An Investigation of Early Lunar History using Gravity Data

Abstract

The Moon has fascinated humanity since ancient times. Recent technological and scientific advances have allowed us to gain significant information on the state of the lunar interior, revolutionizing our understanding of the Moon and its history. One such key method in studying the lunar interior is using gravity data. Gravity data can reveal anomalous structures that have minimal topographical expressions on the surface, regardless of whether the anomalous structure is a located at a few kilometers beneath the surface, or beneath the crust-mantle boundary. In this thesis, I present new advances in the understanding and interpretation of the Moon and its history through the analysis of gravity measurements. The Gravity Recovery and Interior Laboratory (GRAIL) mission has revealed scattered linear to arcuate gravity anomalies in both the lunar near and far sides, as well as a polygonal anomaly in the nearside that surrounds the Procellarum KREEP Terrane (PKT), an area of high radioactive element abundance. My investigations in Chapters 2 and 4 revealed fundamental differences in nature between the two types of anomalies, as the linear anomalies are found to be most consistent with dike-like features located at roughly 15 km beneath the crust, while the PKT anomalies are most consistent with remnants of ilmenite-bearing cumulates (IBCs) that did not sink during last stages of lunar mantle overturn. The global mantle overturn is a critical theory that has been used to address fundamental questions about the Moon such as the degree-1 asymmetry in radioactive elements, and my work provides the first ever physical evidence of the global mantle overturn. In Chapter 3, I investigated the mechanisms responsible for the deficit in the crater size frequency distributions of the nearside mare accounting for buried craters observed in the gravity data, in comparison to those of the farside highlands. This deficit is proposed to be related to the missing gravitational signatures of the rings of the Imbrium basin where they are buried beneath the maria. Thermal erosion of substrate topography due to mare flooding was found to be the explanation that is most consistent with the observations. These findings greatly contribute to our understanding of the Moon's thermal and tecto-magmatic evolution, shedding light on crucial parts of lunar history.