Magnetic Field Signatures of Intermediate-sized Impact Craters on Mars

Magnetic field signatures over impact craters provide constraints for the history of the Martian dynamo. Due to limitations of the spatial resolution of magnetic field models, previous studies primarily focused on large impact craters (mostly >= 500 km in diameter). To fill the impact crater age gaps of previous studies, we investigate the magnetic field signature of 23 intermediate-sized craters (150-500 km in diameter) on Mars using both MAVEN data and a magnetic field model. Ten impact craters located in the South Province, the unmagnetized primordial crust, exhibit no or weak magnetic field signatures. The other 13 impact craters produce stronger magnetic anomalies, with the ratio of the averaged magnetic field inside and outside the craters (B in/B out) ranging from 0.4 to 1.2. The B in/B out values exhibit correlation coefficients of -0.54, -0.57, and -0.69 with the diameters of craters, calculated from the MAVEN data, the crustal field model at the surface, and 150 km altitude, respectively. A B in/B out larger than 1.0 usually appears in craters with smaller diameters, which is also demonstrated by the forward modeling in this study. Furthermore, the results of the forward modeling indicate that the craters of stronger magnetizations show a larger B in/B out. According to this, the Martian dynamo can be associated with the magnetization of craters of different ages, and the characteristic time of the dynamo can be limited. Our study supports the hypothesis that the Martian dynamo weakened or ceased at similar to 4.0 Ga and a late dynamo was perhaps active at similar to 3.7 Ga.