Analysis of Martian soil by Nasa's Mars Curiosity rover has exposed a surprising amount of water, as well as a chemical that would make a search for life more complicated, according to scientists.
A scoop of fine-grained sand collected by the rover in August last year showed the soil contained about 2 per cent of water by weight.
"It was kind of a surprise to us," said Curiosity scientist Laurie Leshin of Rensselaer Polytechnic Institute. "If you take a cubic foot of that soil you can basically get two pints of water out it," she said. "The soil on the surface is really a little like a sponge for sucking stuff out of the atmosphere."
Scientists announced last week that so far the planet's atmosphere showed no signs of methane, a gas which on earth is strongly tied to life. Plumes of methane had been detected over the past decade by Mars orbiters and ground-based telescopes.
Methane, which should last about 200 years under Martian photochemistry, can also be produced by geologic events.
The water was found by heating a tiny bit of soil to 835 degrees Celsius inside Curiosity's chemistry laboratory and analysing the resulting gas releases.
Scientists found that in addition to water, sulphur dioxide, carbon dioxide and other materials, the sands of Mars also contain reactive chemicals known as perchlorates.
The US space agency's now-defunct Phoenix lander had found perchlorate in the planet's northern polar region, but scientists did not know until Curiosity's analysis that the chemical was apparently widespread.
"They seem to accumulate on the surface [of Mars], almost like snow," said lead Curiosity scientist John Grotzinger of the California Institute of Technology.
That is important to know because looking for organic material on Mars may now require a new approach.
"The tried-and-true technique on earth is to heat the sample and take a look at the gases that are produced.
"But the heat can cause perchlorate to break down, in the process degrading the organic compounds scientists are looking for."
The presence of perchlorate in soil samples could explain why scientists have so far had a hard time finding organic material on Mars.
Even if life never evolved on Mars, the planet should have organic carbon deposits left by crashing asteroids and meteors, scientists believe.
The results of Curiosity's first 100 days on Mars, published in the journal Science this week, also revealed the presence of a rock with a far more complicated chemical history than scientists expected to find on Mars.