Small-scale density variations in the lunar crust revealed by GRAIL

Abstract

Data from the Gravity Recovery and Interior Laboratory (GRAIL) mission have revealed that ~98% of the power of the gravity signal of the Moon at high spherical harmonic degrees correlates with the topography. The remaining 2% of the signal, which cannot be explained by topography, contains information about density variations within the crust. These high-degree Bouguer gravity anomalies are likely caused by small-scale (10's of km) shallow density variations. Here we use gravity inversions to model the small-scale three-dimensional https://ntrs.nasa.gov/search.jsp?R=20170003155 2019-06-07T05:41:52+00:00Z 2 variations in the density of the lunar crust. Inversion results from three non-descript areas yield shallow density variations in the range of 100-200 kg/m 3 . Three end-member scenarios of variations in porosity, intrusions into the crust, and variations in bulk crustal composition were tested as possible sources of the density variations. We find that the density anomalies can be caused entirely by changes in porosity. Characteristics of density anomalies in the South PoleAitken basin also support porosity as a primary source of these variations. Mafic intrusions into the crust could explain many, but not all of the anomalies. Additionally, variations in crustal composition revealed by spectral data could only explain a small fraction of the density anomalies. Nevertheless, all three sources of density variations likely contribute. Collectively, results from this study of GRAIL gravity data, combined with other studies of remote sensing data and lunar samples, show that the lunar crust exhibits variations in density by ±10% over scales ranging from centimeters to 100's of kilometers.

Ring faults and ring dikes around the Orientale basin on the Moon

“…However, the random background variations in the gravity gradients are of comparable magnitude to the signal of interest at the rings. These background variations are not noise in the data, but instead reveal seemingly random, small-scale density anomalies within the upper crust (Jansen et al, 2017). This variability poses a challenge to any study of small-scale (<100 km) subsurface structures using the GRAIL data.…”

“…However, this limiting resolution nevertheless resulted in loss of signal from the crust-mantle interface at shorter wavelengths. Maps of the residual gravity anomaly not included in the GRAIL global crustal thickness models show anomalies in the range of ±40 mGal in the vicinity of the major basins (e.g., Figure 1b of Jansen et al, 2017). As a lower…”

“…Global crustal thickness models are limited by the pervasive small-scale gravity anomalies which arise from shallow density anomalies in the crust (Jansen et al, 2017). During the downward continuation of the Bouguer gravity to the depth of the crust-mantle interface (e.g., Wieczorek, 2007), shorter wavelengths (higher degrees) are amplified more than longer wavelengths (lower degrees), and the small-scale gravity anomalies arising from shallow density anomalies are unstably amplified.…”

Density Structure of the Rümker Region in the Northern Oceanus Procellarum: Implications for Lunar Volcanism and Landing Site Selection for the Chang'E‐5 Mission

“…Lunar subsurface structures have also been recovered using three‐dimensional (3D) density inversion methods based on the Li and Oldenburg (, ) algorithm (Jansen et al, ; Liang et al, ). Jansen et al () used this algorithm in Cartesian coordinates to model the small‐scale 3D density variations in the lunar crust ranging from 100 to 200 kg/m 3 . Although mafic intrusions can explain most of the variations in the density anomalies, Jansen et al () also suggested that they can equally be explained by variations in rock porosity.…”

“…Jansen et al () used this algorithm in Cartesian coordinates to model the small‐scale 3D density variations in the lunar crust ranging from 100 to 200 kg/m 3 . Although mafic intrusions can explain most of the variations in the density anomalies, Jansen et al () also suggested that they can equally be explained by variations in rock porosity. Liang et al () modified the 3D density inversion of Li and Oldenburg (, ) and used it to resolve the subsurface density structure of the lunar crust, including mass concentration and lunar basins, in spherical coordinates.…”