Planetary geologists made the discovery by combing through recent photos taken by the Mars Global Surveyor spacecraft. The images show the smallest features ever observed from martian orbit, about the size of a sport-utility vehicle.
Sites mostly in the south
"These are new landforms that have never been seen before on Mars," principal investigator Michael Malin said. "We see features that look like gullies formed by flowing water and the deposits of soil and rocks transported by these flows."
Mostly in high latitudes and the southern hemisphere, the sites include gullies and alluvial fans within the walls or rims of pits, valleys and impact craters. More than 90 percent occur south of the equator and most occur on slopes that face away from the sun.
"We were quite surprised and confused by it. It didn't fit our model of what Mars is like," said Malin of the Surveyor data. Malin Space Science Systems operates a camera onboard the NASA satellite, which has orbited Mars since 1997.
Scientists have long thought Mars' surface coursed with water billions of years ago, based on evidence of liquid erosion and signs of ancient channels and seas. But the water all but disappeared as the planet cooled and its atmosphere thinned.
'Dragged kicking and screaming'
Water is known to exist today as ice in the northern polar cap and as vapor in faint clouds. But the presence of liquid water near the surface could strengthen the theory that life exists or once existed on Mars.
"If life ever did develop there, and if it survives to the present time, then these landforms would be great places to look," Weiler said.
Liquid water on Mars would also make travel to the planet easier. Astronauts could convert water into hydrogen and oxygen, using both as rocket fuel and the second for breathing gas.
Ken Edgett, co-author of the Science magazine report, was at first skeptical that water formed the unusual features.
"I was dragged kicking and screaming to this conclusion," but the visual evidence linking groundwater discharges as the cause was too strong, he said.
The water is thought to exist about 100 to 400 meters (300 to 1,300 feet) below the surface. Malin estimated the volume of a typical flow at about 2,500 cubic meters (90,000 cubic feet), "which would fill half a dozen or so swimming pools."
Troubling questions remain
Still some troubling questions remain. Why do the discharges take place in the coldest areas on Mars, near the poles and on slopes facing the poles?
Malin suggested that the groundwater could be some exotic salty brine with unknown physical properties.
U.S. Geological Survey scientist Kenneth Tanaka suggested in Science that frozen carbon dioxide, flashing into a liquid and then gas, could be the culprit.
NASA plans to launch an orbiter in 2001 that will examine the seepage sites for evidence of water-related minerals. The agency is considering two options for an unmanned mission to Mars in 2003.
The Mars Polar Lander was to search for martian water in 1999. The doomed robot ship was to dig beneath the surface and study the atmosphere but went silent as it entered the atmosphere of the red planet.
Correspondent Miles O'Brien contributed to this report.