Reull Vallis, the river-like structure, 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.
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NASA has released a new panorama from its Mars Exploration Rover Opportunity, showing the terrain where the robot spent the four-month Martian winter. The full-circle scene combines 817 images shot by the panoramic camera (Pancam).