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# 9.12 Planetary Magnetic Fields

As Earth's magnetic field and its interaction with the solar wind became more well-understood, observations of other planets indicated they might share the same characteristics. Just two years after the first satellite was launched, an astronomer named Frank Drake detected electromagnetic radiation from electrons around Jupiter. He speculated the existence of a magnetic field around the planet that would trap electrons and ions, just as the Earth's magnetic field traps charged solar wind particles in its Van Allen belts. Not until 1973, when Pioneer 10 passed by Jupiter, was the enormous size and strength of Jupiter's magnetic field realized. Jupiter's magnetic field is 20,000 times as strong as Earth's, and reaches far out into space. If we could see Jupiter's magnetosphere from Earth, it would appear 16 times bigger than the full Moon in the night sky.

The formation of a magnetic field requires an electrical current in the interior of a planet. In order to generate this, the planet must have two things: a liquid, electrically conductive core, and a significant rotation rate. In the terrestrial planets, the conductive core is most likely metallic iron, probably mixed with some sulfur or nickel that lowers the melting point. The metallic hydrogen in the interiors of the gas giant planets is similarly conductive. The planet's rotation, when combined with convective motions, causes eddies in the core that drive the magnetic field.

Because magnetic fields drop off as the cube of distance (rather than the square of distance, like radiation or the force of gravity), satellites that are close to a planet are often affected strongly. Jupiter's satellite Io, for example, orbits just six Jupiter radii away from the planet. Intense volcanism on Io releases sulfur, oxygen, and other gases. As Io moves through Jupiter's magnetic field, the particles are swept off and ionized. Io leaves a trail of charged particles in its wake, forming a donut-shaped torus"" of radiation dangerous to orbiting spacecraft. The movement of Io through Jupiter's magnetic field creates an enormous electrical current between the planet and its satellite - generating approximately two trillion watts of power! When charged particles from Io follow this current, they crash into the poles of the giant planet, creating huge aurorae visible from Earth.

Saturn is the only planet whose magnetic field is aligned with its rotation axis. Like Earth's, most magnetic fields are tilted with respect to the planet's rotation axis. Although not nearly as strong as Jupiter's, Saturn's field is still almost 600 times stronger than Earth's. The large satellite Titan also affects Saturn's magnetic field, similar to the interaction between Io and Jupiter. Nitrogen in Titan's upper atmosphere becomes ionized and trapped in Saturn's magnetosphere, creating belts of charged particles.

Voyager discovered magnetic fields at Uranus and Neptune as well, although the source of these fields is unknown. The planets aren't large enough to have metallic hydrogen in their cores, so there must be some other conductive material in their interiors. These magnetic fields are oriented very oddly — they're tilted far from the planets' rotational axes (60° for Uranus and 46° for Neptune). Their magnetic fields are also offset from the centers of the planets — Uranus's by almost a third of its radius and Neptune's by half its radius.