"While this clearly doesn't tell us anything about whether life does exist at present or ever has in the past, I think it's the smoking gun that says there's liquid water and Mars meets all of the environmental requirements to support life," said Bruce Jakoski, director of the Center for Astrobiology at the University of Colorado.
NASA's Mars Global Surveyor spacecraft has snapped more than 65,000 pictures of Mars over the past two years. In about 250 of those pictures, scientists have found channels, gullies and delta-like "aprons" that appear to have formed in the recent geologic past.
In a paper appearing in the June 30 issue of Science magazine, Michael Malin, principal investigator with the Global Surveyor camera, and Kenneth Edgett, a staff scientist with Malin Space Science Systems, conclude the channels were caused by "processes associated with ground-water seepage and surface runoff."
"We see features that look like gullies formed by flowing water and the deposits of soil and rocks transported by these flows," Malin said in a NASA statement. "The features appear to be so young that they might be forming today. We think we are seeing evidence of a ground water supply, similar to an aquifer."
There is little doubt liquid water flowed on Mars in the distant past, carving huge canyons and flood plains billions of years ago when the atmosphere was thicker and the temperature warmer. Water still exists in the polar regions of the planet, locked up in icy soil.
But few scientists held out any hope of finding liquid water today because of Mars' extreme, ultra-cold environment and low atmospheric pressure. Water exposed to such conditions would instantly boil away.
Conventional wisdom has long held the only place liquid water could exist today on Marsif at allwould be well below the surface in relatively warm layers near the equator.
But the features found by the Mars Global Surveyor were discovered primarily within 30 degrees of the north or south pole. And in nearly every case, the channels face away from the equator toward one of the poles.
"Together, those two attributesbeing poleward of 30 degrees and pole facingmean that these features form on the coldest locations at any given latitude, which is exactly opposite of what you would expected for something to be conducive to liquid water," Malin said.
But Malin and Edgett have developed a hypothesis to explain the unusual observations.
Imagine a penetrated layer of rock within a half kilometer or so of the martian surface that has been exposed to the atmosphere by a meteorite impact or some other geologic process.
On slopes facing the sun, that is, toward te equator, water would simply boil off at the surface in a tenuous cloud of hard-to-detect steam. But on slopes facing away from the warmth of the sun, the picture might be different.
"We've come up with a model to explain these features and why the water would flow down the gullies instead of just boiling off the surface," Edgett said in a NASA statement. "When water evaporates it cools the groundthat would cause the water behind the initial seepage site to freeze."
"This would result in pressure building up behind an 'ice dam,'" he said. "Ultimately, the dam would break and send a flood down the gully."
Michael Carr, a planetary scientist with the United States Geological Survey and an expert on Mars, said the imagesand Edgett's explanationare compelling. But he remains skeptical.
"Carl Sagan used to warn us about our terrestrial chauvinism," Carr said. "And this becomes a particular problem in photo interpretation in that we tend to interpret photos in light of our experiences here on Earth. Conditions are very, very different on Mars. Perhaps there are processes on Mars that do not occur on Earth."