RENEE MONTAGNE, host:
Scientists have discovered features on Mars that could be signs of running water. If that's true, it would be big news for scientists looking for signs of life on Mars. After all, practically everywhere on Earth where there's running water, there's also life.
NPR's Richard Harris has the story.
RICHARD HARRIS: First off, we already know that Mars has water on it, lots of water. But Phil Christensen, a long-term Mars watcher from Arizona State University, says that knowledge isn't all that exciting to biologists.
Dr. PHIL CHRISTENSEN (Geophysicist, Arizona State University): Much of that water is in the form of ice in the polar regions or the high latitudes. And at those places, that water is going to be frozen throughout the year.
HARRIS: Living organisms, as we understand them at least, need at least some liquid water to do their thing. So, Christensen is excited to learn that NASA's Mars reconnaissance orbiter has made an intriguing discovery closer to the Martian equator.
At a news conference yesterday, Alfred McEwen from the University of Arizona described dark streaks that run down several steep crater walls and come and go with the seasons.
Dr. ALFRED McEWEN (Lunar and Planetary Laboratory, University of Arizona): So these form and grow. They darken. Some of them start fading while new lineaments are forming and growing still and eventually they completely disappear.
HARRIS: Looking at them, it's easy to imagine that they could be water pouring from springs in the bedrock and flowing down the crater walls. They even shape the ground around them, the way running water makes features on Earth. They leave bright, smooth areas behind.
Dr. McEWEN: And that appears to be some sort of deposit or residue left behind by these slope flows.
HARRIS: What's especially intriguing is that these flows occur only during the warm seasons. In fact, this area of Mars even warms up above freezing at times. So the conditions are actually right for running water, McEwen says. But if it is water, it's probably not pure water. It's probably brine, which happens to be harder to freeze.
And yet, with all this tantalizing evidence, Alfred McEwen is not jumping up and down on his chair celebrating a major discovery of liquid water on Mars for a simple reason.
Dr. McEWEN: We have no direct detection of water.
HARRIS: The provocative images aren't proof. And when the spacecraft in orbit around Mars pointed an instrument down to look at the chemical composition of the streaks, it did not register the presence of water. That may simply be that the dark streaks are pretty narrow, as little as a few feet wide. And the satellite can't take readings with pinpoint accuracy. Or maybe the streaks aren't salty water at all.
McEwen and his colleagues who have published these results in the latest issue of Science magazine say they can't come up with an explanation that's more plausible than water.
Dr. McEWEN: But there may be people out there that are more clever than us and it's definitely worthwhile to keep thinking about multiple alternate explanations.
HARRIS: And if liquid water is the most promising hypothesis, that means the scientists may have stumbled upon an oasis where life could exist on Mars.
Astrobiologist Lisa Pratt from Indiana University says, life on Earth certainly exists in similar extreme environments such as the Arctic permafrost.
Dr. LISA PRATT (Astrobiologist, Indiana University): I think this is an eye-opening discovery that will really help us begin the planning process for future missions, specifically looking for signs of life on present-day Mars.
HARRIS: And a discovery like that would surely rank among the most exciting in all of science.
Richard Harris, NPR News.
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