Outflow areas seen by Viking Orbiter 2.

Back in the 1970s, three Mars orbiters sent back images that revealed landscape shapes apparently formed by flowing water in the distant past. NASA's Mariner 9, Viking 1 and Viking 2 spacecraft showed us Martian channels carved as if by rivers and outwash plains scoured as if by floods.

Geologists estimate that astoundingly heavy flows, equal to thousands of Mississippi Rivers, would have been necessary to shape some of the surface features on Mars. Yet Mars' atmosphere is too thin and cold for water to remain liquid at the surface. Instead of melting, warmed water ice on Mars turns directly into vapor, the way carbon-dioxide "dry" ice does on Earth. To account for the signs of copious water flows in the past, scientists figure that long, long ago, Mars had a thicker atmosphere than it does now.

Pink clouds consist of water ice condensed on reddish dust particles.

Water ice is present on the surface of Mars' north polar cap and in ice-crystal clouds. Plenty of frozen water probably persists in permafrost layers underground, near the surface at the poles and farther buried at lower latitudes. If some underground areas are warm, they might even hold liquid water in the pores between grains of rock.

In June 2000, a headline-making discovery raised the possibility that liquid water may still sometimes come to the surface of Mars. Gullies that begin partway down the slopes of several Martian crater walls and hillsides appear to have formed recently, according to scientists analyzing some of the highest-resolution pictures from NASA's Mars Global Surveyor, which has been orbiting Mars since 1997. Scientists hypothesize that liquid water underground may build enough pressure to burst out and cut the gullies before evaporating. Scientists suggest these features look so recent that the process may still be active.

Layered rock on Candor Chasma.

Mars Global Surveyor has also discovered two types of sites that apparently stayed wet for long periods in the past, which could make them targets to search for fossil evidence of life. Pictures released in December 2000 reveal areas where bedrock shows the layered texture usually formed by accumulation of sediments in lakes or seas, although accumulation of volcanic ash is another possible explanation. Also, the orbiter's tool for mapping the mineral makeup of Mars' surface identified deposits of coarse-grained hematite, a mineral that usually forms in wet environments, such as hot springs on Earth.

MGS instrument (left) and Odyssey instrument (right).

However, Mars Global Surveyor has not found other types of wet-birthed minerals, such as carbonates, that some scientists had anticipated as indicators of bygone surface water. Those may be present in patches too small for Mars Global Surveyor to discern. If so, NASA's newest red planet mission, the 2001 Mars Odyssey, may find them. That orbiter, launched April 7 2001, carries a mineral-identifying instrument with about 30 times sharper resolution than the one on Mars Global Surveyor. Another instrument on Mars Odyssey can detect frozen or melted water as much as a meter (three feet) below the surface.

In 2003, NASA will send two rovers to inspect areas selected for signs of past or present water. Later, the search for water will guide choices about where robots will gather samples of Martian rock and soil for mailing back to Earth.

Water Everywhere

Europan Evidence