On the secular retention of ground water and ice on Mars

Abstract

Tropical ground ice on Mars undergoes long‐term sublimation and likely exospheric escape. Without restriction of sublimation, the cryosphere would eventually breach, leading to massive loss of any underlying groundwater. We seek to understand the conditions under which the ground‐ice seal, groundwater, and subsurface habitability are preserved. Using multireservoir models for the evolution of deuterium‐to‐hydrogen ratios, we derive a median estimate of the Hesperian‐Amazonian H2O loss of 60 m (interquartile range 30–120 m) global equivalent layer (GEL), neglecting magmatic degassing. Most of the loss may have been early, with consequent low loss in the “modern” cold and dry climate. We modeled global H2O transport within Mars following an assumed abrupt transition to modern conditions at 3 Ga. Sublimation is retarded (in order of decreasing priority) by higher obliquity, smaller porosity, higher tortuosity, lower heat flow, and smaller pore radius. Higher obliquity reduces H2O loss by decreasing tropical surface temperatures and raising global atmospheric water vapor. Time‐variable obliquities do not overly influence outcomes as long as the total duration <30° obliquity is <1 Gyr, with no interval longer than a few hundred million years. Decreased solar luminosity or a thicker CO2 atmosphere can also retard loss of subsurface H2O, whereas lower atmospheric water vapor accelerates sublimation. If Mars' post‐Noachian crustal H2O inventory was a few hundred meters GEL or more, then the modest loss since then implies that the cryospheric seal has been maintained following top down freezing and that groundwater likely exists globally on Mars today.

Squeezing Marsquakes Out of Groundwater

“…Freezing of aquifers may allow pore pressure to approach lithostatic pressure at the base of the cryosphere and hence to rupture the cryosphere, leading to groundwater discharge on the Martian surface (e.g., Gaidos, 2001;Wang et al, 2006). It remains uncertain, however, whether pressurization of Martian aquifers by gradual freezing can create sufficiently high pore pressure to rupture the cryosphere (Hanna & Phillips, 2005), though water loss has been low enough that groundwater should at least persist globally (Grimm et al, 2017). Pressure in aquifers confined by a cryosphere may also be elevated if they are recharged at higher elevation (e.g., Andrews-Hanna & Lewis, 2011;Harrison & Grimm, 2004).…”

The Density of the Medusae Fossae Formation: Implications for its Composition, Origin, and Importance in Martian History

“…A lack of ice in the MFF is not surprising, as ice is highly unstable at the surface in the equatorial latitudes of Mars (Mellon & Jakosky, 1993). Any remnant subsurface ice should have sublimated on a much shorter time scale than the depositional age of the MFF (Mellon & Jakosky, 1993), unless it is buried at great depths or covered by an impermeable layer (e.g., Grimm et al, 2017).…”

A Maximum Subsurface Biomass on Mars from Untapped Free Energy: CO and H2as Potential Antibiosignatures

“…The lower latitudes will be desiccated due to the instability of ground ice with respect to the water vapor content of the atmosphere (Clifford et al 2010;Dundas et al 2014;Feldman et al 2004). Our calculations are relevant to the low to mid-latitudes because subsurface aquifers could be stable there over geological timescales (Grimm et al 2017;Grimm and Painter 2009). As subsurface life presumably requires a liquid groundwater table, which is typically estimated around ≥5 km depth (Clifford et al 2010), the potential diffusive flux may be smaller than our calculated maximum biogenic surface sinks at these depths and further limit the maximum allowable biomass unless a minimum water activity level for life is maintained by a minimum H2O layer thickness in the pore space somehow.…”