Assessment of Geophysical Methods for Groundwater Detection and Characterization on Mars

Renewed interest in accessing the martian deep subsurface calls for a review of geophysical methods to identify prospective locations for groundwater and to guide the drill. Clifford et al. (JGR, 2010) estimated tropical cryospheric thicknesses 0-9 km for melting temperatures 252 K and higher: the median was 3-4 km. Single-station geophysical methods to search for groundwater are ranked by their ability to penetrate to at least this depth and by their sensitivity to a model aquifer.

Ground-Penetrating Radar (GPR) provides high resolution at low power. However, penetration on Mars is poor except in ice- or ash-rich deposits. Even low-frequency surface radars are unlikely to image deeper than a few km. Aquifer reflectivity power is 3% and GPR cannot fully probe conductive, saline aquifers. Single-station methods have been demonstrated only on glaciers and not in complex geological environments.

Seismology can assess fluid content by the ratio of compressional to shear wave velocities; this has 40% contrast for the model aquifer and reflectivity power 3%. A single seismometer can use distant marsquakes to construct P and S velocity profiles in the crust from multiple mode conversions (receiver functions). The mode-converted power reflectivity at the top of the model aquifer is 0.3%. This method can be tested, in principle, by InSight.

Low-frequency electromagnetic (EM) methods have very high sensitivity to groundwater, increasing with salinity: aquifer reflectivity power is 20%, 65%, and 85% for salinities of 1, 35, and 360 g/l, respectively, assuming a background conductivity 0.001 S/m. Furthermore, skin depths can be large enough to fully penetrate and thus jointly characterize aquifer salinity and thickness. A single ground station measuring both electric and magnetic fields ( 2 kg) is sufficient, provided useful waves are propagated to the surface from the ionosphere. Alternatively, magnetometers alone are sufficient if fields are measured simultaneously at the surface and in the ionosphere: this method, too, can be tested by joint analysis of InSight and MAVEN data. If ambient energy is weak, a transmitter can be used (<10 kg) to probe to depths of several km.

Saline groundwater on Mars is an ideal EM exploration target and thus EM methods are recommended for first-generation groundwater reconnaissance.