Images from the visible light camera on NASA’s Mars Odyssey spacecraft, combined with images from NASA’s Mars Global Surveyor, suggest melting snow is the likely cause of the numerous eroded gullies first documented on Mars in 2000 by Global Surveyor. The now-famous martian gullies were created by trickling water from melting snow packs, not underground springs or pressurized flows, as had been previously suggested, argues Dr. Philip Christensen, the principal investigator for Odyssey’s camera system and a professor from Arizona State University in Tempe.