 for the mixing fraction smaller than about 15–20% and ξ < 0 for the mixing fraction larger than about 20–30%. This is not significantly dependent on temperature and pressure in molten region and H/C ratio in the building stone. The estimated mixing fraction of the volatile-rich component, about 10% for the Earth and 35% for Mars, is consistent with the observed difference in volatile distribution between the surfaces of both planets. |
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