User:Robertinventor/O2 solubility in Martian near-surface environments and implications for aerobic life - notes
These are notes to help with background research for the article: Cold salty water on Mars may have enough oxygen for air breathing microbes, possibly even simple animals like sponges
It is a theoretical rather than an experimental study, based on previously published research about the properties of salty brines on the surface of Mars. The relevance of the salts is that they let the brines remain liquid at very low temperatures as low as 140 K, that's -133 C. Even though there is little oxygen in the atmosphere, such low temperatures make it easier for it to enter the water.
Some highlights from my notes[edit]
Some highlights from my notes, many of them not mentioned in the articles about their research or in the press release:
- 6.5% of the Mars surface, current Mars, would be able to support sponges with supercooling (a process that has been confirmed in experiments with brines mixed with regolith - the brines stay liquid right down to -133 C).
- Some areas even would have oxygen levels the same as for modern Earth oceans
- Mars has been tilted optimally for oxygen in the brines for the last 5 million years and will continue like that for at least another 10 million years.
- When the axial tilt is more than 45 degrees (which happens with Mars occasionally, leading to equatorial ice sheets and ice free poles) then the levels of oxygen are not high enough for sponges.
- For the last 20 million years there has always been enough oxygen for sponges according to their models, if the brines are there
- Modern Mars would have more oxygen in its brines than early Earth did before the Great Oxygenation event
- The oxygen in these brines may explain some weathering processes on Mars and even the methane seeps detected by Curiosity.
- They only studied processes for the top few cms of the Mars surface and oxygen mixing with water in those layers (below that, then the ground would be permanently frozen except at geological hot spots or subglacial lakes.
- They do not speculate about subglacial lakes so this is speculation by commentators on the article, not by the authors themselves and their research would not seem to apply directly to that situation so I think we shouldn't mention it, or if we do, as a question, not a suggestion.
Importance of oxygen for life[edit]
They open out by talking about why aerobic life (i.e. based on oxygen) is important
Existing research on brines[edit]
They then summmarize existing research on brines on Mars and on the oxygen levels needed for oxygen metabolism microbes (a millionth of a mole per cubic meter) and for primitive sponges (0.02 moles of oxygen per cubic meter).
Combining brine studies with a simplified general circulation model for Mars[edit]
They take this experimental and theoretical work about the brines and they combine it with a general circulation model for the Mars atmosphere. In this model they use temperatures averaged out throughout the day and year, ignoring seasonal and diurnal variations. So it is rather a simplified model but a first look at the situation there:
For the brines they go into a lot of theoretical detail about how they work out the solubility of oxygen into a brine taking account of the temperture and the various anions and cations in the water. This is theoretical, but it is also in agreement with experimental data - basically this section is justifying their model. I won't go into it in these notes. We can assume they got it right as it is straightforward non controversial chemistry and physics, and it passed peer review.
Eutectic mixtures and supercooling[edit]
They then look at eutectic mixtures and supercooling. Eutectics describe how if you mix salts with water then some of the salt may stay out of solution but the solution will automatically evolve to the optimal concentrations to stay liquid at the lowest possible temperatures as you cool it down. Salt moves in or out of solution as you cool it to keep it optimal.
Then with supercooling it can get even colder. Down to 140 K or -133 C. This is a quote from that section of the paper:
They then give evidence that eutectic mixtures can form on Mars and may for instance be present in the recurring slope lineae - and that even when mixed with regolith (as they would be) can still be supercooled down to that 140 K or -133 C.
Levels of oxygen enough for microbes everywhere - levels for sponges with supercooling near the poles and can even reach concentrations typical of sea water[edit]
They do two versions of their calculations, with, or without supercooling. In both calculations they find that all the brines on Mars, no matter where they are, can potentially have oxygen levels high enough for microbes that use oxygen.
The question then is whether it can ever reach the 0.02 moles per cubic meter needed for sponges. They find that without supercooling it never reaches high enough concentrations for this. The best value at the south pole, without supercooling, is about a five thousandth of a mole per cubic meter (from the colour coding of their Figure 1 b)
The text summary says
With supercooling they find that there are many oases where the oxygen levels on present day Mars can rise to levels high enough for sponges. There the thing about sponges is that they are made up of fine networks of sheets of cells. The salty water flows in and out of the sponge and the thin layers only one cell thick have salty oxygenated water passing by them on either side. So a small sponge a few mms in size can take in oxygen from the water rather readily. They cite a paper that says this can happen at 0.02 moles of oxygen per cubic meter. Not only that, the oxygen levels can even reach levels comparable to Earth's oceans today.
There the highest value there they give, 0.2 moles per cubic meter of dissolved O2 in polar regions, is comparable to the levels in modern oceans on Earth.
MY COMMENT HERE: Remarkable! Modern fish would in principle get enough oxygen to live there - if they could withstand the perchlorates and the ultra cold temperatures at night of course, which modern Earth life couldn't do.
6.5% of the Martian surface could support enough oxygen for primitive sponges in the oxygenated brines[edit]
They find that 6.5% of the surface of present day Mars could have oxygen levels high enough for aerobic respiration by primitive sponges:
This is where I assume they mean that the brines absorb the oxygen while supercooled and then retain it as they warm up to more habitable temperatures because the temperatures they consider, -133 C are very low, usually the limit for Earth life is -20 C. Alternative biochemistries could get to much lower temperatures by using a metabolism with perchlorates inside the cells, and hydrogen peroxide but they don't have anything to suggest they are talking about such biochemistries here.
Better for oxygen than early Earth before the great oxygenation event[edit]
They make a comparison with Early Earth before the "Great Oxygenation Event" that added oxygen to our atmosphere, and they find that Modern Mars, very surprisingly, will have better oxygenated brines than early Earth. Mainly because of the cold conditions there and the way that the perchlorates can be liquid at such low temperatures.
Chemical weathering[edit]
They comment that at the levels they calculated, oxygen in the brines could contributed to the chemical weathering of rocks, and also the release of methane, both surprising recent and hard to explain observations on Mars.
Mars' axis is currently optimally tilted for oxygen rich brines[edit]
They find that Mars currently has the potential for far higher oxygen levels than normal. For the last 5 million years they would have been particularly oxygen rich. The 15 million years before then would have seen oxygen levels 200 times lower than they are today. This is due to changing tilt of the Mars axis (our axis is stabilized by the Moon while Mars soemtimes tilts so far, beyond 45 degrees tilt, that it has equatorial ice sheets and ice free poles).
They show that when tilted by more than around 45 degrees - when Mars loses its polar ice caps and its atmosphere becomes much thicker, then the oxygen levels are no longer high enough for sponges. However this hasn't happened in the last 20 million years. Projecting forwards, it should continue at its current high levels similarly to the last 5 million years for at least 10 million years into the future (from their figure 4).
They conclude that this is a new way to get levels of oxygen on a planet that are high enough for aerobes, without the need for photosynthetic life to create it.