HR: 09:40h
AN: P41E-05 INVITED     [Abstracts]
TI: Comparative Late Accretionary Histories of the Earth, Moon and Mars: Implications for the Timing and Nature of the Final Stages of Terrestrial Planet Growth
AU: * Walker, R J
EM: rjwalker@geol.umd.edu
AF: Department of Geology, Univeristy of Maryland, College Park, MD 20815 United States
AB: The comparatively high abundances and chondritic relative proportions of the highly siderophile elements (HSE) in the terrestrial mantle have been cited as evidence that late accretion, subsequent to the last substantive core-mantle interactions, resulted in the addition of approximately 0.5 percent of the mass of the Earth to the mantle. Recent high resolution smooth particle hydrodynamic models of the putative Moon-forming impact suggest that a sufficiently fine redistribution and slow re-segregation of metal following the impact may have caused a final stripping of HSE from the silicate Earth. If so, the late accretionary periods of both the Earth and Moon may have begun at essentially the same time. Recent studies have suggested that the lunar mantle may contain far lower concentrations of HSE than would be expected for a period of late accretion similar to that of the Earth. One possible explanation for this is that much of the late accreted materials to the Moon are present in portions of the mantle that have not yet been sampled or sufficiently well characterized with regard to HSE. Another possible explanation is that the bulk of the materials added to the Moon by late accretion were added relatively late (after crust formation) and still reside in the crust. Studies of lunar impact melt rocks suggest that at least some crustal rocks contain quite high abundances of HSE. In some instances, the abundances can be used to constrain the nature of the impactors. Mars provides a critical point of comparison to the Earth-Moon system. The HSE abundances in SNC meteorites indicate that the Martian mantle may have HSE abundances approximately 2-3 lower than in the terrestrial mantle. This would be consistent with Earth and Mars receiving a similar flux of late-stage impactors. The nature of the Martian veneer has yet to be determined.
DE: 1025 Composition of the mantle
DE: 1026 Composition of the moon
DE: 1027 Composition of the planets
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
SC: Planetary Sciences [P]
MN: Fall Meeting 2005