Asymmetric magnetic anomalies over fresh impact craters on Mercury

Magnetic field data for Mercury acquired by MESSENGER at low altitudes show that portions of the crust are magnetized. Several isolated magnetic anomalies are found to correlate with complex impact craters. Mercury's surface has a low iron abundance, which is consistent with the hypothesis that some impactors brought magnetic carriers to account for the crustal magnetic fields. However, some of the crater-related magnetic anomalies are slightly or totally asymmetric with respect to their corresponding crater's center. Here, an analysis of magnetic anomaly morphology and the impact crater geomorphology is carried out to seek geological evidence of any correlation between the impactor materials and the anomalies. We focus on the two complex craters, Rustaveli (peak-ring) and Stieglitz (central peak). These fresh craters retain a well-preserved ejecta blanket and visible secondary crater chains. By analyzing the slight asymmetries in the ejecta distribution, we find that the location of the impact melt is well-correlated with magnetic anomaly morphology. However, we also note that both impact angles were likely > 45° above horizontal, for which no preferential distribution of the melt is expected. Instead, the melt possibly migrated to the topographic lows during crater formation and probably recorded the past magnetic field of Mercury after quenching.

The first author acknowledges funding from the Italian Space Agency (ASI) under ASI-INAF agreement 2017-47-H.0.