A Serenitatis origin for the Imbrian grooves and South Pole-Aitken thorium anomaly

The northwest corner of the Moon's South Pole-Aitken (SPA) basin contains a high abundance of thorium and a unique Imbrian aged geomorphologic unit that consists of “grooves and mounds” (referred to here as the Imbrian grooves). Because the location of these features are almost antipodal to the Imbrium basin, where high-thorium ejecta and seismic energy are expected to have converged, an Imbrium origin for these units has long seemed certain. By modeling the deposition of impact ejecta on the Moon, we have investigated whether the convergence of Imbrium's ejecta at its antipode could be the origin of both the Imbrian grooves and SPA thorium anomaly. As a result of the Moon's rotation, our results show that ejecta from this basin should converge more than 12° west of its antipode. Both the Imbrian grooves and thorium anomaly within SPA, however, reside slightly to the east of Imbrium's antipode. In an attempt to reconcile this disparity, the effects of a putative oblique Imbrium impact have been qualitatively investigated. While this model can distribute ejecta in the general vicinity of the Imbrian grooves, the planform of our modeled antipodal ejecta is distinctly different from that which is observed. As an alternative explanation for the origin of these features, we find that the modeled distribution of ejecta from an oblique Serenitatis impact is surprisingly similar to the planform of the Imbrian grooves, with the exception that it is offset directly to the east. This eastward offset is likely to be an artifact of our not being able to properly include the effects of the Moon's rotation in our oblique impact models. We conclude that the Imbrium grooves and SPA thorium anomaly are most consistent with having an origin from the convergence of ejecta antipodal to the Serenitatis basin. If this conclusion can be substantiated once quantitative ejecta scaling relations for oblique impacts are determined, then this implies that (1) the Serenitatis target contained a high abundance of thorium and (2) the convergence of seismic energy at the antipodes of either the Imbrium or Serenitatis basin was not sufficient to cause substantial surface modification. Extrapolating this result to Mercury suggests that the “hilly and lineated” terrain antipodal to the Caloris basin was formed by the convergence of ejecta, and not seismic waves.