Effect of lunar rotation on antipodal ejecta deposition

Abstract

The Moon's crust exhibits strong magnetic anomalies that likely record ancient magnetic fields produced by a core dynamo. Lunar materials have low magnetic susceptibility, but impactor material with high susceptibility may record the Moon's ancient dynamo. Some magnetic anomalies are antipodal to lunar basins and it is hypothesized that the impact ejecta accumulates at the basin antipode and records the ancient lunar magnetic field. Previous impact simulations, which ignored the rotation of the Moon, support this hypothesis. Here, we reevaluate the deposition of antipodal ejecta by considering the rotation of the Moon. Assuming that the rotational period of the Moon is the same as the current day, we find that the rotation of the Moon does not strongly affect the deposition of the antipodal ejecta if impacts occur near the poles. In contrast, rotation has a considerable effect on antipodal deposits for impacts occurring near the equator. An eastward impact at a 45 degree incidence angle results in almost no antipodal ejecta, however, an eastward impact with a 30 degree incidence angle accumulates more antipodal ejecta than the non-rotating case. It is hard to determine the impactor's direction and location, for basins without an ellipsoidal shape and/or an asymmetric ejecta blanket. Nevertheless, our work demonstrates that care must be taken in interpreting the magnetic anomalies antipodal to lunar basins and that the effect of rotation should not be ignored.