NASA scientists made the judgement based on images taken in 2004 and 2005 revealing bright new deposits seen in two gullies on Mars that suggest water carried sediment through them sometime during the past seven years.

    "These observations give the strongest evidence to date that water still flows occasionally on the surface of Mars," said Michael Meyer, lead scientist for NASA's Mars Exploration Program.

    Scientists had previously found evidence of ice and water vapor existing on Mars. Liquid water is considered to be necessary for life.

    "The shapes of these deposits are what you would expect to see if the material were carried by flowing water," said Michael Malinof Malin Space Science Systems, San Diego, who is principal investigator for the Mars Orbiter Camera on the Mars orbiter and lead author of a report about the findings published in the journal Science.

    The atmosphere of Mars is so thin and the temperature so cold that liquid water cannot persist at the surface. It would rapidly evaporate or freeze. Researchers propose that water could remain liquid long enough, after breaking out from an underground source, to carry debris downslope before totally freezing. The two fresh deposits are each several hundred meters or yards long.

    The light tone of the deposits could be from surface frost continuously replenished by ice within the body of the deposit. Another possibility is a salty crust, which would be a sign of water's effects in concentrating the salts, NASA scientists said on Wednesday.

    Mars Global Surveyor has discovered tens of thousands of gullies on slopes inside craters and other depressions on Mars. Most gullies are at latitudes of 30 degrees or higher. Malin and his team first reported the discovery of the gullies in 2000.

    To look for changes that might indicate present-day flow of water, his camera team repeatedly imaged hundreds of the sites. One pair of images showed a gully that appeared after mid-2002. That site was on a sand dune, and the gully-cutting process was interpreted as a dry flow of sand.

    Today's announcement is the first to reveal newly deposited material apparently carried by fluids after earlier imaging of the same gullies. The two sites are inside craters in the Terra Sirenum and the Centauri Montes regions of southern Mars.