Magnetic anomalies on the young craters of Mercury
The crustal magnetic field of Mercury overlapped on the geological map of the Stieglitz crater. Credits: V. Galluzzi et al./Geophysical Research Letters
Rome, Tuesday 24 February 2021 - It is possible to find a point of convergence between geophysics and planetary geology, and a group of researchers led by Valentina Galluzzi, a young researcher from the Italian National Institute for Astrophysics (INAF), did so by analyzing the crustal magnetic field of the planet Mercury, focusing on some anomalies identified nearby two recently formed craters. Their peculiarity is that, while corresponding to the craters, these anomalies are not perfectly centered on them, but they are asymmetrical. The results of this study have been published online in Geophysical Research Letters.
The crust of rocky planets (Earth, for example) may include magnetic elements such as iron, that can register the local magnetic field. It is also known that Mercury’s surface has a low iron abundance. The first author of the study explains: "From a geological point of view, we wanted to verify the possibility that these anomalies were generated by magnetic elements like iron carried by impactors. We verified it by analyzing the impact dynamics that helped us understand the direction and angle of the meteorite that formed the craters. This type of analysis can be performed on young craters, whose geological characteristics are well preserved and allow to establish the downrange zone, the region of the crater located in the impact direction, which typically shows appreciable topographic and textural variations".
The craters featured in the article are Stieglitz and Rustaveli, two young impact craters on Mercury associated with overlapping crustal magnetic anomalies: these are the only two craters that allowed this kind of comparison. The study of Stieglitz and Rustaveli craters allowed the team led by INAF to identify a series of stagnations of molten material in the downrange direction. “This molten material creates the only morphological asymmetry clearly visible for the two craters. Asymmetric magnetic anomalies lie off-center in exactly the same direction. An additional geophysical analysis was necessary to identify the exact position of the magnetized material (considering an error at the surface of 30 km, given by the resolution gap between the anomalies maps and the base maps used for cartography). The magnetic dipoles thus obtained are placed close to the molten material, in the downrange region”, Galluzzi continues.
Other authors already observed on the Moon that some magnetic elements could be brought to a planetary surface by the impactors, but for the first time, the results offer evidence that the magnetic elements were brought by the impactors to Mercury. Galluzzi concludes: “On Mercury, as also observed on the Moon, impacts are one of the causes of the existence of these localized anomalies. The fusion of the impactor made of magnetic elements and its consequent cooling process allow to permanently register the magnetic field of the planet, hence the anomaly, in the rock. This also allows us to confirm that Mercury's magnetic dynamo was active at the time these impacts occurred (less than 1.7 billion years ago)”.
Other helpful data for this type of study (such as new details on the topography of the impact melt areas) will be provided in the future by the BepiColombo mission (ESA/JAXA), for which the authors of this study are working.
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