Magellan image mosaic Fortuna region. The image is centered at about 40 degrees north latitude, 18 degrees longitude. As the name suggests, arachnoids are circular to ovoid features with concentric rings and a complex network of fractures extending outward. In this image, the arachnoids range in size from approximately 50 kilometers (29.9 miles) to 230 kilometers (137.7 miles) in diameter. Since arachnoids are similar in form but generally smaller than coronae (circular volcanic structures surrounded by a set of ridges and grooves as well as radial lines), one theory concerning their origin is that they are a precursor to coronae formation. The radar-bright lines extending for many kilometers may have been caused by an upwelling of magma from the interior of the planet which pushed up the surface to form "cracks." Radar-bright lava flows are present in the central part of this image, also indicative of volcanic activity in this area. Some of the fractures cut across these flows, indicating that the flows occurred before the fractures appeared; such relations between different structures provides good relative age dating of events. At present, arachnoids are found only on Venus and can now be more closely studied with the high resolution (120 meter/0.07 mile) radar imagery from Magellan.Arachnoids, one of the more remarkable features found on Venus, are seen on radar-dark plains in this
