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 (318 times Earth),
if it had been only about 100 times more massive at birth (not so much by astronomical standards) it would have become a star instead of a planet. Then the Solar System might be a double star system instead of a single star with a planetary system.
Jupiter is the fourth brightest object in the sky (after the Sun, the Moon and Venus; at some times Mars is also brighter).
Jupiter has features very different from terrestrial planets. Its composition is more like that of stars, and if it has any solid surface it is hidden deep at its center: Jupiter is apparently almost entirely gas and liquid.
It also has an internal energy source and enormous magnetic fields. Finally, the 4 largest moons of Jupiter (the Galilean Moons) are sufficiently interesting in their own right that they are among the most studied objects in the Solar System. We shall devote a separate section to studying their properties.
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Orbit:778,330,000 km (5.20 AU) from Sun
Diameter: 142,984 km (equatorial)
Mass:1.900e27 kg
Density: 1,326 kg/m^3
Minimum Distance from Sun: 741 million km (460 million miles)
Maximum Distance from Sun: 817 million km
Minimum Distance from Earth: 588 million km (365 million miles)
Rotation Period about Axis: 9.8 hours
Revolution Period about the Sun: 12 years
Tilt of Axis: 3.1 Degrees
Surface Gravity: 23.12 m/s^2 (2.64 x Earth's)
Name in Roman/Greek Mythology: Jupiter/Zeus
Rotation Period about Axis: 9.8 hours
Average Cloud Top Temperature (K): 123K
Distance across the Great Red Spot: 40,000 km (24,860 miles)
Temperature at Cloud Tops: -150 Degrees C (-101 Degrees F)
Average Cloud Top Temperature (K): 123K
Rings: 1, very thin
Satellites: 16 known
Jupiter was first visited by Pioneer 10 in 1973 and later by Pioneer 11, Voyager 1, Voyager 2 and Ulysses. The spacecraft Galileo is currently in orbit around Jupiter and will be sending back data for at least the next two years.
The planet Jupiter is 5.2 A.U. from the Sun on average, which corresponds to an orbital period of 11.9 Earth years. It is the most massive planet, being 318 times as massive as the Earth, and about 2 1/2 times the mass of all other planets combined.
Jupiter is massive because it has a large diameter (11.2 times that of Earth). Its density is only 1.33 g/cc, which is 1/4 that of Earth, and only slightly more than the 1 g/cc characteristic of water. This very low density compared with the terrestrial planets is because it is composed mostly of hydrogen and helium( Astronomer Rupert Wildt researched the planet and found the atmosphere is mostly hydrogen and helium.), in ratios similar to that found in stars (approximately 82% hydrogen and 17% helium by mass).
Furthermore, Jupiter is almost entirely gas and liquid. Therefore, Jupiter and the related planets Saturn, Uranus, and Neptune are sometimes called Gas Giants (these are also called the Jovian Planets, since Jove was another name for Jupiter).
We guess that Jupiter probably has a small rocky core comparable in size to terrestrial planets and perhaps with a similar composition, but we have no direct evidence of this.
Because Jupiter is not solid its parts do not rotate at exactly the same velocity. However, measurements of the rotation of the magnetic field give a rotation period of 9 hours and 55 minutes. This rapid rotation and the gas and liquid structure give rise to a pronounced equatorial bulge.
The atmosphere has clouds with a colorful and complex structure. These features are partially associated with the effect of very high velocity winds in the Jovian atmosphere (as much as 900 km/hr).
It also has an large, complex, and very intense magnetic field, and has a system of 16 moons that is a kind of miniature Solar System unto itself. It has an internal heat source, since it emits more radiation than it receives. This is thought to be associated with residual heat left over from the formation of the planet. Finally, it even has a faint ring.
Jupiter has a very complex atmosphere. It is dominated by colorful bands and turbulent swirls, as illustrated in the following two images.
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| Storms in the atmosphere of Jupiter | Jupiter and Io with Ganymade's shadow on Jupier |
Zones and Belts
All that we see is the top of the atmosphere. The light bands are called zones and the darker bands are called belts. The zones tend to be white or yellow, while the belts are often some shade of reddish brown.
Temperature measurements by the Pioneer spacecraft (1973) established that the temperature of the dark belts is higher than that of the light zones, implying that the former are lower in the atmosphere.
