Impact demagnetization at the moon and Mars: new results from hydrocode simulations and multiple altitude magnetic field data

The magnetic field signatures of large demagnetized impact basins on the moon and Mars offer a unique opportunity to study the magnetic properties of the crust and the processes of basin formation and impact shock demagnetization. The prime determining factors for the post-impact magnetic field signature are: 1) the energy of the impact (and hence the volume of crust excavated and location of peak shock pressure and temperature contours), 2) the manner in which the crustal minerals demagnetize under shock pressure and 3) the statistical properties of the pre-existing crustal magnetization including its dominant direction, strength, thickness and vertical and horizontal coherence wavelengths. We present a framework for calculating radial magnetic field profiles measured above an impact-demagnetized basin for a range of impact and crustal magnetization properties. Due to inherent nonuniqueness in the relationship between crustal magnetization and measured magnetic field, we opt for a statistical approach, comparing/fitting the resulting radial magnetic field profiles with azimuthally averaged crustal magnetic field magnitude measured at multiple altitudes by the Mars Global Surveyor and Lunar Prospector Electron Reflectometer and Magnetometer experiments respectively. We find that the magnetic signature in and near the Argyre basin on Mars can be explained by: 1) a dominant lateral size of coherently magnetized pre-impact crust of ~500 km (magnetization wavelength of ~1000 km), 2) a 2.4 x 10^27 J impact event and 3) a magnetic mineral that does not demagnetize appreciably when subject to shock pressures below ~2 GPa. We will present analogous results for several more Martian and lunar basins. Post-impact temperature (a) and pressure (b) and pressure-demagnetization curves (f) leads to demagnetization (c, d), from which we calculate azimuthally-averaged radial magnetic field profiles which are fit to magnetic field data at 185 km and 400 km near the Argyre basin (e).