Early History of Jupiter

Given the planet's large proportion of hydrogen and its huge mass, it has been traditional to assume that Jupiter formed by condensation from the primordial solar nebula. This hypothesis implies that the elements should all be present on Jupiter in the same proportions that they occur in the Sun. However, the most recent evidence indicates that the elemental proportions on Jupiter differ from the solar values.

Current models for Jupiter's origin suggest instead that a solid core of about 10 Earth masses formed first as a result of the accretion of icy planetesimals. This core would have developed an atmosphere of its own as the planetesimals released gases during accretion. As the mass of the core increased, it would have become capable of attracting gases from the surrounding solar nebula, thus accumulating the huge hydrogen-helium envelope that constitutes Jupiter's atmosphere and fluid mantle. The accumulating envelope would have mixed with the outgassed atmosphere from the core. Thus, the presently observed enrichment of the most abundant heavy elements in this envelope, compared with solar values, reflects the concentration of such elements in the core.

The mass spectrometer on the Galileo probe showed that these heavy elements are enriched by the same factor of about three. For this enrichment to include volatile substances like argon and molecular nitrogen requires that the icy planetesi-mals must have formed at temperatures of 30 K (-400 °F, -240 °C) or less. Just how this happened remains a puzzle, and its solution may ultimately help explain the presence of giant planets that have been detected very close to their stars in other planetary systems.

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