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

Abstract

Abstract. The northwest comer 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 ø 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.

The lunar‐wide effects of basin ejecta distribution on the early megaregolith

“…We correct our ejecta estimates to account for the spherical nature of the Moon which leads to antipodal thickening of ejecta (Moore et al 1974;Wieczorek and Zuber 2001). Antipodal thickening is evident in our model results as small areas of increased ejecta or deeper mixing outside of basins Figs.…”

On estimating contributions of basin ejecta to regolith deposits at lunar sites

“…The increase beyond the minimum occurs for two reasons: 1) The circumference of the ring of SOIs decreases past the halfway point to the antipode (2730 km), so the surface density of PriFrags increases from the minimum value despite the decrease in total PriFrag mass with increasing distance from the primary crater; 2) The higher velocity ejecta that reach greater distances from the primary crater are more efficient excavators than their lower velocity counterparts (Equation 10). Special effects on deposit thickness are expected in the vicinity of the antipode (5460 km) because of long flight times and the rotation of the Moon, so the calculations are not carried out to that distance (e.g., Dovrobolskis 1981; Haskin, McKinnon, and Benner 1996;Wieczorek and Zuber 2001). If the ejection angle θ is steep (e.g., 55°), ejecta with higher velocities cannot approach the antipode but are lost from the Moon.…”

New observational evidence of global seismic effects of basin‐forming impacts on the Moon from Lunar Reconnaissance Orbiter Lunar Orbiter Laser Altimeter data

“…These studies were inspired by the work by Schultz and Gault [1975], who attributed some specific morphologies in a region antipodal to Caloris basin on Mercury and possibly in regions antipodal to Imbrium and Orientale basins on the Moon to seismic effects. At the present state of knowledge, however, the balance between seismic waves and the convergence of basin ejecta [e.g., Stuart-Alexander, 1978;Wieczorek and Zuber, 2001, and references therein] is not clear in terms of the formation of different morphologies in the antipodal regions. On the other hand, as noted by Hughes et al [1977], even outside the antipodal regions, seismic waves can produce stresses high enough to disrupt rocks and accelerations high enough to exceed gravity, and thus there is a possibility that basinforming impacts have a global effect on morphology not limited to the antipodal region.…”