Study of impact demagnetization at Mars using Monte Carlo modeling and multiple altitude data

摘要

The magnetic field signatures of large demagnetized impact basins on Mars offer a unique opportunity to study the magnetic properties of the crust and the processes of basin formation and impact shock demagnetization. We present a framework for determining the effects on such signatures due to the dominant direction, strength, thickness, and vertical and horizontal coherence wavelengths of the surrounding crustal magnetization, as well as the demagnetization radius and the width of the demagnetization gradient zone caused by impact shock. By comparing model results with observed magnetic field profiles at 185 km and 400 km over the five largest apparently demagnetized impact structures, we find that (1) the dominant lateral size of coherently magnetized regions of crust falls in the range ～325 km to 600 km, (2) the magnetic field observed over a circular demagnetized region is such that clear demagnetization signatures should only be visible in magnetic field maps at 185 km and 400 km altitude for demagnetization diameters larger than ～600 km and ～1000 km, respectively, (3) demagnetization radii can be meaningfully constrained despite relatively poor constraints on associated demagnetization gradient zone widths, (4) the ratio of demagnetization diameter to the outer topographic ring diameter is close to 0.8 for the Isidis, Hellas, Argyre, and Utopia basins, suggesting that similar basin-forming and shock demagnetization processes occurred in each of these four ancient impacts, and (5) if used in conjunction with impact simulations, such modeling may lead to improved constraints on peak pressure contours and impact energies for these basins.