By Helen Briggs
BBC science reporter, Boston
The Red Planet was too salty to sustain life for much of its history, according to the latest evidence gathered by one of the US rovers on Mars' surface.
Experts said the findings 'tightened the noose' on hopes of life on Mars
High concentration of minerals in water on early Mars would have made it inhospitable to even the toughest microbes, a leading Nasa expert says.
Clues preserved in rocks that were once awash with water suggest the environment was both acidic and briny.
The observations were made by the US space agency's Opportunity rover.
It has spent months examining rocks on an ancient Martian plain.
'Ghost of a chance'
Dr Andrew Knoll, a member of the rover science team, and a biologist at Harvard University, Cambridge, US, said the finding "tightens the noose on the possibility of life".
Speaking at the annual meeting of the American Association for the Advancement of Science (AAAS) in Boston, he said conditions on Mars in the past four billion years would have been very challenging for life.
"It was really salty - in fact, it was salty enough that only a handful of known terrestrial organisms would have a ghost of a chance of surviving there when conditions were at their best," he explained.
The quest for life on Mars will go on with the next generation rover
The US Mars rovers - Opportunity and its twin, Spirit - have now spent more than 1,400 days on the Martian surface.
As their work comes to an end, Nasa has its hopes set on the Phoenix lander, which is due to reach Mars on 25 May.
The Phoenix mission will land near the planet's north pole, and aim to dig under the frozen surface in search of signs of microbial life, past or present.
The next-generation rover, the Mars Science Laboratory (MSL), is set to leave Earth in 2009, and land in 2010.
Twice as long and three times as heavy as Spirit and Opportunity, it will collect Martian soil and rock samples, and analyse them for organic compounds.