From http://www.jpl.nasa.gov/galileo/

   
   
            Possible Internal Structures of the Galilean Satellites
                                       
   
   
   NOTE: This JPEG image is made available in order to share with the
   public the excitement of new discoveries being made via the NASA/JPL
   Galileo spacecraft. Galileo scientists are in the process of
   calibrating and validating this data. The full digital image necessary
   for scientific analysis will be released within one year of receipt of
   this orbit's last data.
   
   This image is available only on the WWW; it is not available in
   hardcopy or other forms.
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   Possible Internal Structures of the Galilean Satellites
   
   Cutaway views of the possible internal structures of the Galilean
   satellites. Ganymede is at the lower left, Callisto at the lower
   right, Io on the upper left, and Europa on the upper right. The
   surfaces of the satellites are mosaics of images obtained in 1979 by
   NASA's Voyagerspacecraft, and the interior characteristics are
   inferred from gravity field and magnetic field measurements by NASA's
   Galileo spacecraft. The satellites are shown according to their actual
   relative sizes. Ganymede's radius is 2634 kilometers (km); Callisto's
   is slightly smaller at 2403 km; Io's radius is 1821 km, similar to the
   1738 km radius of our Moon;Europa's radius is 1565 km, not too much
   smaller than our Moon's radius. With the exception of Callisto, all
   the satellites have metallic (iron, nickel) cores (shown in gray)
   drawn to the correct relative size. Again, with the exception of
   Callisto, all the cores are surrounded by rock (shown in brown)
   shells. Io's rock or silicate shell extends to the surface, while the
   rock layers of Ganymede and Europa (drawn to correct relative scale)
   in turn surrounded by shells of water in ice or liquid form (shown in
   blue and white and drawn to the correct relative scale). Callisto is
   shown as a relatively uniform mixture of comparable amounts of ice and
   rock. The surface layers of Ganymede and Callisto are shown as white
   to indicate that they may differ from the underlying ice/rock layers
   in a variety of ways including, for example, the percentage of rock
   they contain. The white surface layer on Europa could have similar
   significance, although it could also suggest an ice layer overlying a
   liquid water ocean. Galileo images of Europa suggest that a liquid
   water ocean might now underlie a surface ice layer several to ten
   kilometers thick; however, this evidence is also consistent with the
   existence of a liquid water ocean in the past. It is not certain if
   there is a liquid water ocean on Europa at present.
   
   The figure was created by Zareh Gorjian and Eric DeJong of the Jet
   Propulsion Laboratory Digital Image Animation Lab. The Jet Propulsion
   Laboratory, Pasadena, CA manages the Galileo mission for NASA's Office
   of Space Science, Washington, DC. JPL is an operating division of
   California Institute of Technology (Caltech).
   
   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://www.jpl.nasa.gov/galileo. Background information and
   educational context for the images can be found at URL
   http://www.jpl.nasa.gov/galileo/sepo