Depth of the Martian cryosphere: Revised estimates and implications for the existence and detection of subpermafrost groundwater

The Martian cryosphere is defined as that region of the crust where the temperature remains continuously below the freezing point of water. Previous estimates of its present thickness have ranged from ∼2.3–4.7 km at the equator to ∼6.5–12.5 km at the poles. Here we revisit these calculations, review some of the assumptions on which they were based, and investigate the effects of several parameters, not previously considered, on the cryosphere's thermal evolution and extent. These include astronomically driven climate change, the temperature‐dependent thermal properties of an ice‐rich crust, the potential presence of gas hydrate and perchlorate‐saturated groundwater, and consideration of recent lower estimates of present‐day global heat flow (which suggest a mean value roughly half that previously thought, effectively doubling the potential thickness of frozen ground). The implications of these findings for the continued survival of subpermafrost groundwater and its potential detection by the MARSIS radar sounder onboard Mars Express are then discussed. Although our estimates of the maximum potential thickness of the cryosphere have significantly increased, consideration of the likely range and spatial variability of crustal heat flow and thermal properties, in combination with the potential presence of potent freezing point depressing salts, may result in substantial local variations in cryosphere thickness. The locations that appear best suited for the detection of groundwater are those that combine low latitude (minimizing the thickness of frozen ground) and low elevation (minimizing the depth to a water table in hydrostatic equilibrium). Preliminary results from a MARSIS investigation of one such area are discussed.