Rapid decrease in Martian crustal magnetization in the Noachian era: Implications for the dynamo and climate of early Mars

The magnetic signatures and crater retention ages of the 19 largest (>1000 km diameter) impact basins on Mars are examined to constrain the history of the acquisition of crustal magnetization during the Noachian era. The 5 most clearly impact‐demagnetized basins are younger than the 14 basins within which lies substantially re‐magnetized crust. Poisson analysis shows that the most likely time of this magnetization cessation was 4.115–4.13 Ga (model age) and that it occurred quickly, taking less than 20 Ma. A global decrease in effective crustal magnetic susceptibility due, e.g., to a decrease in the rate of hydrothermal alteration, is one possible explanation. Alternatively, the cessation of post‐impact magnetization reflects the rapid death of the Martian dynamo.