NASA's sextet of Mars landers and rovers may have missed signs of life for over three decades because they're not looking far enough beneath the red planet's surface. That's the conclusion of a team of British and Swiss researchers, who say the only way Martian life could have adapted to the harsh conditions would have been to burrow at least several meters beneath the soil.
One of the main reasons Earth has hosted life for the last 3 billion years is that our planet's atmosphere and magnetic field deflect cosmic radiation. Another is the abundance of liquid water, which permits cellular metabolism and reproduction. That's not the case on the red planet. "The last time liquid water was widespread on Mars was billions of years ago," says astrobiologist Lewis Dartnell of University College London. And even when there was water, he notes, "the hardiest cells we know of could not possibly survive the cosmic radiation levels near the surface."
In Tuesday's Geophysical Research Letters, Dartnell and colleagues describe how they calculated radiation exposures for three typical Martian surface configurations: dry soil, frozen soil containing layered permafrost, and ice. Based on those calculations, and allowing for microbes’ ability to survive extremely harsh conditions, the researchers conclude that any Martian bacteria or spores that ventured too near the surface would long ago have been annihilated by the freezing temperatures and radiation.
Instead, if life did evolve and persist on Mars, it likely settled at least 2 meters below the surface, the researchers say. That depth is far beyond the reach of the instruments aboard Spirit and Opportunity, the twin rovers still rolling across the Martian landscape. But it may be within range of the European Space Agency's ExoMars mission, currently planned to land in 2013 and begin drilling below the Martian surface. If future missions can bring back organisms from those depths, researchers might be able to reanimate them if they have grown dormant, says Dartnell. "The Holy Grail ... is finding a living cell that we can warm up, feed nutrients, and reawaken for study."
It's an "excellent preliminary study," that could help mission controllers zero in on landing sites for future missions that feature, say, surface ice of sufficient depth, says astrobiologist Andrew Steele of the Carnegie Institution in Washington, D.C. One important area for further study, he says, is radiation doses at various depths within Martian rocks, because organisms tend to work their way into rocks when confronted by extremely harsh conditions. Steele adds that finding a living organism or fossil does not need to be the only goal of Mars missions. "If we find that Martian chemistry progressed toward life but didn't produce it, that's as valuable as finding life," he says.