By Jonathan Amos
There is a surprising amount of water bound up in the soil of Mars, according to an analysis done onboard the US space agency's (Nasa) Curiosity rover.
When it heated a small pinch of dirt scooped up from the ground, the most abundant vapour detected was H2O.
Curiosity researcher Laurie Leshin and colleagues tell Science Magazine that Mars' dusty red covering holds about 2% by weight of water.
This could be a useful resource for future astronauts, they say.
"If you think about a cubic foot of this dirt and you just heat it a little bit - a few hundred degrees - you'll actually get off about two pints of water - like two water bottles you'd take to the gym," Dr Leshin explained.
"And this dirt on Mars is interesting because it seems to be about the same everywhere you go. If you are a human explorer, this is really good news because you can quite easily extract water from almost anywhere."
The dean of science at the Rensselaer Polytechnic Institute, New York, has been describing her work with Curiosity in this week's Science In Action programme on the BBC.
The revelation about the amount of water chemically bound into the fine-grained particles of the soil is just one nugget of information to come from a series of five papers in the respected journal describing the early exploits of the rover.
Some of this data has been reported previously at science meetings and in Nasa press conferences, but the formal write-up gives an opportunity for the wider research community to examine the detail.
'Good and bad'
Dr Leshin's and colleagues' publication concerns a sample analysis done at "Rocknest", a pile of wind-blown sand and silt about 400m from where Curiosity touched down on the floor of Gale Crater in August 2012.
The robot used its tools to pick up, sieve and deliver a smidgeon of this Martian dirt to the Sam instrument hidden away inside the belly of the vehicle.
Sam has the ability to cook samples and to identify any gases that are released. These products are diagnostic of the different components that make up the soil.
So, for example, Curiosity saw a significant proportion of carbon dioxide - the likely consequence of carbonate minerals being present in the sample. Carbonates form in the presence of water.
And it saw oxygen and chlorine - a signal many had expected to see following similar studies in Mars' "High Arctic" by Nasa's Phoenix lander in 2008.
"[We think these] are break-down products from a mineral called perchlorate, and that's there at about a half-a-percent in the soil," said Dr Leshin.
"If the water was the good news for the astronauts, this is the bad news. Perchlorate actually interferes with thyroid function, so it could be a problem if humans were to ingest some of the fine dust on Mars. It's just something we need to know about now so we can plan for it later."
Three of the other Curiosity papers in the Science Magazine release also concern themselves with the nature of the Martian soil.
The fifth is a report that describes a pyramid-shaped rock found in the vehicle's path. This striking block was dubbed Jake Matijevic, in honour of a recently deceased Nasa engineer.
The team led by Prof Ed Stopler from Caltech, Pasadena, can now confirm that Jake_M is a rock not seen before on the Red Planet.
It is most like a mugearite, says the group - a type of rock found on islands and rift zones on Earth.
"On Earth, we have a pretty good idea how mugearites and rocks like them are formed," said co-worker Prof Martin Fisk from Oregon State University, Corvallis.
"It starts with magma deep within the Earth that crystallises in the presence of 1-2% water.
"The crystals settle out of the magma and what doesn't crystallise is the mugearite magma, which can eventually make its way to the surface as a volcanic eruption."
Mugearite was first identified on Earth by British petrographer/petrologist Alfred Harker. The name references a local croft, Mugeary, on the Isle of Skye, just off the Scottish mainland.
The Curiosity rover is currently engaged in some hard driving in Gale Crater. Since early July, it has been rolling tens of metres a day.
The robot is trying to reach the foothills of the large mountain that dominates the centre of the deep, equatorial impact bowl.
Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos
Source and special thanks to the BBC: http://www.bbc.co.uk/news/science-environment-24287207