Porosity and hydrous alteration of the Martian crust from InSight seismic data

The composition and layering of the Martian crust provide important con- straints on planetary crustal evolution as well as on present-day conditions, e.g., with regard to the presence of liquid water or ice. The seismic data of the InSight mission yielded new and critical information on crustal structure at several locations on Mars. Here, we use rock physical models to investigate the range of lithologies, porosities and alteration scenarios compatible with seismic P- and S-wave velocities as well as vP /vS ratios from InSight. We find that present-day crustal porosity extends to 20-25 km depth at all sam- pled locations, with large Noachian impacts as main drivers for the creation of porosity, and viscous pore closure as likely agent of removal of porosity at depth. Heterogeneous seismic velocities can be related to differences in porosity that could be caused by subsequent localized magmatic activity. A felsic component to the crust, e.g. due to impact-generated buoyant par- tial melts, cannot be excluded, but would not be present globally. At the InSight landing site, hydrated minerals as traces of aqueous alteration are present throughout the crust, though the water within these minerals could be fairly limited at 0.3 wt% or less. The most likely types of minerals are also consistent with a post-depositional environment that was limited in water. Seismic and geological constraints for the depth range of approximately 200 m to 2000 m beneath the lander strongly favour basaltic Noachian sediments saturated with a mixture of up to 10% ice and brine. However, the lateral extent of this present day aquifer is not constrained by the available data.