OFFICE OF PUBLIC INFORMATION
JET PROPULSION LABORATORY 
CALIFORNIA INSTITUTE OF TECHNOLOGY 
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 
PASADENA, CALIFORNIA 91109. TELEPHONE (2l3) 354-5011 
  
PHOTO CAPTION                                      Voyager 1
                                                   P-21287C
                                                   March 13, 1979 
  
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.
  
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