NASA Images Provide Possible Evidence of Water on Mars

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• JEFFREY BROWN:

  The tantalizing evidence comes from a series of photos taken by the Mars Surveyor spacecraft. Astronomers are intrigued by pictures like this one, that shows a ravine and a crater that seems to have been altered in just the last several years by flowing liquids.

  Rick Weiss of the Washington Post has been covering the story and joins me now. Welcome to you.

• RICK WEISS, Washington Post:

  Thanks.

• JEFFREY BROWN:

  Help us understand what scientists are seeing in these photos that suggests recent flowing water.

• RICK WEISS:

  Well, what they've got is a series of before-and-after shots, some taken five, six years ago, some taken in the last year or so. And there's remarkable differences, when they took a close look at a few of these gullies, gullies that were already there but that look very different now than they did a few years ago.

• JEFFREY BROWN:

  All right. Here's one we've just put up. So this is the before-and-after. Explain what we're seeing. (Data and images courtesy of Science Magazine.)

• RICK WEISS:

  Well, here you don't have to be a rocket scientist to see you've got a gully here. But if you really looked with fine grain precision, as they can, you've got swirling patterns, moving around obstacles. You've got a little delta at the bottom with fingers of drainage going out.

  And you can even tell by calculating the movement of how these swirls went around obstacles — what was the stuff that was going down here? Was it dust? Was it sand? Was it water? And the only thing that really fits the model with the right viscosity and so on is water, probably with some sediments.