Cassini passed by Hyperion at a range of 514 kilometres on 26 September 2005, revealing it to have a spongey appearance. The arcuate ridge called Bond-Lassell Dorsum proved to be circular, and the rim of an impact crater of a diameter only a little less than the longest dimension of the irregularly shaped moon, complete with a raised central mound. Landslides on the steep interior walls had submerged some of the small craters on the floor of the main cavity. A 200-metre-wide crater on a patch of the dark material that occupied a topographical hollow had created bright rays, demonstrating that the dark material was a thin mantle, perhaps only tens of metres thick, over bright material that was presumably representative of the icy moon's constitution.
A small part of Hyperion imaged on 26 September 2005 at a resolution of 26 metres per pixel showing that dark material has collected in topographical lows.
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