"When I was a kid," says John Grant, "the big thing was: there are billions of stars in our own Milky Way, what are the odds that life doesn't exist?"
Grant, no longer a kid in stature if still in spirit, now plays a substantial role in setting those odds. The geologist at the Center for Earth and Planetary Studies, part of the National Air and Space Museum, is one of a half dozen scientists in charge of creating itineraries for Spirit and Opportunity, the two NASA rovers that since early 2004 have explored Mars for signs of life, past or present.
Researchers designed the rovers to gather images of rocks and terrain where water, the presumed prerequisite of life, might have flowed. Opportunity's success came soon after touching down at Meridiani Planum, Spirit's a while after landing among the volcanic rocks of Gusev Crater. But the rovers' life-detection skills are limited. They lack the equipment to analyze organic compounds or examine fossils. (The mission's running joke, says Grant, is that a rover will spot a dinosaur bone and be unable to retrieve it.) These tasks are reserved for the Mars Science Laboratory mission, scheduled for 2010.
The search for life in the universe, however, isn't confined to the rovers' path. For that matter, it's no longer limited to Mars, or even the Earth's solar system. More and more, astronomers at labs and observatories around the world are finding evidence for the foundations of life—foremost, water—in our planet cluster and beyond.
"As we get more data about places outside of Earth, we're starting to see conditions where you've got to scratch your head and say, 'This is a potentially habitable environment,'" says Grant. "It's not proof, but you're doing the statistics and they're all going in the category of: In Favor of Life."
That column received another check in mid June, when a group of scientists revived the idea that a vast ocean once existed on the northern hemisphere of Mars. A couple decades ago, scientists analyzed images of this region and found what seemed to be a shoreline. But an ocean shoreline has a uniform elevation, and later topographical tests revealed great variation—in some places, more than a mile separated the terrain's peaks and dips.
The new research, published in the June 14 Nature, argues that, in the past billion or so years, Mars has changed the way it spins on its axis. In the process, much of the planet's mass has shifted in a manner that accounts for the alternation of the once-level shoreline.
The ocean, of course, no longer ebbs and flows along this boundary. But it's unlikely that all the water escaped into the universe, says the study's lead author, J. Taylor Perron of Harvard University.
"We know that life, as we're familiar with it, seems to require liquid water," says Perron. "That basic requirement may have been satisfied on Mars, either when the ocean existed on the surface, or subsequently deeper within the crust."
Whether scientists can dig into the planet's surface and find evidence of water—and with it signatures of life—remains to be seen. Whether they can Massachusetts Institute of Technology, who was not associated with the study, in an accompanying commentary. "The result hints … that the understanding of the 'blue' history of the red planet is far from complete."