Origin of the Moon - The Collision Hypothesis

Abstract

In 1871, during his presidential address to the British Association in Edinburgh, Sir William Thompson (later Lord Kelvin) discussed the impact of two Earth-like bodies, asserting that "when two great masses come into collision in space, it is certain that a large part of each is melted" [see Arrhenius (1908, p. 218) for the complete quotation]. Although he did not go on to speculate about lunar origin, it must have been remarkable to see one of the creators of the bastions of nineteenth century conservative science discuss such an apocalyptic event and the debris issuing from it. It is equally remarkable that until recently, lunar origin myths have usually centered around three possibilities (fission, capture, and binary accretion) that exclude any important role for giant impacts. The Origin of the Moon Conference held in Kona, Hawaii, on October 13-16, 1984, saw a megaimpact hypothesis of lunar origin emerge as a strong contender, not because of any dramatic new development or infusion of data, but because the hypothesis was given serious and sustained attention for the first time. The resulting bandwagon has picked up speed (and some have hastened to jump aboard). Most significantly, efforts have been made to simulate giant impacts using three-dimensional hydrodynamic codes on supercomputers. Although all this effort is promising, a sober reflection on the problem after two years suggests that a lot more work is needed. It is not yet clear whether the collision hypothesis satisfies the observational facts.