Scientists say if life ever existed on Mars, the evidence might be found in the vast fields of gypsum on the red planet's surface.
The claim was made at a NASA conference in Houston, Texas held to coincide with the Astrobiology Science Conference.
Professor JWilliam Schopf of the University of California, Los Angeles told the meeting that microscopic fossils have been discovered in gypsum deposits on Earth, and may also exist in similar deposits on Mars.
Schopf says fossilised remains of plankton, diatoms and cyanobacteria, or 'pond scum', have been discovered in gypsum in the northern Italian Alps.
He says the deposits formed some 5.6 million years ago when the Mediterranean Sea dried up after being cut from the Atlantic Ocean by tectonic plate movement.
Gypsum is a common calcium sulphate dihydrate mineral. It precipitates out of sea water and is usually found in evaporated beds in association with sedimentary rocks.
It's very soft and until recently, was not considered the sort of place where fossil evidence for life was likely to be found.
Schopf says if fossils trapped in gypsum can last on Earth for millions of years, they might also be found on the dried up sea beds of Mars as well.
Images of the Martian surface taken from space by NASA's Mars Reconnaissance Orbiter indicate the existence of gypsum dunes in the northern polar region of the red planet.
Dr Steve Squyres, principal investigator for the science payload on NASA's Mars Exploration Rover Mission, told the meeting that the rover Opportunity, which landed near the Martian equator, was also in an area carpeted with sulphate rocks.
Mars rover record
Opportunity touched down on the red planet on 25 January 2004, 21 days after its sister rover Spirit.
Opportunity has now been operational for more than 6 years and 3 months, well beyond its original 90-day designed mission life.
The achievement has now surpassed the record for Mars surface missions set by NASA's Viking 1 lander.
It touched down on a dried out Martian flood plain called the Chryse Planitia back on the 20 July 1976 and continued operating until November 1982.
One of the aims of the Viking 1 mission was to search for signs of extraterrestrial life.
On board was the Labelled Release Experiment, designed to test for any biological reactions from a sample of Martian soil. Although the experiment produced result, scientists concluded the reaction was chemical rather than biological.