By Rebecca Morelle
BBC News science reporter
Mineral maps based on data from Europe's Mars Express probe are helping scientists piece together a detailed picture of the Red Planet's history.
The research provides new sites for future Mars rovers to explore
Life on Mars was most likely during the planet's infancy, the data suggests.
The maps show the planet had three distinct geological eras. The team believes the earliest of these would have been the most hospitable for life.
Future missions may use the information to target these ancient areas in the hunt for life, Science journal reports.
Unlocking the past
The European Space Agency's (Esa) Mars Express mission was designed to shed new light on the planet's atmosphere, structure, geology and composition.
The spacecraft carries a payload of seven science instruments. The team carrying out this research used data from Omega, an imaging spectrometer which uses visible and infrared light to determine the composition of minerals on the Martian surface.
The Omega team, led by Professor Jean-Pierre Bibring, of the Institut d'Astrophysique Spatiale in Orsay, France, used a Martian year's worth of data, covering 90% of the planet's surface.
"Through the minerals we can discover the processes that these minerals were made from," explained Professor Bibring.
"And if you have a given mineral, it means you have a given environment at a given time. So, for the first time, we can see the history of Mars as derived from the minerals we have detected."
The researchers define the planet's history in three distinct geological periods, corresponding to the dominant minerals that were present.
The first age, the Phyllocian era, lasted from just after the planet's birth to about four billion years ago. Ancient rocks show the presence of clay-rich minerals - phyllosilicates - which to form would have required a water-abundant alkaline environment.
The second era emerged after a dramatic shift in the Martian climate. Now sulphate minerals dominated and the researchers have labelled this the Theiikian era, named after the Greek for sulphate.
The team believes the change in mineral composition was caused by volcanic activity around four billion years ago.
"When you have lava pouring out you also have a huge amount of gases. Among these gases you have a lot of sulphur, and the sulphur makes the environment very acid," said Professor Bibring. "The interaction of water that came to the surface with the sulphur created sulphates."
Omega has mapped the clays (shown in blue) at Marwth Vallis
The US space agency's Mars Exploration Rovers (Mer), Spirit and Opportunity, both landed in sulphate-rich regions.
The third era, which continues to the present day, began roughly 3.5 billion years ago. Minerals during this time were not formed in the presence of water.
"All the water disappeared apart from the two big polar caps, and the third era began," said Professor Bibring.
It is essentially categorised by the formation of ferric oxides, he said, which are not hydrated. The team has labelled this time period the Siderikan era.
It is unclear how the new eras will fit with the already-well established way of dividing Martian geology; the Noachian, Hesperian, and Amazonian eras are based on counting impact craters on the surface. Very broadly, there are similarities; but the cut-off periods reveal distinct differences.
The team's analysis led it to conclude that water is not responsible for Mars' red colour. Instead, said Professor Bibring, a slow oxidation of the minerals with small levels of peroxides in the atmosphere created the red-coloured ferric oxides, rather than liquid water.
Life on Mars
Professor Bibring and his team say that the findings point to the time when life formation on Mars was most likely.
"The three eras are important because they tell the story of Mars.
"If one is now looking for a moment during Mars' history during which water may have played a role, in particular for life to have emerged, you have to focus on the very early clay-rich period - the Phyllocian era."
The team hopes its findings will give rise to future missions which can explore the areas where ancient rocks containing clay-rich minerals are present.
These Martian regions include: Marwth Vallis, Arabia Terra, Terra Meridiani, Syrtis Major and Nili Fossae.
"We are building a new instrument, called micrOmega, which would do in situ the same Omega does from orbit," explained Professor Bibring.
"It will be put, I hope, on the European ExoMars mission (planned for 2011). We would be planning to land ExoMars in a clay-rich area, and with MicrOmega we will look for samples that are rich in hydrated clays because we think that they are the most favourable to host, at a microscopic level, potential bio-relics."