A dark feature about 200 km across has been named Olbers in honor of Vesta 's discoverer , but it does not appear in elevation maps as a fresh crater would , and its nature is presently unknown , perhaps an old basaltic surface . A dark albedo feature about 200 k across has been named Olbers in honor of Vesta 's discoverer , but it does not appear in elevation maps as a fresh crater would . Its nature is presently unknown ; it may be an old basaltic surface .