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Callisto has a very weak magnetic field that causes definite perturbations in the magnetic field of Jupiter as it passes in its orbit. Magnetic fields require currents, and current in turn require either metal or an electrolytic solution.<ref name=solarviews/> The most likely explanation is that Callisto has a saltwater ocean, which according to current models lies 200 km deep to the surface and is 10 km deep.<ref name=interior>"[http://photojournal.jpl.nasa.gov/catalog/PIA01478 Catalog page for Image PIA01478]." [[Jet Propulsion Laboratory]], [[NASA]], October 21, 1998. Accessed March 1, 2008.</ref> Further evidence in favor of this saltwater ocean lies in the relatively flat nature of the features of the Valhalla basin. Dr. David A. Williams of [[Arizona State University]] has long held that such an ocean must exist and help absorb the shock of meteoric impacts; the recent photographs of the region support this hypothesis.<ref name=Williams>"[http://www.jpl.nasa.gov/releases/2001/release_2001_230.html Ocean Inside Jupiter's Moon Callisto May Have Cushioned Big Impact]." [[NASA]] news release, November 29, 2001. Accessed March 1, 2008.</ref>
Callisto has a very weak magnetic field that causes definite perturbations in the magnetic field of Jupiter as it passes in its orbit. Magnetic fields require currents, and current in turn require either metal or an electrolytic solution.<ref name=solarviews/> The most likely explanation is that Callisto has a saltwater ocean, which according to current models lies 200 km deep to the surface and is 10 km deep.<ref name=interior>"[http://photojournal.jpl.nasa.gov/catalog/PIA01478 Catalog page for Image PIA01478]." [[Jet Propulsion Laboratory]], [[NASA]], October 21, 1998. Accessed March 1, 2008.</ref> Further evidence in favor of this saltwater ocean lies in the relatively flat nature of the features of the Valhalla basin. Dr. David A. Williams of [[Arizona State University]] has long held that such an ocean must exist and help absorb the shock of meteoric impacts; the recent photographs of the region support this hypothesis.<ref name=Williams>"[http://www.jpl.nasa.gov/releases/2001/release_2001_230.html Ocean Inside Jupiter's Moon Callisto May Have Cushioned Big Impact]." [[NASA]] news release, November 29, 2001. Accessed March 1, 2008.</ref>
== Problems for Uniformitarian Theory ==
Callisto and Ganymede are, according to [[uniformitarianism|uniformitarian]] theories, about the same age, and almost as old as the [[solar system]]. But the composition and surface features of Callisto and Ganymede are vastly different. Ganymede has a metallic core; Callisto has none. Ganymede's ocean is far deeper than is Callisto's. Ganymede's surface features are also far more pronounced than those of Callisto. This poses a problem for theories that insist that all processes that act on celestial bodies do so in a uniform fashion.<ref name=arnett/>
== Exploration ==
== Exploration ==