Worlds of David Darling
Encyclopedia of Science
Home > Encyclopedia of Science


Jupiter taken by the Cassini spacecraft from 16 million km on Dec. 29, 2000

  • atmosphere
  • rotation
  • interior
  • magnetic field
  • other facts
  • statistics
  • The largest planet in the Solar System and the fifth in order from the Sun. Jupiter is a gas giant, with 11 times the diameter of Earth, and two and half times the mass of all the other planets and satellites combined. It has an extensive family of satellites and a faint ring system. In July 1994, Comet Shoemaker-Levy 9 collided with Jupiter with spectacular results.


    Jupiter'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.

    Great Red Spot
    Jupiter taken by Voyager 1 on March 1, 1979, from a distance of 4.3 million km (2.7 million miles). The photo shows Jupiter's Great Red Spot (top) and one of the white ovals that can be seen in Jupiter's atmosphere from Earth.


    Jupiter 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.


    Jupiter interior

    Jupiter radiates about 2½ times more heat than it receives from the Sun, indicating a substantial source of internal heat, almost certainly gravitational contraction, to account for the surface temperature. At its center, Jupiter is thought to have solid metal-rock core, similar in composition to Earth, with a diameter of about 24,000 km and a mass of 10 to 15 Earth-masses. Surrounding this, out to a diameter of about 100,000 km, is a metallic mixture of hydrogen and helium. On Earth we know these two as gases; in Jupiter's interior the pressure is so high that the hydrogen takes up a state in which it behaves like a metal. Outside this metallic hydrogen zone is a shell of liquid molecules, mainly hydrogen and helium, with the cloudy atmosphere, richer in ammonia and methane, about 1,000 km deep, above. Temperatures range from -130C at the top of the clouds, to 30C about 70 km below, to perhaps 20,000 K at the planet's center.

    Magnetic field

    Jupiter 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.

    Jupiter and Earth compared
    Jupiter and Earth compared

    Other facts

    Other 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.

    distance from Sun 778.3 million km (483.7 million miles, 5.20 AU)
    equatorial diameter 142,984 km (88,864 mi.)
    equatorial diameter (Earth=1) 11.209
    mass (Earth=1) 317.9
    mean density 1.3 g/cm3
    axial period 9.8 hours
    axial inclination 3.1°
    orbital period 11.86 years
    orbital inclination 1.3°
    orbital eccentricity 0.048
    gravity at cloud-tops (Earth=1) 2.64
    escape velocity 60.2 km/s (216,720 km/h, 134,692 mph)
    atmospheric composition ~90% hydrogen, ~10% helium, traces of methane, etc
    mean temperature (cloud tops) -150°C (-238°F)
    albedo 0.43
    number of moons 63

    Related entries

       • Great Red Spot
       • moons of Jupiter
       • rings of Jupiter
       • life on Jupiter?
       • Jovian
       • Galileo Galilei
       • Pioneer 10
       • Pioneer 11
       • Voyager 1
       • Voyager 2
       • Galileo spacecraft

    Related category