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 than 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 like 140-150K while staying liquid, and cooler liquids can dissolve more gas.