This image provided by NASA shows long, narrow gullies along the walls of a crater on the giant asteroid Vesta taken by the NASA Dawn spacecraft. Scientists are unclear how these gullies formed and work is underway to determine their origin. (AP Photo/NASA)

Associated Press
This image provided by NASA shows long, narrow gullies along the walls of a crater on the giant asteroid Vesta taken by the NASA Dawn spacecraft. Scientists are unclear how these gullies formed and work is underway to determine their origin. (AP Photo/NASA)
This image provided by NASA shows long, narrow gullies along the walls of a crater on the giant asteroid Vesta taken by the NASA Dawn spacecraft. Scientists are unclear how these gullies formed and work is underway to determine their origin. (AP Photo/NASA)
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