The stability of liquid-water below the South Polar Cap of Mars

The presence of a liquid deposit of water below the South Polar Layered Deposits (SPLD) Region has been reported based on analysis of  MARSIS radar data in the Planum Australe area (Orosei et al., 2018). These radar data show bright subsurface reflections that have been interpreted to be due to liquid water buried at a depth of 1.5 km (Orosei et al., 2018). The presence of such water would have important implications for the present-day thermal state of the region. Previous work based on the flexure of the lithosphere yielded a present surface heat flow of about 20–30 mW m^-2 (Ruiz et al., 2010; Parro et al., 2017) at the south polar cap. Global thermal evolution models that included variations in crustal thickness and heat production estimated a global surface heat flow of 23.2–27.3 mW m^-2 (Plesa et al. 2018). These estimations are difficult to reconcile with the high heat flows, in excess of 72 mW m^-2 (Sori and Bramson, 2019), that are required to explain the existence of liquid water at 1.5 km deep. In this work, we aim to re-analyze the thermal state of the region and the thermal properties that are required to stabilize liquid water under the SPLD.

In order to study the thermal conditions that are compatible with the liquid water deposit, we first recalculated the depth of the bright radar reflections using a temperature-dependent relative permittivity for the water ice (Fujita et al., 2000). The depth to the putative liquid water is important, as deeper depths require lower heat fluxes to reach the melting temperature, and vice versa. We obtained a new depth to the bright reflector of 1.7 km, assuming a surface temperature of 160 K and a melting temperature of 200 K, which is appropriate if calcium perchlorate is present in the area.

Then, we calculated surface heat flows and subsurface temperatures by solving the stationary heat conduction equation. We assume that the composition of the SPLD region is a mixture of 85% of water ice and 15% of dust with a density of 1220 kg m^-3 (Zuber at al., 2007). We use an ice thermal conductivity that is dependent on temperature following:

where T is the temperature in Kelvins (Fukusako, 1990). For the dust component, we assume a thermal conductivity of 2 W m^-1 K^-1, and calculate the thermal conductivity of the dust-ice mixture as a geometric mean (Beardsmore and Cull, 2001). We also include a superficial layer of CO₂ with a thickness of 1 m, a density of 1600 kg m^-3, and a thermal conductivity of 0.02 W m^-1 K^-1. We find that a surface heat flow of 61 mW m^-2 is needed to obtain melting at 1.7 km depth. This result is still higher than the values previously estimated from lithosphere flexure in the region, but somewhat lower than that reported by Sori and Branson (2019). Additionally, fractures or voids near the surface of the SPLD at this site could reduce the thermal conductivity of the region and lower the required heat flow even further. In order to account for this possibility, we calculated surface heat flows for additional models which include an intermediate layer of lower thermal conductivity placed between the CO₂ and the more conductive dust-ice mixture. Models with an intermediate insulating layer 15 m thick and with thermal conductivities between 0.05 and 0.1 W m K^-1 yield surface heat flows of 41 and 50 mW m^-2, respectively, to stabilize liquid water at 1.7 km depth. Further work will realize a more complete assessment of this location and of the regional heat flow context.

References

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