Opportunity opened its eyes to a bizarre, alien landscape
Data from Nasa's Mars rover Opportunity shows its unique landing site is a prime spot for a return mission to look for life, scientists say.
The robot was not designed to find evidence of biology on Mars and did not detect any during nearly a year spent exploring the Meridiani Planum region.
But writing in Science, team members claim the site may have been habitable for long periods of Mars' history.
And locations on Earth with similar conditions do host microbial life.
As Opportunity's first images steadily downloaded onto screens at the US space agency's Jet Propulsion Laboratory in January, it was clear Meridiani Planum was like no other place seen on Mars.
Viewers around the world were transfixed as pictures revealed a thoroughly alien landscape of dark, rippled soil and stark, pale slabs of exposed bedrock.
Opportunity had struck lucky. Investigations carried out over the ensuing month proved the pale slabs were part of a larger sequence of sedimentary rock laid down in the presence of liquid water.
Professor Colin Pillinger, lead scientist on the ill-fated European lander Beagle 2, agreed that Meridiani was a first-rate location to search for life.
"The most exciting thing was seeing those sedimentary formations," he commented.
"It meant there had been big bodies of water on the surface. There is a historical record in a sedimentary formation and for us that is where you might find the remnants of life."
Intriguingly, the Rio Tinto, an acidic river in south-western Spain that resembles Meridiani chemically, is home to specialised communities of microbes.
The Sun has not yet set on the rover missions
"Sample return of Meridiani rocks might well provide more certainty regarding whether life developed on Mars," rover team members write in Science.
They point out that iron oxide deposits at Rio Tinto contain beautifully preserved fossils of tiny microbes.
At the moment, the scientists cannot prove whether Meridiani could have hosted biology, but water is a crucial requirement for life.
Beside the seaside
While researchers are confident the layered bedrock was laid down in the presence of water, some uncertainties remain.
"There are people who think [the site] was on the margin of an ancient sea," Professor Ray Arvidson, deputy principal investigator for the rovers told the BBC News website.
"The other line of thought is that this is a big, layered stack of volcanic sediment, which came out very wet and with a lot of sulphur. If you look at what I call the etched terrain (site to the south of Opportunity's present position) I think you can see evidence for individual ash flows."
Another issue which needs to be resolved is the age of the rock sequence beneath Meridiani's soil.
Ancient rocks are just below the soil
"The oldest terrain [on Mars] is the ancient crater terrain, which is 4 to 4.5 billion years old. Opportunity is sitting on top of about 300m-worth of layered rock," said Professor Arvidson.
"The layered rock sits right on top of the ancient crater terrain. So we know it's younger than 4 to 4.5 billion years. I personally think these rocks were buried and then exhumed by wind erosion.
"But it's very difficult to date because the surface we are sitting on has very few craters, and that may be because it has eroded so quickly."
Professor Monica Grady, a planetary scientist at London's Natural History Museum, believes that getting good dates must now be a prime objective - and an excellent reason to try to return samples of Martian rock to Earth for analysis.
"Until we get absolute ages we can't say when a particular volcanic eruption stopped or a body of water disappeared," she said.
A raft of Mars missions are currently in the pipeline. The next one to reach the surface will probably be a low-cost "scout" mission called Phoenix in 2007.
Future landing missions could speed up the process of selecting interesting targets in the landscape by using artificial intelligence (AI).
"The rovers used quite advanced technology to explore Mars. But there is only a small amount of AI in the robots. Most of the geology is done by a large team of people," said Dr Patrick McGuire of the Centre for Astrobiology in Madrid, Spain.
The rovers use artificial intelligence to avoid hazards
Dr McGuire and colleague Jens Ormö have devised a wearable computer system which uses intelligent software to select interesting rocks. They have named the system the cyborg astrobiologist.
"One of the ideas we had was to offload some of the burden on to the robot itself. This would relieve the humans from doing low-level mineralogy interpretation and scene interpretation," Dr McGuire said.
In recent field tests, the cyborg astrobiologist was able to find interesting points in real-time in the geological scenery.