NASA / SCIENCE PHOTO LIBRARY NASA / SCIENCE PHOTO LIBRARY
Arachnoid structures in Fortuna Regio, Venus. The data for this image were gathered by the Magellan radar-mapping spacecraft in early 1991. Arachnoids are the large circular features with radiating fault lines, and are thought to be an early stage of caldera formation. At this point, it is thought that large magma chambers have developed below the surface, forcing the surface material to rise and crack. When the magma eventually retreats, the surface material collapses into the space left behind to form the caldera.
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