Wednesday, 23 February 2011


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 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.

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 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.

Jupiter Statistics

Mass (kg)  1900e+27
Mass (Earth = 1) 3.1794e+02
Equatorial radius (km) 71,492
Equatorial radius (Earth = 1) 1.1209e+01
Mean density (gm/cm^3) 1.33
Mean distance from the Sun (km) 778,330,000
Mean distance from the Sun (Earth = 1) 5.2028
Rotational period (days) 0.41354
Orbital period (days) 4332.71
Mean orbital velocity (km/sec) 13.07
Orbital eccentricity 0.0483
Tilt of axis (degrees) 3.13
Orbital inclination (degrees) 1.308
Equatorial surface gravity (m/sec^2) 22.88
Equatorial escape velocity (km/sec) 59.56
Visual geometric albedo 0.52
Magnitude (Vo) -2.70
Mean cloud temperature -121°C
Atmospheric pressure (bars) 0.7
Atmospheric composition
Helium  10%    

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