Nanedi Valles valley raises questions

A new set of images, taken by the High Resolution Stereo Camera (HRSC) on board ESA's Mars Express spacecraft, show the 800-kilometre valley long Nanedi Valles valley system. This steep-sided feature may have been formed in part by free-flowing water, but this is open to debate. Some researchers suggest the impressive feature may have been caused by 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.