Origin of the Moon: The precipitation hypothesis

Abstract

The basaltic rocks returned by Apollo 11 were probably formed by a small degree of partial melting in the lunar interior. Their compositions overlap the terrestrial basalt ranges for most elements; however, there are important differences in the abundances of another group of elements and also in oxidation states. These similarities and differences are believed to be characteristic of the source regions of the lunar basalt and terrestrial basalts. In particular they demonstrate the occurrence of a fractionation between lunar and terrestrial mantles dominated by selective volatility of some elements. They are not readily explained by current versions of fission, binary planet and capture hypotheses. It is therefore necessary to examine other theories of lunar origin. Öpik has argued that the moon formed by coagulation of a terrestrial “sediment-ring” but did not explain the origin or composition of the latter. A possible explanation arises from an extension of the single-stage hypothesis for the origin of the earth previously advanced by the author. This maintains that during the later stages of accretion of the earth, a massive primitive atmosphere developed which was hot enough to selectively evaporate a substantial proportion of the silicates which were accreting upon the earth. Subsequently, the atmosphere was driven away by particle radiation from the sun as it passed through a T-Tauri phase. The relatively non-volatile silicate components were precipitated close to the earth to form a swarm of planetesimals or moonlets, as the atmosphere was dissipated, and the moon accreted from these chemically fractionated planetesimals. The more volatile components of the terrestrial atmosphere were precipitated at lower temperatures, further from the earth, as fine smoke particles and were lost from the earth-moon system with the escaping gases. The hypothesis appears capable of explaining the low density of the moon, the inferred fractionation of relatively volatile elements between earth and moon, and the different oxidation states of the terrestrial and lunar mantles. The above “precipitation” hypothesis thus implies a close genetic relationship between earth and moon. The precipitation and fission hypotheses are also closely connected since the material now in the moon is regarded as having been derived ultimately from the earth — not from the solid mantle, but from the massive primitive terrestrial atmosphere.