EXCAVATION OF THE MOON’S MANTLE BY THE SPA IMPACT BASIN

The 2500 km diameter South Pole-Aitkin (SPA) basin is the largest and most ancient impact structure now recognizable on the Moon. Its elliptical shape indicates that it was created by the oblique impact of an asteroid between 200 and 300 km in diameter moving roughly from South to North that struck at an angle of 30° to 45° from the horizon. Like all impacts, SPA excavated material from below the surface and ejected it far from the point where the asteroid touched down. Because SPA is the largest basin on the Moon, it also excavated most deeply, so its ejecta surely expose the most deep-seated material to be found on the Moon. Most craters excavate material from about 1/10 of their diameter, but oblique impacts do not dig as deeply as vertical ones. We simulated the SPA impact using the 3-D version of the iSALE hydrocode. We have currently completed high-resolution models using a flat-Moon approximation and a lower resolution spherical Moon model. We find that SPA almost certainly excavated the upper mantle of the Moon and ejected it beyond the rim of the crater, implying that much of the rock debris underlying the farside highlands of the Moon originated from the Moon’s upper mantle. However, multispectral reflectance data of small craters that excavated the highlands are dominated by orthopyroxene (OPX), not the olivine expected from current models of the Moon’s upper mantle. OPX is expected as a late differentiate from a crystallizing magma ocean, but current models propose that it is denser than olivine and sinks in a massive overturn event. Our finding suggests that either this overturn did not occur beneath the SPA basin, or that the magma ocean models need refinement.