Gravity constraints on the age and formation of the Moon's Reiner Gamma magnetic anomaly

The Moon once possessed a dynamo that produced numerous magnetic anomalies in the lunar lithosphere. Determining the ages and magnetization strengths of these anomalies would constrain the timing and strength of the dynamo. Unfortunately, most anomalies are not associated with dated geologic features. Here we report that the northeastern portion of the Reiner Gamma magnetic anomaly overlies a relative-negative Bouguer gravity anomaly that we interpret to be a buried impact crater (consistent with Evans et al., 2016). Reiner Gamma is associated with sinuous optical markings known as swirls, which are likely correlated with the underlying magnetization. These markings provide a higher resolution map of the magnetization than possible with existing field data. We present two arguments that suggest the crater overprinted by the Reiner Gamma swirl formed in a pre-existing flood basalt layer. Hence, we constrain the Reiner Gamma source body's age to be bounded by the approximate duration of mare volcanism, ~3.3 Ga to ~3.9 Ga. This range includes the putative high-field epoch of the Moon's dynamo (~3.56–3.9 Ga), which could explain Reiner Gamma's high magnetization. Finally, we calculate the potential range of positions of the rim of the buried crater, and find that the main body of Reiner Gamma is likely outside the buried rim. This finding may be useful for constraining the origin of the Reiner Gamma magnetic source bodies.