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Credit: MALIN SPACE SCIENCE SYSTEMS

Mystery veils the origin of the 2.5-km-wide canyon of Nanedi Vallis, one of the Martian valley systems — here pictured by the Mars Orbiter Camera. The picture was taken last month during the 87th orbit of the Mars Global Surveyor spacecraft.

Rocky outcrops occur along the upper walls, and weathered debris is found on the lower slopes and along the floor of the canyon, which cuts through cratered plains in the Xanthe Terra region of Mars.

Some features, such as terraces within the canyon and the 200-metre wide channel — both evident near the top of the picture — suggest it arose from continual fluid flow. Others, such as the lack of a contributing pattern of smaller channels on the surface surrounding the canyon, the shape of tributaries and the size and tightness of the apparent meanders, indicate that it was formed by collapse.

It is now thought likely that both continual flow and collapse were responsible for the canyon's appearance. Further observations are now planned to help to distinguish the relative effect of these and other potential formation and modification processes.