Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009dps....41.5802o&link_type=abstract
American Astronomical Society, DPS meeting #41, #58.02
Astronomy and Astrophysics
Astronomy
Scientific paper
In the late 18th century, Laplace discovered that the Moon's shape, described by its radius along three axes, is not the expected shape based on rotational flattening and tidal stretching. A frozen tidal bulge alone cannot produce the observed lunar shape even if viscously relaxed. We model the redistribution of mass caused by the South Pole-Aitken basin forming impact. The basin itself becomes isostatically equilibrated, but a thick global ejecta blanket is supported by the lunar lithosphere. We use the semi-empirical model for ejecta thickness by Housen et al. (1983) and include an antipodal correction factor to calculate the global ejecta resulting from the basin-forming impact. We superpose this ejecta blanket on a fossil tidal bulge frozen at distances from 105 km to the current Earth-Moon distance and allow the impact site relative to the tidal bulge to vary. The superposition of impact ejecta on a fossil tidal bulge is consistent with the observed degree-2 gravity coefficients to within 10% for bulge formation distances of 1/2 to 3/4 of the current Earth-Moon distance and impacts along the lunar limb. As a result of the massive global ejecta blanket produced by the South Pole-Aitken basin, the Moon experiences a massive reorientation that places the impact basin near the South Pole where we observe it today. This work is supported by the National Science Foundation Graduate Research Fellowship.
Melosh H.
Ong Lissa C. F.
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