Scientists once speculated the boiling cauldrons on Io ejected mostly sulfur material. Closer examination of Galileo data proved otherwise. Maybe a few are, but we realize now that most have a rocky composition. Callisto's pockmarked surface is the most cratered in the solar system But serious puzzles remain hidden near the two largest planets. Jupiter has a magnetosphere twice the size physicists would expect.
Scientists theorize charged particles from Io are the culprit.
Every second Jupiter's magnetic field strips from Io about one ton of material, which becomes ionized and forms a thick ring of intense radiation. Ions escaping the ring create aurora storms in Jupiter's atmosphere and inflate the planet's magnetic field, according to NASA scientists. A joint robot expedition later this year should shed light on the matter. Cassini, a NASA probe headed to Saturn, is expected to team up with Galileo in December to study the intense, interconnected magnetic fields in the planet's system. Galileo images revealed unknown surface features on Europa, another large moon around Jupiter.