Oxygenated brines on Mars/Guide to paper

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Technical details - guide to the paper
A few notes may help for those of you who wish to read the paper. You can read it via the link provided in the author's website through the Nature sharing initiative.

They explain that their work proceeded by modeling the physics of the solubility of oxygen in the magnesium / calcium perchlorate brines on Mars. Those were the mixes that permitted the lowest temperatures and so the highest oxygen concentrations. As mixtures of salts are cooled down, any excess of one or the other of the two salts will come out of solution leaving a "eutectic mixture" that has an optimal mix of calcium and magnesium perchlorates to keep the brines liquid at the lowest possible temperature or eutectic point.

The brines can be cooled down below this theoretical lowest temperature without freezing, in a process known as supercooling. Experiments with Mars soil (regolith) simulants show that even with soil mixed in with the brines, they can still be supercooled to temperatures as low as -123 to -133 °C before transitioning to a glassy state.

They studied oxygen solubility with and without supercooling. They also studied two ways of modeling the physics, a best case, which matches the available data within a few percent, and a worst case simulation which gives a thermodynamic lower limit, however their "best case" is most likely close to the actual situation on Mars, and in their Methods section they remark that


 * Our worst case provides the logic for a conservative lowermost bound on O2 solubility, and it is important to note that the true solution is probably much greater and closer to our best-estimate scenario.

The main points in their research are summarized in their figure 3 which shows two versions of the map, with and without supercooling. The upper figure is the one with supercooling (note the colourcoding is different for the two maps). The dotted lines in that diagram show the limit for sponges, above 75 degrees North and South. The paper says that 6.5% of the surface area of Mars could have oxygen concentrations suitable for primitive sponges. The white and purple coloured regions close to the south pole is a region that could have oxygen solubilities similar to Earth's oceans. The south pole region is extremely cold.


 * ''"For our best estimate (including supercooling), the results dis-play large gradients in O2 solubility for Ca- and Mg-perchlorates across Mars, with polar regions having the greatest potential to har-bour near-surface fluids at 2 ×  10−1 mol m−3 of dissolved O2, and the  least O2-rich environments in the tropical southern highlands  at ~2.5 ×   10−5 mol m−3 of dissolved O2. The O2 solubility of near-surface brines across Mars today could vary by five orders of magnitude. This trend results from lower temperatures at higher latitudes promoting O2 entry into brines.