Jupiter, in astronomy
Introduction
Sections in this article:
Its Moons and Rings
Astronomers have discovered 79 satellites orbiting Jupiter, but five of those, small satellites that were identified in 2003 and 2011 but have not been found since then, are considered lost. Jupiter's satellites are divided into six main groups (in order of increasing distance from the planet): Amalthea, Galilean, Himalia, Ananke, Carme, and Pasiphae. The first group is comprised of the four innermost satellites—Metis, Adrastea, Amalthea, and Thebe. The red color of
The four largest satellites—Io, Europa, Ganymede, and Callisto—were discovered by Galileo in 1610, shortly after he invented the telescope, and are known as the Galilean satellite group.
The eight inner satellites are regular, that is, their orbits are relatively circular, near equatorial, and prograde, i.e., moving in the same direction as the planet's rotation. The remainder are irregular in that their orbits are large, elliptical, inclined to that of the planet, and, in the case of nearly all the moons beyond Carpo, retrograde.
The Ananke group comprises 20 satellites, which share similar orbits and range from .6 to 3 mi (1–5 km) in diameter except for two:
The most distant of the groups from the planet is the Pasiphae, which comprises 16 widely dispersed satellites that, except for three, also range from .6 to 3 mi (1–5 km) in diameter:
Jupiter has three rings—
Astronomical and Physical Characteristics
Jupiter's orbit lies beyond the asteroid belt at a mean distance of 483.6 million mi (778.3 million km) from the sun; its period of revolution is 11.86 years. In order from the sun it is the first of the giant outer planets—Jupiter, Saturn, Uranus, and Neptune—very large, massive planets of relatively low density, having rapid rotation and a thick, opaque atmosphere. Jupiter has a diameter of 88,815 mi (142,984 km), more than 11 times that of the earth. Its mass is 318 times that of the earth and about 2
The atmosphere of Jupiter is composed mainly of hydrogen, helium, methane, and ammonia. However, the concentration of nitrogen, carbon, sulfur, argon, xenon, and krypton—as measured by an instrument package dropped by the space probe
Jupiter has no solid rock surface. One theory pictures a gradual transition from the outer ammonia clouds to a thick layer of frozen gases and finally to a liquid or solid hydrogen mantle. Beneath its has been suggested that Jupiter may have a core of rocky material with a mass 10–15 times that of the earth. The spot and other markings of the atmosphere also provide evidence for Jupiter's rapid rotation, which has a period of about 9 hr 55 min. This rotation causes a polar flattening of over 6%. The temperature ranges from about −190℉ (−124℃) for the visible surface of the atmosphere, to 9℉ (−13℃) at lower cloud levels; localized regions reach as high as 40℉ (4℃) at still lower cloud levels near the equator. Jupiter radiates about four times as much heat energy as it receives from the sun, suggesting an internal heat source. This energy is thought to be due in part to a slow contraction of the planet. Jupiter is also characterized by intense nonthermal radio emission; in the 15-m range it is the strongest radio source in the sky. Jupiter has a huge asymetrical magnetic field, extending past the orbit of Saturn in one direction but far less in the direction of the sun. This magnetosphere traps high levels of energetic particles far more intense than those found within earth's Van Allen radiation belts. Nine space probes have encountered the Jovian system:
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