Origin of the Moon-The Collision Hypothesis

Abstract

In 1871, during his presidential address to the British Association in Edinburgh, \nSir William Thompson (later Lord Kelvin) discussed the impact of \ntwo Earth-like bodies, asserting that "when two great masses come into \ncollision in space, it is certain that a large part of each is melted" [see \nArrhenius (1908, p. 218) for the complete quotation]. Although he did not \ngo on to speculate about lunar origin, it must have been remarkable to see \none of the creators of the bastions of nineteenth century conservative \nscience discuss such an apocalyptic event and the debris issuing from it. It \nis equally remarkable that until recently, lunar origin myths have usually \ncentered around three possibilities (fission, capture, and binary accretion) \nthat exclude any important role for giant impacts. The Origin of the \nMoon Conference held in Kona, Hawaii, on October 13-16, 1984, saw a \nmegaimpact hypothesis of lunar origin emerge as a strong contender, not \nbecause of any dramatic new development or infusion of data, but because \nthe hypothesis was given serious and sustained attention for the first time. \nThe resulting bandwagon has picked up speed (and some have hastened \nto jump aboard). Most significantly, efforts have been made to simulate \ngiant impacts using three-dimensional hydrodynamic codes on supercomputers. \nAlthough all this effort is promising, a sober reflection on the \nproblem after two years suggests that a lot more work is needed. It is \nnot yet clear whether the collision hypothesis satisfies the observational \nfacts. \n