Arachnoids, among the more remarkable features found on Venus, are seen on radar-dark plains in this Magellan image mosaic in the Fortuna region. The image is centered at about 40 degrees north latitude, 18 degrees longitude. Arachnoids are circular to avoid features with concentric rings and a complex network of fractures extending outward. The top image is a mosaic of Magellan radar images with a resolution of 120 meters. The MGN image is illuminated from the left at an angle of 35 degrees off vertical. The bottom image was obtained by the Soviet Venera 15/16 radar system. The Venera image has a resolution of about 1.5 kilometers (.9 mile). The bottom image is illuminated from the right side at 10 degrees off vertical. This image illustrates the importance of multiple mapping angles for some areas. Notice in the top image, the bright, irregular lava flows, just to the left of bottom center. The rough lava surface is bright at the Magellan incidence angle. The lava flows are not detected in the Venera image at the higher incidence angle which is less sensitive to roughness variations. However, the high angle Venera image reveals the topography which is not easily seen at the Magellan incidence angle. In the 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.
