WASHINGTON, June 22 -- There may be water near the surface of Mars, pictures taken by the Mars Global Surveyor satellite show.
Researchers said Thursday that surface features on the red planet look remarkably like the gullies and deltas carved by small-scale floods on Earth. Strikingly, the landforms appear to be relatively recent, increasing the hope that usable water may still exist on Mars.
Liquid water on Mars would also boost the chances of finding life there, the researchers think.
But other scientists cautioned that, although the satellite images are "compelling," it's still hard to understand how liquid water could have existed within the past million years, since the planet has been bitterly cold throughout that time.
The research is to be publishedin the June 30 edition of the journal Science, but the data were released in advance after some details leaked out and were published on the Internet.
"Let me start by saying what (the researchers) have not found, despite recent reports," said NASA spokesman Ed Weiler. "They have not found lakes and rivers flowing on Mars, they have not found hot springs spouting out of Mars, they certainly have not found a hot tub with a Martian swimming in it."
But he said the research is the first time scientists have found evidence of water near the surface in recent times -- within the past million years and possibly still today.
Earlier images have suggested Mars once had enormous floods, billions of years ago. But the water in those floods has long since vanished and Mars is now thought to be largely desert, although there is evidence of water ice at the poles.
Michael Malin, who's the principal investigator on the Mars Orbiter camera aboard the satellite, said the new features are rare -- they only show up on between 200 and 250 of the more than 65,000 pictures the camera has transmitted to Earth.
The images show what appear to be collapsed cliffs, above channels or gullies apparently carved by flowing water. At the base of the channels are "aprons" -- delta-like areas that appear to be material carried by the flowing water.
"Had this been on Earth," Malin said, "there would have been absolutely no question there was water involved."
The features create a number of puzzles, he said, and it's possible that the coming scientific debate will find other explanations than liquid water for such features on Mars.
One of the puzzles is that most of the features are found in the frigid regions within 30 degrees of latitude of one of the poles. "The equatorial region...is devoid of these features," Malin said.
They are also facing away from the sun, meaning they get little of what solar heat is available. "These features form in the coldest locations on the planet," Malin said, "which is exactly opposite from what we would have expected."
Ken Edgett, the other author of the Science paper, said a key aspect of the research is that the features appear to be very young, because they show little evidence of weathering or meteor impacts.
They also show polygonal patterns -- which also occur in Earth's polar regions -- that resemble the pebbled surface of a basketball. On Earth, he said, "polygons like that would only be a few thousand years old."
Some of the images show evidence that airborne dust, which is constantly being deposited on the Martian surface, is being cleared away in some way. "That's what really blows my mind," Edgett said. "Some process is going on right now."
Edgett said he and Malin are suggesting that a layer of porous rock about 500 meters below the surface is filled with liquid water. Where that rock layer ends in a cliff, the water freezes, forming an ice barrier.
But, according to their senario, the pressure of water behind the barrier eventually breaks the ice and liquid water breaks through, pouring down the cliff and carving the channels seen by the Global Surveyor.
At several of the sites, the gullies group together and form clusters -- something that may point to a more defined Martian groundwater system, similar to natural rock-bound aquifers on Earth.
Geologist Michael Carr of the U.S. Geological Survey, author of a book called "Water on Mars," said the images are "compelling", but added there are two ways hopes for liquid water could be disappointed -- the researchers could be misinterpreting the pictures orthere may be some other mechanism that creates the gullies and deltas.
"I'm skeptical," Carr said, "because of what we know about conditions on Mars, which make it hard to have water near the surface." He said the average temperature in the areas where the features are found is between minus 70 and minus 100 Celsius.
"We expect the ground to be frozen three to six kilometers deep," he said. "It's really incredibly cold."
He cautioned against what he called "terrestrial chauvinism" -- the tendency to interpret photographs of Mars in the light of Earthly experience. But he added that the pictures "look just like many of the features that we see flying over the (American) West."
And he said there may be other ways the gullies and deltas could have come into existence, although it's hard to come up with such a mechanism.
But, Carr said, within a few weeks of today's announcements, "There will be all kinds of proposals as to how we could make them without water."
The presence of liquid water is regarded as one of the essential elements needed for life to exist. The others are organic molecules and energy.
"On Earth, essentially everywhere you have liquid water below the boiling point, you have life," said Bruce Jakosky, director of the Center for Astrobiology at the University of Colorado.
If these images do indicate the presence of liquid water, he said, it will be the "smoking gun" that shows that Mars has all the ingredients needed for life to exist.
Weiler said the discovery does not increase the pressure on NASA to send humans to Mars, bit it does increase the pressure to know more about Mars and about how humans can deal with the rigors of a voyage to and from the red planet.
"Before we send humans to Mars, there's a lot more homework we have to do," Weiler said.
But Malin said a manned mission may be the only way to get a final answer. "This won't be settled definitively until someone goes up to one of these cliffs and digs into it," he said.
