The sparse occurrence of the mineral olivine in reflection spectra of the Moon’s surface, as well as in deep-seated lunar rocks, has long puzzled lunar geologists because it is expected that the largest impact basins, particularly the 2500-km-diameter South Pole–Aitken (SPA) basin, should have exposed the lunar mantle. Using three-dimensional numerical impact simulations, we show that, even in our most conservative cases, the SPA basin must have sampled the Moon’s upper mantle to depths of 100 km and inevitably spewed this material onto the farside highlands. The spectra of this region are, however, dominated by the signature of low-calcium pyroxene (LCP), not olivine. We thus argue that, in contrast to the upper mantle of Earth, the Moon’s upper mantle is dominated by LCP. This signature also characterizes the mantle of the large main belt asteroid 4 Vesta and suggests that the current paradigm of olivine-rich planetary upper mantles may require revision.

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