Vashan Wright, UCSD / Scripps Institute of Geophysics and Planetary Physics

Searching for Evidence of Water in the Martian Crust

Liquid water existed on Mars in rivers, lakes, and as groundwater. The water is hypothesized to have subsequently been buried as ice or liquid water, incorporated in minerals, or lost to space. Geophysical measurements have the potential to test the first of these three hypotheses because water affects the physical properties of rocks, such as elastic moduli and bulk density. We use a forward modeling approach and a Markov Chain Monte Carlo inversion scheme to identify combinations of rock type, water distribution, porosity, and pore shape consistent with the seismic velocities and gravity within 50 km of the InSight lander on Mars. The shear Vs and compression Vp wave velocities within the upper 300 m are consistent with a crust composed of minimally cemented (< 2 % of the pores) sediments. Fractured rocks at this depth could host ice within 20% of the pore space. Vs within the upper 8 km of the crust are too low for a fully ice-saturated cryosphere. Using Vs, lithology, and gravity-derived bulk density data, we find that the lower crust (11.5-22 km below the surface) beneath InSight is mafic and highly porous or felsic and less porous. The addition of Vp data leads to the conclusion that a lower crust composed of igneous rock with thin fractures filled with liquid water can explain the existing data, though available seismic attenuation data provide a challenge to these findings. Our results have implications for understanding Mars’ water cycle from the Noachian to the present, determining the fates of past surface water, searching for past or extant life, and assessing in-situ resource utilization for future missions.