Images from the visible light camera on NASAs Mars Odyssey spacecraft, combined with images from the Mars Global Surveyor (MGS), suggest melting snow is the likely cause of the numerous eroded gullies first documented on Mars by the Mars Orbiting Camera in 2000 by the MGS orbiter.The now-famous Martian gullies were created by trickling water from melting snow packs, not underground springs or pressurized flows, as has been previously suggested, argues Dr. Philip Christensen, the principal investigator for Odysseys camera system and a Professor at Arizona State University in Tempe. He proposes gullies are carved by water melting and flowing beneath snow packs, where it is sheltered from rapid evaporation in the planets thin atmosphere. His paper is in the electronic February 19 issue of Nature.Looking at an image of an impact crater in the southern mid-latitudes of Mars, Christensen noted eroded gullies on the craters cold, pole-facing northern wall and immediately next to them a section of what he calls "pasted-on terrain." Such unique terrain represents a smooth deposit of material that Mars researchers have concluded is "volatile" (composed of materials that evaporate in the thin Mars atmosphere), because it characteristically occurs only in the coldest, most sheltered areas. The most likely composition of this slowly evaporating material is snow. Christensen suspected a special relationship between the gullies and the snow.
http://www.nasa.gov/HP_news_03075.html
