Composition
S
cientific 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.
Introduction Structure & Composition Important numbers Satellites & Moons
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