By Jonathan Amos
BBC News science reporter, San Francisco
A French scientist believes Europe's next mission to Mars should target some of the oldest rocks on the planet if it wants to find evidence of past life.
ExoMars is awaiting approval from space ministers
Jean-Pierre Bibring has identified areas that were in contact with water just after the planet's formation.
In one such region, known as Marwth Vallis, conditions could have been stable long enough for life to start.
Prof Bibring is pushing for Europe's ExoMars rover, an 580m-euro robotic vehicle, to be sent there in 2011.
He said: "Marwth Vallis is a good site, too, because the altitude is close to zero.
"You have to have a site very low on Mars for the parachutes to work."
The scientist from the Institute of Space Astrophysics, Orsay, was speaking here at the Fall Meeting of the American Geophysical Union.
He made his remarks as Europe's space ministers gathered in Germany to approve the ExoMars rover. An official announcement signing off the project will be made on Tuesday.
ExoMars will carry a drill and a suite of instruments to study surface materials for evidence of past or present biology.
And when mission managers come to decide where to send the rover, they will be listening closely to Prof Bibring's views.
He is the principal investigator on Omega, an instrument on the Mars Express orbiter that is mapping the minerals in the Red Planet's surface rocks.
It can see materials that were formed over long time periods in the presence of large amounts of liquid water.
What is fascinating is that these hydrated minerals - so called because they contain water in their crystalline structure - were produced in the first few hundred million years after the planet was created. In other words, the rocks they make up are more than four billion years old.
Benign for life
Crucially, these are not the sulphate minerals seen by the US Mars rovers but a different class of hydrated minerals, known as phyllosilicates - more familiarly called clay minerals.
In Bibring's opinion, it is far more likely that ExoMars will find evidence of life laid down in these rocks than if it were to look at the sulphates documented by the US vehicles.
"Phyllosilicates trace the moment when liquid water was perennial and persistent - something not necessary to make sulphates. To make clay minerals requires long-standing bodies of water and [for life to form] you need that - at least with the experience we have from Earth."
This puts Marwth Vallis and other clay locations - such as Arabia Terra, Terra Meridiani, Syrtis Major, and Nili Fossae - high on the list of possible ExoMars targets.
And it pushes down the list the sulphate locations such Meridiani Planum and Gusev Crater currently being inspected by the US Mars rovers. Their sulphates were formed in acidic conditions - a challenging environment for any lifeform to evolve.
It is a point echoed last week by US rover scientist Dr Andrew Knoll of Harvard University.
He observed: "Life that had evolved in other places or earlier times on Mars, if any did, might adapt to Meridiani conditions, but the kind of chemical reactions we think were important to giving rise to life on Earth simply could not have happened at Meridiani."
Jean-Pierre Bibring says the instruments on ExoMars should be equipped to look for large carbon molecules in amongst the clays of Marwth Vallis as a possible signature of past life.