Thermophysical arguments against basal melting in the south polar region of Mars

Abstract

The south polar layered deposits (SPLD) on Mars is primarily composed of H2O ice, with smaller quantities of impurities such as CO2 ice and dust. The presence of these impurities can affect the thermal structure of the SPLD, increasing the basal temperature and potentially leading to basal melting. Recently, bright basal reflectors in radargrams from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) of the SPLD at Ultimi Scopuli have been interpreted as evidence of a briny subglacial liquid water layer. However, this interpretation is difficult to reconcile with the low Martian geothermal heat flow and the frigid surface temperature at the Martian south pole. Here, we conduct a comprehensive thermophysical evolution modeling analysis to investigate the conditions that could enable basal melting at Ultimi Scopuli. Using numerical models that describe time-dependent firn densification and the thermal properties of porous solid mixtures, we first constrain the thermophysical parameters of the SPLD. We then run high-resolution thermal models that account for heat transfer and phase change to ascertain if basal melting can occur. Even when considering SPLD at Ultimi Scopuli to consist of 30% dust, basal melting does not occur. Considering the average surface temperature of the SPLD to be between 160 and 170 K and a maximum surface heat flow of 30 mW m−2, we find that the bulk thermal conductivity of the SPLD needs to be <1.5 W m−1 K−1 for basal melting to occur at Ultimi Scopuli. The combination of H2O ice, CO2 ice, and dust that can yield kbulk <1.5 W m−1 K−1 is at odds with current constraints on the composition of the SPLD. We also model the average dimension of a magmatic intrusion that can enable basal melting at Ultimi Scopuli. We find that an intrusion must be <15 km from the surface and at least 5 km wide to create a 20–30 km wide subglacial liquid water layer. We also find that subglacial liquid water sustained by magmatic intrusion will eventually refreeze over 1–2 million years unless continually sustained by newer intrusions. Given the difficulty in forming and sustaining subglacial liquid water on Mars, we suggest that alternative interpretations of the bright radar reflections merit additional consideration.