Jupiter, Earth, Mercury, and Saturn have external magnetic fields which extend beyond the planets themselves. This means they act something like a big electromagnet. The magnetic field of the Earth is what helps the compass-carrying traveler find his way.

Jupiter's magnetosphere is like a giant bubble, larger than the Sun, around the planet. If we could see Jupiter's magnetosphere from Earth, it would appear several times larger than the Moon. It extends beyond the orbit of Saturn - at least 650 million miles (that's over 2,700 times the distance between the Earth and the Moon!). Electrical activity in Jupiter is so strong that it pours billions of watts into Earth's own magnetic field every day. Jupiter's magnetic field could emanate from large liquid hydrogen core.

Planetary magnetic fields are dynamic items - constantly changing, it is difficult to understand them without studying them over time. Try to imagine describing all of the weather on Earth by just seeing a few seconds of one thunderstorm! The Galileo orbiter carries six fields and particles instruments designed to study the many different processes in Jupiter's magnetosphere over the period of several years.

Along the way, the spacecraft also detected a large "hole" in Jupiter's magnetic field near Io, leading to speculation about whether Io possesses its own magnetic field. If so, it would have been the first planetary moon known to have one. However, Ganymede was not to be one-upped! Tidal forces cause surprising results on the solar system's largest moon. Galileo revealed that Ganymede is first moon known to have its own magnetic field. A molten core of iron or iron sulfide generates Ganymede's magnetic field.