By Victoria Gill Science reporter, BBC News
Researchers have identified rocks that they say could contain the fossilised remains of life on early Mars.
The team made their discovery in the ancient rocks of Nili Fossae.
Their work has revealed that this trench on Mars is a "dead ringer" for a region in Australia where some of the earliest evidence of life on Earth has been buried and preserved in mineral form.
They report the findings in the journal Earth and Planetary Science Letters.
The team, led by a scientist from the Search for Extraterrestrial Intelligence Institute (Seti) in California, believes that the same "hydrothermal" processes that preserved these markers of life on Earth could have taken place on Mars at Nili Fossae.
The rocks there are up to four billion years old, which means they have been around for three-quarters of the history of Mars.
When, in 2008, scientists first discovered carbonate in those rocks the Mars science community reacted with great excitement; carbonate had long been sought as definitive evidence that the Red planet was habitable - that life could have existed there.
Carbonate is what life - or at least the mineral portion of a living organism - turns into, in many cases, when it is buried. The white cliffs of Dover, for example, are white because they contain limestone, or calcium carbonate.
The mineral comes from the fossilised remains shells and bones and provides a way to investigate the ancient life that existed on early Earth.
In this new research, scientists have taken the identification of carbonate on Mars a step further.
Adrian Brown from the Seti Institute, who led the research, used an instrument aboard Nasa's Mars Reconnaissance Orbiter called Crism to study the Nilae Fossae rocks with infrared light.
Then he and his team used exactly the same technique to study rocks in an area in north-west Australia called the Pilbara.
"The Pilbara is very cool," Dr Brown told BBC News. "It's part of the Earth that has managed to stay at the surface for around 3.5 billion years - so about three quarters of the history of the Earth."
"It allows us a little window into what was happening on the Earth at its very early stages."
And all those billions of years ago, scientists believe that microbes formed some distinctive features in the Pilbara rocks - features called "stromatolites" that can be seen and studied today.
"Life made these features. We can tell that by the fact that only life could make those shapes; no geological process could."
This latest study has revealed that the rocks at Nili Fossae are very similar to the Pilbara rocks - in terms of the minerals they contain.
And Dr Brown and his colleagues believe that this shows that the remnants of life on early Mars could be buried at this site.
"If there was enough life to make layers, to make corals or some sort of microbial homes, and if it was buried on Mars, the same physics that took place on Earth could have happened there," he said. That, he suggests, is why the two sites are such a close match.
'Geological olympics' Dr Brown and many other scientists had hoped that they would soon have the opportunity to get much closer to these rocks. Nili Fossae was put forward as a potential landing site for Nasa'a ambitious new rover, the Mars Science Laboratory, which will be launched in 2011.
The site was championed by other geologists, including John Mustard from Brown University in Rhode Island, whose team made the case to Nasa to have it included in the landing site shortlist for MSL.
But Nilae Fossae was eventually deemed too dangerous a landing site and it was finally removed from the list in June of this year.
"The rover is being landed remotely - so there's no human pilot involved; it's all up to the robot. And [that's] a very dangerous thing," said Dr Brown. "You need 20km of smooth terrain and unfortunately at this site it is pretty rocky - those ancient rocks are pretty weathered and the surface is rocky and uneven."
"It will be visiting another interesting site when it lands, but this is the place that we should be checking out for life on early Mars."
John Grant, a scientist from the Smithsonian Institution in Washington DC, and a member of the planetary sciences panel that advises Nasa on the MSL mission, spoke to BBC News earlier this year about the choice of landing site.
He said that the objective of mission was a search for "habitability". It was not, he said, a life detection mission.
"[It] entails looking at geologic environments that may not only have been habitable but where signals associated with that habitability have been preserved," he told BBC News in February.
But that does not alleviate the disappointment that many feel over having Nili Fossae and all its secrets taken off the table for the mission.
And what makes Mars Science Laboratory even more of a crucial mission for scientists is the fact that it will be the last rover to explore the surface of Mars until 2018 - partly because funding the mission has been so extraordinarily expensive.
Dr Brown described the experience of having his favoured landing site removed from the shortlist as the geological equivalent of having "your city's Olympic bid rejected".
"I also see a race happening here," he said. "It might take us a couple of decades to build our capability to land [unmanned] rovers somewhere geologically interesting on Mars.
"And in those decades, human space flight capabilities are going to develop and we could have the capability to send humans to Mars."
So in this race of the human versus the robots, which will win?
"It's my personal belief," said Dr Brown, "that by the time real human geologists get to go to Mars, the question of whether there is life on Mars will still be open."
http://www.bbc.co.uk/news/science-environment-10790648
