Constraining the Early History of Mercury and Its Core Dynamo by Studying the Crustal Magnetic Field

International audienceKey Points: 9 • We analyze crustal magnetic anomalies that are likely thermoremanent and ob-10 tain the corresponding paleopole positions. 11 • All best fitting paleopoles are found in the Southern Hemisphere. 12 • Our study strongly suggests that Mercury has evolved with time. Abstract 14 Low altitude magnetic field data acquired by MESSENGER over a small portion of Mer-15 cury's surface revealed weak crustal magnetic field signatures. Here we study the crustal 16 magnetic anomalies associated with impact craters on Mercury. We assume that the sources 17 of these anomalies consist of impact melt, enriched in impactor iron. We assume that 18 the subsurfaces of Mercury's impact craters have cooled in the presence of a constant 19 global magnetic field, thus becoming thermoremanently magnetized. We invert for the 20 crustal magnetization direction within five craters using a unidirectional magnetization 21 model which assumes that the melt impact rocks recorded the constant core magnetic 22 field present when the crater was formed, and that the crater's magnetization has not 23 been altered since its formation. From the best fitting magnetization direction we then 24 obtain the corresponding north magnetic paleopole position assuming a centered core 25 dipolar field. Results show that all five magnetic paleopoles lie in the Southern Hemi-26 sphere but are not required to be located near the present-day magnetic pole, which lies 27 near the south geographic pole. Accounting for the uncertainties, we show that our re-28 sults all agree in a common small region that excludes the current magnetic pole. This 29 strongly suggests that the dynamo has evolved with time. Our results represent valu-30 able information for understanding the evolution of Mercury, and emphasize the impor-31 tance of including more anomaly analyses to complete and refine our conclusions. 3