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Numerical Simulations of the South Pole-Aitken Impact

Wednesday, February 18, 2004
Collins (Lunar & Planetary Lab., University of Arizona)

The South Pole-Aitken (SPA) basin, on the far side of the Moon, is the largest and oldest impact structure still preserved in the solar system. The crater is about 2500 km in diameter and formed in the Pre-Nectarian era of lunar history, approximately 4 Gyr ago. At this time, the thermal state of the Moon was much hotter than it is today. Accretional energy from the rapidly forming Moon melted the outermost few hundred kilometers of the Moon. As this magma ocean differentiated and cooled a 60-100-km thick low-density crust formed at the surface; below this the residual melt, with a higher density, cooled to form the lunar mantle.
The giant SPA impact event punctured the Moon some time during the cooling of the magma ocean and thus provides a unique window for studying the lunar interior and the early formative processes of the Moon. The impact excavated otherwise inaccessible samples of the deep crust and (possibly) upper mantle, which has inspired proposed sample return missions. Furthermore, the thermal and rheologic state of the early Moon played a role in shaping the final structure of the basin. I will discuss some numerical simulations of the SPA impact event and their implications regarding site selection for future sample return missions to the SPA basin, and the thermal state of the Moon at the time of impact

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