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                                      1-77 
                                                   P-21207C
                                                   March 5, 1979 
  
This picture was taken on March 4, 1979 at 2:30 A.M. PST by
Voyager 1 from a distance of 2.6 million kilometers (1.6 million
miles). Ganymede is Jupiter's largest satellite with a radius of
about 2600 kilometers, about 1.5 times that of our Moon. Ganymede
has a bulk density of only approximately 2.0 g/cc almost half that
of the Moon. Therefore, Ganymede is probably composed of a mixture
of rock and ice. The features here, the large dark regions, in the
northeast quadrant, and the white spots, resemble features found
on the Moon, mare and impact craters respectively. The long white
filaments resemble rays associated with impacts on the lunar
surface. The various colors of different regions probably
represent differing surface materials. There are several dots on
the picture of single color (blue, green, and orange) which are
the result of markings on the camera used for pointing
determinations and are not physical markings. JPL manages and
controls the Voyager project for NASA's Office of Space Science.
  
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