the outer Planets 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 planetesimals This core would have developed an atmosphere of its own as the planetesimals 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 Crescent view of Jupiter a composite composite of three images taken by Voyager 1 on March 24 1979 10
GIant JuPIter outgassed atmosphere from the core Thus the presently 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 Jupiter has an internal heat source it emits more energy than it receives from the Sun The pressure in its deep interior is so high that the hydrogen there exists in a fluid metallic state This giant has the strongest magnetic field of any planet with a magnetosphere so large that if it could be seen from Earth its apparent diameter would exceed that of the Moon Jupiter’s system is also the source of intense bursts of radio noise at some frequencies occasionally occasionally radiating more energy than the Sun Despite all its superlatives however Jupiter is made almost entirely of only two elements hydrogen and helium and its mean density is not much more than the density of water Knowledge about the Jovian system grew dramatically dramatically after the mid-1970s as a result of explorations by three spacecraft missions Pioneers 10 and 11 in 1973 74 Voyagers 1 and 2 in 1979 and the Galileo orbiter and probe which arrived at Jupiter in December 1995 The Pioneer spacecraft served as scouts for the Voyagers Voyagers showing that the radiation environment of Jupiter was tolerable and mapping out the main characteristics of the planet and its environment The greater number and increased sophistication of the Voyager instruments 11
the outer Planets provided so much new information that it was still being analyzed when the Galileo mission began The previous missions had all been flybys but Galileo released a probe into Jupiter’s atmosphere and then went into orbit about the planet for intensive investigations of the entire system until September 2003 In July 2016 the Juno orbiter arrived at Jupiter for a mission expected to last two years In February 2018 it completed its tenth orbit of Jupiter and went on to explore the planet’s south pole There it discovered massive massive cyclones unlike any other in solar system In March 2018 Juno discovered that Jupiter’s winds run deep into its atmosphere Other looks at the Jovian system were provided in late 2000 and early 2001 by the flyby of the Cassini spacecraft on its way to Saturn and in 2007 by the flyby of the New Horizons spacecraft on its way to Pluto Observations of the impacts of the fragmented nucleus of Comet Shoemaker-Levy 9 with Jupiter’s atmosphere in 1994 also yielded information about its composition and structure JUPITER BY THE NUMBERS Jupiter has an equatorial diameter of about 143,000 kilometres kilometres 88,900 miles and orbits the Sun at a mean distance 12
GIant JuPIter distance of 778 million km 483 million miles Of special interest are the planet’s low mean density of 1 33 grams per cubic cm in contrast with Earth’s 5 52 grams per cubic cm coupled with its large dimensions and mass and short rotation period The low density and large mass indicate that Jupiter’s composition and structure are quite unlike those of Earth and the other inner planets Three rotation periods all within a few minutes of each other have been established The two periods called System I 9 hours 50 minutes 30 seconds and System II 9 hours 55 minutes 41 seconds are mean values and refer to the speed of rotation at the equator and at higher latitudes respectively as exhibited by features observed in the planet’s planet’s visible cloud layers Jupiter has no solid surface the transition from the gaseous atmosphere to the fluid interior occurs gradually at great depths Thus the variation in rotation period at different latitudes does not imply that the planet itself rotates with either of these mean velocities In fact the true rotation period of Jupiter is System III 9 hours 55 minutes 29 seconds This is the period of rotation of Jupiter’s magnetic magnetic field first deduced from Earth-based observations at radio wavelengths and confirmed by direct spacecraft measurements This period which has been constant for 30 years of observation applies to the massive interior of the planet where the magnetic field is generated 13
