Signs of recent water on Mars.

Pictures showing fresh deposits of bright material on two Martian gullies provide the most compelling evidence yet that water flowed on parts of the Red Planet during the past few years, researchers say. If further evidence links bright deposits with water, the findings would indicate new places to look for signs of past or present life on Mars.

The gully images, taken in 2004 and 2005 by the recently deceased Mars Global Surveyor spacecraft (SN: 12/.9/06, p. 382), show bright streaks that weren't there in 1999. A team led by Mike Malin, who built Surveyor's camera, argues in the Dec. 8 Science that the deposits probably formed when groundwater broke through the surface and flowed downhill. Before the newest observations by Surveyor, Malin's team could assert only that water had flowed on gullies as recently as l0,000 years ago (SN: 7/1/00, p. 5).

Both the colors and shapes of the new-found streaks indicate a watery flow similar to a mudslide, assert Malin and his collaborators at Malin Space Science Systems in San Diego. The bright color could be either frost or a salty crust deposited by water, they suggest. The features weren't caused by sliding dry dust, the researchers say, because that would have exposed dark patches of subsoil, as rovers have done on Mars.

Other researchers disagree. Slumping dust might have exposed brighter material that happened to lie beneath, says Michael H. Carr of the U.S. Geological Survey in Menlo Park, Calif. "Formation of a bright streak on a slope does not mean water is involved;' he says.

Several instruments on NASA's Mars Reconnaissance Orbiter, which arrived at Mars earlier this year, have begun looking for changing brightness within the same Martian gullies. The craft's spectrometer might also determine the composition of the streaks, and a radar detector could discern structures beneath gully soil.--R.C.