Mineral maps are providing unprecedented insights into the history of Mars, using data from the OMEGA instrument aboard Europe's Mars Express spacecraft.

The maps suggest Mars went through three distinct eras, changing from a relatively hospitable, water-rich world to an acidic one after a period of active volcanism, before finally falling into an extended deep-freeze. Researchers say future rovers should search for signs of life at sites where the most ancient minerals have been exposed.

Since the 1970s, spacecraft have beamed back images of deep channels and canyons that suggest water once flowed across the Red Planet. "But we have always been on the lookout for the mineralogical evidence," says Ray Arvidson, a Mars researcher at Washington University in St. Louis, Missouri, US.

NASA's twin rovers (see our special report), now exploring opposite sides of the planet, have each revealed minerals that have interacted with water. But researchers did not have global maps of these minerals because the spectrometers on all previous spacecraft have operated at relatively long wavelengths and low spatial resolutions. These parameters are best-suited to map the composition of the basaltic materials and dust that dominate the planet's surface.

Now, the OMEGA near-infrared spectrometer on Mars Express has used shorter wavelengths and the highest-ever spatial resolution to map these water-altered minerals across 90% of the planet's surface. OMEGA (Observatoire pour la Mineralogie, l'Eau, les Glaces et l'Activit?) has operated for one Martian year, or two Earth years.

Water and rocks

The OMEGA team, led by Jean-Pierre Bibring of the Institut d'Astrophysique Spatiale in France, found about two dozen sites rich in clay minerals, which form in water and in conditions of low acidity. These sites are scattered around the planet in ancient craters and where an overlying layer of volcanic cover or wind-blown sand and dust has been removed.

That suggests the clays formed early in the planet's history, says Bibring: "The clays may have been formed on a large scale but we only see them where they have been exposed by erosion, outflows, or impacts."

The spectrometer also found sulphate minerals, such as gypsum and grey haematite, concentrated in a few places. These locations include Valles Marineris ? the Red Planet's "Grand Canyon" ? and Meridiani, where NASA's Opportunity rover landed and also found sulphate-rich rocks. Sulphates require water to form, but some detected by OMEGA must also have been created in acidic conditions.

Finally, OMEGA found minerals rich in ferric oxides that had not been altered by water. These minerals are found over most of the planet and are thought to be caused by the slow weathering of rocks through chemical interactions with the atmosphere.

Hot core

The team says these three mineral classes trace out three different epochs in the history of Mars. For approximately the first 600 million years of the planet's 4.6-billion-year history, the planet may have boasted a denser atmosphere and large bodies of water on its surface that helped create the clays.

Then, heat that had built up inside the planet by the decay of radioisotopes would have led to a period of active volcanism. Around the planet, volcanoes erupted and lava flowed up through the surface. "This resulted in a pulse of sulphur-rich gases that formed sulphuric acid that settled out on the surface," says OMEGA team member John Mustard at Brown University in Rhode Island, US.

Ground water interacting with these acidic surface materials led to the formation of the sulphate minerals over about 500 million years. If Mars ever had a denser atmosphere, researchers believe it also lost most of it during the latter part of this acidic era, when its interior stopped producing a global magnetic field and volcanism died down. The solar wind was then able to slam into the atmosphere without deflection, eventually stripping it away almost entirely.

"This put Mars into a deep freeze and it became very dry and very cold," says Arvidson, who is also an OMEGA team member. Over the next 3.5 billion years, Mars has remained in this state, gradually growing redder as its iron-rich surface rusts by interacting with the atmosphere.

Chemistry of life

The new results represent "a bold new picture of the history of Mars", says planetary scientist Mark Bullock of the Southwest Research Institute in Colorado, US, who is not part of the OMEGA team. "It isn't so much rewriting the history of Mars as it is adding a new layer to the story - one based on chemistry."

But while he agrees with the overall conclusions of the new work, he says it may be overly simplistic. "I doubt that enough analysis has been done to really determine that the clays always pre-date the sulphates," he told New Scientist. That could mean that non-volcanic processes, such as the interaction of water with pyrite, might create some of the sulphates detected by OMEGA, he says.

"But if the clays are old rocks, then they are excellent targets for astrobiological research," Bullock says. "We know the clays contain water, have contained water since they were formed, and are excellent sites to promote the chemistry that life appreciates."

NASA's next rover, the Mars Science Laboratory, may be sent to one of the exposed clay regions identified by OMEGA, says Arvidson. These regions will be studied in closer detail with the CRISM spectrometer aboard NASA's Mars Reconnaissance Orbiter, which is due to begin scientific observations in November 2006. CRISM boasts 10 times the spatial resolution of OMEGA ? resolving patches of land just 18 metres across.

"I think of OMEGA as the pathfinder," Arvidson told New Scientist. "And then with MRO we will narrow down the places where we have lots of materials exposed and find where we could land and drive up to them."

Journal reference: Science (vol 312, p 400)