Changes

Except for Callisto, all the moons appear to have metallic (iron, nickel) cores. Again, except for Callisto, all the cores are surrounded by rock shells. Io's rock or silicate shell extends to the surface, while the rock layers of Ganymede and Europa are in turn surrounded by shells of water in ice or liquid form. Callisto is shown as a relatively uniform mixture of comparable amounts of ice and rock. Recent data, however, suggests a more complex core than has been previously supposed.

Except for Callisto, all the moons appear to have metallic (iron, nickel) cores. Again, except for Callisto, all the cores are surrounded by rock shells. Io's rock or silicate shell extends to the surface, while the rock layers of Ganymede and Europa are in turn surrounded by shells of water in ice or liquid form. Callisto is shown as a relatively uniform mixture of comparable amounts of ice and rock. Recent data, however, suggests a more complex core than has been previously supposed.

The surface layers of Ganymede and Callisto may differ from the underlying ice/rock layers in a variety of ways, including the portion of rock they contain. Europa's surface could be similarly composed, or it could have an ice surface overlying a liquid water ocean. Galileo images of Europa suggest that a liquid water ocean might now underlie a relatively thin surface ice/rock layer. The thinness of the surface was the key fact that informed the decision to crash Galileo into Jupiter rather than risk an unplanned crash landing on Europa.<ref name=rense>"[http://www.rense.com/general41/galui.htm Galileo Crashes into Jupiter]." ''ABC News Online'', Australian Broadcasting Company, September 21, 2003. Accessed February 20, 2008.</ref><ref name=Macey>Macey, Richard. "[http://www.smh.com.au/articles/2003/09/14/1063478069136.html Galileo's death leap onto Jupiter]." ''Sydney Morning Herald Online'', September 15, 2003. Accessed February 20, 2008.</ref>

The surface layers of Ganymede and Callisto differ markedly from the underlying ice/rock layers in a variety of ways, including the portion of rock they contain and definitely the relative relief of the surfaces. Ganymede has very high relief while Callisto has very low relief and looks flat by comparison. Europa's surface is one of brittle ice overlying either another, much thicker ice layer or a liquid water ocean. Galileo's recent flyby data, including magnetometer measurements, suggest that a liquid water ocean might now underlie a relatively thin surface ice/rock layer. The thinness of the surface was the key fact that informed the decision to crash Galileo into Jupiter rather than risk an unplanned crash landing on Europa.<ref name=rense>"[http://www.rense.com/general41/galui.htm Galileo Crashes into Jupiter]." ''ABC News Online'', Australian Broadcasting Company, September 21, 2003. Accessed February 20, 2008.</ref><ref name=Macey>Macey, Richard. "[http://www.smh.com.au/articles/2003/09/14/1063478069136.html Galileo's death leap onto Jupiter]." ''Sydney Morning Herald Online'', September 15, 2003. Accessed February 20, 2008.</ref>

 

Much of the preceding is taken from the original caption to the image shown, written by JPL/NASA staff, but revised in light of additional data received subsequent to the composition of this image.<ref>"[http://photojournal.jpl.nasa.gov/catalog/PIA01082 Catalog page for Image PIA01082]," ''NASA/JPL Photojournal''. Accessed February 29, 2008.</ref>

Much of the preceding is taken from the original caption to the image shown, written by JPL/NASA staff.<ref>"[http://photojournal.jpl.nasa.gov/catalog/PIA01082 Catalog page for Image PIA01082]," ''NASA/JPL Photojournal''. Accessed February 29, 2008.</ref>

== Io ==

== Io ==