Ever since spacecraft began sending back close-up pictures of Mars, scientists have suspected that water once flowed on the planet, eroding the surface enough to leave formations that look like dry riverbeds and huge canyons.

Since then the evidence has become much stronger and planetary scientists are now convinced that Mars once had not only snow and ice but flowing rivers and vast lakes. Although no liquid water remains on the frozen surface, it is now believed there is liquid water half a mile or so beneath the surface, where the planet's inner heat warms the rocks.

According to reports at "Water on Mars Workshop" held at NASA's Ames Research Center in California, Mars was probably a much warmer place millions of years ago, when its atmosphere of carbon dioxide was much thicker than now. Carbon dioxide is a "greenhouse gas," that lets sunlight reach a planet's surface but retards the subsequent radiation of heat back to space.

It may have been the flowing water that, indirectly, turned Mars cold, the National Aeronautics and Space Administration meeting was told. As the water weathered Martian rocks, scientists speculate, it would have made minerals available to react with the atmospheric carbon dioxide, forming carbonate compounds that are solid. As the atmosphere lost carbon dioxide, it became too thin to produce a greenhouse effect.