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

The Gravity Recovery and Interior Laboratory (GRAIL) mission to the Moon was designed to determine the structure of the lunar interior from crust to core and to advance the understanding of the Moon's thermal evolution by mapping the gravity field of the Moon globally at high resolution. The mission consisted of two spacecraft with Ka-band inter-satellite tracking complemented by tracking from the Earth provided by NASA's Deep Space Network (DSN). The mission had two phases: a primary mapping mission from March 1 until May 29, 2012 at an average altitude of 50 km, and an extended mission from August 30 until December 14, 2012, with an average altitude of 23 km before November 18, and 20 and 11 km from November 18 to the end of GRAIL's mission on December 17, 2012. Global and local models of high resolution have been determined from these data, and one of the main findings of the GRAIL mission was the high correlation between topography and gravity, indicating a highly fractured crust. These high correlations have been exploited to infer both lateral and vertical variations in crustal density. Laterally varying density of the uppermost crust together with loading effects result in correlations close to but different from unity. Yet at small scales, the correlations between gravity and topography decrease rapidly due to factors such as the geographically varying sensitivity of the data and the type of constraint used in the determination of the gravity field model

We present our latest high-resolution GRAIL gravity model of degree and order 1200 in spherical harmonics, where we apply a constraint based on topography information. This constraint has been applied to Mars with orbiter tracking data, and we showed that we can robustly determine the bulk crustal density from the data, independent of choices made for the constraint. Our new lunar gravity model shows high correlations between gravity and topography for the entire range of spherical harmonic degrees, as well as a stable effective density spectrum (an indication of crustal density). We use this model to derive lateral and vertical variations in the density of the lunar crust.