Ocean worlds and Mars: A cosmochemical perspective on the liquid brines "problem"

Bright basal reflections detected on Mars by radar sounder MARSIS (Mars Advanced Radar for Subsurface and Ionospheric Sounding) have been interpreted to indicate the presence of liquid perchlorate brines [1-2] in Ultimi Scopuli (193°E; 81°S) a marginal area of the South Polar Layered Deposits (SPLD). This is the first (and only) report of extant bodies of liquid water on Mars, although this interpretation is not universally accepted. Other authors have suggested that the bright reflections may be caused by clays [3], hydrated salts [4], basalt [5], or that they are produced by constructive interference of radar waves [6-7]. These alternatives to the liquid brine interpretation have been investigated and found to be implausible [8-11].

We are not yet close to a definitive explanation of the mechanisms of formation and persistence of liquid brines in the Martian south polar regions, however. Even though basal temperatures could conservatively be estimated to be as high as 193 K [12], a value close to the eutectic temperature of Ca-perchlorate (198.5 K; [13]), the commonly accepted tenet that the south polar region of Mars is too cold for the presence of large bodies of liquid water [14] has not shifted. Liquid brines could form metastably at sub-eutectic temperatures, but it is not clear whether they could persist over geologically significant timescales [15]. Recent geophysical and petrological evidence points to a heterogeneous Martian interior and suggests the possibility of higher heat flows than previous estimates [16-17], but these results have not translated into recalculations of SPLD basal temperatures. The presence of chemical species that could act as antifreeze (such as ammonia or methanol; [18]) or of clathrate hydrates [19] has been proposed, but not yet adequately modeled because of lack of data.

Stimulated by the complexity and current paucity of geophysical evidence to progress further, we reframe the problem from a cosmochemical perspective: if briny oceans exist beneath the frozen crust of small planetary bodies in the outer solar system, under what circumstances could small and contained bodies of subglacial liquid water exist on Mars? Here, we consider data and models of:  solar system formation;  element condensation temperatures;  relationship between planetary noon temperature, gravity, and atmospheric composition; Mars’s volatile budget; chemical reaction cycles in the Martian atmosphere; atmosphere-lithosphere processes. We identify current gaps in data, and highlight future work to fill the gaps.

References. [1]10.1126/science.aar7268. [2]10.1038/s41550-020-1200-6. [3]10.1029/2021GL093618. [4]10.1029/2021GL093880. [5]10.1029/2021GL096518. [6]10.1038/s41550-022-01775-z. [7]10.1126/sciadv.adj9546. [8]10.1016/j.epsl.2022.117370. [9]10.1016/j.icarus.2022.115163. [10]10.1029/2022JE007398. [11]10.1029/2022JE007513. [12]10.1038/s41467-022-33389-4. [13]10.1007/s11167-005-0306-z. [14]10.1029/2020GL091409. [15]10.1073/pnas.2321067121. [16]10.1016/bs.agph.2022.07.005. [17]10.1029/2023GL103537. [18]10.1089/ast.2024.0075. [19]10.1002/2014RG000463.