A thunderstorm, 1000 kilometres across, climbs above the surrounding cloud deck.

THE EVOLUTION OF JUPITER

New information was obtained by the Galileo probe, which entered Jupiter's upper atmosphere on December 7th, 1995. It didn't take any pictures but sent back information about the top 150 kilometres of atmosphere. Though the probe went down a just short way, it provided fresh insights into the overall composition of Jupiter. The findings are supported by the Galileo Orbiter's measurements using a device called a neutral mass spectrometer to analyse gas molecules.

The proportion of hydrogen and helium present are crucial to the debate on Jupiter's origin. The amount of helium (about 24%) found by the Galileo probe closely matches the proportions present in the Sun (24.85%). From this it appears that Jupiter has changed little since it coalesced 4 or 4.5 thousand million years ago from the original solar nebula which formed the planets. There was more carbon and sulphur than previously thought, but less organic compounds and water.

The presence of more helium than previously recognised, a very light element, must be balanced out by a greater abundance of heavier elements. This is indeed the case as carbon, nitrogen, and sulphur are all found in higher concentrations in Jupiter than the Sun. This is probably because of the contribution to Jupiter made by meteorites and comets.

Jupiter may have begun as a small rocky planet. In such a scenario, the original rocky or terrestrial body forms a nucleus for the gas giant. As it became larger, continually accreting, sweeping up rocky debris, Jupiter would have been able to retain gases released from its interior. Ever more gravitationally powerful, Jupiter was able to attract and incorporate gases from a considerable region of the Solar System, so forming its enormous atmosphere.

Jupiter has often been called a failed star. This is because a star is formed through the mechanism of gravitational collapse of a gas cloud. The collapse causes heating, and when temperatures climb high enough thermonuclear burning takes over and the result is a star. Jupiter is slowly collapsing, although at a very slow rate. Nevertheless, this slow contraction causes Jupiter to radiate more energy than it receives from the Sun. Had Jupiter been more massive, the contraction would have raised temperatures sufficient to convert Jupiter from a planet to a star.