By Dr David Whitehouse
BBC News Online science editor
If the 25 January landing on Mars of the Opportunity rover is successful then the next target should be an extinct volcano, say scientists.
Shrouded in cloud - Apollinaris Patera
Opportunity follows Spirit's successful landing at Gusev Crater on 4 January.
"If both of these landers survive then it blows the doors wide open for future Mars landing sites in more dramatic regions," says Nasa's Dr Tracy Gregg.
Gregg - of the group choosing future landing sites - favours Apollinaris Patera, an ancient extinct volcano.
'Yeah, right, good luck'
"With the success of Spirit, I feel so much more confident about future Mars landers," she says. "The airbags seem to be able to withstand quite a bit of trauma."
But the idea of cushioning a lander using airbags was initially received with scepticism.
Gregg remembers attending a conference presentation a few years ago by Dr Matt Golombek of the US space agency's (Nasa) Jet Propulsion Laboratory and, at the time, the principal investigator on the Mars Pathfinder mission, in which he proposed the airbag landing technology.
Dr Tracy Gregg - looking for lava flows
"He listed the 15 steps that had to happen at exactly the right time and in exactly the right way in order for this technology to work. The general mood in the lecture hall was, 'Yeah, right, good luck,'" Gregg says.
But that attitude changed when Mars Pathfinder got down safely on to the surface of the planet in 1997.
"Well, the next year, he got up to a standing-room-only crowd at a meeting of the same organisation and he described all of the same steps that the Pathfinder had successfully completed on Mars. He got a standing ovation."
Complex balancing act
The selection of Mars landing sites is a complex balancing act, Gregg says, where the potential for important scientific discoveries has to be balanced against the requirement that sites be absolutely safe so that the rovers can perform well and send data back to Earth.
Both Gusev Crater, where Spirit landed, and Sinus Meridiani, where Opportunity is scheduled to land, were chosen because they are not expected to have large boulders, steep cliffs or deep craters that could pop an airbag or swallow up the lander preventing the transmission of radio signals.
"If Opportunity survives the landing on [25 January GMT], there is a high possibility that we will get to see layers of ancient rock deposited when Mars was warm and wet and could have supported life," she says.
"Evidence of river channels, which we expect to see at Sinus Meridiani, could be remnants of that early, warm history."
When pictures start coming back from Opportunity, Gregg will look for layers in the walls of the dried-up river channels.
"Those layers could be lava flows," she says, noting that often the best place to look for evidence of life on any planet is near volcanoes."
"That may sound counterintuitive, but think about Yellowstone National Park, which really is nothing but a huge volcano," she says.
Gregg hopes that a future landing Mars site will be near a volcano, particularly one called Apollinaris Patera.
Apollinaris Patera rises 5 kilometres above the surrounding plain and is a huge shield volcano that is particularly interesting to geologists because of its variety of lava flows and collapsed central caldera.
"A landing site near a volcano might be possible, now that the airbag technology has worked so wonderfully," says Gregg.