For the first time, scientists believe they have evidence of liquid water on Mars. It's only a tiny bit, and only in certain seasons, but this salty water makes the hunt for past life on Mars all the more exciting.

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We know Mars has water, and we also know that Mars once had liquid water (a whole ocean, in fact) but now it seems we may have evidence of liquid water today. Or during the planet's spring and winter months, anyway. In a study published Monday in Nature Geoscience, researchers report that the Curiosity rover has found signs that thin layers of super-salty water could form and evaporate frequently on the surface of Mars.

The problem with water on Mars is that it doesn't act like water on Earth. The planet's atmospheric pressure is so low that water acts more like dry ice would on Earth: It goes right from solid to gas, skipping the liquid step we're used to seeing in the middle.

[New NASA research says Mars once had a large ocean]

But some water on Mars may get so flooded with salt that its freezing temperature lowers significantly.

Curiosity detected salts called perchlorates, which can absorb water from the atmosphere and turn it into briny liquid.

"These can decrease the freezing point of water by more than 70 degrees," explained study author Morten Bo Madsen, associate professor and head of the Mars Group at the Niels Bohr Institute at the University of Copenhagen. "And they attract water quite violently. This can result in salty water moving up and down the surface."

Day/night hypothetical water cycle in Mars.
(Martín-Torres and Zorzano)

So with the perchlorates there, liquid water is possible under the right conditions. The team used Curiosity's weather-monitoring equipment to look for those conditions and found that they occur every day in months throughout winter and spring. They suggest that overnight and before sunrise, some of the frost that forms on the planet's surface interacts with the strong salts and turns liquid, seeping into the soil. This lines up with previous studies, which have detected geographic features that suggest flowing water.

All signs indicate that the Gale crater was once a shallow lake. Now it seems it may still harbor liquid water, though only in small amounts. (NASA/JPL-Caltech/MSSS)

The results come from the Gale Crater, which is itself too cold to support microbial life — even with liquid water present. But the study authors believe this phenomenon could occur anywhere on the planet, and may actually be more common in areas closer to the polar regions. Still, Mars is a pretty desolate place, and the amount of water we're talking about is minimal at best.

"There's so little water that you can't even see it visibly," Morten Bo Madsen said.

But more and more evidence supports the theory that Mars was once a very wet planet, before its stripped atmosphere exposed it to the ravages of space. Once upon a time, Mars might have been much more hospitable to life than it is now. Our rovers will just have to find the signs that life left behind.

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