In its current form, Mars isn’t the best place to host life — but some of its subsurface oases may be pretty welcoming, a new study reports.

Credits: NASA.

In order to sustain life, a planet (or any celestial body) must fulfill quite a lot of conditions. You need sufficient gravitational pull, an atmosphere with a specific chemical makeup, a Goldilocks temperature — and that’s just the start of it. All in all, the stars need to align just right to provide the necessary conditions for life to evolve.

Mars definitely fulfills some of those requirements. It’s a rocky planet, lies at a reasonable distance from its star, and shows signs of hosting water. Its atmosphere, however, isn’t conducive to life as we know it. Aside from being very thin (just 6% the density of Earth’s atmosphere on average), it comprises 96% carbon dioxide and only contains traces of free oxygen. Due to the scarcity of oxygen, Mars has been assumed to be incapable of producing environments with sufficiently large concentrations of O2 to support aerobic respiration, researchers write in a new study.

However, there are some places where Mars could, in fact, host life: underground. But the story isn’t that straight-forward.

Here on Earth, oxygen and life go hand in hand. Photosynthesis evolved at least 2.3 billion years ago, and aerobic (or oxygen-breathing) life evolved with it. We haven’t seen any sign of that on Mars, however. Furthermore, life requires liquid water — and with an average surface temperature of -81 degrees Fahrenheit (-63 Celsius), liquid water is a scarce commodity on the red planet. There’s no clear evidence that liquid water exists (or could exist) on Mars’ surface, but there are some hints that liquid water could exist underground, in the form of brine (very salty water).

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Water mixed with salt freezes at lower temperatures than freshwater. While pure water will freeze at 32 degrees Fahrenheit (0 Celsius), salty water will still be liquid at that temperature. But salt also reduces the amount of oxygen that water can store, while low temperatures increase the amount of oxygen. So salt, temperature, and oxygen are trapped in a constant push and tow.

To study this dance, Vlada Stamenković and colleagues calculated how much molecular oxygen could be dissolved in subsurface Martian liquid brines. They found that molecular oxygen concentrations are particularly high in the polar regions, and remarkably, some of them could even contain enough oxygen to support aerobic life.

These findings also fit with surface observations, particularly oxidized rocks observed by rovers exploring Mars’ surface.

“That’s the thing of habitability; we never thought that environment could have that much oxygen,” says Stamenković, a planetary scientist and physicist at NASA’s Jet Propulsion Laboratory.

This completely changes our understanding of the potential for life on current-day Mars, he adds. Of course, it doesn’t mean that there is life on Mars — but the fact that could be is already pretty exciting.

The study has been published in Nature Geoscience.

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