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Diameter: 139,822 km
Distance from Sun: 778,500,000 km
Surface area: 61,418,738,571 km²
Length of day: 0d 9h 56m
Gravity: 24.79 m/s²
Circumference: 449,200 km
Mass: 1.898E27 kg (317.8 Earth mass)
Length of year: 12 years
Surface temp: -108 C

Jupiter is the fifth planet from the Sun and by far the largest. Jupiter  is more than twice as  massive as all the other planets combined (the mass of Jupiter is 318 times that of Earth).

Jupiter (a.k.a.  Jove; Greek  Zeus) was the King  of the Gods, the ruler  of Olympus and the patron of the Roman state.  Zeus was the son of Cronus (Saturn).

Jupiter is the fourth brightest object in the sky (after the Sun, the Moon and Venus).  It has been known since prehistoric  times as a bright “wandering star”.  But in 1610 when Galileo first pointed a telescope at the  sky he  discovered  Jupiter’s four  large moons Io,  Europa,    Ganymede  and  Callisto  (now known as the  Galilean moons) and recorded their motions  back and forth around  Jupiter.  This was  the first discovery of a center of motion not apparently centered on the Earth. It was a  major point in  favor of Copernicus’s  heliocentric theory of the  motions of the planets (along with other new evidence from his telescope:  the phases of Venus and the mountains on the Moon).  Galileo’s outspoken support of the Copernican  theory got him in trouble with the Inquisition. Today anyone can repeat  Galileo’s observations (without fear of retribution  using binoculars  or an inexpensive telescope.

Jupiter was first visited by  Pioneer 10 in 1973 and later by Pioneer 11,  Voyager  1,  Voyager 2 and Ulysses. The spacecraft Galileo  orbited Jupiter for eight years.  It is still regularly observed by  the  Hubble Space Telescope.

The gas planets do not  have solid surfaces, their gaseous material simply gets denser with depth  (the radii and diameters quoted for the planets are for levels corresponding to a pressure of 1  atmosphere. What we see when looking at these planets is the tops of clouds  high  in their atmospheres (slightly above the 1 atmosphere level).

Jupiter's red spot

Jupiter is about 90% hydrogen and 10% helium  (by numbers of atoms, 75/25% by mass)  with traces of methane,  water, ammonia and “rock”.  This is very close to the composition of the  primordial Solar Nebula from  which the  entire solar system was formed. Saturn has a  similar composition, but Uranus and Neptune have much less hydrogen and helium.

Our knowledge of the interior of Jupiter (and the other gas planets) is  highly indirect and likely to remain so for some time.  (The data from Galileo’s atmospheric  probe goes down only about 150 km below the cloud tops.)

Jupiter probably has a core of rocky material amounting to something like 10 to 15 Earth-masses.

Above the core lies the main bulk of the planet in  the  form of liquid metallic hydrogen.  This exotic form of the most  common of  elements is possible only at pressures exceeding 4 million bars, as is the case in the interior of  Jupiter (and Saturn). Liquid metallic hydrogen consists of ionized  protons  and electrons (like the interior of the Sun but at a far lower   temperature).  At the temperature and pressure of Jupiter’s interior hydrogen is a liquid, not a gas. It is an electrical conductor and the  source of  Jupiter’s magnetic field.  This layer probably also  contains some helium and traces of various “ices”.

The outermost layer is composed primarily of ordinary molecular hydrogen and helium which  is liquid in the interior and gaseous further out. The  atmosphere we see is just the very top of this deep layer. Water,  carbon dioxide, methane and other simple molecules are also present in  tiny amounts.

Recent experiments have shown that hydrogen does not change phase suddenly. Therefore the interiors of the Jovian planets probably have indistinct  boundaries between their various interior layers.

Jupiter's cloud layer

Three distinct layers of clouds are believed to exist  consisting of ammonia ice, ammonium      hydrosulfide and a mixture of ice and  water.  However, the preliminary results    from the Galileo probe show only faint indications of  clouds (one instrument seems to have detected the topmost layer while  another may have seen the second). But the probe’s entry point (left)  was unusual —  Earth-based telescopic observations and more  recent observations by the Galileo orbiter suggest that the probe entry site may well have been  one of the warmest and least cloudy areas on  Jupiter at that time.

Data from the Galileo atmospheric probe also indicate that there is much less  water than expected.  The expectation was that Jupiter’s atmosphere would  contain about twice the amount of oxygen (combined with  the abundant  hydrogen to make water) as the Sun.  But it now appears that the actual  concentration much less than the Sun’s.  Also surprising was the high  temperature and density of the uppermost parts of the atmosphere.

Jupiter small image

Jupiter and the other gas planets have high velocity winds which are confined in wide bands of       latitude. The  winds blow in opposite directions in adjacent bands. Slight chemical and  temperature differences between these bands are responsible for the  colored bands that dominate the planet’s appearance. The light  colored bands are called zones; the dark ones belts. The  bands have been known for some time on Jupiter, but the complex  vortices in the boundary regions between the bands were first seen by  Voyager. The data from the Galileo probe indicate that the winds are even      faster than expected (more than 400 mph) and extend down into as far as  the probe was able to observe; they may extend down thousands of kilometers  into the interior. Jupiter’s atmosphere was also found to be quite  turbulent. This  indicates that Jupiter’s winds are driven in large part by  its internal heat rather than from solar input as on Earth.

