Are the Moon's Nearside-Farside Dichotomies the Result of a Giant Impact?
Abstract
The dichotomy of the lunar crustal thickness is one of the most striking features of the Moon. Recent observations from the GRAIL mission indicate a crustal thickness of 30-40 km on the nearside lowlands and 50-60 km on the farside highlands. The highland crust may be composed of two layers: a primary anorthositic crust with a thickness 30-50 km, and an uppermost mafic-rich layer with a thickness of 10 km. Several mechanisms have been proposed to explain the formation of the crustal thickness dichotomy, but the origin remains controversial. Giant impacts are thought to have happened frequently in the ancient solar system and the collision of a large body with Mars has been suggested as an explanation for the north-south crustal dichotomy on Mars. Therefore, it is possible that the ancient Moon suffered from a giant impact, which could have formed the Moon's crustal thickness dichotomy and farside highlands. In this work, we conduct a series of numerical models of giant impacts on the Moon to test quantitatively the large collision hypothesis for the formation of crustal dichotomy and farside highlands. 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 dichotomy and structures comparable to those observed on the current Moon, including the nearside lowlands and the farside thick mafic-rich layer on top of the primordial anorthositic crust. Our model shows that the excavated deep-seated KREEP 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, forming its observed distribution. In addition, our model implies a significant 182W anomaly in the Moon before the dichotomy-forming impact, as predicted if the Moon was created through a giant collision with the proto-Earth.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.P31G3781W
- Keywords:
-
- 1027 Composition of the planets;
- GEOCHEMISTRYDE: 3672 Planetary mineralogy and petrology;
- MINERALOGY AND PETROLOGYDE: 6299 General or miscellaneous;
- PLANETARY SCIENCES: SOLAR SYSTEM OBJECTSDE: 5455 Origin and evolution;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS