But the water has since retreated, presumably underground, and appears to have occasionally reached the surface in large floods.
(Read related story: "Mars's Water Could Be Below Surface, Experts Say" [January 25, 2007].)
Now that it is widely agreed that water once flowed along extensive networks of cracks, a new clue falls into place in a lingering mystery.
"One of the problems [was] how, if the water is underground, you get so much of it to the surface at once," Clifford said.
Other Ingredients of Life
Water appears to have been very common at various times on Mars's surface. So in the search for life it's now time to do more than seek out places where the red planet was once wet, said Tori Hoehler, an astrobiologist with NASA's Ames Research Center in Moffett Field, California.
Instead, scientists need to start thinking about other possible requirements for life, such as energy, Hoehler said.
For example, he said, microorganisms might be able to harness chemical energy from Martian rocks, as some earthly bacteria appear to do.
"This is the next thing we should be looking for on Mars," he said.
But expanding the search for life requires studying the geology in more detail, the scientists said.
NASA's Opportunity rover is currently on the rim of Victoria Crater, where scientists have observed outcrops of rock similar to that seen in Okubo's photos. (Related photos: Mars rovers).
"If we can get to [those outcrops]," Hoehler said, "it would be a great to visit them."
But, Okubo added, "right now, they're on the far side of the craters, so it may take a while for Opportunity to get there."
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