This image, taken by the High Resolution Stereo Camera (HRSC) on board ESA's Mars Express spacecraft, shows the Nanedi Valles valley system, a steep-sided feature that may have been formed in part by free-flowing water.
They show the region of Nanedi Valles, a roughly 800-kilometre valley extending southwest-northeast and lying at approximately 6.0Â° North and 312Â° East in the region of Xanthe Terra, southwest of Chryse Planitia.
Here, Nanedi Valles ranges from approximately 0.8- to 5.0-kilometre wide and extends to a maximum of about 500 metres below the surrounding plains. This valley is relatively flat-floored and steep-sloped, and exhibits meanders and a merging of two branches in the north.
The origin of these striking features remains heavily debated.
Some researchers point to sapping (erosion caused by ground-water outflow), while others suggest that flow of liquid beneath an ice cover or collapse of the surface in association with liquid flow is responsible for the valley's formation.
While the debate continues, it seems likely that some sort of continuous flow rather than a single flooding event created these features.