Oxygenated brines on Mars/Guide to paper
(hidden from indexing) 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 colour-coding is different for the two maps). The dotted lines in that diagram show the limit for sponges, roughly 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 poles are regions that could have oxygen solubilities similar to Earth's oceans.
- "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 harbour 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."
The effects of the variations in the tilt of Mars' axis (obliquity) are summarized in their figure 4. Their figure 4a shows how the oxygen solubility varies with the tilt. The global maximum for supercooling and the best estimate is shown as a red line with purple dots and shows potential for sponges at angles up to around 45 degrees. Then figure 4c shows the effect of the variation in tilt for the last 20 million years and the next ten million years. The supercooling best estimate is shown at the top, the gray shaded boxes are times of atmospheric collapse. They observe in the paper that oxygen solubility levels have been particularly high for the last five million years and will continue in the same way for at least ten million years. They have been high enough for simple sponges for at least twenty million years. You can see how this works from this figure.