Mars is mostly a red pile of mysteries. In its youth, it was clearly a very different place, even hosting oceans and lakes of liquid water. But apart from wondering whether anything living ever made Mars its home, figuring out how the Red Planet got warm enough to thaw all that water has turned out to be no small thing.

The evidence shows there probably wasn’t enough CO 2 to warm up the early Martian greenhouse above the freezing point of water on its own. So might other gases have contributed? One option is simple hydrogen gas (H 2 ). Although two-atom molecules like this typically aren’t greenhouse gases, hydrogen can absorb some infrared radiation in the moment it bounces off other molecules. And it can also react with CO 2 to make some methane, which is a potent greenhouse gas.

But was there a source of hydrogen gas on Mars? A team led by Nicholas Tosca of the University of Oxford decided to follow a lead from the rocks beneath the wheels of the Curiosity rover—the mineral magnetite in a rock typical of lake-bottom sediments. The magnetite (which, not shockingly, is magnetic) would have formed within the mud, and that process can produce hydrogen.

It’s all chemistry

The researchers experimented with the likely chemistry at the bottom of that ancient lake to see if that might work. With tons of basalt rock around, the chemistry of the water would have reflected that rock—high in iron, magnesium, and silica. Given a higher pH and lack of oxygen, the researchers measured the precipitation of bits of an iron hydroxide mineral that would turn into magnetite within a few days. As it happens, some hydrogen gas is produced from the surrounding water during that process.

Repeating that experiment with a lot of CO 2 around shows a potential problem, though. That CO 2 makes the water more acidic, causing the iron to prefer grabbing dissolved CO 2 to form a nice carbonate mineral. So, in order for the hydrogen-producing process to play out on Mars, something would have had to push the pH in the other direction.

Here the researchers turned to a computer simulation. Their idea was that water in the lake could have met with groundwater coming through the basalt. Groundwater in basalt would lose all its CO 2 and attain a high pH through reactions with the rock. So while water in the lake would contain CO 2 from the atmosphere, groundwater coming through the lake bottom would have a different chemistry, making for an interesting mixture in the lake-bottom mud.

The simulations showed that this mixture could trigger magnetite formation—and also the production of hydrogen gas. And given plausible flows of groundwater, the model produced about as much magnetite as Curiosity has seen in the rocks in a thousand years or so.

What’s more, if all of Mars’ equatorial lakes did this, enough hydrogen gas would be released to push the climate above the freezing point.

Making a greenhouse

So here’s the scenario the researchers envision: low-altitude areas near the equator would have been the warmest places and could have hosted some liquid water at times. If a small warming event melted ice from surrounding highlands, flows of groundwater toward the lakes would have been possible. Once that happened, the magnetite reaction in the lake-bottom muds could have released hydrogen gas that caused additional warming and maintained above-freezing temperatures in a broader area of the planet.

If the loss of hydrogen to space (and methane to atmospheric reactions) overtook the production of hydrogen in the lakes, the warm period would come to an end.

The researchers say that this scenario is compatible with all the evidence the Curiosity rover has turned up, so it’s at least a plausible piece of the puzzle. They also note that having hydrogen gas around would provide some of the same ingredients that early life on Earth survived on—that’s a tantalizing thought for those holding out hope for ancient life on Mars. Fortunately, we still have a presence on the Red Planet, and it’s scratching in the Martian dirt for more clues.

Nature Geoscience, 2018. DOI: 10.1038/s41561-018-0203-8 (About DOIs).