The vivid colors seen in Jupiter’s clouds are probably the result of  subtle chemical reactions of the trace elements in Jupiter’s  atmosphere,  perhaps involving sulfur whose compounds take on a wide variety of colors,  but the details are unknown.

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 holes in the upper ones.

Jupiter's giant red spot

The Great Red Spot (GRS)  has been  seen by Earthly observers for more than 300 years (its discovery is  usually attributed to Cassini, or Robert  Hooke in the 17th century). The GRS is an oval about 12,000 by 25,000 km,  big enough to hold two Earths.  Other smaller but similar spots have been  known for decades.  Infrared observations and the direction of its  rotation indicate that the GRS is a high-pressure region whose cloud tops  are significantly  higher and colder than the surrounding regions.  Similar structures have been seen on Saturn and Neptune. It is not known how such structures can persist for so long.

Jupiter radiates more energy into space than it receives from the Sun. The interior of Jupiter is hot: the core is probably about 20,000 K.  The  heat  is generated by the Kelvin-Helmholtz  mechanism, the slow gravitational compression of the planet.  (Jupiter does NOT produce energy by nuclear  fusion  as in the Sun; it is much too small and hence its interior  is too cool to ignite nuclear reactions.)  This interior heat  probably  causes convection deep within Jupiter’s  liquid layers and is probably responsible for the complex motions we see  in the cloud tops.  Saturn and Neptune are similar to Jupiter in this  respect, but oddly, Uranus is not.

Jupiter is just about as large in diameter as a gas planet can be.  If more material were to be added, it would be compressed by gravity such that the overall radius would increase only slightly.  A star can be larger only  because of its internal (nuclear) heat source. (But Jupiter would  have to be at least 80 times more massive to become a star.)

Jupiter has a huge magnetic field, much stronger than Earth’s. Its magnetosphere extends  more than 650  million km (past the orbit of Saturn!).  (Note that  Jupiter’s  magnetosphere is far from spherical — it extends “only” a few  million  kilometers in the direction toward the Sun.)  Jupiter’s moons therefore lie  within its magnetosphere, a fact which may partially explain some of the  activity on Io. Unfortunately for future space  travelers and of real concern to the designers  of the Voyager and Galileo  spacecraft, the environment near Jupiter  contains high levels of energetic  particles trapped by Jupiter’s magnetic field.  This “radiation” is  similar to, but much more intense than, that  found within Earth’s Van  Allen belts.  It would be immediately fatal to an unprotected human  being. The Galileo atmospheric probe discovered a new intense radiation belt  between  Jupiter’s ring and the uppermost atmospheric layers. This new belt is  approximately 10      times as strong as Earth’s Van Allen radiation belts.  Surprisingly, this new belt was also found to contain high energy helium  ions of unknown origin.

Jupiter's ring

Jupiter has rings like Saturn’s, but much  fainter and  smaller (right). They were totally unexpected and  were only discovered when two of the Voyager 1 scientists insisted that  after traveling 1 billion km it was at  least worth a quick look to  see if any rings might be present. Everyone else thought that the  chance of finding anything was nil, but there they were. It was a major coup.  They have since been  imaged  in the infra-red from ground-based observatories and by  Galileo.

Unlike Saturn’s, Jupiter’s rings are dark  (albedo about .05).  They’re probably composed  of very small grains of rocky material.  Unlike Saturn’s rings, they seem to  contain no ice.

Particles in Jupiter’s rings probably don’t stay there for long (due  to  atmospheric and magnetic drag).   The Galileo spacecraft found clear  evidence that the rings are continuously resupplied by dust formed by  micro-meteor impacts on the four inner moons, which  are very energetic because of Jupiter’s large gravitational field. The inner  halo ring is broadened by interactions with Jupiter’s magnetic field.

Comet Shoemaker-Levy 9 collided with Jupiter

In July 1994, Comet Shoemaker-Levy 9 collided with  Jupiter with spectacular results (left).  The effects were  clearly visible even with amateur telescopes.  The debris  from the collision was visible for nearly a year afterward with HST.

When it is in the nighttime sky, Jupiter is often the brightest “star” in the  sky (it is second only to Venus, which is seldom visible in a dark sky).  The four Galilean moons are easily visible with binoculars;  a few bands  and the Great Red Spot can be seen with a small astronomical      telescope. There are several Web sites that   show the current position of Jupiter (and the other planets) in the sky. More  detailed and customized charts can be created with a planetarium  program.

Jupiter’s Satellites

Jupiter's satellites

Jupiter has 63 known satellites  (as of Feb 2004): the four large Galilean moons  plus many more small ones some of which have not yet been named.




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