This is the Web Edition of "A Trip Into Space", a Coimbra-based electronic book on space science. Both the texts and the photos are by courtesy of National Aeronautics and Space Administration.

A Trip Into Space Jupiter Satellites Jupiter's Satellite Callisto - Close Up	Index
See also: Jovian Satellite Fact Sheet, Jupiter's Satellite Callisto, Jupiter's Satellite Callisto - Surface View, Jupiter's Satellite Callisto - Valhalla, Jupiter's Satellite Callisto - Asgard

Jupiter's Satellite Callisto
Close Up

This picture of Callisto was taken by Voyager 1 Tuesday, March 6, during its approach to Jupiter's outer large satellite. The spacecraft was 350,000 kilometers from Callisto at the time and shows features about 7 kilometers across on the surface. Callisto is a little smaller than Ganymede (about the size of the planet Mercury) and is apparently composed of a mixture of ice and rock. The darker color of Callisto (about half as reflective as Ganymede but still twice as bright as the Moon) suggests that its upper surface is "dirty ice" or water rich rock frozen at Callisto's cold surface (approximately 120 degrees K or -243 degrees F at the equator). Callisto shows far more craters on its surface than Ganymede leading scientists to believe that its surface is the oldest observed so far in the Galilean system, possibly dating back to final accretional stages of planet formation 4-4.5 billion years ago. The prominent bullseye type feature in this picture is believed to be a large impact basin, similar to Mare Oriental on the Moon and the Caloris Basin on Mercury. The brighter circular spot is about 600 kilometers across. The outer rings is about 2600 kilometers across. This is the first recognized basin in the Jovian system and supports the concept of an old surface for Callisto. The lack of relief (high ridges, ring mountains, or a large central depression) suggests that the impacting body may have come close to penetrating Callisto's crust. The differences in appearance of this basin compared with those of the inner solar system may result from fracturing of Callisto's crust. The lack of obvious basins on the side of Ganymede observed by Voyager 1 and smaller number of craters on Ganymede's surface suggest to scientists that Callisto may have a thicker crust than Ganymede.

Last Update: 2005-Nov-29