NASA probe finds evidence of liquid water on Mars
BY WILLIAM HARWOOD
Posted: June 22, 2000
KENNEDY SPACE CENTER, Fla. -- Images from NASA's Mars Global Surveyor spacecraft show "compelling evidence" that liquid water has flowed on the red planet's surface in the geologically recent past and may exist today in what amounts to a water table just below the frigid surface.
Ed Weiler, NASA's associate administrator for space science, said the interpretation of the images, if proved correct, has "profound implications for the possibilities of life on Mars."
"If we ever had the desire to send humans eventually to Mars, I'd say that desire should be much stronger today," he said. "This is one heck of an interesting planet."
There is little doubt water once flowed on Mars, carving huge channels in the planet's surface billions of years ago when the atmosphere was thicker and temperatures higher. And water is still locked up in the icy soil near the martian poles.
But scientists held out little or no hope that liquid water could exist today or even in the recent geological past because of the planet's extreme+ environment. The average temperature in the polar regions, for example, ranges from -70 to -100 degrees Centigrade.
But in about 250 of the 65,000 images snapped by the Mars Global Surveyor's camera over the past two years, scientists found channels on the sides of craters and other steep slopes that appear to have been cut by flowing water or mud in the recent past. The processes may even be going on today.
Michael Malin, principal investigator for the Global Surveyor's high-resolution camera, and Kenneth Edgett, a staff scientist with Malin Space Science Systems, conclude in the June 30 issue of Science magazine that the features are best explained 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."
The age of the features, most of which are located in the southern hemisphere, is inferred from their relationship with surrounding terrain. For example, the delta-like "aprons" at the base of many of the channels, where debris from the flow fans out, cover up features like sand dunes that change shape and location on short time scales.
At a news conference today, Malin unveiled a variety of photographs that show numerous examples of freshly formed channels and aprons.
The features are found almost exclusively poleward of 30 degrees latitude, that is, most of the channels and gullies are found within 30 degrees of the north or south pole. In nearly every case, the channels face away from the equator toward one of the poles.
"So the equatorial region, which is the area we always thought would be the location where you would find the most evidence of water on Mars, is devoid of these features," Malin said.
"The second attribute that was also very puzzling is that they are found predominantly on surfaces that face away from the equator," he added. "So in the south, they're found on south-facing slopes and in the north, they're found on north-facing slopes.
"Together, those two attributes - being poleward of 30 degrees and pole facing - mean 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 theory nonetheless.
Imagine a permeable layer of rock within a half kilometer of the martian surface. Now imagine a depression, perhaps caused by a meteor impact, tectonic activity or some other geological process, that exposes the water-bearing layer to the atmosphere.
On slopes facing the sun, that is, toward the equator, water would simply boil off at the surface in a tenuous cloud of cold steam not detectable by current instruments. But on slopes facing away from the warmth of the sun, the story might be different.
In those regions, "it's very cold, so what we think happens the water reaches that surface and freezes and forms like a barrier of ice," Edgett said. "But the water pressure builds up behind that barrier, that barrier eventually breaks and you get a flow of mud, debris and rocks down the slope."
Malin and Edgett said their hypothesis is preliminary and could be in error. And Carr, while impressed with the images, cautioned that other explanations might be more likely and that a definitive answer is not yet in sight.
"On the one hand, I find the interpretation that these are water worn very compelling," he said. "On the other hand, I'm skeptical just because of the conditions that we know prevail on Mars and knowing how difficult it is to have liquid water near the surface. So we have a problem."
Carr said the average temperature in the polar regions should keep the surface solidly frozen to a depth of 3 to 6 kilometers. While scientists have no problem envisioning subsurface ice at high latitudes, liquid water is another matter.
"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."
Even so, scientists are clearly intrigued by the possibility of liquid water on Mars today.
"What's new here is that we see evidence for geologically recent liquid water in the crust and water that's much nearer to the surface and more accessible than we thought," said Bruce Jakosky, director of the Center for Astrobiology at the University of Colorado.
"So 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 Weiler: "Nobody in the community before these results were shown even proposed that water might be flowing today on Mars or even a million years ago.
"Just about any place they find liquid water below the boiling point, organic molecules and energy, they find life, whether it's on the surface of the Earth, whether it's 10,000 feet below the Earth, whether it's in radiation dumps," Weiler said.
"Life seems to have a way of finding a way to exist. Finding possible evidence for liquid water at some point in the near term on Mars has profound implications for astrobiology."
Ed Weiler, NASA's associate administrator for space science, describes the surprising discovery of recent water flows on Mars.
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Weiler explains what the water discovery means and the profound impact it could have on the search for life.
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