EXPLORATION DESIGN TO TEST THE GIANT IMPACT HYPOTHESIS FOR MOON'S ORIGIN

The idea of a giant impact origin of the Moon has become entrenched in scientific literature, popular scientific press, and most academic institutions. The unquestioned assumption of a giant impact origin within the first ~30 m.y. of solar system history underlies most geochemical and geophysical interpretation of new data reported in the lunar science literature. The estimated iron content of the Moon, as well as hafnium/tungsten systematics, further constrain about ninety percent of the Moon's parent to be the impactor rather than differentiated Earth's mantle. This latter constraint essentially turns the giant impact hypothesis into an "impact assisted capture" hypothesis. An additional critical constraint on this computer-based hypothesis is that the impactor needed to have evolved in the same oxygen isotopic reservoir as did the Earth. The major problem with the giant impact hypothesis lies with information on the interior of the Moon below about 550km. Velocity data suggest that the lower lunar mantle is more aluminous than overlying upper mantle and melting and fractional crystallization of a magma ocean was limited to the highly differentiated upper mantle. Also, the non-glass components of Apollo 17 and 15 pyroclastic glasses and Apollo 15 have chondritic signatures for tungsten, lead, and siderophile and chalcophile elements. Volatiles associated with the glasses are enriched over associated basalts by factors greater than 100 in Cl, F, Br, Zn, Ge, Cd, Tl, and Ag and by factors greater than 10 in Pb, Ga, Sb, Bi, In, Au, Ni, Se, Te, and Cu. Inconsistencies between the constraints imposed by Apollo exploration and sample analyses and the geochemical and geophysical implications of a giant impact origin of the Moon can be tested. Future lunar field exploration and sampling of pyroclastic glasses and long-lived geophysical measurements that will better define the nature of the lower lunar mantle should be a scientific priority. If the giant impact hypothesis cannot explain the spectrum of geological evidence about the nature of the lower mantle outlined above, alternatives to that hypothesis should be considered. The most plausible alternative to giant impact appears to be the non-catastrophic capture of the Moon as an independently evolved planetesimal.