The computer-generated map, based on topographic data from NASA satellites, also shows that the network of valleys on the red planet is at least twice as extensive as previously estimated.

"The relatively high values over extended regions indicate the valleys originated by means of precipitation-fed runoff erosion -- the same process that is responsible for formation of the bulk of valleys on our planet," said Wei Luo, geography professor at Northern Illinois University who co-authored the report.

"A single ocean in the northern hemisphere would explain why there is a southern limit to the presence of valley networks," Luo said.

"The southernmost regions of Mars, located farthest from the water reservoir, would get little rainfall and would develop no valleys. This would also explain why the valleys become shallower as you go from north to south, which is the case."

Rain, said Luo, "would be mostly restricted to the area over the ocean and to the land surfaces in the immediate vicinity, which correlates with the belt-like pattern of valley dissection seen in our new map."

The report appears in the Journal of Geophysical Research.

Since NASA's Mariner 9 space probe discovered the ancient Martian valley networks in 1971, debate has raged over whether the valleys resulted from water erosion -- meaning that there was humidity and rainfall -- or through groundwater sapping erosion, which can happen in cold and dry conditions.

Luo worked is co-author of the report along with Tomasz Stepinski, a staff scientist at the Lunar and Planetary Institute in Houston, Texas.

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