High‐Resolution Gravity Field Models from GRAIL Data and Implications for Models of the Density Structure of the Moon's Crust

Abstract

We present our latest high‐resolution lunar gravity field model of degree and order 1200 in spherical harmonics using Gravity Recovery and Interior Laboratory (GRAIL) data. In addition to a model with the standard spectral Kaula regularization constraint, we determine models by applying a constraint based on topography called rank‐minus‐one (RM1). The new models using this RM1 constraint have high correlations with topography over the entire degree range by design. The RM1 models allow the determination of apparent crustal densities at all spatial scales (called effective density) covered by the model, whereas the Kaula‐constrained model can only be used globally up to spherical harmonic degree 700. We find that the effective density spectrum has a smaller slope for the high degrees when compared to the medium degrees. We interpret this as indicative of a global average surface density, as opposed to an ever‐decreasing effective density as one approaches the surface. We use the RM1 models to derive maps of lateral and vertical density variations in the lunar crust. These models allow us to increase the resolution of this analysis compared to previous studies, by increasing the degree range over which to fit theoretical models of vertical density variations, and by decreasing the size of the spherical caps used in a localized analysis. Several regions on the Moon, such as South Pole‐Aitken and Mare Orientale, are distinct from their surroundings in terms of surface densities. The RM1 models are especially valuable in (localized) spectral studies of the structure of the lunar crust.

GRAIL-identified gravity anomalies in Oceanus Procellarum: Insight into subsurface impact and magmatic structures on the Moon

“…The GRAIL nominal and extended mission data (Goossens et al, 2018) analyzed and discussed here permit higher resolution gravity modeling than in previous studies (e.g., Kiefer et al, 2013;Evans et al, 2016). These data demonstrate that the amplitude of the four PBGAs cannot be explained by mare-filled craters alone, as inferred by Evans et al (2016), and instead require an additional density contrast.…”

“…Four positive Bouguer gravity anomalies in Oceanus Procellarum. GRAIL-derived Bouguer spherical harmonic solution to degree 6-660 is displayed on the left, with an assumed density correction of 2800 kg/m 3 , windowed from a global, locally patched, constrained solution (GRGM1200B_RM1_1E1; Goossens et al, 2018). Surface elevations, measured from the LOLA 1060 instrument (Smith et al, 2010), are shown on the right.…”

“…We use GRAIL gravity data (Zuber et al, 2013a), which at long wavelengths largely indicate variations in compensation state and thickness of the crust (Neumann et al, 1996;Wieczorek and Phillips, 1998;Wieczorek et al, 2013). The latest solutions from the GRAIL extended mission (e.g., GRGM1200B_RM1_1E1) use data from all altitudes in the primary mission (average altitude ~50 km) and the extended mission, which exploit satellite-to-satellite tracking at altitudes as low as 8 km in this region (average altitude ~28 km) to achieve maximum resolution of degree and order 1200 (Goossens et al, 2018). Spacing of ground tracks and small-scale density variations within the uppermost crust limit the increase in resolution and correlation with topography so we consider a maximum degree of 660, equivalent to ~8 km resolution, and discretize our forward models at roughly 2x2 km intervals.…”

“…GRAIL data are now available from both the primary and extended missions (Goossens et al, 2018). The solutions presented here from the sha.grgm1200b_rm1_1e1 model are of spherical harmonic degree 1200 windowed to degree 660 (Goossens et al, 2018), and are higher resolution than the solutions used by Kiefer (2013) (degree 110) and Evans et al (2016) (degree 900 windowed to degree 600).…”

“…GRAIL data are now available from both the primary and extended missions (Goossens et al, 2018). The solutions presented here from the sha.grgm1200b_rm1_1e1 model are of spherical harmonic degree 1200 windowed to degree 660 (Goossens et al, 2018), and are higher resolution than the solutions used by Kiefer (2013) (degree 110) and Evans et al (2016) (degree 900 windowed to degree 600). With these new data, with an estimate of the density contrast between the crust in Marius Hills and mare basalts that is specific to this location, and with recent geologic analyses of the generation, ascent, and eruption of magma on the Moon (Head and Wilson, 2017;Wilson and Head, 2017), we are motivated to examine crustal density models in order to reevaluate the subsurface structures in the Marius Hills region.…”