<!--intro-->Some channels visible on the surface of Mars may have been gouged by ice, rather than by catastrophic flooding, as is generally believed. That is the view of Dr. Baerbel K. Lucchitta of the U.S. Geological Survey in Flagstaff, Arizona, who compared the Martian features with strikingly similar ones on the Antarctic sea floor. Her findings are reported in the February 1 issue of Geophysical Research Letters, a publication of the American Geophysical Union. <!--/intro--> Outflow channels on Mars may be tens of kilometers [miles] wide and hundreds of kilometers [miles] long, as are some that Lucchitta studied in Antarctica. Ice flows in streams within Antarctica's ice sheets before merging with ice shelves in the surrounding ocean; the ones she studied flow from West Antarctica into the Ross and Ronne Ice Shelves. The martian channels arise suddenly from chaotic terrains or fractures and terminate in the northern plains, where there may once have been an ocean. Both the Antarctic ice streams and some martian channels are based below sea level, which on Mars is defined as the average surface elevation of the hypothetical ancient northern plains ocean. The Antarctic channels were mapped using recently available sonar imagery. Lucchitta demonstrates that martian channels, especially one known as Kasei Valles, display similar characteristics to those of Antarctic channels known to have been carved by ice streams. She compares the Rutford Ice Stream at its confluence with the Ronne Ice Shelf, where it diverges around an ice rise, formed of more stable ice than the adjacent flow, with Ares Vallis. The latter diverges around an island and displays similar curved flow lines where it enters the hypothetical ocean. The configuration of these two streams is identical, she writes. Lucchitta infers that Ares Vallis was filled by material that had the characteristics of flowing ice that entered an ice covered body of water. She believes that dust covered ice may persist in Ares Vallis or that rocky material left an expression of the flow forms after the ice evaporated. "The observations strongly support the notion that an ocean once existed in the northern plains of Mars," she says. Another similarity between Antarctica and Mars noted in the study is that some streams and channels rise in altitude in the downstream direction. On Earth, uphill flow at the base of ice is common, because the surface gradient drives the ice, whereas water does not flow uphill for extended distances. There are differences between Antarctica and Mars regarding the origin of ice in ice streams. On Earth, the streams flow from ice sheets, while on Mars, it derived from fluids erupting from below the surface. Also, on Earth, the ice flows between ice walls, while on Mars it flowed between rock walls, but the width to depth ratio on Mars is more like that of ice streams than of mountain glaciers on Earth, Lucchitta notes.