A century after scientists first thought they saw canals on Mars, astronomers on Thursday released detailed photographs showing dozens of gullies and channels where water may indeed still be flowing on the red planet, providing a possible habitat for life.
The conclusion, spurred by striking images from the Mars Global Surveyor, would make Mars the only world besides Earth where liquid water is known to exist in the open. If the researchers are right, Mars may have active aquifers just beneath the surface.
Scientists had seen evidence of water on Mars before in the images sent from a roving robot that showed ancient waterways carved perhaps billions of years ago. Astronomers also know there is ice–frozen water–at the poles of Mars.
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But the new photographs appear to show fresh gullies carved into young geological features, like dunes, that can change year to year.
Saying that water etched those paths causes a host of new problems for scientists, however, for it contradicts the long-held belief among many experts that water could never flow in today’s bitterly cold and thin Martian atmosphere.
Even members of the team from the National Aeronautics and Space Administration that analyzed the photos at first refused to believe what their eyes were telling them.
“I was dragged kicking and screaming to this conclusion,” said Ken Edgett, a staff scientist at the company that operates the probe’s imaging equipment for NASA.
But Edgett and numerous independent geologists who have examined the photos said they bear an uncanny resemblance to water-related features in the mountains of Colorado or the hills of Antarctica.
“This is some of the most exciting data we’ve had concerning this issue,” said Mike Carr, a planetary geologist with the U.S. Geological Survey and author of the book “Water on Mars.”
Even Ralph Harvey, who was among the first experts in 1996 to challenge claims that NASA scientists had found signs of past life in a meteorite from Mars, said he agrees with the interpretation that the channels were cut by water.
“I think it’s great,” said Harvey, a geologist at Case Western Reserve University. “It means water has been around on Mars for a while.”
Experts emphasized that the findings, to be published next week in the journal Science, still must pass the test of independent scientific scrutiny and alternative explanations, such as the possibility that some form of carbon dioxide gas or volcanic lava formed the channels.
The lure of Martian water dates back to the late 1800s, when Italian astronomers reported seeing features they called “canali” on the planet’s surface. American skygazers such as Percival Lowell fancied they could see artificial canals on the planet surface–though the Italian word can simply mean “channels.”
The first real evidence that water once gurgled on Mars came from the Mariner 9 probe in the 1970s, which revealed the Valles Marineris, a huge, ancient waterway the size of the continental U.S.
But until now, most experts thought the era of open-air water belonged to a distant past when Mars was warmer and wetter, more than 3 billion years ago.
Astronomers spotted the first hints of new channels two years ago, when the Global Surveyor first went into orbit around Mars. Those photos, however, could not make out features any smaller than 60 feet across. Resolution in the new images is good enough to see an automobile.
With the improved photo quality, scientists found signs of surface runoff at more than 120 locations around the planet. The features are most plentiful in valleys such as Nirgal Vallis and Dao Vallis, and the Gorgonum Chaos region of Mars’ southern hemisphere.
The formations looked just like what happens here when water bursts from an underground aquifer on a mountainous slope, leaving deep channels and an apron of sediment below. In many instances, it appeared rivulets had flowed in several places from a single layer of subsurface water.
“Had this been on Earth, there would be no question that water was associated with the formation of this feature,” said Michael Malin, principal investigator on the Mars orbital camera and head of San Diego-based Malin Space Science Systems.
But on Mars, where temperatures can plunge to nearly 150 degrees Fahrenheit below zero, many scientists believed any water would be frozen solid to nearly 2 miles below the surface.
Even more puzzling, the runoff appeared to happen mostly near the planet’s poles, on cliffs facing away from the equator.
“It means these features form at the coldest locations, which is exactly the opposite of what you would have thought,” Malin said.
Malin’s theory is that water percolating from subsurface reservoirs evaporates quickly on rocks that get direct sunlight, but can slowly build up frost and ice in cooler surface areas. Increasing pressure from the reservoir and perhaps a spike in temperatures may cause a sudden outflow.
“Maybe it happens one day out of the Martian year, maybe once in a century,” Harvey said.
Water in the form of ice is plentiful in the solar system, having been found or theorized to exist on comets and most moons, and even in shadowed craters at the poles of Mercury.
Aside from Earth and Mars, the only likely home for liquid water is Jupiter’s moon Europa, which experts believe may hide a huge ocean beneath a crust of fractured ice.
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Readily available water near the Martian surface could greatly increase the chances for indigenous life or human colonization of the planet, experts said.
“One would want water in the liquid state in order to find the sorts of organisms we have here on Earth,” said James Staley, a microbiologist and director of the astrobiology program at the University of Washington. “That would be the minimum criterion for life.”
After the embarrassing failures of two Mars probes last year, the tantalizing signs of water suggest a focus for future missions, said Bruce Murray, a professor of planetary science and geology at the California Institute of Technology.
“All of a sudden, nature comes through and provides an appealing target,” said Murray, who had planned experiments for both of the lost probes.
Although NASA officials said they must learn much more about the effects of space travel on the body before sending people on the long voyage to the red planet, Thursday’s report may raise the stakes for such a trip.