Are the Moon's nearside-farside asymmetries the result of a giant impact?

TL;DR

This study uses numerical modeling to test if a giant impact on the early Moon can explain its nearside-farside asymmetries in elevation, crustal composition, and structure, supporting the impact hypothesis for lunar formation.

Contribution

The paper provides quantitative modeling evidence that a giant impact can produce the observed lunar asymmetries and crustal features, linking impact processes to lunar geology.

Findings

A large impactor can create a mega-basin and crustal asymmetries.

Impact excavates and redistributes KREEP material across the lunar surface.

Pre-impact lunar W isotope anomalies may have existed, supporting a giant impact origin.

Abstract

The Moon exhibits striking geological asymmetries in elevation, crustal thickness, and composition between its nearside and farside. Although several scenarios have been proposed to explain these asymmetries, their origin remains debated. Recent remote sensing observations suggest that (1) the crust on the farside highlands consists of two layers: a primary anorthositic layer with thickness of ~30-50 km and on top a more mafic-rich layer ~10 km thick; and (2) the nearside exhibits a large area of low-Ca pyroxene that has been interpreted to have an impact origin. These observations support the idea that the lunar nearside-farside asymmetries may be the result of a giant impact. Here, using quantitative numerical modeling, we test the hypothesis that a giant impact on the early Moon can explain the striking differences in elevation, crustal thickness, and composition between the nearside and farside of the Moon. We find that a large impactor, impacting the current nearside with a low velocity, can form a mega-basin and reproduce the characteristics of the crustal asymmetry and structures comparable to those observed on the current Moon, including the nearside lowlands and the farside's mafic-rich layer on top of a primordial anorthositic crust. Our model shows that the excavated deep-seated KREEP (potassium, rare-earth elements, and phosphorus) material, deposited close to the basin rim, slumps back into the basin and covers the entire basin floor; subsequent large impacts can transport the shallow KREEP material to the surface, resulting in its observed distribution. In addition, our model suggests that prior to the asymmetry-forming impact, the Moon may have had an 182W anomaly compared to the immediate post-giant impact Earth's mantle, as predicted if the Moon was created through a giant collision with the proto-Earth.