Atmospheric oxygen both enables complex life and is a hallmark of it. Aerobic respiration is so energetically efficient that it's thought to have been needed for multicellularity. There is hardly any oxygen in the atmosphere surrounding Mars, raising questions about the prospects for complex life there.

But it's possible some oxygen is dissolved in any watery brines that could exist at or just below the Red Planet’s surface. Geologists at CalTech and Harvard have analyzed exactly how much oxygen these brines could hold and found that it might be just enough.

A few conditions made them think there was even a chance. First, the Martian atmosphere has a tiny bit of oxygen, which is very different from it having none. (“There’s a big difference between mostly dead and all dead.”) Second, the chemistry of Martian meteorites and rocks indicates that oxygen played a role in the formation of the Martian crust. Third: the brines on or just beneath the Martian surface have a much lower freezing point than pure water because of the magnesium and calcium perchlorate salts they contain. They can supercool down to 140 to 150 Kelvin while staying liquid, and cooler liquids can dissolve more gas.

And finally: recent experiments have shown that the simplest multicellular life—microorganisms and sponges—need much less dissolved oxygen for aerobic respiration than most microbial life and simple animals do. So it would take only a minimum amount of oxygen to support something like these organisms.

So the researchers calculated the solubility of molecular oxygen in brines close to the Martian surface (since the oxygen is in equilibrium with the atmosphere) with varying levels of different salts at temperatures ranging from 140 to 300 Kelvin. They examined the brines not only of modern Mars but also Mars of 20 million years ago and 10 million years hence by noting how the conditions would change as a function of axial tilt.

Worst-case estimates

Even under their worst-case estimates, the brines could hold twice as much oxygen as microbes need for aerobic respiration. At the poles, where it is colder and the brines are more likely to sporadically freeze, they could hold a million times more—enough for multicellular sponges to breathe. These oxygen levels are comparable to those in Earth’s oceans today and orders of magnitude higher than the oceans of early Earth. Comparing these results with different axial tilts over time showed that Mars has supported environments much richer in oxygen over the past five million years.

Here on Earth, photosynthesis likely put enough oxygen in the atmosphere to pave the way for aerobic respiration. But just because that’s the way aerobic life arose here doesn’t mean that’s the only way it can arise or that it has to use that route. Even though there’s only a tiny bit of oxygen in Mars’ atmosphere, this work shows that the possibility exists that there may be (or maybe once was or maybe could be) aerobic life on Mars.

Nature Geoscience, 2018. DOI: 10.1038/s41561-018-0243-0 (About DOIs).