Timing the Martian Dynamo II: Impact Basins, Edge Effects and Volcanic Edifices

The discovery by Mars Global Surveyor of strongly magnetized regions within Mars' crust, demonstrated the past existence of an Earth-like dynamo operating in its core and which is now extinct. Because of its intensity Mars' crustal magnetization is attributed to thermoremanence (TRM) as the molten material cooled in the presence of the field. A determination of the epoch when this dynamo ceased to operate provides a strong constraint for models of the interior of Mars and its thermal evolution. The close correlation that exists between the magnetized terrain and its age, derived from the cratering record and accepted Lunar/Martian chronology estimates, strongly suggests that the dynamo had ceased to operate when the northern lowlands and dichotomy boundary were formed. Moreover, the absence of magnetization within the Hellas, Argyre and Isidis impact basins also suggests that the dynamo was extinct when they were formed, very early in the planet's history. Did the Mars' dynamo die and restart at a later epoch when an inner core solidified as some authors have suggested based on Lunar paleomagnetic data? This paper will review the different arguments and present analyses of Mars Global Surveyor, Lunar Prospector and paleomagnetic data which support (or not) the interpretations.