The absence of crustal magnetic fields above the martian basins Hellas, Argyre, and Isidis is often interpreted as proof of an early, before 4.1 billion years (Ga) ago, or late, after 3.9 Ga ago, dynamo. We revisit these interpretations using new MAVEN magnetic field data. Weak fields are present over the 4.5-Ga old Borealis basin, with the transition to strong fields correlated with the basin edge. Magnetic fields, confined to a near-surface layer, are also detected above the 3.7-Ga old Lucus Planum. We conclude that a dynamo was present both before and after the formation of the basins Hellas, Utopia, Argyre, and Isidis. A long-lived, Earth-like dynamo is consistent with the absence of magnetization within large basins if the impacts excavated large portions of strongly magnetic crust and exposed deeper material with lower concentrations of magnetic minerals.
Bibliographical noteFunding Information:
We acknowledge support from the Natural Sciences and Engineering Research Council of Canada and the Canadian Space Agency (A.M. and C.L.J.). B.L. was partly supported by CNES in the frame of the InSight mission and by the NEWTON project, funded by the European Union's Horizon 2020 research and innovation program under grant agreement 730041. C.L.J. also thanks the Green Foundation for Earth Sciences for support during a leave at the Institute of Geophysics and Planetary Physics, 2019-2020.