Endowed with 16 known moons, a ring system, and an immense, complex atmosphere, Jupiter is the innermost of the 4 giant planets (the others are Saturn, Uranus, and Neptune), and clearly the most dynamic. Its atmosphere bristles with lightning and swirls with huge storm systems including the Great Red Spot, a storm that has persisted for at least 100---and perhaps as long as 300---years. With its dynamism, huge energy output, and entourage of satellites, Jupiter is in many ways like a small sun, and the Jovian system resembles a miniature solar system. Although Jupiter is a stellar composition----most of its mass is hydrogen and helium---it does not burn like the Sun. Models of star formation suggest that Jupiter's mass is only about one-eightieth of the mass needed for ignition, which occurs due to heating from internal gravitational collapse. Jupiter's smaller size leaves its center too cool to ignite, sustaining instead internal masses of liquefied gas.
Jupiter holds clues to many of the mysteries of the early solar system. About 4.5 billion years ago, when the solar system formed out of a swirling mass of gases and dust called the solar nebula, Jupiter's core probably began as a solid mass of ice and rock about 15 times the bulk of Earth. The ice content of Jupiter's mass was high because it formed in the colder outer region of the solar system, where the nebula contained a lot of ice particles, principally water and methane. Probably because icy masses can adhere and compress into a single large body faster than plain rock can, the outer planets formed their cores before the rocky inner planets did. The gravity of the large icy cores of Jupiter and Saturn attracted most of the light hydrogen and helium gas in the nebula, and it is these gasses that we find dominating their atmospheres today. Jupiter, the innermost of the icy giants, "grew" the largest atmosphere. In fact, the face of Jupiter that we see is really just the top of its atmosphere. What goes on inside is even more intriguing.
Jupiter's massive atmosphere creates tremendous pressures as you move closer to the center of the planet. The substances inside the atmosphere are subject to extreme conditions, leading to exotic chemistry. For example, scientists have reason to believe that the inner layers of hydrogen in Jupiter's atmosphere, under the pressure of the atmosphere above, may have formed into a planet-encircling layer of what is called liquid metallic hydrogen. Not exactly an ocean, not exactly atmosphere, this layer of hydrogen would have properties that stretch our understanding of chemistry. Instead of the simple, free-moving and loosely bonded behavior of gaseous hydrogen, liquid metallic hydrogen is a strange matrix capable of conducting huge electrical currents. The persistent radio noise and improbably strong magnetic field of Jupiter could both emanate from this layer of metallic liquid. Some scientists theorize that beneath this layer there is no solid mass at the center of Jupiter, but that the unique temperature and pressure conditions sustain a core whose density is more like liquid or slush.
Farther from the planet's core, in what we can more certainly call the atmosphere, we see gases behaving in a more familiar manner, moving in general planetary circulations driven primarily by the rotation of the planet. Jupiter is believed to have three cloud layers in its atmosphere. At the top are clouds of ammonia ice; beneath that ammonium-hydrogen sulfide crystals; and in the lowest layer, water ice and perhaps liquid water. Jupiter is noteworthy for its turbulent cloudtops, and its long-standing storm, the Great Red Spot. The origins of these colorful features are uncertain, but scientists believe that they are caused by plumes of warmer gases that rise up from deep in the planet's interior. The plumes' colors are probably caused by their chemical content. Although the amount of carbon, for example, in the Jovian atmosphere is very small, carbon readily combines with hydrogen and trace amounts of oxygen to form a variety of gases such as carbon monoxide, methane, and other organic compounds. The orange and brown colors in Jupiter's clouds may be attributable to the presence of organic compounds, or sulfur and phosphorus.
Jupiter is one of the planets visible to the naked eye, and its path through the night sky has been traced for thousands of years. In 1610, the Italian astronomer Galileo Galilei discovered four large moons -- Io, Europa, Ganymede, and Callisto (known as the Galilean satellites) -- orbiting Jupiter. This was one of the earliest astronomical discoveries made with a telescope. It fueled the controversial argument of the time that the Sun and not the Earth was the center of the solar system. In the centuries since then, as telescopes improved, Jupiter because known primarily for its size and the Great Red Spot, which was imagined to be an island in a Jovian sea. As Earth-based astronomy continued to improve, we came to realize that the few fuzzy "surface" features we could see were constantly changing in position, size, and color, suggesting that they were atmospheric phenomena. With the advent of radio astronomy, we discovered that Jupiter is a source of strong radio-frequency noise, suggesting electrical activity. Out fascination with Jupiter increased.
In March 1972, NASA launched the Pioneer 10 spacecraft to observe the asteroid belt and Jupiter. Arriving at Jupiter in December 1973, Pioneer 10 revealed Jupiter's intense radiation output, its tremendous magnetic field, and the probability of a liquid interior. One year later, Pioneer 10's sister spacecraft, Pioneer 11, flew by Jupiter on its way to Saturn, and provided even more detailed imagery and measurements, including our first close-up look at the giant planet's polar regions. Then, in August and September 1977, NASA launched the two Voyager spacecraft to the outer solar system. The Voyagers' 1979 encounters with Jupiter provided us with startling, beautiful imagery of the king of the planets, revealing thousands of features never before seen. Swirling multicolored turbulence surrounded the Great Red Spot. Rising plumes and spinning eddies formed and dissipated, suggesting a strong source of heat bubbling up from within the planet.
The Voyager imagery, with its exposure of so many small details, told us that Jovian dynamics were much more complex than previously imagined. Yet many of the features resemble effects we know of in out own and other planetary atmospheres, magnified by the enormity and extremity of the Jovian environment. In studying Jupiter we can learn more about atmospheric effects and interactions that are subtle on Earth, such as magnetosphere-atmosphere interactions.
Subsequent missions to the giant planet will help us improve models of atmospheric dynamics, and will help us understand the chemistry and behavior of Earth's own relatively thin, but very precious, atmosphere.
-- Excerpted from Discovering Jupiter, published by NASA's Solar System Exploration Division
For more facts and information about Jupiter, please check: http://seds.lpl.arizona.edu/nineplanets/nineplanets/jupiter.html.
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The Jovian Satellites
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