Magnetic Properties of Martian Impact Basins Including the Effect of Mantle Excavation

Abstract The weak magnetism of many impact basins on Mars has historically been interpreted as evidence of the martian dynamo's cessation before ∼4.0 Ga. However, expected magnetic properties of these impact basins are poorly known because basin formation exhumes and recrystallizes mantle material, which has not been sampled by martian meteorites. Potential depletion of magnetic minerals in these mantle‐derived rocks, as observed when comparing terrestrial mantle xenoliths to typical crustal material, may lead to weak magnetization even in the presence of a strong, stable dynamo. A reversing or intermittently active dynamo may also permit higher magnetic mineral concentrations while remaining consistent with the weak detected magnetic field signal. To explore the relationship between ancient martian dynamo behavior and orbitally observed magnetic fields, we quantify the expected magnetic properties of large impact basins by modeling the crystallization of candidate impact melt compositions. Our results suggest maximum impact basin magnetization intensities in the 0.5–4 A/m range assuming a non‐reversing dynamo. Although these values are low compared to the 1–20 A/m range expected for unperturbed martian crust, they would still yield stronger magnetic fields than those observed above the large martian impact basins Hellas, Utopia, Isidis, and Argyre. This ultimately suggests that a reversing or intermittent dynamo may be necessary to explain the weak magnetism of these martian impact basins. A martian dynamo which persisted until 3.9 to 3.7 Ga and reversed at rates >5 Myr −1 remains the most straightforward explanation for all available orbital magnetic observations and paleomagnetic measurements.