Chemical compositions of the Moon, Earth, and eucrite parent body

Abstract

Model compositions of the Moon and Earth were calculated on the assumption that these planets had experienced the same nebular fractionation processes as the chondrites. The proportions of 7 basic components (early condensate, metal, etc.) were estimated from geochemical constraints, such as K/U, bulk U and Fe abundances, etc., and used to construct abundance tables for 83 elements. When lunar and terrestrial basalt data are normalized to these model compositions (to cancel differences in bulk composition), the abundance patterns become strikingly similar. This would seem to demonstrate the essential sameness of igneous processes on both planets. The model correctly predicts the abundance ratios of certain volatile/ refractory element pairs (e.g. Cd/Ba, Ga/La, Sn/Th, and Pb/U), the density of the Moon, and the major rock types. The model is also used, in the reverse direction, to reconstruct the composition of the eucrite parent body. It resembles the Moon to a remarkable degree, except for a lower content of refractory elements. Because of this similarity, it is unlikely that the Moon acquired its composition by some unique chance event, such as disintegrative capture. More likely, such compositions represent the natural outcome of nebular fractionation processes, which may have been more extreme in the inner solar system than in the asteroid belt. All input data required by this model can be obtained by unmanned spacecraft or ground-based observations. Thus, if this model proves viable, it will permit construction of a detailed geochemical profile of a differentiated planet after a single visit by an unmanned spacecraft.