Small scale gravity anomalies observed in GRAIL gravity data

Abstract

The Gravity Recovery and Interior Laboratory (GRAIL) data has revealed an unparalleled high-resolution gravity map of the Moon. This high-resolution data enables us to look at small-scale (10’s of km) shallow gravity anomalies and attempt to interpret them in terms of density. In chapter 2 the background variability is investigated and explained by using gravity inversions to model the small-scale three-dimensional variations in the density of the lunar crust. In an attempt to explain the density variations, we interpret them in terms of three end-member scenarios of variations in porosity, intrusions into the crust, and variations in bulk crustal composition. Though it is most likely that all three end-member scenarios contribute to the background variability, we find that the density anomalies can be caused entirely by changes in porosity. In chapter 3 we investigate small-scale gravity anomalies radiating out from the Orientale basin, shown in GRAIL data. These radial gravity lineations are sometimes associated with secondary crater chains or catenae. Here we use gravity inversions, hydrocode modeling, and observations to investigate the radial gravity anomalies in more detail. Density inversion models show that the gravity can be matched by solutions ranging from broad low amplitude anomalies to shallow high amplitude anomalies. Hydrocode models show that the impacting material remains as a thin layer within the secondary craters and this can explain the linear gravity anomalies. However, this does not explain the majority of linear gravity features that do not have secondary crater chains associated with them. Therefore, we conclude that the majority of radial gravity anomalies represent the structure of the ejecta blanket. This density variability within the ejecta of basins likely contributes substantially to the density variability of the shallow upper crust as a whole.