Origin of the Earth—Moon system

Abstract

At present, hypothesis of the formation of the Moon from material of the Earth-mantle is a paradigm. It suggests collision of the Earth with a large (Mars-size) body during the flrst 30{50million years of the Earth history. This hypothesis, known as the Giant Impact Hypothesis (GIH), was coined in the middle of the 1970s and gained acceptance because it provided a simple solution for a number of dynamic and geochemical problems (Hartmann and Davis 1975; Cameron and Ward 1976). Primarily, it explains iron deflciency in the Moon as the Moon is proposed to have originated from the Earth’s mantle after the planetary iron had already concentrated in the Earth’s core. However, during the course of time some inconsistencies of the impact hypothesis have surfaced. It is not the purpose of this article to make a critical review of Giant Impact Hypothesis. Instead, we would like to show another mechanism of the formation of the Moon, difierent from the collision model (GIH), which considers the geochemical constraints. First of all, it should be stressed that there exists a signiflcant geochemical similarity of the lunar and Earth’s mantle material, including their oxygen and chromium isotope ratios. Both the Earth and Moon samples follow the same 16 O- 17 O- 18 O isotope fractionation line, have the same 53 Cr= 52 Cr isotope ratios (Lugmair and Shukolyukov 1998; Clayton and Mayeda 1975). The discovery of such a similarity invalidates any propositions which consider the Earth and Moon as alien bodies, for instance, the hypothesis of the capture of the Moon by the Earth. At the same time, there are some important difierences between the composition of the Earth and that of the Moon. The flrst major difierence is depletion of iron content in the Moon. It is explained in the GIH by proposing that the Moon was formed from the Earth’s mantle material, after most of the iron sank to the core. Indeed, while the Earth contains about 32.5% iron, the Moon has only about 10{15%. However, concentration of FeO in the Earth’s mantle is about 8%, compared to 13% in the Moon. In GIH, it has been proposed that the additional iron might have come from the impactor. Recent versions of the impact model admit contribution from the impactor in the lunar material to be predominant. However in such a case, the protolunar material should have been enriched in associated siderophile elements, whereas, in fact, the Moon is depleted in siderophile elements. Moreover, the idea that the Moon mainly inherited the impactor’s material of unknown composition devalues the geochemical arguments and is contrary to the main features of similarity between the Moon and the Earth, including their similarity in oxygen and chromium isotope composition, which in cosmochemistry, plays the same role as DNA plays in the identiflcation of genetic relation of organisms.