Planetary Science

The Two Faces of the Moon

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Science  23 Nov 2012:
Vol. 338, Issue 6110, pp. 1010-1011
DOI: 10.1126/science.338.6110.1010-e

The nearside and farside of the Moon look very different. The farside, which we cannot see from Earth but has been imaged by satellites, almost completely lacks the large basaltic plains (mare) that are so prominent on the nearside. A giant impact early in the Moon's history is one of the possible explanations for this asymmetry, which manifests itself not only in mare basalt distribution but also in crustal thickness and concentrations of radioactive elements. Nakamura et al. used spectral data from the Japanese Kaguya mission to look for the signatures of such an impact. Specifically, they searched for low-calcium pyroxene, a mineral that can be associated with a large impact, because it would either be excavated from the upper mantle and exposed at the surface or produced by the melting of a mixture of crust and mantle materials after the impact. Concentrations of low-calcium pyroxene were found to occur around the two largest impact basins on the Moon: the South Pole–Aitken basin, on the farside, and the Imbrium basin, on the nearside. The mineral was also found around the Procellarum basin, a 3000-kilometer-diameter basin on the nearside, whose origin has been equivocal. A large impact on the Procellarum region could explain the asymmetry between the two faces of the Moon.


Nat. Geosci. 5, 775 (2012).

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