MELISSA BLOCK, Host:
From NPR News, this is ALL THINGS CONSIDERED. I'm Melissa Block
ROBERT SIEGEL, Host:
And I'm Robert Siegel.
Compared to Mars, the driest desert on earth is a tropical paradise. So the last thing you'd expect to find is an indication that some kind of liquid had flowed down a Martian gully in the last five years. But in a news conference at NASA headquarters this afternoon, that is just what scientists say they found.
NPR's Joe Palca reports.
JOE PALCA: A NASA spacecraft called Mars Global Surveyor went into orbit around Mars in 1997. Two years later, it began snapping pictures of the Martian surface. Even though it was only supposed to last a few years, it kept working, so scientists got lots more pictures.
At today's news conference, Kenneth Edgett remembers how he felt when he saw one particular picture.
KENNETH EDGETT: Hey, what's this?
PALCA: Edgett is a scientist at Malin Space Science Systems in San Diego. What Edgett was looking at was a picture of a gully taken in April 2005. And what was surprising was that the gully's appearance had changed since it was first photographed in 2001. There was some kind of bright material along the length of the gully, and it looked like the material had been carried there by a liquid.
EDGETT: And you're always kind of scared like, no, this can't really be what it looks like.
PALCA: That's because most scientists think it's been millions, if not billions, of years since there was liquid water on the surface of Mars. But a few months later, Edgett found another picture showing another gully with fresh deposits. And Edgett is convinced the deposits got there courtesy of flowing water.
EDGETT: It could be acidic water. It could be briny water. It could be water carrying all kinds of sediment. It could be slushy. But H2O is involved.
PALCA: There are several reasons to think H2O is involved. The first is the shape of the deposits. They appear to curve around obstructions just like a stream would curve around a rock. The eponymous Michael Malin of Malin Space Science Systems points out another reason - if the deposits were put there by a rock slide, the Martian surface would have been disturbed.
MICHAEL MALIN: Wherever Mars' surface is disturbed, it is always dark and yet this disturbance is bright. That means that there's something associated with this flow that caused it to be brighter.
PALCA: So if there is water, where is it coming from? Malin thinks it's from an underground aquifer. He says a possible scenario is when the underground water initially reaches the surface, it freezes, forming an ice dam. Then more water builds up behind the dam.
MALIN: And then eventually that dam breaks and a brief outflow of the dam ice - I don't mean damn ice, but the dam ice - the rock in which that ice was embedded and the water behind that comes bursting out. It picks up more debris as it goes down a very steep slope and then it flows down the channel.
PALCA: It would be a pretty spectacular event, says Malin's colleague Ken Edgett.
EDGETT: If you were there and this thing was coming down the slope, you'd probably want to get out of the way. If you've ever been in a desert in a flash flood situation, water coming down an arroyo, you kind of want to get out of the way.
PALCA: Edgett and Malin's findings appear in this week's edition of the journal Science.
Planetary scientist Phil Christiansen of Arizona State University was also at today's news conference. Christiansen was not associated with the research, and he's impressed.
PHIL CHRISTENSEN: I think that the evidence for this recent water is compelling. Certain questions still remain.
PALCA: Christiansen says it's not clear what the light-colored debris is made of. And the pictures Mars Global Surveyor was able to take aren't high enough resolution to see the kind of details that would be absolutely convincing that water was at work.
Still, Christiansen says the new research proves that Mars may look dead, but things are still happening there. And when scientists find liquid water, they usually think they've found a good place to search for life.
Joe Palca, NPR News, Washington.
SIEGEL: And you can see images of NASA's evidence for water on Mars at NPR.org.