Low atmospheric pressure on Mars, it was thought, meant that water goes straight from solid to gas, without passing through a liquid phase like it does on Earth. However, the rover has found evidence that during particularly cold nights, thin layers of (rather salty) water can form on the planet's surface as frost, then be absorbed into the soil, before evaporating again with sunrise.
The presence of a particular salt — calcium perchlorate — detected by the Rover, was key.
"When night falls, some of the water vapor in the atmosphere condenses on the planet surface as frost, but calcium perchlorate is very absorbent and it forms a brine with the water, so the freezing point is lowered and the frost can turn into a liquid," wrote Morten Bo Madsen, one of the lead researchers for the paper published in Nature Geoscience.
However, the researchers say they are talking about trace amounts of water, not enough to sustain even microbial life. And the highly variable temperatures and high radiation levels on the planet still make the environment hostile to living organisms.
Astronomers have long known of the existence of solid water on the planet in the form of Mars' polar ice caps. And the geography of the planet — with features that look like they were carved by water — has made researchers almost certain that the planet used to have a liquid ocean covering about 20% of Mars' surface.
— Tring Astro (@Tring_Astro) December 10, 2013
Recent photos taken by Curiosity of Gale Crater — the area it landed in for it's exploratory mission in August of 2012 — also suggest the remnants of an ancient lake. Certain sediment deposits detected by the rover were likely formed by waters flowing downhill, collecting sediment, and settling in a lake, researchers hypothesize.
— EarthSky (@earthskyscience) December 13, 2014
Curiosity was launched from Cape Canaveral, Fla. on November 26, 2011 and travelled 563,000,000 km to study the geology and climate of the red planet.
— Milky way scientists (@MilkyWay_Earth) February 4, 2015