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Keywords:

  • Mars;
  • dynamo;
  • crater;
  • magnetic;
  • age;
  • history

Abstract

[1] Large impacts simultaneously reset both the surface age and the magnetization of the entire depth of crust over areas comparable to the final size of the resulting craters. These properties make large impact craters (>300 km in diameter) ideal “magnetic markers” for constraining the history of the Martian core dynamo. However, the relationship between crustal magnetization and magnetic field measured in orbit is nonunique, making the measured magnetic field signature of an impact crater only a proxy for the magnetization (or lack thereof) below. Using Monte Carlo Fourier domain modeling of subsurface magnetization, we calculate probability distributions of the magnetic field signatures of partially and completely demagnetized craters. We compare these distributions to measured magnetic field signatures of 41 old impact craters on Mars larger than 300 km in diameter and calculate probabilities of their magnetization state. We compare these probabilities to cratering densities and absolute model ages and in this manner arrive at a robust time history of Martian large-crater magnetization and hence of the Martian dynamo. We conclude that the most likely scenario was a Mars dynamo active when the oldest detectable basins formed, ceasing before the Hellas and Utopia impacts, between 4.0 and 4.1 Ga (in model age) and not thereafter restarting. The Mars atmosphere was thereafter exposed directly to erosion by the solar wind, significantly altering the path of climate evolution. Further improvements to the history of the Martian dynamo will require better crater age estimates and lower altitude magnetic field data.