Image: Courtesy of AGU

Lineations on the plains of Mars show evidence that they were carved by glacial ice, not floods. Using the Antarctic as her model, Baerbel K. Lucchitta has concluded that the flutes and drumlines may have been gouged by ice streams. Several regions in the Antarctic seafloor (in the Ross Sea, just off the Ross and Ronne Ice Shelves), which were recently imaged with sonar, are "strikingly similar" to outflow channels in the Kasei Valles and Ares Vallis, Lucchitta writes in an article to be published February 1 in the Journal of Geophysical Research Letters.

As in the Antarctic, the channels left by ice streams seem to be "grounded" below the shoreline of the hypothetical ocean that once filled Mars¿s northern plains (see images above). The channels are also inclined downstream, which is typical of ice flows that can push up inclines. Such gouge marks are unlikely to be made with moving water, which cannot flow uphill for extended distances, even during large-scale flooding.

Lucchitta, a geologist at the U.S. Geological Survey in Flagstaff, Ariz., speculates that the ice was melted in the ground, perhaps due to a change in the planet¿s atmospheric pressure or because of heating by the Tharsis volcanoes, which are in the vicinity of the Kasai Valles. The meltwater would have shot up like a geyser and frozen again, building up until it could flow in an icestream. Another possible source, she suggests, is an ice cap that may have sat on the Tharsis rise.

Lucchitta¿s final conclusions are that large-scale catastrophic floods may not have been necessary to carve the surface of Mars: smaller floods may have given rise to ice streams that inexorably flowed over debris and dust, in the same way ice flows on the Antarctic seafloor.