IT IS a question that has been posed by minds as diverse as HG Wells and David Bowie.
Is there life on Mars?
Now it seems that scientists may finally have an answer, and it doesn't look encouraging for those hoping to make contact with little green men.
The Red Planet was too salty to sustain life for much of its history, accord
ing to the latest evidence gathered by one of the US rovers on Mars' surface.
High concentration of minerals in water once present would have made it inhospitable to even the toughest microbes, a leading Nasa expert has claimed.
Clues preserved in rocks that were once awash with water suggest the environment was both acidic and briny, which does not bode well for the existence of Martian life.
The observations were made by the US space agency's Opportunity rover, which has spent months examining rocks on an ancient Martian plain.
Dr Andrew Knoll, a member of the rover science team and a biologist at Harvard University, 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 in Boston, he said conditions on Mars in the past four billion years would have been very challenging.
"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.
"By the time the rocks that we analysed formed around three and half to four billion years ago, the planetary surface on Mars was arid, acidic and oxidising. That's not a very pleasant place to live and it's a worse place to try to do the chemistry that is generally thought to have given rise to life on this planet."
He claimed future probes would have to look elsewhere for evidence of Martian life – either deep underground or in a place where rock layers from the planet's earliest history were exposed.
He insisted that all hope was not lost. "Probably the best place to look for evidence of life is in Mars's earliest history – the first 500 or 600 million years.
"We know these places exist as they have been characterised from orbit."
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 May 25. The Phoenix mission will land near the planet's north pole, aiming to dig under the frozen surface in search of signs of microbial life, past or present.
The next generation rover, the Mars Science Laboratory, 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.
Last month Nasa unveiled a photograph which appeared to show a human-like figure crouching behind a Martian rock. But they dismissed the image as an oddly-shaped rock formation.
Previously the detection of methane in the Martian atmosphere was taken as a sign that life could still exist today on the planet.
Meanwhile, Nasa has expressed an interest in backing a UK-led mission to the Moon.
The American agency has indicated its support for the Moonlite mission, which aims to send a robot-operated probe to their lunar surface.
Vital signsThe first picture of a 'face' on Mars was taken by a Viking 1 Orbiter in 1976. The chance photo of a shadowy rock formation then became a kind of pop icon in the US.
• A 1996 report by Nasa scientists claimed to have found evidence of primitive bacterial life on Mars. They said a meteorite found in Antarctica contained possible microfossils of bacteria.
• In 2006, a study of a Martian meteorite found a series of microscopic tunnels not unlike the tracks found on earth rocks by feeding bacteria. Researchers were not able to extract DNA from the meteorite, but said that does not discount the possibility.
• Another 2006 study by an international team of scientists found that Mars could have had plenty of water, hospitable temperatures and acid levels during its first 600 million years.
• In 2007, the Mars Spirit rover has discovered "the best evidence yet" of a past habitable environment on the planet. Silica-rich soil, a main environment for willowy grass, was found.
