Magnetic Signatures of Martian Volcanoes: Evidence for a Second Dynamo Episode?

Using electron reflection magnetometry, we investigate the magnetic anomaly signatures of 20 Martian volcanoes. Most show magnetic lows characteristic of thermal demagnetization of crust in the presence of no significant global magnetic field. However, one of the oldest volcanoes, Hadriaca Patera, appears to be a magnetic source, suggesting thermoremanent magnetization following its last major period of magmatism. We investigate the likelihood that a global magnetic field was present at this time and find a distinct possibility that such a field was present. We compare the relative ages (according to the Hartmann & Neukum cratering time-scale) and magnetic signatures of these 20 volcanoes and 7 giant impact basins and conclude that, ~300 Myr after the cessation of an early dynamo, a second dynamo episode may have started around ~3.85 Gyr ago and lasted for 100-350 Myr. We speculate that the second episode may have been due to one or more large mantle plumes that, in removing heat from the lower mantle, temporarily increased the core-mantle thermal gradient to restart the core dynamo. We discuss possible geophysical causes for such plumes and consider the implications of a second dynamo, and its associated magnetic shielding, for astrobiology.