Callisto

Why does Jupiter's moon Callisto alter the magnetic field of Jupiter in its
vicinity? Callisto itself does not have a strong magnetic field. One possible answer is
that Callisto harbors sub-surface oceans of electrically conducting salt-water. This
hypothesis was bolstered recently by a new analysis of how Callisto creates and dissipates
heat. Callisto is thought to create heat by the radioactive decay of internal rock -- a
process that keeps the Earth's mantle molten. Callisto may not be able to dissipate this
heat very efficiently, however, as it has thick layers of ice and rock on its surface.
Perhaps this heat is enough to keep sub-surface water from freezing into ice. With this
hypothesis, Callisto joins two other of Jupiter's moons, Europa and Ganymede, in candidates
for sub-surface oceans. Callisto's oceans, however, might prove too hostile to support
Earth-like life.