Physics
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.v41d..01s&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #V41D-01
Physics
1025 Composition Of The Mantle, 1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
Scientific paper
Introduction: There are two main scenarios that account for the accretion of water to the terrestrial planets: either the water originated outside of the inner solar system and was later delivered to the terrestrial planets by means of some suitable mechanism, or the source of water was local, i.e. it came from the same feeding zone of the Earth's rocky material, and was concomitantly accreted to the planet (Drake, 2004). These two end member scenarios distinguish water both with respect to its source (exogenous - comets, asteroids, phyllosilicates [Cielsa and Lauretta, 2004]), or endogenous - hydrous minerals, water adsorbed on grains [Stimpfl et al. 2004]) and timing of addition to the terrestrial planets (after accretion or concomitant with accretion). If the provenance of water was related to exogenous sources, the terrestrial planets would have accreted from dry material and subsequently obtained water from comets, asteroids, or phyllosilicates. On the contrary, if water came from local sources, then the terrestrial planets would have accreted from wet materials and therefore water would have been present early in the history of these planets. It is important to distinguish between these possibilities because the presence or absence of water while the planets were differentiating would have profound implications in the geochemical evolution of the planetary bodies. For example, it would affect the partitioning behavior of siderophile elements, the melting temperature of the rocks, and of course it could control the evolution of the oxidation state of the planets. Oxidation state of terrestrial planets: An extremely reducing environment characterized the early solar system. Based on the ratio H2O/H2 Jurewicz (1995) proposed a fO2 of about IW-5 for the solar nebula; under this condition very little Fe2+ can exist. However, because planetary material contain Fe2+ and Fe3+ there must have been some mechanism at work that oxidized the metallic iron. If water was present during early differentiation, it could represent the oxidizing agent that might have moved the initial fO2 from solar value to the current one inferred for the mantle of Moon, Vesta, Mars, Venus ( ˜ IW -1) and Earth (QFM) as recently summarized by Jones (2004). It has been proposed that the reaction: Fe(metal) + H2O(melt) = FeO(silicate melt) + 2H(core) (1) could simultaneously oxidize the mantle and also, in the case of the Earth, explain the density deficit of the core (Okuchi 1997, Righter and Drake 1999). Wet Earth: Stimpfl et al., (2004) have recently shown that adsorption of water gas onto dust grains in the accretion disk could account for 3.5 times the amount of water stored in the terrestrial oceans (5 x 10E24 g). However, if water is used by reaction (1) to oxidize the mantle the initial volatile budget of the Earth needs to be much greater. Righter (2004) recently proposed that ~50 Earth's oceans are required to raise the fO2 of the terrestrial mantle from one consistent to 2% FeO (IW-3 or IW-2) to the proposed 8% FeO for the primitive upper mantle (QFM). However, the results presented by Stimpfl et al. (2004) are preliminary and that more sophisticated modeling could sustain higher adsorption on the grains. Likewise, the results of Righter (2004) do not take into account the unknown effects of pressure and temperature. Coupled with the results of Rubie et al. (2004), accretion of water-bearing grains could plausibly contribute to the current oxidation states of planetary mantles.
Drake Michael J.
Lauretta Dante S.
Stimpfl Marilena
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