Almost as soon as Mars Global Surveyor slipped into orbit around the Red Planet in late 1997, its main camera began snapping pictures that made scientists do a double take. The walls of some otherwise nondescript craters in Mars's southern hemisphere looked as if they had been chiseled out recently by running water. But that was impossible. The atmosphere of Mars is so thin and so cold, say the textbooks, that surface water would immediately vaporize or freeze. Eager to spy more of the enigmatic features, Michael Malin and Kenneth Edgett of Malin Space Science Systems, which built and operates the camera, scrutinized the thousands of photos that Surveyor snapped during its 1998 orbits. Nothing. But in March 1999 the camera began shooting images 10 times sharper. And there they were again: deep gullies, winding channels and deltas of debris. Malin still wouldn't believe that he was seeing evidence of running water. Instead, he tried to explain away the weird landscapes as the handiwork of dry landslides, avalanches--anything but water. Finally, he could no longer avoid a conclusion that threatens--or promises--to turn decades of conventional Martian wisdom on its little green head. "We see features that look like gullies formed by flowing water," Malin said last week. "We think we are seeing evidence of a groundwater supply, similar to an aquifer."
NASA has lost more than a billion dollars' worth of hardware on and around Mars since 1993--but finding water there might right the balance sheet. The driving principle of the Mars program has been clear: follow the water. The mundane rationale is that a supply of water would make human exploration of the Red Planet far more feasible. Water is not only useful for keeping humans alive and for growing crops. It can also be broken apart, with high-school chemistry, into oxygen (good for breathing and, in liquid form, a rocket propellant) and hydrogen (which also makes a dandy rocket fuel). The more visionary rationale is that finding liquid water on Mars would take scientists a step closer to answering whether we are and always have been the lone forms of life in the vast universe. The hint of water-carved gullies on Mars, says Bruce Jakosky, director of the astrobiology program at the University of Colorado, "is the smoking gun that says there's liquid water and that Mars meets the environmental requirements to support life."
Rumors about the discovery stirred such a frenzy last week that NASA had to swat down speculation that Surveyor had spotted bubbling hot springs full of Martians. The discovery was to be announced in a paper in the June 30 issue of Science, but leaks forced the journal to release the news nine days early. What excited space enthusiasts was the possibility that Surveyor had found evidence not of just any old water flows, but of recent water flows. The notion that Mars coursed with rivers and floods billions of years ago dates back to 1972, when the Mariner 9 spacecraft returned photos of giant flood channels. But today Mars is an arid, windblown desert. "Ever since [Mariner 9], Mars science has focused on the question 'Where did the water go?' " says Edgett. "The new pictures from Global Surveyor tell us part of the answer: some of that water went underground, and quite possibly it's still there." The gullies seem to be relatively recent decor: they are not scarred by impact craters, freeze cracks or windblown deposits, as a Martian landform more than a few million years old presumably would be. "These gullies could be on the order of a million years old," says Malin, "or they could have formed yesterday."
Scientists are far from unanimous on that, however. Because any liquid water reaching the Martian surface should either boil away instantly or freeze, Mars expert Michael Carr of the U.S. Geological Survey remains unconvinced that Surveyor's gullies were carved by water. But Malin and Edgett have a hunch about how Mars might have achieved an aqueous mission impossible. Pent-up groundwater, they say, might rise to the surface from porous rock 300 to 1,300 feet below. The first water to break the surface might freeze, forming a small ice dam. So far, so by-the-textbook. Now for the ingenious part: the groundwater would keep seeping toward the surface, building up pressure behind the dam until the water burst out and thundered down the slope like flash flood in a desert arroyo. As it happens, more than 90 percent of the rivulets lie near Mars's south pole. In these frigid areas, the cold may slow down evaporation enough for ice dams to form.
NASA is being careful not to overplay its life-on-Mars card. Assistant administrator Ed Weiler harrumphed last week that the discovery of gullies "has nothing to do with the possibility of life on Mars." But other scientists can hardly contain their enthusiasm. "Mars may have had a groundwater system for most of its 4 billion years," says astrobiologist Jack Farmer of Arizona State University. "There could be a whole biosphere going on under the surface." John Priscu, a biologist at Montana State whose study of microbes frozen in Antarctic lakes has given him respect for the tenacity of life, notes that "ice can be a great refugium for bacteria; they can stay frozen for a long time and still survive."
The hints of water coursing over Mars has given a much-needed morale boost to NASA's beleaguered Mars team. These are the guys who lost the $165 million Mars Polar Lander last December (too few people, too much work, too many miscalculations), the $125 million Climate Orbiter last September (that amazing English/metric units mistake) and the $1 billion Mars Observer in 1993 (reasons still murky). The snafus forced NASA to scale back its ambitious schedule of Mars launches. But the missions still on track can, as luck would have it, build on the latest find. An orbiter due for launch next April will search for water, hopefully confirming Surveyor's find. The 2003 mission is far enough away that it can be reconfigured to exploit the hints of water even more directly. One possibility is to look for water-related minerals at the Martian surface. A more tantalizing one is to deploy a little roving lab to scoot around. If life ever did get going on Mars, the gullies would be great places to look for it--as fossils or, maybe, something livelier.