Scientific knowledge of the Jupiter system increased enormously in 1979 with the successful visits by the Voyager 1 and 2 spacecraft launched by the United States National Aeronautics and Space Administration. Spectroscopic observations from Earth had demonstrated that most of Jupiter’s atmosphere is molecular hydrogen, H2. Infrared studies from the Voyager spacecraft indicated that 87 per cent of the molecules in Jupiter’s atmosphere are H2, with helium, He, constituting most of the remaining 13 per cent. Because the helium molecule has about twice the mass of the hydrogen molecule, these figures indicated that the mass of helium present is about a quarter of the total mass. The interior must have essentially the same composition as the atmosphere in order to yield the observed low 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 a large amount of information about Jupiter when fragments of Comet Shoemaker-Levy 9 crashed into the planet in July 1994. The collisions stirred up the planet’s atmosphere, heating interior gases to incandescence and bringing them to the surface. Astronomers captured detailed images of these gases with telescopes on Earth and in space. They used spectroscopes to analyse the gases in order to verify and expand their knowledge of the composition of the planet’s atmosphere.
Still more knowledge was gained from the entry into Jupiter’s atmosphere of a sub-probe that was part of the Galileo unmanned mission to Jupiter. In December 1995 the Galileo spacecraft went into orbit around Jupiter after a circuitous six-year flight in which it had flown past Venus and twice past Earth, using a gravity assist on each occasion to boost it on its way. Also in December 1995 the sub-probe separated from the main craft and entered the atmosphere, deploying parachutes. For an hour it transmitted data to the mother craft orbiting above, as it descended approximately 160 km (100 mi) below the visible cloud-tops before being crushed by the atmospheric pressure. The data were relayed over a period of months from the orbiter to Earth. The Galileo main probe continued its journey on a tour of the Jovian system.
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. This is the way in which stars form. However, Jupiter would need to be almost 100 times as massive to produce temperatures at its centre high enough to release nuclear energy in reactions like those that power the Sun and other stars.
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