Low‐frequency electromagnetic exploration for groundwater on Mars

Water with even a small amount of dissolved solids has an electrical conductivity orders of magnitude higher than dry rock and is therefore a near‐ideal exploration target on Mars for low‐frequency, diffusive electromagnetic methods. Models of the temperature‐ and frequency‐dependent electrical properties of rock‐ice‐water mixtures are used to predict the electromagnetic response of the Martian subsurface. Detection of ice is difficult unless it is massively segregated. In contrast, liquid water profoundly affects soundings, and even a small amount of adsorbed water in the cryosphere can be detected. Subcryospheric water is readily distinguishable at frequencies as low as 100 Hz for fresh water to 10 mHz for brines. These responses can be measured using either natural or artificial sources. ULF signals from solar wind and diurnal‐heating perturbations of the ionosphere are likely, and disturbances of regional crustal magnetic fields may also be observable. Spherics, or ELF‐VLF signals from lightning discharge, would provide optimal soundings; however, lightning may be the least likely of the possible natural sources. Among the active techniques, only the time‐domain electromagnetic (TDEM) method can accommodate a closely spaced transmitter and receiver and sound to depths of hundreds of meters or more. A ground‐ or aircraft‐based TDEM system of several kilograms can detect water to a depth of several hundred meters, and a system of tens of kilograms featuring a large, fixed, rover‐ or ballistically deployed loop can detect water to several kilometers depth.