ESA's Mars Express imaged the striking upper part of the Reull Vallis region of Mars with its high-resolution stereo camera last year.
Reull Vallis, the river-like structure in these images, is believed to have formed when running water flowed in the distant martian past, cutting a steep-sided channel through the Promethei Terra Highlands before running on towards the floor of the vast Hellas basin.
This sinuous structure, which stretches for almost 1500 km across the martian landscape, is flanked by numerous tributaries, one of which can be clearly seen cutting in to the main valley towards the upper (north) side.
The new Mars Express images show a region of Reull Vallis at a point where the channel is almost 7 km wide and 300 m deep.
The sides of Reull Vallis are particularly sharp and steep in these images, with parallel longitudinal features covering the floor of the channel itself. These structures are believed to be caused by the passage of loose debris and ice during the 'Amazonian' period (which continues to this day) due to glacial flow along the channel.
The structures were formed long after it was originally carved by liquid water during the Hesperian period, which is believed to have ended between 3.5 billion and 1.8 billion years ago.
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