Claims have re-emerged that the US space agency (Nasa) found signs of life on Mars during the historic Viking landings of 1976.
By Helen Briggs
BBC News Online science reporter
Dr Gil Levin, a former mission scientist, says he now has the evidence to prove it, just days before the US and Europe send new expeditions to the Red Planet.
The United States and Russia have spent billions since the 1960s on a handful of space craft designed to land on Mars.
An image of Mars from the Viking II lander
Only three have succeeded so far: the two Viking probes in the 1970s and Mars Pathfinder in 1997.
In 1976, the world was gripped by excitement when a robotic spacecraft touched down on Mars for the first time in history.
Biology experiments detected strange signs of activity in the Martian soil - akin to microbes giving off gas.
Before announcing the news that life had been found on another planet, Nasa carried out more tests to look for evidence of organic matter.
The Viking experiments failed to find this essential stuff of life and it was concluded that Mars was a dead planet.
Dr Levin, one of three scientists on the life detection experiments, has never given up on the idea that Viking did find living micro-organisms in the surface soil of Mars.
He continued to experiment and study all new evidence from Mars and Earth, and, in 1997, reached the conclusion and published that the so-called LR (labelled release) work had detected life.
Beagle is looking for life
He says new evidence is emerging that could settle the debate, once and for all.
He told BBC News Online: "The organic analysis instrument was shown to be very insensitive, requiring millions of micro-organisms to detect any organic matter versus the LR's demonstrated ability to detect as few as 50 micro-organisms."
Dr Levin, now president and CEO of US biotechnology company Biospherix, has a new experiment that he says "could unambiguously settle the argument".
But it was rejected by both Nasa and the European Space Agency (Esa) to go on-board this summer's Mars missions.
The British-built Beagle 2, which will be deposited on the Martian surface by Esa's Mars Express space craft, is going with the main purpose to hunt for life. This is a risky strategy, claims Dr Levin.
"Strangely, despite its billing, Beagle 2 carries no life detection experiment!" he said. "Neither its GCMS (organic detector) which is claimed to be more sensitive than Viking's, nor its isotopic analysis instrument can provide evidence for living organisms."
Nasa's mission to Mars is taking a more circumspect approach to the big life question.
Its two identical rovers will roam the ancient plains of Mars acting as robot geologists.
Mark Adler, deputy mission manager, said the main science objective was to understand the water environment of Mars not to search for life.
He told BBC News Online: "What we learnt from Viking is that it is very difficult to come up with specific experiments to look for something you don't really know what to look for."
Claims of life on Mars have always proved highly contentious. Twenty years after Viking, microbe-like structures discovered inside a Martian meteorite found in Antarctica led to more claims that were later rejected.
As the astronomer Carl Sagan once said, extraordinary claims require extraordinary evidence. And there is no reason to believe that anything found this time will be any different.
"It's going to take a number of missions if we want to know whether there is life on Mars or not," said Dr Charles Cockell, a Mars biologist at the British Antarctic Survey in Cambridgeshire, UK.
"If we find no evidence of life on Mars it may just mean we have looked in the wrong place."