Galileo and the oceans of Callisto

The unmanned NASA/JPL space probe Galileo makes a remarkable find at Callisto, the outermost of Jupiter’s four large “Galilean” moons: evidence that a saltwater ocean may lie beneath the moon’s pockmarked surface. Even more unusually, it may be the catalyst for Callisto’s magnetic field (a rarity for a satellite – not even all of the solar system’s planets have magnetic fields). Galileo’s instruments raise the possibility that the subsurface ocean may be conducting electricity and helping to generate that field (which current scientific models say Callisto should be too small to generate on its own). Scientists do not, however, believe that Callisto is a strong candidate to support life, unlike Europa.