The Jovian Magnetospheres

Jupiter's magnetosphere is the most powerful in the solar system. Its extent reaches some 18,600,000 miles (30 million km) north to south. Saturn has a magnetosphere that extends about 600,000 miles (1 million km) toward the sun. The magnetospheres of Uranus and Neptune are smaller, weaker, and (strangely) offset from the gravitational center of the planets.

Io (as we'll see in the next chapter) is volcanically active. Due to its small mass (Io is just slightly smaller in diameter than the earth's moon), its eruptions have enough velocity to send some charged particles into orbit around Jupiter. These particles are trapped by Jupiter's enormous magnetic field.

Io (as we'll see in the next chapter) is volcanically active. Due to its small mass (Io is just slightly smaller in diameter than the earth's moon), its eruptions have enough velocity to send some charged particles into orbit around Jupiter. These particles are trapped by Jupiter's enormous magnetic field.

The rapid rate of rotation and the theorized presence of electrically conductive metallic hydrogen inside Jupiter and Saturn account for the generation of these planets' strong magnetic fields. While Uranus and Neptune also rotate rapidly, it is less clear what internal material generates the magnetic fields surrounding these planets, since they are not thought to have metallic hydrogen in their cores. With charged particles trapped by their magnetospheres, the jovian planets experience Aurora Borealis, or "Northern Lights," just as we do here on Earth. These "lights" occur when charged particles escape the magnetosphere and spiral along the field lines onto the planet's poles. The Hubble Space Telescope has imaged such auroras at the poles of Jupiter and Saturn.

The Least You Need to Know

V The jovian planets are Jupiter, Saturn, Uranus, and Neptune.

V While the jovians are the largest, most massive planets in the solar system, they are on average less dense than the terrestrial planets. Their outer layers of hydrogen and helium gas cover a dense core.

V Uranus was discovered in 1781, partly because of a bit of numerology called the Bode (or Titius-Bode) Law.

V Neptune was discovered in 1846, partly because astronomers were searching for an explanation of Uranus's slightly irregular orbit. Newton's theory of gravity had an explanation—the mass of another planet.

V The jovian planets all have in common thick atmospheres, ring systems, and strong magnetic fields.

V The missions to the outer planets, Voyager 1, Voyager 2, and Galileo have had a huge impact on our understanding of the jovian planets. The Cassini mission to Saturn and its moons, which will arrive in 2004, is likely to have a similar effect.

Chapter 15

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