GRAIL Bouguer Gravity Anomalies of Complex Lunar Craters: Determined Through Spectral Filtering

The lunar Gravity Recovery And Interior Laboratory (GRAIL) mission has provided a global gravity field of unparalleled resolution. There has been a strong scientific effort to understand the Bouguer anomalies (BA) of complex lunar craters (cf., impact basins) so that they can be used as probes of both the lunar crust and the cratering process itself [Phillips et al., 2012]. One of the key challenges in understanding the signal associated with craters has been separating out the background anomaly. We use a database of >1000 craters with the gravity and topography cutoff at degree and order 600. We focus on intermediate-sized craters with a diameter range of ~30-150 km. With this database, we compare different methods of isolating the crater anomaly, including subtracting the mean BA of a background annulus and spectral high-pass filtering. Specifically we set the minimum degree of expansion, lmin, to (2πR0)/(n·D) where R0 is the lunar radius, D is the crater diameter, and n is an integer between 3 and 6. We find that by scaling the background annulus and n as shown in Table 1, the two methods are in very close agreement. Table 1

Background Annulus

n

1.0r ↔ 1.5r

3

1.5r ↔ 2.0r

4

2.0r ↔ 2.5r

5

2.5r ↔ 3.0r

6

Monte Carlo simulations are used to test the hypothesis that the crater Bouguer anomalies are due to the pre-existing density structure of the crust or processing artifacts. These simulations have shown that the magnitude of a Bouguer anomaly (by either method of background signal removal) scales with the diameter used and is not diagnostic of the cratering process. We infer this behavior is due to the strong redness (power increases with wavelength) of the Bouguer spectrum. We have also found robust regional differences, including a high predominance of craters with associated negative Bouguer anomalies in South Pole Aitken basin. Phillips, R. J., et al. "Bouguer Gravity Anomalies Associated with Lunar Craters: Initial Results from the GRAIL Mission." AGU Fall Meeting 2012.