By Olivia Johnson
BBC News, Dublin
Scientists have found evidence that large amounts of water-ice hide within massive sand dunes on Mars.
One of the dunes, called Kaiser Dune, which spans 6.5km and rises 475m above the Martian surface, is among the largest in the Solar System.
The icy dunes could be a valuable resource for any future manned missions to the planet, said Dr Mary Bourke.
The researcher presented her results to a science conference in Dublin.
"If you're looking for a source of water for any future landers," said Dr Bourke, "I am advocating that you march up to your nearest sand dune.
"Perhaps you'll be lucky enough to find a reservoir that can be used to produce fuel and to help the survival of humans," she told the British Association's Festival of Science.
The Gamma Ray Spectrometer aboard the Mars Odyssey spacecraft returned the first definitive detection of solid water on Mars in 2002.
Data from this mission allowed scientists to construct a rough map of the water content in the Martian soil. Mars' polar ice caps were found to contain up to 70% water by mass.
Dr Bourke, of the Planetary Science Institute in Arizona, US, first identified sand dunes as another possibly important reservoir of water-ice by overlaying this water content map on detailed geologic maps of Mars.
She found that the regions containing sand dunes in the planet's northern hemisphere contained perhaps 40-50% water by mass.
"It came as quite a surprise to us to realise that there could be some frozen water in these sand dunes," Dr Burke explained.
"On Earth, you do find snow and ice within sand dunes, but they're not well understood or even studied. They are reported as an oddity."
Because the resolution of the Gamma Ray Spectrometer data was not high enough to confirm the water was associated with the dunes themselves, Dr Bourke looked to more detailed satellite imagery.
Examining aerial photographs of sand dunes in Earth's polar regions allowed her to identify distinctive signatures of icy dunes which are visible from above.
"We were keen to identify what sand dunes look like from satellites in Antarctica," Dr Bourke explained, "because all we have for regions on Mars is satellite data."
Earth dunes containing solid water are seen to have overhanging cornices, jagged steps, and exposed ridges which result from the ice and snow binding the sand grains together and strengthening them against erosion.
That similar features are observed in the Martian dunes suggests that they, too, are strengthened by water-ice.
Even stronger evidence comes from signatures of melting, according to Dr Bourke. In regions where ice warms and seeps through the sand, dunes develop cracks and fan-shaped depressions.
Both features have been found in the Martian dunes. One fan-shaped basin seen on Mars spans over 400m, suggesting it has lost an extremely high fraction of water.
Mars' largest dune, Kaiser Dune, may contain over 500 cubic metres of water alone.
While the icy dunes on Mars cannot yet be accurately dated, the lack of cratering from asteroid impacts indicates they are relatively recent.
Dr Bourke speculates they may result from Martian snowfalls occurring in the last 100,000 years, and suggests they may eventually allow investigation of the changing Martian climate.
The dunes may also provide a new venue for the search for life on Mars, as methods have already been developed for studying biological life in similar regions on Earth.
The ability to associate water-ice with surface features as discrete, and as common, as sand dunes may be of significant practical value to any future manned mission to Mars which will need a supply of local water to survive on the planet.