When scientists have looked for evidence of water on the Red Planet, they have looked at formations that are billions of years old, canyons and cliffs etched during a time when Mars looked more like the Earth of today.
New images from a satellite orbiting Mars, however, suggest that large quantities of water gushed from underground springs in the relatively recent past, carving paths down the sides of craters and steep valley scarps.
While the pictures do not show liquid water, as had been suggested by some media reports, they raise the tantalizing prospect that water, likely in the form of ice, might lie relatively close to the desert-like Martian surface.
Such deposits raise new questions about the forces at work on Mars. But more important, they also bolster the possibility that primitive life could exist there. Moreover, they could also provide vital resources that could be used during manned Mars missions.
"This is the smoking gun that says that Mars has all the requirements for life," says Bruce Jakosky, director of the Center for Astrobiology at the University of Colorado at Boulder.
The team of planetary scientists that analyzed the images will publish a fuller review of the data next week in the journal Science.
The researchers emphasize that their interpretation of the images is preliminary. But if they're right, the discovery could help solve a Mars riddle: Where all its water went.
Early in the planet's history, vast amounts of water sculpted enormous valleys, deep canyons, and flood channels.
"The question is, where is that water now?" says James Bell, an astronomer at Cornell University deeply involved in NASA's Mars exploration program.
The planet's ice caps clearly hold some, and much evaporated into space as the planet's climate cooled and its atmosphere thinned. But some planetary scientists say that water may be trapped as subsurface ice.
"If water is seeping out, that lends support to the hypothesis that water lies below the surface, Bell says. "And that makes it available as a resource."
The seepage indicated in the new photos could have happened at any time during the past few million years - recent, by geologic standards. But it points to a potential underground source that may still be present.
The water could be consumed directly by any humans who travel there, or it could be used to make rocket fuel by separating the oxygen and hydrogen in water molecules.
In addition, since water is one of the key ingredients for organic life, icy deposits could hold the frozen remains of past microbial life forms. Depending on their age, they could also be incubators for life today.
"We've found life here on Earth underneath meters of ice," says Darren Williams, an astrophysicist at Penn State University in State College. "Life likes these [environments] just as much as it likes the beaches at Cancun."
The place where the gullies and debris were found is curious. They appear in high latitudes, where the Martian climate is colder, presenting a puzzle for planetary scientists who would have been less surprised if the flows appeared nearer the warmer equator or ancient volcanos where residual heat could melt underground ice deposits.
The new images could give the Mars program a much-needed boost. Last fall, the program suffered back-to-back failures of two high-profile missions.
In September, the Mars Climate Orbiter burned up in the Martian atmosphere instead of entering its designated orbit around the planet. Then, in early December, the Mars Polar Lander was lost.
Although the specific causes for failure differed, an independent study released in March laid the blame more broadly on ineffective management and an overzealous adherence to the NASA mantra of "faster, cheaper, better."
Next month, NASA "has a very big decision coming up," regarding its plans for a Mars mission to be launched in 2003, says Bell.
At issue is whether the mission should focus on another orbiter like the one that sent back the recent pictures, or whether it should send a lander. If water-ice deposits lie close to the surface in accessible locations, that could tip the scale toward a lander.
The current orbiter, the Mars Global Surveyor, was launched in December 1996. The $155 million craft is roughly the size and weight of a Volkswagen Beetle. It has a camera, an infrared sensor to identify soil and rock characteristics, and a laser altimeter to create topographical maps of the planet's landscape.
(c) Copyright 2000. The Christian Science Publishing Society