| COMPOSITION/STRUCTURE/MAGNETIC FIELD | SATELLITES & RINGS |
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Composition, Structure, and Magnetic Field
Scientific knowledge of the Jupiter system increased enormously with visits to the system by spacecraft launched by the National Aeronautics and Space Administration (NASA). In 1979 Voyagers 1 and 2 passed close to Jupiter on their way out of the solar system, taking photographs and measurements much more detailed than any ever taken from Earth. In 1995 the Galileo spacecraft neared Jupiter and launched a probe toward the planet. Before the probe was incinerated in Jupiter's atmosphere, it sent new information about the planet's atmosphere back to Earth. Galileo was scheduled to continue returning data to Earth from orbit around Jupiter until late 1997.Earlier observations from Earth using spectroscopy had demonstrated that most of Jupiter's atmosphere is molecular hydrogen (H2). Samples tested by the Galileo probe indicated that 80 percent of Jupiter's atmosphere is H2, with helium (He) constituting almost all of the remaining 20 percent. Jupiter's interior must have essentially the same composition as its atmosphere in order to yield the low observed density. Apparently, then, this huge world is made mostly from the two lightest and most abundant elements in the universe, a composition similar to that of the sun and other stars. Jupiter may therefore represent a direct condensation of a portion of the primordial solar nebula-the great cloud of interstellar gas and dust from which the entire solar system formed about 4.6 billion years ago.
Scientists also collected new information about Jupiter when fragments of the dying Comet Shoemaker-Levy 9 crashed into the planet in July 1994. The collisions stirred up the planet's atmosphere, heating interior gases to incandescence-that is, the point at which they produce light-and bringing them to the surface. Scientists captured detailed images of these gases with telescopes located on the earth and in space. They used spectroscopy to analyze the gases in order to verify and expand knowledge about the composition of the planet's atmosphere.
Jupiter radiates about twice as much energy as it receives from the sun. The source of this energy is apparently a very slow gravitational contraction of the entire planet rather than the nuclear fusion that powers the sun. Jupiter would have to be almost 80 times larger to have enough mass to ignite a nuclear furnace.
Jupiter's turbulent, cloud-filled atmosphere is therefore cold. With hydrogen so abundant, hydrogen-based molecules, such as methane, ammonia, and water, predominate. Periodic temperature fluctuations in Jupiter's upper atmosphere reveal a pattern of changing winds like that in the equatorial region of Earth's stratosphere. Photographs of sequential changes in Jovian clouds suggest the birth and decay of giant cyclonic storm systems . Galileo probe results indicate that strong winds (faster than 650 km/h, or 400 mph) blow through the atmosphere at all cloud depths, suggesting that the winds are caused by heat escaping from Jupiter's depths-unlike Earth's winds, which are caused by heating from the sun or from condensing water vapor. There is lightning on Jupiter, but it is much different from that on Earth. Lightning strikes about ten times less often on Jupiter, but each Jovian lightning bolt has about ten times more energy than a lightning bolt on Earth. Jupiter appears to be much drier than scientists anticipated, having a lower percentage of water molecules than the sun does.
Two known cloud layers, ammonia and ammonium hydrosulfide, and at least one theorized cloud layer, made of water vapor, occur in Jupiter's atmosphere. Ammonia freezes in the low temperature of Jupiter's upper atmosphere (-125° C, or -193° F), forming the white cirrus clouds-zones, ovals, and plumes-seen in many photographs of the planet transmitted by the Voyager spacecraft. At lower levels, ammonium hydrosulfide can condense. Colored by other compounds, clouds of this substance may contribute to the widespread sand-colored cloud layer on the planet. The temperature at the tops of these clouds is about -50° C (about -58° F), and the atmospheric pressure is about twice the sea-level atmospheric pressure on earth. Through holes in this cloud layer, radiation escapes from a region that may be a layer of water vapor clouds where the temperature reaches 17° C (about 63° F). Still deeper, warmer layers have been detected by radio telescopes that are sensitive to cloud-penetrating radiation.
Scientists had hoped that the Galileo probe would pass through the first three layers of clouds (ammonia, ammonium hydrosulfide, and water vapor), but the probe hit the atmosphere in an unexpectedly clear area where only the ammonium hydrosulfide layer was present.
Although only the barest skin of the planet is directly visible, calculations show that the temperature and pressure continue to increase toward the interior, reaching values at which hydrogen first liquefies and then assumes a metallic, highly conducting state. A core of solid, earthlike material may exist at the center.
The Jovian magnetic field is generated deep within these layers. At the surface of Jupiter, this field is 14 times stronger than earth's. Its polarity is the opposite of earth's, so a terrestrial compass taken to Jupiter would point south. This field is responsible for the huge belts of trapped charged particles that circle the planet out to a distance of 10 million km (about 6 million mi). One of these belts, between Jupiter's ring and the outermost atmosphere, was discovered by the Galileo probe and came as a surprise to scientists. It is about ten times as strong as the earth's Van Allen radiation belts and contains mysterious high-energy helium ions from an unknown source.
Jupiter's Satellites and Rings
Sixteen satellites of Jupiter have so far been discovered. The four largest were discovered in 1610 by Italian astronomer Galileo. They were subsequently named after mythological love partners of Jupiter: Io, Europa, Ganymede, and Callisto. This tradition has been followed in the naming of the other moons. Modern observations have shown that the mean densities of the largest moons follow the trend apparent in the solar system itself. Io and Europa, close to Jupiter, are dense and rocky like the inner planets. Ganymede and Callisto, at greater distances, are composed largely of water ice and have low densities. During the formation of planets and satellites, closeness to the central body (the sun or Jupiter) evidently prevented substances that easily evaporate from condensing.
Callisto is almost as big as Mercury, and Ganymede is bigger than Mercury. If these satellites orbited the sun, they would be considered planets. The icy crusts of these two bodies are marked by numerous craters, the record of an early bombardment, probably by comet nuclei, similar to the asteroidal battering that scarred earth's moon. In contrast, the surface of Europa is extremely smooth. It is covered by a layer of water ice that emerged from Europa's interior after the early bombardment, and a layer of liquid water may lie beneath the ice. An intricate network of shallow cracks covers the icy surface. Observations conducted by the Hubble Space Telescope in June 1994 detected a thin oxygen atmosphere on Europa, making Europa the fourth moon in the solar system known to have an atmosphere.
The most remarkable satellite is unquestionably Io. Its surface has a bizarre appearance: yellowish, brown, and white areas dotted by black features. Io is racked by volcanism that is driven by the dissipation of tidal energy in this satellite's interior. Ten volcanoes were erupting during the Voyager flybys in 1979, and evidence for later eruptions has since been detected. Sulfur dioxide issues from the vents and condenses on the surface, forming a local, transient atmosphere. The white regions are solid sulfur dioxide; the other markings are presumably caused by other sulfur compounds.
The remaining moons are very much smaller and less well studied than the four Galilean satellites. Between Jupiter and Io are the moons Metis, Adrastea, Amalthea, and Thebe. Away from Jupiter, past Callisto, are eight more moons, arranged in two clusters of four moons each, which may be bodies that have been catured by Jupiter's gravity. The inner cluster is made up of Leda, Himalia, Lysithea, and Elara; the outer cluster contains Ananke, Carma, Pasiphae, and Sinope.
Close to the planet, the Voyager spacecraft discovered a faint system of rings. The material in these rings must be continuously renewed since it is visibly moving in toward the planet. It may be produced by the disintegration of small moonlets imbedded within it. The satellite Metis is just at the outer boundary and could be one source of ring material.
