The trio of missions that will touch down on Mars this December and January are "following the water" as they search for signs of life. But new data from instruments orbiting Mars builds a case for a volcanic planet only gently touched by the chemical alteration that water has wreaked on Earth for billions of years.
In the latest development, reported in the 22 August issue of Science, a team led by planetary geologist and spectroscopist Philip Christensen of Arizona State University in Tempe finds much less of the mineral carbonate than would be expected on a wet planet. On Earth, water and atmospheric carbon dioxide combine to form carbonic acid that eats away at rocks and flushes their remains into lakes and oceans. There, dissolved carbonates eventually precipitate to form solid carbonate deposits like the White Cliffs of Dover. Geochemists imagined that wet weathering would produce something like 20% carbonate in martian dust.
Christensen and colleagues analyzed data from the Thermal Emission Spectrometer on the orbiting Mars Global Surveyor spacecraft to estimate the fraction of carbonate. The wiggles and squiggles of the spectrum are "consistent with a small amount of carbonate," Christensen says, about 2% to 3%. That's no more than might form over the eons by the dust's reaction with today's vanishingly low martian humidity, he notes, or the higher humidities possible during the ebb and flow of martian ice ages.
The work adds to growing evidence pointing to a colder, drier Mars (Science, 22 August, p. 1037). And other products of extensive weathering expected under a warm, wet climate, such as clays, are showing up in unexpectedly tiny quantities, if at all. Researchers have also found minerals long exposed on the surface that would have been quickly destroyed by liquid water. In the emerging view, a perpetually frigid climate has kept Mars's modest store of water locked up as ice most of the time. The surface may never have been particularly hospitable to life, at least not for long.
Yet even spectroscopists do not fully trust their squiggly lines to tell them all about Mars. Upcoming missions will take a closer look. Earlier this month NASA announced plans to land a soil and ice analyzer on the ice-laden northern plains of Mars in 2008 (Science, 8 August, p. 743). And three Mars landers--two NASA and one European--are now on their way to the planet. With luck, they'll have a firsthand chance to spy water's work on Mars.