In summer 1994, the biggest news in astronomy was the collision of comet Shoemaker-Levy 9 with Jupiter. During this school year, Jupiter will charge back into the limelight with the arrival of the Galileo spacecraft (see figure 8). On Dec. 7, following a tortuous six-year ramble through the solar system, Galileo will begin its two-year mission studying Jupiter and its gaggle of moons.
Galileo. Photo courtesy of NASA Jet Propulsion Laboratory.
If you thought the Voyager pictures of Jupiter were beautiful, wait till you see Galileo's, which will be taken with a sharper camera and from a closer vantage point. Besides the camera, Galileo will carry instruments capable of identifying minerals on Jupiter's moons. And its atmospheric probe, launched from the main spacecraft in July, will give us our first look inside a gas giant.
Named after the Roman king of the gods, the planet Jupiter is quite a king. It is the largest planet in our solar system, 11 times the diameter and 318 times the mass of our Earth. A view through a backyard telescope readily reveals its four main moons, cloud bands, and Giant Red Spot, thought to be a long-lived hurricane bigger than three Earths (see figure 9).
Jupiter. The black spot is the shadow of Io. Photo courtesy of H.A. Weaver, T.E. Smith, J.T. Trauger, R.W. Evans, and NASA.
Jupiter, along with Saturn, Uranus, and Neptune, is classified as a gas giant because its chief ingredient is hydrogen. Yet most of the planet is actually liquid. The planet's imposing mass creates a crushing gravity so strong that the hydrogen is squeezed into liquid metal. This metallic form of hydrogen is unknown anywhere else in our solar system except Saturn. It lurks 20,000 kilometers (12,000 miles) below the upper cloud decks, far deeper than Galileo's atmospheric probe will penetrate.
The liquid hydrogen, combined with Jupiter's rapid 10-hour rotation, generates a magnetic field 25 times stronger than Earth's. The magnetic field, in turn, traps ions in a huge volume called the magnetosphere. Jupiter's magnetosphere is the largest structure in our solar system, bigger even than the Sun. Its tail extends past the orbit of Saturn. If we could see the magnetosphere, it would appear as large as the Moon in our sky. The ions and electrons within the magnetosphere are a form of radiation 10,000 times more intense than Earth's deadly Van Allen belts. Pioneer 10, the first spacecraft to go through the belts, received a dose of radiation 500 times that needed to kill a person.
The 16 known moons that orbit Jupiter constitute a planetary system in their own right. The four largest are called the Galilean satellites, since it was once believed that Galileo Galilei discovered them; actually the German scientist Simon Marius discovered them at the same time. Larger than some planets, the Galilean satellites undergo not only the usual geologic processes, but also the effects of Jupiter's gravity and magnetosphere. They fall into two groups: Io and Europa are about the size of the Moon and consist largely of rock; Ganymede and Callisto are larger, the size of Mercury, and consist largely of ice -- as indicated by their densities of less than 2 grams per cubic centimeter.
With a surface covered with craters upon craters, Callisto is possibly the oldest landscape in the solar system. The more craters a surface has, the older it is -- just as the older a shooting target is, the more bullet holes it contains. One crater, Valhalla, seems to be surrounded by frozen shock waves that extend 2,000 kilometers from its center, giving it a bull's-eye appearance (see figure 10). All the craters on Callisto are flattened because the surface, largely composed of ice, tends to settle and flow. Not a single volcano or fault line appears on the surface of Callisto.
Callisto as seen by Voyager 1. The bull's-eye on the left is the crater Valhalla. When the meteoroid that formed Valhalla struck the surface, it set off ripples in the ice. These ripples froze into place, forming the bull's-eye rings. The bright circular spot at the center of the bull's-eye is about 600 kilometers (380 miles) across. Image courtesy of NASA Jet Propulsion Laboratory.
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