AtmosphereJupiter's immense atmosphere consists of about 75% hydrogen and 25% helium by mass (90% hydrogen and 10% helium by number of atoms), with trace amounts of methane, ammonia, and other light substances.
The upper atmosphere is striated into wide parallel bands at different latitudes because of a combination of the planet's rapid rotation and extensive convection caused by internal heat rising to the surface. Winds of more than 600 km/h blow in opposite directions in adjacent bands, while slight chemical and temperature differences between the bands are responsible for their different shades of yellow, brown, orange, and red. The light-colored bands are referred to as zones and the dark ones as belts. The zones are at a slightly higher altitude and about 15 K cooler than the belts. Complex vortices in the boundary regions between the bands were first seen by Voyager. The Galileo spacecraft's small descent probe also found turbulence in the Jovian atmosphere, indicating that Jupiter's winds are driven largely by the planet's internal heat rather than by solar radiation as on Earth.
The colors of the surface gases are believed to be due in part to the release of phosphorous and the formation of acetylene. The colors correlate with the cloud's altitude: blue lowest, followed by browns and whites, with reds highest. Sometimes we see the lower layers through gaps in the upper ones. An enormous elliptical region in Jupiter's South Equatorial Belt, known as the Great Red Spot, is though to be a centuries'-old cyclone. Other similar but smaller and less long-lived spots have been known for decades. Intense lightning and powerful aurorae are other features of the jovian atmosphere.
RotationJupiter spins around on its axis once every 9.8 hours (Jupiter's "day"). This is so fast, given Jupiter's size and the fact that it mostly a fluid, that Jupiter is noticeably squashed at its poles.
Measuring longitude values on Jupiter is complicated by the fact that the planet spins more rapidly near the equator than it does at the poles. Three systems are used. Jupiter System I is used for features within about 10° of the equator, where a full rotation takes about 9h 50.5m. Jupiter System II is used for features north and south of this zone (such as the Great Red Spot), where a rotation takes about 9h 55.7m. Finally, Jupiter System III, which is based on the rotation of Jupiter's interior, is used for radio observations and isn't particularly useful for visual observers. This rotation time of 9h 55.5m probably reflects the rate at which the solid core of Jupiter rotates, far below the cloud layers.
Magnetic fieldJupiter has a powerful magnetic field of about 4 gauss (the magnetic axis inclined 15° to the rotational axis and about 0.1 Jupiter radius from the center of the planet) and an immense magnetosphere that extends several million kilometers in a Sunward direction and more than 650 million km away from the Sun – past the orbit of Saturn! In December 1995, the Galileo atmospheric probe discovered a new intense radiation belt between Jupiter's ring and the uppermost atmospheric layers that is about 10 times as strong as Earth's Van Allen belts and contains high energy helium ions of unknown origin.
Other factsOther measurements by the atmospheric probe confirmed that Jupiter and the Sun have almost the same elemental composition and formed therefore from the same primordial mixture in the solar nebula. However, not all of the probe's measurements tied in with theoretical predictions, the biggest surprise being the lack of water in the upper atmosphere. Several ideas have been put forward to explain this finding. Perhaps water concentrations vary with latitude, a suggestion that ties in with the observation that most of Jupiter's lightning (an activity associated with water clouds) occurs at middle latitude whereas the probe descended near the planets' equator. Another possibility is that, during Jupiter's formation, water tended to be confined to the planet's core by the massive overlying blanket of hydrogen and helium.
Jupiter orbits the Sun five times further out than does the Earth, so that each unit area of its surface receives only about one twenty-fifth, or 4%, of the solar energy falling on a similar-sized patch of our own world. Although Jupiter has virtually the same elemental composition as the Sun, it would have needed at least 10 times more mass in order to have become a brown dwarf, and about 80 times more mass in order to have initiated nuclear reactions and shone as a star.
There has been some speculation about the possibility of life existing at some level within Jupiter's atmosphere (see Jupiter, life on). However, the main focus of astrobiological interest in the jovian system at present is the Galilean satellites, most notably Europa.
Related entries Great Red Spot
moons of Jupiter
rings of Jupiter
life on Jupiter?
Related category PLANETS AND MOONS
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