Jupiter is the largest planet in the solar system. As viewed in Earth's skies, it is also usually the second brightest planet, exceeded only by Venus. Naked-eye observations from antiquity had established the duration of its circuit through the background stars as 11.86 Earth years. When Galileo turned his telescope toward Jupiter in January 1610, he first realized that Jupiter had four large moons (henceforth called the “Galilean moons”), a discovery that contributed to the acceptance of the Copernican view of the heavens and to the determination of the speed of light. Application of Newton's 1687 laws of motion helped astronomers to realize that Jupiter was a giant, but low-density planet. By then, improved telescopes enabled astronomers to discern latitudinally confined light and dark bands in the atmosphere of Jupiter, along with an enormous oval storm, possibly the same storm that later became known as the Great Red Spot. Before Pioneer 10’s encounter, Earth-based observations in ultraviolet to radio wavelengths revealed the main atmospheric gas to be hydrogen, with as-yet-undetected helium suspected to be the second most abundant gas. These spectroscopic observations led to the conclusion that the visible “surface” of Jupiter consisted of clouds of ammonia ice crystals, colored by unknown impurities. Minute quantities of methane and ammonia gas were also detected. Before the early 1972 launch of Pioneer 10, Jupiter's moon count had risen to 12, with the 1892 discovery of Amalthea by Edward E. Barnard and seven others by various observers between 1904 and 1951.
Pioneers 10 and 11 encountered Jupiter in December 1973 and 1974, respectively. They detected helium in the atmosphere, measured the intense radiation belts and magnetic field of the giant planet, and provided the best images of the planet up to that time. They also determined that Jupiter's interior was primarily liquid and provided improved masses for the four Galilean moons. Using measurements of mass, volume, rotation, gravitational field, and electromagnetic radiation, mathematical models provided assessments of Jupiter's interior conditions. There is no liquid or solid surface beneath the cloud-tops, although the high pressures and temperatures inside Jupiter compress the materials into liquid-like layers. Below about halfway to the center, hydrogen is compressed into a metallic liquid, somewhat like mercury, and electrical currents flowing in this conductive layer are the likely source for the planet's strong magnetic field. At Jupiter's center there may be an Earth-sized core of molten rocky material. In 1975 measurements from the Kuiper Airborne Observatory detected water vapor in Jupiter's atmosphere.
The Voyager 1 encounter with Jupiter occurred in March 1979; the Voyager 2 encounter followed four months later. They were the first to view Jupiter's ring system and the three small moons (in addition to Amalthea) that were later found to be the source of its material. They verified the internal rotation period of Jupiter as 9.924 hours, consistent with periodic radio bursts measured at Earth. Images of the planet during approach enabled mapping of the atmospheric wind speeds relative to the rotation rate, disclosing both prograde and retrograde winds, some with speeds up to 150 m/s. Helium was found to constitute 23 percent of the mass of the atmosphere above the cloud tops, which was slightly less than that of the Sun; hydrogen occupied 76 percent of the mass, and all other gases combined were less than 1 percent. Io was seen to have volcanoes on its surface, triggered by gravitational interactions with Europa and Jupiter. Europa's crust is almost devoid of impact craters, but is covered with cracks, a probable indication that it harbors an ocean beneath a thick crust of water ice. Ganymede, the largest moon in the solar system, showed heavily cratered dark regions and lighter regions with grooves and troughs. Callisto, the second largest Galilean satellite, is heavily cratered and may also have a layer of liquid water at great depth.
The first orbital mission to Jupiter was Galileo, which was inserted into orbit in December 1995 and ended its life by plunging into the Jupiter atmosphere in early September 2003. The Galileo Probe made the first in situ measurements of the changing atmospheric pressure, temperature, and composition with depth below the cloud tops. The orbiter also studied the structure and dynamics of the magnetosphere of Jupiter, and completed high-resolution mapping of the surfaces of all four Galilean satellites, discovering nearly 100 active volcanic regions on the surface of Io. Galileo also discovered that the Gossamer Ring was divided into an inner (Amalthea) and outer (Thebe) portion and better characterized the Main and Halo Rings of Jupiter. Detection of small intrinsic magnetic fields on Ganymede and Callisto provided evidence of liquid water deep within their interiors.
During its December 2000 encounter with Jupiter, the Cassini Orbiter, whose primary mission was to study the Saturn system, skirted along the outer boundary of Jupiter's magnetic field, greatly increasing the understanding of conditions near that boundary. Cassini also obtained the highest resolution images ever of Jupiter's atmosphere and also studied the cloud motions within that atmosphere for more than four months. It obtained images of Himalia, the only disk-resolved image of any of the myriad of outer satellites of Jupiter known to exist. Due to a combination of spacecraft encounters and much-improved ground-based imaging, by 2007 Jupiter was known to possess at least 63 moons, most of which are captured asteroids in distant inclined and elliptical orbits.
See also: Cassini-Huygens, Pioneer 10 and 11, Voyager
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