It's widely accepted that Mars was once home to streams and rivers, and that parts of its ice cap are composed of frozen water. But could there still be moisture in the planet's soil? After analyzing a sample collected by the Curiosity rover, a team of scientists has found strong evidence that Martian dirt contains traces of water — and that it could be recovered and used in manned Mars missions.

The discovery was made possible by a February mission to collect a soil sample from Mars. A month earlier, Curiosity had used a brush to sweep up dust from the planet's surface, but the material didn't lend itself to deep analysis. So the bot made its way to the Gale Crater, a site that's thought to have held water in the past. From there, Curiosity drilled a 2.5-inch-deep hole in the ground, scooping up the cylinder of dirt inside. The real work, however, had just begun. To figure out what was in the dirt, the Mars Science Laboratory Team used a device known as the Sample Analysis at Mars, or SAM. As lead author Laurie Leshin, dean of Rensselaer Polytechnic Institute, puts it, a baby aspirin-sized piece of the sample was fed into a tiny cup in Curiosity, then heated to temperatures of 835 degrees Celsius (over 1,500 degrees Fahrenheit.) The gases that came off revealed the composition of the soil inside.

"You don't need to go to the polar caps. You don't need to dig way down deep."

The 8-hour process revealed gaseous carbonite, which forms in the presence of water, and overall, the analysis suggests that about 2 percent of Martian soil is composed of water. By Earth standards, that's still incredibly dry. But finding even these amounts on the surface of Mars is a coup. "It really validates that you should be able to get water," says Leshin. "You don't need to go to the polar caps. You don't need to dig way down deep." While the Gale Crater is cited as a promising location for water, the team believes that this sample is representative of the larger Martian surface. "Mars has kind of a global layer, a layer of surface soil that has been mixed and distributed by frequent dust storms," Leshin says. "So a scoop of this stuff is basically a microscopic Mars rock collection."

Curiosity was launched in part with the goal of finding evidence of life on Mars, but this research could mean more for the planet's future than the past. While you'd never get much water from this tiny sample, a larger chunk of soil is a different story. A cubic foot of soil, Leshin says, could provide a couple of pints of condensation when heated. For something like the 2021 manned Mars mission, that's two pints less that would have to be carried from Earth (or, as has been suggested, from an asteroid.)

It's not all good news for astronauts. Analysis of the sample also revealed the presence of perchlorate, which can limit thyroid hormone production. That risk is manageable, but Leshin says knowing about it is helpful. And there's still more research to be done, both with Curiosity's sample and with future probes. Likewise, SAM found no sign of life on Mars. The sample didn't seem to contain living microbes or dead ones, though some traces of organic material from Earth were found. The success of the rest of SAM's analysis, though, tells Leshin that at least it isn't missing traces of life that are there: even if it doesn't shed light on the red planet's biggest mystery, "it does show us our instrument is working pretty well."