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JUPITER

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Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass one-thousandth that of the Sun, but two-and-a-half times that of all the other planets in the Solar System combined. Jupiter is one of the brightest objects visible to the naked eye in the night sky and has been known to ancient civilizations since before recorded history. It is named after the Roman god Jupiter. When viewed from Earth, Jupiter can be bright enough for its reflected light to cast visible shadows and is on average the third-brightest natural object in the night sky after the Moon and Venus.

Jupiter is primarily composed of hydrogen with a quarter of its mass being helium, though helium comprises only about a tenth of the number of molecules. It may also have a rocky core of heavier elements, but like the other giant planets, Jupiter lacks a well-defined solid surface. Because of its rapid rotation, the planet's shape is that of an oblate spheroid (it has a slight but noticeable bulge around the equator). The outer atmosphere is visibly segregated into several bands at different latitudes, resulting in turbulence and storms along their interacting boundaries. A prominent result is the Great Red Spot, a giant storm that is known to have existed since at least the 17th century when it was first seen by telescope. Surrounding Jupiter is a faint planetary ring system and a powerful magnetosphere. Jupiter has 79 known moons including the four large Galilean moons discovered by Galileo Galileo in 1610. Ganymede, the largest of these, has a diameter greater than that of the planet Mercury.

Pioneer 10 was the first spacecraft to visit Jupiter, making its closest approach to the planet on December 4, 1973; Pioneer 10 identified plasma in Jupiter's magnetic field and also found that Jupiter's magnetic tail was nearly 800 million kilometres long, covering the entire distance to Saturn. Jupiter has been explored on a number of occasions by robotic spacecraft, beginning with the Pioneer and Voyager flyby missions from 1973 to 1979, and later by the Galileo orbiter, which arrived at Jupiter in 1995. In late February 2007, Jupiter was visited by the New Horizons probe, which used Jupiter's gravity to increase its speed and bend its trajectory en route to Pluto. The latest probe to visit the planet is Juno, which entered into orbit around Jupiter on July 4, 2016. Future targets for exploration in the Jupiter system include the probable ice-covered liquid ocean of its moon Europa.

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INTERNAL STRUCTURE 

Jupiter was expected to either consist of a dense core, a surrounding layer of liquid metallic hydrogen (with some helium) extending outward to about 78% of the radius of the planet, and an outer atmosphere consisting predominantly of molecular hydrogen, or perhaps to have no core at all, consisting instead of denser and denser fluid (predominantly molecular and metallic hydrogen) all the way to the centre, depending on whether the planet accreted first as a solid body or collapsed directly from the gaseous protoplanetary disk. However, the Juno mission, which arrived in July 2016, found that Jupiter has a very diffuse core, mixed into the mantle. A possible cause is an impact from a planet of about ten Earth masses a few million years after Jupiter's formation, which would have disrupted an originally solid Jovian core.

Above the layer of metallic hydrogen lies a transparent interior atmosphere of hydrogen. At this depth, the pressure and temperature are above hydrogen's critical pressure of 1.2858 MPa and a critical temperature of only 32.938 K. In this state, there are no distinct liquid and gas phases—hydrogen is said to be in a supercritical fluid state. It is convenient to treat hydrogen as gas extending downward from the cloud layer to a depth of about 1,000 km, and as a liquid in deeper layers. Physically, there is no clear boundary—the gas smoothly becomes hotter and denser as one descends. Rain-like droplets of helium and neon precipitate downward through the lower atmosphere, depleting the abundance of these elements in the upper atmosphere. Rainfalls of extraterrestrial diamonds have been suggested to occur, as well as on Saturn and the ice giants Uranus and Neptune.

The temperature and pressure inside Jupiter increase steadily inward, due to the Kelvin–Helmholtz mechanism. At the pressure level of 10 bars (1 MPa), the temperature is around 340 K (67 °C; 152 °F). At the phase transition region where hydrogen—heated beyond its critical point—becomes metallic, it is calculated the temperature is 10,000 K (9,700 °C; 17,500 °F) and the pressure is 200 GPA. The temperature at the core boundary is estimated to be 36,000 K (35,700 °C; 64,300 °F) and the interior pressure is roughly 3,000–4,500 GPA.

Prior models of Jupiter's interior suggested a core radius of 0.08 to 0.16 of Jupiter's overall radius. Early results from the Juno mission suggest the core may be larger and more diffuse with a radius from 0.3 to 0.5 of the planetary radius.