Hopes of finding evidence that organic life gained a toe-hold on Mars appear to be brightening.
To be sure, no one yet has uncovered direct evidence for life there. An experiment designed to look for it, the European Space Agency's Beagle 2 lander, failed on arrival at Mars in December 2003.
But results from US and European orbiters circling the Red Planet and the discovery of another new species of ice-happy microbes on Earth leave many researchers convinced that it is just a matter of time - and money - before they come up with the "smoking guns."
On Friday, the European Space Agency wrapped up a week-long meeting in Noordwijk, Netherlands, focusing on the first year's results from its Mars Express orbiter. Planetary scientists reported evidence for volcanic eruptions within the planet's recent past. Others have spotted what they interpret as a large, dust- covered frozen sea. Still others cite the presence of methane and formaldehyde in the Martian atmosphere as evidence for current biological activity.
"Hints of life on Mars are getting stronger," notes Vittorio Formisano, whose research group detected the two compounds in the planet's tenuous atmosphere using instruments on Mars Express.
Indeed, an informal survey of participants at the meeting indicated that 75 percent of the researchers present agreed that bacteria once lived on Mars. Roughly 25 percent agreed that bacteria inhabit the planet today.
Meanwhile, a meeting in Houston scheduled for next month will highlight a range of results from Mars Express and from the US Mars Global Surveyor and Mars Odyssey orbiters for near-surface ice and glacial activity in Mars' recent past.
Results pointing to likely ice floes on a frozen sea and to other near-surface ice deposits are "tremendously exciting," says Richard Hoover, an astrobiologist at the National Aeronautics and Space Administration's Marshall Space Flight Center in Huntsville, Ala. If there's life on Mars today, a frozen sea "would be the ideal place to look for it."
The evidence for a frozen sea near Mars' equator comes from Mars Express images analyzed by a team led by John Murray, a researcher at the Open University in Milton Keynes, England. Placed side by side with a picture of Antarctic pack ice, the Mars image looks different only in its signature rust-colored dust.
The researchers say the water that formed the sea probably came from within the crust. They estimate that the floods covered the region somewhere between 2 million and 10 million years ago The team theorizes that a layer of dust and volcanic ash is all that stood between the sea, its ice, and quick vaporization in Mars' thin atmosphere.
The researchers reckon that the sea is still there. The few craters in the area are very shallow, indicating that once an impactor hit, water quickly refilled the crater. Moreover, the surface surrounding the ice floes is extremely flat, compared with hints of an underlying "sea bottom," implying a frozen sea.
Meanwhile, researchers from Cornell University, NASA's Johnson Space Center, and Lockheed-Martin describe a layer of ice some 40 meters thick, thought to be at least 5 million years old. The layer is actively reshaping the surface in a mid- latitude region known as Western Utopia.
These results and others leave researchers champing at the bit for a sample-return mission. Both space agencies are looking at the prospect for launching such missions between 2011 and 2014.
For astrobiologists such as NASA's Dr. Hoover, missions to bring back samples can't come soon enough. Last month, he and colleagues from NASA, the University of Alaska, and the University of Alabama published their discovery of a new species of bacteria, revived after a 32,000-year nap. It had been locked in permafrost formations beneath the Alaskan tundra.
Hoover notes that colleagues elsewhere claim to have revived bacteria from 100 million-year-old salt crystals, and some varieties "freeze dry" well at liquid-nitrogen temperatures - only to be thawed and cultured later. So finding ancient bacteria on Mars no longer seems far-fetched.
And if a future mission does bring bacteria back, dead or alive? "Even if they were all dead, if you bring back the frozen cells, you could answer some very crucial, fundamental questions," he says. "For example if there's life on Mars, does it have DNA and RNA? What would the genes look like? Do they have similar structures to genes on Earth or are they dramatically different?"
And the prospect of answering such questions, he adds, make this "one of the most exciting times for NASA and ESA."
• Material from the wires was used in this report.