Probing the source of ancient linear gravity anomalies on the Moon

A large set of linear to arcuate gravity anomalies which are not associated with any surface expressions was revealed by lunar gravity data from the GRAIL mission. The anomalies can be categorized into two types: a set of large anomalies that border the Procellarum KREEP terrane (PKT) region, and the linear gravity anomalies (LGAs) that are scattered throughout the lunar highlands. In this study, we use band-passed gravity gradient maps and localized power spectral analyses to characterize the nature of and the differences between the two types of anomalies. The results show that the PKT border anomalies are primarily long wavelength features whose power spectra are not clearly distinguishable from the background spectrum of the surrounding region, while the linear gravity anomalies are short wavelength features whose power spectra in at least one case rises significantly above the background spectrum over a wide range of degrees. This difference in gravitational signature suggests a fundamental difference in the nature of the sources of the two types of anomalies. We then used a Markov chain Monte Carlo model to test different interpretations for the most prominent LGA by comparing the observed power spectrum to modeled spectra for elliptical, triangular, and T-shaped intrusive geometries. We find multiple geometries are able to fit the power spectrum of the LGA equally well. Analogs of comparable scale to the LGAs originate from extensional stress regimes, supporting an inferred period of global expansion in the early Moon, though the cause of that expansion remains uncertain. If the depth extent of the intrusions were governed by neutral buoyancy, we find that intrusions on the nearside similar to those on the farside would have been eruptive and may have contributed to the earliest mare volcanism. The total volume of the intrusions is similar to 20% of the mare volume, revealing a lower magma production rate on the farside than on the nearside.

Automated summary

The GRAIL mission's lunar gravity data revealed a large set of linear to arcuate gravity anomalies on the Moon, categorized into anomalies bordering the Procellarum KREEP terrane (PKT) region and scattered linear gravity anomalies in the highlands. Analyzing band-passed gravity gradient maps and power spectral analyses helps characterize the nature and differences between these anomalies.