<!--intro-->Mars may once have been a very wet place. A host of clues remain from an earlier era, billions of years ago, hinting that the Red Planet was host to great rivers, lakes and perhaps even an ocean. But some of the clues are contradictory -- they don't all fit together in a coherent whole. Little wonder, then, that the fate of water on Mars is such a hotly debated topic.<!--/intro--> The reason for the intense interest in Martian water is simple: Without water, there can be no life as we know it. If it has been 3.5 billion years since liquid water was present on Mars, the chance of finding life there is remote. But if water is present on Mars now, however well hidden, life may be holding on in some protected niche. Based on what we have observed so far, Mars today is a frozen desert. It's too cold for liquid water to exist on its surface and too cold to rain. The planet's atmosphere is also too thin to permit any significant amount of snowfall. Even if some internal heat source warmed the planet up enough for ice to melt, it wouldn't yield liquid water. The Martian atmosphere is so thin that even if the temperature rose above freezing the ice would change directly to water vapor. Signs of Heavy Flooding But there must have been water, and plenty of it, in Mars's past. That is evident from the massive outflow channels that are found, mostly, in the northern lowlands. The intensity of the floods that carved these channels was tremendous, perhaps reaching discharge rates as high as 10,000 times that at which the Mississippi, when flooded, pours into the Gulf of Mexico. What caused these giant floods? Was it a climate change, perhaps brought about by a change in Mars's orbit? Or was the planet's own internal heat responsible? And, whatever mechanism caused the floods in the first place, where has all that water gone? Was it absorbed into the ground where it remains today, frozen? Or did it dissipate into the Martian atmosphere, where it was subsequently lost to space? No-one knows for certain the answers to these questions. Some scientists believe that the catastrophic floods that carved the outflow channels occurred nearly simultaneously, releasing such vast quantities of water that they merged into an ocean that covered the northern lowlands. Tim Parker of NASA's Jet Propulsion Laboratory first proposed such an idea in 1989. Parker, examining images taken by the Viking Orbiters, found what he believed were remnants of two ancient ocean shorelines, which he called "contacts," one inside the other, in the Martian north. Expanding on this notion, in 1991 Vic Baker of the University of Arizona, suggested that Mars might not be geologically dead and permanently frozen. Instead, he proposed, Mars might undergo cycles, or pulses -- first heating up, releasing groundwater and forming an ocean in the north, then dissipating the ocean back into the planet's crust and re-freezing. More recently, Jim Head and colleagues at Brown University, found evidence that is consistent with a shoreline that might indeed have existed at the inner of Parker's two proposed contacts, contact 2. Head and colleagues examined elevation data gathered by the Mars Orbiter Laser Altimeter (MOLA) on board the Mars Global Surveyor (MGS) and found that the elevation at points along contact 2 were much closer to a straight line than those at contact 1. They also found that the terrain below this elevation was smoother than the terrain above it. Both of these findings are consistent with the former presence there of an ocean. But the story doesn't end there. Shortly after Head and colleagues published their findings, Mike Malin and Ken Edgett of Malin Space Systems used the Mars Orbital Camera (MOC) aboard MGS, to take a series of high-resolution images of contact 2 terrain. Their conclusion: there's nothing there. And the debate continues. Says Mike Carr of the U.S. Geological Survey, author of the book Water on Mars, "We're getting all this new data from MGS, and I think a lot of it is just not understood yet. It's very hard to understand. The whole business of the oceans, the evidence is so contradictory." Mysterious Valleys Mars's small-valley networks, which occur mainly in the southern highlands, pose another perplexing problem. Scientists who first studied images of these valleys thought they resembled river valleys on Earth. So, they reasoned, a similar process, the runoff of rainwater, must have formed them. For Mars to be warm enough to rain, however, it would have needed a much thicker atmosphere than it has today. And no-one has come up with a clear-cut explanation for how such an atmosphere could have formed. One alternative theory is that a process known as sapping, or collapse caused by the softening of the soil by groundwater, created the valleys. Yet another notion is that perhaps glaciers covered the regions around the valleys, and that glacial meltwater carved them. As with Mars's other watery mysteries, however, the question of how the valley networks formed remains unanswered. And if these vexing problems weren't enough, recent images from MOC reveal a startling new puzzle. In nearly a dozen different locations on Mars - all of them far from the equator - there are signs that water has been seeping out of the walls of valleys and craters, forming small gullies. Some scientists speculate that this activity is very recent, perhaps occurring within the past 10 years; others say 10 million years is more likely. Yet many aspects of these seepage gullies defy common sense. "They sure look like water-worn features," says Mike Carr, "but they seem to contradict what we know about the stability of water." They occur not only in the coldest regions on Mars, but on slopes facing away from the Sun, where the temperature rarely gets above minus 50 degrees Centigrade. Yet the water appears to be seeping out from only 100 meters below the surface, a depth at which scientists previously believed Mars's crust to be frozen solid. Scientists are busily working to devise an explanation for this phenomenon. There is one additional thorn in the side of those who study water on Mars. No evidence of carbonates has yet been found anywhere on the planet. Carbonates are minerals that form readily when liquid water reacts with carbon dioxide in the atmosphere. If Mars had abundant liquid water in its past, carbonates should be detectable in the Martian rock record. The Thermal Emission Spectrometer (TES) instrument aboard MGS was designed to look for just such a signature. But so far it has found none. Perhaps other evaporites, such as sulfates (as detected in Martian meteorites and interpreted from landing site analyses), are the dominant material of this type on Mars. What Next? New debates will undoubtedly emerge as data from Mars Global Surveyor is digested. In 2001, NASA will send a new orbiter to Mars, which will include a higher-resolution spectrometer to search for carbonates. In 2003, NASA will send two rovers to Mars to hunt for water's signatures in rocks and soil on the surface. But many questions about the history of water on Mars are likely to remain unanswered until samples are returned from the Red Planet for examination on Earth. Says Carr, "I think the sample return is what we want, particularly of sediments. And if we could get samples of things like this back on Earth, I think it would do an awful lot to help us understand what's going on [on Mars]."