Lunar Megaregolith Structure Revealed by GRAIL Gravity Data

Abstract

We use gravity data from NASA's GRAIL mission to characterize the porosity structure of the upper lunar crust. We analyze the gravitational anomalies produced by the porosity of craters with diameters D between 10 and 30 km. We find that the gravitational signature of craters changes significantly at $D=16.4_{-0.6}^{+1.4}$ km, which is related to a discrete change in porosity at a depth ∼3-5 km. We propose that this discrete porosity change reveals the location of the boundary between large-scale basin ejecta and the deeper less porous portion of the megaregolith, known as the structurally disturbed crust. The ejecta thickness can help constrain models of material transport and mixing on the Moon and, because the ejecta layer acts as an insulating blanket, models of heat flow and magmatism.

Keywords: Bouguer; Moon; ejecta; gravity; megaregolith; porosity.

Figure 1

Residual Bouguer anomaly (RBA) of 20,730 lunar craters with diameters between 10 and 30 km and our best fit model. (a) Full range of the RBA data. (b) Zoom in to the −10 to 10 mGal range. The best fit model of the RBA data is a two‐slope model with a positive slope before the breakpoint and a negative slope after it. This breakpoint is located at $D=16.4_{-0.6}^{+1.4}$ km.

Figure 2

Schematic of the lunar megaregolith modified from Hörz et al. (1991). The location of the boundary between the large‐scale ejecta and the structurally disturbed crust layers, as inferred in this work, is shown in red.

Figure 3

Residual Bouguer Anomaly (RBA) data and best fit models of the Highlands, South Pole‐Aitken (SPA), and Mare regions. The Highlands and SPA regions have two‐slope models with breakpoints at similar locations. Within uncertainty, the inferred vertical porosity profile in these regions is indistinguishable from each other and from one of the global RBA data in Figure 1. The RBA data of the Mare region is best fit by a one‐slope model, likely due to the sparsity of craters in the region. Within uncertainty, the slope could be positive, zero, or negative, making it impossible to infer a porosity profile in this region. See Section 4.3 for details.