Thus, the belts appear to be regions of descending gas and the zones are regions of rising gas.
Each hemisphere has around 6 bands with winds blowing at very high velocities in opposite directions.
This accounts for the extensive shear and turbulence at the boundaries between these regions.Large lightning bolts and extensive aurora have been observed in the Jovian atmosphere, as we will discuss further when we consider Jupiter's magnetic field.
Jupiter with two moons seen against its surface*** Jupiter's clouds near the Great Red Spot ***
The most famous feature of Jupiter is the Great Red Spot. This violent storm has been watched from Earth since the 1600's. But it wasn't until the 1930's that we knew what Jupiter was made of.
Beginning in 1973, the United States sent four spacecraft to observe this giant planet. Pioneer 10 and 11, along with Voyager I and II, sailed past Jupiter.
Jupiter's moons are too! Galileo discovered four of the larger moons: Io, Europa, Ganymede and Callisto in 1610. Most of the other moons were found by the Voyager spacecraft during its flyby.
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| Jupiter with two moons seen against its surface | jJupiter's clouds near the great Red Spot |
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| A Galileo "true color" image of the great Red Spot in 1996 | Turbulence near the great Red Spot |
Most of the interior of Jupiter is liquid (primarily hydrogen and about 10% helium). The central temperatures are thought to lie in the 13,000-35,000 degree Celsius range, and the central pressure is about 100 million Earth atmospheres. We infer indirectly that the small core (perhaps a few tens of Earth masses) is rocky.
Liquid Metallic Hydrogen
The inner layers of highly compressed hydrogen are in a state that has never been produced on the Earth. Normally, hydrogen does not conduct heat or electricity very well, which are defining characteristics for a metal.
Thus, under normal conditions hydrogen is not a metal. Under the extreme pressure found deep inside Jupiter, theory suggests that the electrons are released from the hydrogen molecules and are free to move about the interior.
This causes hydrogen to behave as a metal: it becomes conducting for both heat and electricity.
The intense magnetic field of Jupiter is thought to result from electrical currents in this region of metallic hydrogen that is spinning rapidly and thought to compose 75% of the planet's mass.
We are limited in our understanding of Jupiter's interior by the fact that we have no laboratory knowledge of the behavior of hydrogen under such conditions and are forced to rely on theory. Conversely, the properties of Jupiter provide a stringent test of our understanding of matter under such extreme conditions.
Internal Source of Energy
Jupiter radiates 1.6 times a much energy as falls on it from the Sun.
Thus, Jupiter has an internal heat source. It is thought that much of this heat is residual heat left over from the original collapse of the primordial nebula to form the Solar System, but some may come from slow contraction (liquids are highly incompressible, so Jupiter cannot be contracting very much).
This internal heat source is presumably responsible for driving the complex weather pattern in its atmosphere, unlike the Earth where the primary heat source driving the weather is the Sun. A Large Planet or a Failed Star? Calculations suggest that Jupiter lacks the mass to initiate hydrogen fusion reactions in its core and become a star by about a factor of 100. Since factors of 100 are not so large on astrophysical mass scales, it is legitimate at some level to view Jupiter as either a very large planet, or a failed 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.
Features of the Magnetic Field
The following figure shows the field lines and some other features of Jupiter's magnetic field.
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| Jupiter's magentic field | Jupiter magnetosphere and interaction with the solar wind |
Jupiter's Auroras
Intense auroras have been observed on Jupiter. The following figure shows Hubble Space Telescope images of auroras near the Jovian poles.
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Jupiter's Ring
At one time, Saturn, with it's beautiful ring system, was thought to be an anomaly in the Solar System: why did Saturn, and no other planet, have rings? As we have explored the outer planets in more detail, we now know that there are multiple ring systems in the Solar System. None are as spectacular as that of Saturn, but we now view rings as common occurences.
The adjacent image shows that Jupiter also has a ring, though it is extremely faint. The ring was discovered by Voyager I in 1979 and is at a radius of about 1.8 times the radius of the planet (which is inside the radius of the innermost moon). It seems to be composed of small (micron size) particles, probably from Io's volcanoes and from debris knocked off the inner moons by meteor impacts.
Jupiter's Satellite:-
Jupiter has 16 known satellites, the four large Galilean moons and 12 small ones plus one more small one discovered recently but not yet confirmed.