Computer Science
Scientific paper
Jan 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998lpico.957...11h&link_type=abstract
Origin of the Earth and Moon, Proceedings of the Conference held 1-3 December, 1998 in Monterey, California. LPI Contribution N
Computer Science
Earth-Moon System, Lunar Evolution, Solar System Evolution, Planetary Evolution, Computerized Simulation, Moon, Chondrites, Hafnium Isotopes, Tungsten, Selenology
Scientific paper
Some recent models for the giant impact theory of lunar origin involve a collision between a proto-Earth that is perhaps only half the present Earth mass and an impactor that is larger than those previously considered. Such new models have a direct bearing on the interpretation of W-isotopic data for the Earth and Moon. If the Earth was only half accreted at the time of the giant impact and this took place about 50 m.y. after the start of the solar system, the accretion rate of the Earth must have been about 5x slower than that predicted by Wetherill. The slower the rate of terrestrial accretion and core formation, the smaller the isotopic effect of excess t82W produced in the silicate Earth by the decay of Hf-182 (T(1/2 = 9 m.y.). If such accretionary rates are correct, it is not difficult to explain why the silicate Earth, should have a chondritic W-isotopic composition, even if terrestrial core formation commenced at the start of the solar system. The W- and Pb- isotopic compositions of the silicate Earth expected as a consequence of slow accretion can be explored with continuous core-formation models. It is assumed that the Earth's core grew in its present proportion during progressive accretion and that the accreting material, including any giant impactor, had (on average) chondritic Hf-W and W-isotopic composition. The accreting material mixes and isotopically equilibrates with the silicate portion of the proto-Earth, and further concomitant metal segregation results in growth of a core in proportion to the increased mass of the Earth. The resultant change in the W-isotopic composition of the silicate Earth as a result of loss of W to the core, dilution with chondritic material, and decay of Hf-182 is calculated for the first 200 m.y. of Earth's history. Changes in w partitioning between silicate- and metal rich Earth reservoirs as a result of changes in oxidation state do not greatly affect the final outcome. Three scenarios are considered: (1) Growth of the Earth (and its core) at an exponentially decreasing rate; (2) Growth of the Earth at an exponentially decreasing rate for 50 m.y. terminated by a giant impact; (3) A giant impact followed by further later growth of the Earth.
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