(P-7) Jupiter

Jupiter holds many records among planets (see tables): most massive, most magnetic, most powerful radiation belt, and the largest number of moons. Two of those moons figure in the image on the right, taken 10 June 1979 by Voyager 2: on the right is the innermost moon Io, while the dark spot on the left is the shadow of Ganymede.

Four of Jupiter's moons are comparable to Earth's Moon or bigger. They were discovered by Galileo in that memorable year 1609, at once suggesting to him a Copernican system of planets, though on a smaller scale. As of now, Jupiter is credited with 63 satellites, plus a thin ring.

It rotates in just under 10 hours (for details, see tables) and the rapid rotation gives the planet a distinctly elliptic cross-section. This led to a famous controversy between Isaac Newton and Giovanni Cassini, director of the Paris observatory, in which Newton made his point by an ingenious thought-experiment.

Jupiter also has a giant magnetic field--with an equivalent central magnet about 20,000 times the Earth's, its field at the planet's visible edge about 10 times more intense than ours. Its radiation belt is also much more intense, enough to cause some damage to Pioneer 10, the first space probe to encounter it. The plasma trapped by this field carries a huge ring current, causing a region of weak fields near the equator of the "magnetosphere" of Jupiter, somewhat similar to the middle of the Earth's magnetotail.

Jupiter aurora

The structure of the radiation belt is not smooth but contains dips due to moons which absorb some of the trapped particles. One dip close to the planet (found by Mario Acuña) seemed not to correspond to any moon: examination of images showed at that distance a thin planetary ring, much thinner than Saturn's but of similar nature. Jupiter also has a bright polar aurora, observed by the Hubble telescope, caused (like Earth's) by energetic electrons colliding with atoms of the upper atmosphere.

Jupiter itself is gas, mostly hydrogen, with colorful clouds of ammonia (see image), bands and giant vortices, some of them quite large and long-lived--like the famous "red spot" (a smaller "red spot" was also noted, and several smaller round features). The equator rotates faster than the poles (though the difference is not as pronounced as it is on the Sun), suggesting a flow near the surface from higher latitudes, returning polewards deeper down.

An unsolved puzzle is the reason why Jupiter (and to a lesser degree, other gaseous planets) should be magnetic. Certainly, the dynamo process associated with the Earth's core does not seem likely. Size tends to stretch the time scales of magnetic changes--though size isn't everything, as the relatively fast magnetic cycle of the Sun suggests. One factor is that under the high pressure in Jupiter's center, hydrogen may become a conducting metal, perhaps a resistance-free "superconductor." One would expect hydrogen with its single outermost electron to behave like the metal sodium--but the formation of the molecule H₂ interferes with that behavior. Under very high pressure, however, that molecule breaks down.

Questions from Users: Why don't gas planets just evaporate?

Next Objects: #P-8 Io and Other Moons of Jupiter

Next Stop (following "The Planets"): #9c The Discovery of the Solar System, from Copernicus to Galileo

Timeline Glossary Back to the Master List

Author and Curator: Dr. David P. Stern
Mail to Dr.Stern: stargaze("at" symbol)phy6.org .

Last updated: 3 April 2014

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(P-7) Jupiter

Above is background material for archival reference only.