Magnetization signatures of craters on Mercury and the contribution of impact-delivered iron.

Magnetic field data from the MESSENGER mission have revealed the presence of crustal magnetization on Mercury, at least some of which is thought to have been acquired in an ancient field. Magnetic fields over craters are particularly interesting because they can elucidate the magnetic history of the planet (i.e., of its dynamo). If the magnetization is remanent, crater signatures, together with information on the relative age of the craters, can be used to constrain the history of the dynamo. At Mercury, craters with an enhanced interior magnetization compared with their surroundings can provide a record of an ancient global field that was stronger than the present-day field.

We show that the crater record at Mercury provides only limited information about the relative timing of magnetization acquisition. For Mercury, magnetic field signatures at craters have previously been examined for only the largest impact basins. Here, we use a crustal magnetization model and examine 202 craters over Mercury’s Northern Hemisphere with diameters 70 km < D < 400 km and center locations between 40° and 75° N, to investigate the presence (or absence) of clear magnetic signatures associated with them. The craters considered have a range of degradation states and hence relative ages: we investigate whether there are correlations between crater magnetic signatures and their degradation class. Although some individual craters suggest the existence of an ancient, stronger field, the results do not provide a clear picture of temporal evolution in strength of the dynamo field.

We also analyze craters in the two main geographical units in the Northern Hemisphere separately, the Inter-Crater Plains (ICP) and the Smooth Plains (SP), characterized by a different iron content. Overall, we find that 59/202 craters have an enhanced interior magnetization. We find that craters in the SP are more likely to have an enhanced magnetization. An alternative explanation to a strong ancient dynamo for strong magnetizations is local enhancements in iron content of the crust, of either endogenic or exogenic (impact-delivered) origin. Accordingly, we use impact scaling relationships to calculate magnetizations that can be acquired in the present-day field by impact-delivered iron.

We find that the magnetization at 66% of the enhanced craters can be explained by a local increase in crustal iron content delivered by a fully iron impactor. Furthermore 39% of enhanced craters do not require an iron-rich impactor: their magnetization can be explained by extra iron delivered by an impactor with 50% iron by mass. 

These results suggest that most of the enhanced magnetization can be acquired in the present-day field, and the overall low iron content of Mercury’s crust makes the effects of impact-delivered iron magnetically detectable especially in the SP.