Groundwater Production from Geothermal Heating on Early Mars as a Potential Solution to the Faint Young Sun Paradox

Abstract:

In explaining extensive evidence for past liquid water, the debate on whether Mars was primarily warm and wet or cold and arid 4-Ga ago has continued for decades. The view of long-term warm and wet surface conditions on Mars is problematic, if not incompatible, with our present-day understanding of the early Martian climate powered by a faint young Sun. An alternative view posits that Mars was mainly frozen, with aquifer discharge or intermittent snow and ice melt by high geothermal heat flow as the main source of liquid water. However, the feasibility of basal melting on Noachian Mars remains mostly unknown as a function of the surface temperature, thickness of the ice sheets, and surface heat flow. Here, we employ thermophysical ice evolution models to ascertain the thickness of the ice and surface heat flow required for basal melting. Once those parameters are estimated, we assess if surface heat flow necessary for basal melting would have been available in Noachian Mars. We use the latest geochemical maps (i.e., derived from Gamma Ray Spectrometer data), geophysical data (gravity derived crustal thickness models), and estimates of the present-day mantle heat flux to provide a first order estimate of the Noachian surface heat flow. As in prior works, we use the current decimeters-deep regional bulk chemistry to represent its ancient counterpart. For the primarily cold and arid (mean Ts = 230 K) Noachian Mars model, we first estimate the volume of liquid water from basal melting of globally deposited ice sheets by our estimated Noachian geothermal heat flow. We show that readily realizable geothermal heat flux, more than 60 mW m^-2, could have significantly melted the base of ice sheets thicker than 1.5 km 4-Ga ago providing a possible solution to the faint young Sun paradox. Additionally, we show that geothermal heat flow in the shallow subsurface of Mars could have sustained hydrothermal alteration regardless of the Noachian climate.