Asgard Multi-Ring Structure on Callisto
This mosaic shows the Asgard multi-ring structure on Callisto, Jupiter's second largest icy moon. The Asgard structure, centered near 30 degrees north latitude, 142 degrees west longitude, is approximately 1700 km across (1,056 miles) and consists of a bright central zone surrounded by discontinuous rings. The rings are fractures that formed when Callisto's surface was struck by a large comet or asteroid. Previous analysis of Asgard identified three major zones: 1) interior bright plains in the center, 2) a zone of inward facing cliffs and, 3) a zone of discontinuous concentric troughs. The mosaic combines high resolution data of 88 meters per picture element (pixel) taken on the tenth orbit of the Galileo spacecraft around Jupiter in September 1997, with low resolution data of 1.1 kilometers (km) per pixel obtained on the third orbit in November 1996. The improved resolution of images obtained by the Solid State Imaging (SSI) system on board NASA's Galileo spacecraft allows for new insights into the Asgard multi-ring system.
Galileo images show that the bright central plains includes a young dome crater, named Doh, located on its southwestern margin (at the top of the high resolution strip). Doh is about 50 km (30 miles) in diameter. Dome craters contain a central mound instead of a bowl shaped depression or the central mountain typically seen in craters. The inner rings of Asgard appear to be degraded ridges in the high resolution data, rather than inward-facing cliffs or scarps as previously interpreted from lower resolution images. In the outermost rings, dark non-ice material that slid down the walls of the troughs has made their floors darker than the surrounding cratered plains.
North is to the top of the picture. The high resolution images were obtained with the clear filter of the Solid State Imaging (SSI) system when NASA's Galileo spacecraft was less than 9,500 kilometers from Callisto. There appears to be a diffuse darker stripe, beginning near the middle and continuing down the strip of higher resolution frames. This darkening is due to the processing used to place the higher resolution frames into the background context.
The Jet Propulsion Laboratory, Pasadena, CA manages the Galileo mission or NASA's Office of Space Science, Washington, DC.
This image and other images and data received from Galileo are posted on the World Wide Web, on the Galileo mission home page at URL http://galileo.jpl.nasa.gov. Background information and educational context for the images can be found at http://www.jpl.nasa.gov/galileo/sepo.