The Origin of the Moon and Geophysical Consequences*

Abstract

Summary The classical theory of tidal friction developed by Sir George Darwin has been used by a number of investigators to gain information about the origin of the Moon by tracing its orbital evolution backward in time. The present paper extends tidal theory further by making the dissipation function frequency-dependent, where frequency is defined as the relative angular rate of the satellite and planet. In order to test this modified theory empirically, it is first applied to a study of the origin of Phobos and Deimos, the two satellites of Mars. Contrary to earlier results which maintained their present low eccentricity orbits, it is found that their initial orbits are highly elongated ellipses (suggesting an origin in terms of captured cometary nuclei). When the frequency-dependent theory (even in a two-dimensional version) is applied to the more complicated Earth-Moon system, it leads to a set of dynamical constraints on the origin of the Moon which can be used together with geophysical evidence and evidence of the lunar surface structure. The most likely origin of the Moon is found to be capture as a separate body, rather than its formation in the vicinity of the Earth from smaller planetesimals. Earlier developments of tidal theory had required an initial orbit of very high inclination (90° or even greater) and initial obliquity of the Earth's axis of 90°; all of which would have made capture extremely unlikely and, further, required the dissipation of unreasonable amounts of rotational kinetic energy. Urey had favoured capture of the Moon for geochemical reasons; we now support this hypothesis because of dynamical arguments. Evidence is presented to support the occurrence of intensive heating and volcanism, both in the Earth and in the Moon, approximately four billion years ago. Thus, capture of the Moon may explain the apparently anomalously early development of the Earth's atmosphere and oceans.