RENEE MONTAGNE, Host:
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.
PHIL CHRISTENSEN: 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: 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.
ALFRED MCEWEN: 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.
MCEWEN: And that appears to be some sort of deposit or residue left behind by these slope flows.
HARRIS: 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.
MCEWEN: We have no direct detection of water.
HARRIS: 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.
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: Astrobiologist Lisa Pratt from Indiana University says, life on Earth certainly exists in similar extreme environments such as the Arctic permafrost.
LISA PRATT: 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: Richard Harris, NPR News.
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MONTAGNE: This is NPR News. Transcript provided by NPR, Copyright NPR.