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Jupiter's Satellite, Callisto
Callisto [kah-LISS-toe] is the second largest moon of Jupiter. It has no atmosphere and is about the same size as Mercury. It is composed of approximately equal proportions of ice-water and rock. Its crust is very old and heavily cratered with remnant rings of enormous impact craters. The largest craters have been erased by the flow of the icy crust over geologic time. Craters and the associated concentric rings are about the only features to be found on Callisto.
The moon is probably composed of a large rocky core surrounded by water and ice, giving it a dark color. Meteorites have punctured holes in the crust, causing water to spread over the surface and forming bright rays and rings around the crater.

      MEAN RADIUS:               2404 km
      MASS:                      0.0179 (Earth=1)
      DENSITY:                   1.81 (gm/cm)
      GRAVITY:                   0.124 (Earth=1)
      ORBIT PERIOD:              16.689 (Earth days)
      ROTATION PERIOD:           16.689 (Earth days)
      SEMIMAJOR AXIS OF ORBIT:   1,883,000 km
      ECCENTRICITY OF ORBIT:     0.007 (Earth=0.017)