Liquid water – a key ingredient for organic life – appears to have flowed over the surface of Mars at least twice in the past seven years.
The discovery points to underground deposits of liquid water, researchers say – vaulting the red planet back into the uppermost ranks of places to hunt for current, if simple, forms of life beyond Earth.
A team of US scientists announced their conclusion at a NASA briefing Wednesday after a painstaking search through thousands of images of Martian gullies. They took repeated images of these features between 2000 and last March, using cameras aboard NASA's now-silent Mars Global Surveyor orbiter.
The research team found what it interprets as fresh flow patterns along gullies etched into the walls of two craters. Water, or debris heavily laced with the liquid, appears to have flowed in one gully sometime between 2001 and 2004. The other outpouring occurred between 1999 and 2004.
For years, evidence from Mars has supported the idea that billions of years ago, large amounts of water flowed on the planet. Surface-penetrating radar on Europe's Mars Express orbiter has found large ice deposits several kilometers below the surface.
But the strongest evidence for potential watery habitats today had come from NASA's Galileo orbiter and from the Cassini orbiter, which is currently touring Saturn and its moons. Galileo's evidence points to a slushy ocean beneath the thick ice crust of Jupiter's moon Europa.
Cassini tracked watery geysers bursting from Enceladus, suggesting that this moon of Saturn holds reservoirs of liquid water.
But now, Mars is back on the leader board.
"I think they've gotten it right," says Bruce Jakosky, director of the Center for Astrobiology at the University of Colorado at Boulder, of the new results.
The presence of liquid water below the Martian surface doesn't prove that organisms live there, he cautions. But it does change the discussion.
"People have talked about ancient life, given the evidence for ancient liquid water" turned up by orbiters and surface rovers, Dr. Jakosky explains. And evidence continues to mount that liquid water has been present in the red planet's geologically recent past. "This is the first piece of evidence that says 'now,' not 'a million years ago,' " he says.
It also highlights the potential value of extending space missions well beyond their initial schedules.
Long-term observations give planetary scientists a better shot at detecting changes on the planet's surface and interior.
But the issue generates some tension among space scientists, because extensions also soak up money that could be spent on new missions.
The Mars Global Surveyor was wrapping up its third extension when it when died last month.
Martian gullies have peaked the interest of the research team, led by Michael Malin of Malin Space Science Systems in San Diego, since it first spotted them after the Mars Global Surveyor reached the red planet in 1997.
At the time, Dr. Malin and his colleagues noted that the sharply scored gullies bear a strong resemblance to those found on Earth. The shapes, the branching, and the end-of-the-line aprons of material suggested either liquid water or mudflow-like debris lubricated with water.
But others weren't so sure. Some colleagues argued that a sudden release of carbon dioxide near the surface of the slope could launch the debris downhill. Or the flow patterns could merely represent slumped soil.
Still others suggested that the planet's current climate would inhibit new gully formation today. Yet recent calculations have indicated that the gully channels are consistent with flowing water under current atmospheric conditions – leaving gullies a good place to look for evidence of current flows.
Another aspect of the team's results, which will appear in Friday's edition of the journal Science, involves the impact craters themselves.
In January, a fresh gouge roughly a kilometer across appeared in new images of the surface. The crater formed sometime after November 2004. This triggered a detailed search that has yielded 20 impact "events" within the past nine years, involving meteoriods ranging from tens of centimeters across to up to three meters in diameter.
The find is expected to help refine planetary scientists' use of craters on Earth's moon as a kind of geological clock. By comparing crater rates on the moon with that of other planets, including Mars, scientists can estimate the geological age of surface features.