Timing of the Martian core dynamo

Abstract

Using the radial component magnetic data acquired by Mars Global Surveyor during its mapping phase, a reliable magnetic anomaly map of Mars is derived. Many weak but reliable magnetic anomalies are now detected in the northern lowlands. Their source bodies have likely been partially demagnetized by the lowlands formation processes, and there has been no core field in the later times to remagnetize the bodies. No consistent anomalies are observed in the Hellas basin, implying that the core dynamo has been inactive at least since the impact event. The topography, the positive Bouguer anomaly at the center, and the strong negative Bouguer anomalies over the outskirts of Thaumasia plateau suggest that the plateau was the site of a large impact basin prior to the formation of the plateau. The lack of magnetic anomalies in the central part and the strong magnetic anomalies over the outskirts of the plateau resemble the magnetic signature of Hellas basin, implying a preexisting giant impact basin beneath the plateau and the absence of the core field since the impact time. The lack of magnetic edge effects of Valles Marineris, the absence of thermal demagnetization of Tharsis plain by the shield volcanoes, and the substantial distance between the paleomagnetic poles and the present rotation axis imply that a major part of Tharsis bulge was formed in the absence of the core field.