Jupiter

Moons - Orbital and Rotational Detail - Atmosphere - Interior - Magnetic Field - The Star that Never Was - Dust Rings

Atmosphere

The Great Red Spot. NASA Voyager 1 false color image.

The Great Red Spot was first spotted by Cassini in 1665. It is so large, that two Earths can fit inside. It is situated above the cloud tops, and is a high pressure region located in an already higher than average pressure zone. It rotates once every 12 days.

The atmosphere contains mainly hydrogen and helium in the same proportion as the Sun.

Interior

Because Jupiter's interior is so hot and compact, we cannot visit it. Thus, all interior has to be inferred from the knowledge about its density, rotation rate, composition of the atmosphere, its heat ratio and its influence on the orbits of both natural and artificial satellites. With this information, Jupiter's interior is thought to be as follows:

A strong concentration of mass is located in the center, or core; it is thought to be made up of rock and ice. It may contain compounds of metals, oxygen, silicon, and heavy volatile elements. The mass of the core is 10-15 times the Earth's mass, or about 4% of Jupiter's mass. Temperatures in the center reach 25,000 K, and pressures, 100 million atmospheres (atm).The rocky core extends up to 6700 km and is surrounded by the icy core, which extends up to 13,440 km, slightly larger than Earth's diameter. At this point, the temperature has decreased to 19,000 K.

Above the core, a layer of liquid metallic hydrogen extends up to 59,000 km. The pressures in this region are so great, that the electrons are freed from the hydrogen nuclei and are able to flow and conduct electricity like a metal. This state of hydrogen has never been achieved on Earth. The pressures are around 3 million atm, and the temperatures about 11,000 K. The rotation of Jupiter, along with its metallic hydrogen interior are thought to be the cause of its strong magnetic field.

Above the metallic H, liquid molecular hydrogen is present, up to 71,400 km, where the pressure is thought to be 10 atm.

The last layers belong to the atmosphere because the temperatures and pressures are such as to allow everything present to be in the gaseous state.

Jupiter's magnetic field is generated by its metallic hydrogen core coupled with its high rotation rate. Jupiter's magnetic field is the strongest of all the planets and was first detected in 1955 when radio noise was measured at 20 megahertz (15 meter wavelength). The radio noise can be measured from Earth or from spacecraft and is produced when protons and electrons from the solar wind spiral around the magnetic field lines and produce radio waves. As in Earth, when these electrically charged particles interact with the atoms or molecules in Jupiter's atmosphere, they create incredible auroral displays which have been photographed.

In addition to the continuous source of radio noise, Jupiter radiates bursts of radio emission that at times can make Jupiter brighter than the Sun at wavelengths of the order of 10 m. These bursts appear to be generated by the motion of the moon Io through Jupiter's magnetic field; as seen from Earth, they are correlated with Io's position in its orbit. This is due to the fact that Io's volcanic eruptions contribute energetic charged particles (like sulfur and oxygen ions) which interact with Jupiter's inner magnetic field. The volcanic eruptions also produce neutral sodium, but this one does not interact with the field. Jupiter's own atmosphere and the surface of other moons also contribute heavy ions that interact with Jupiter's magnetic field. Ions and neutral atoms are released from the moons' surfaces by sputtering.

Like Earth's, the magnetic field of Jupiter is like a bar magnet, but unlike Earth, it is oriented in the opposite direction, so a compass would point south, not north. Jupiter's magnetic field is tilted 10 degrees with respect to its axis of rotation, compared to a 12 degree tilt for Earth. Jupiter's magnetic field is 19,000 times intrinsically stronger than Earth's, but since Jupiter's diameter is 11 times that of Earth, the field strength on its equator is measured to be 4.3 gauss, compared to 0.3 gauss on the surface of the Earth. The strong field produces a huge magnetosphere which extends to 3 million km in the sun-facing side and reaches all the way to Saturn in the opposite direction.

The difference between a star and a planet is that the light of a star is produced by nuclear fusion reactions happening in its core, whereas a planet shines mostly because of light reflected from the Sun. Nuclear reactions take place in the core of stars when the temperature is high enough, and this only happens when the star has enough mass to provide the necessary pressure. Thus, a minimum mass is required for an object to become a star: 0.08 times the mass of the Sun, or about 75 times the mass of Jupiter. It is interesting to note that if Jupiter had been more than ten times as massive as it is now, gravity would have made it smaller in size.

Imagine that Jupiter was 80 times more massive. We would live in a binary star system! What about Jupiter's moons and the possibility of life on them?

Jupiter's rings cannot be seen from Earth because they are made up of dark and tiny silicate dust particles, which are so thinly distributed, that they block very little sunlight. In fact, they absorb less light than clear glass. They were discovered in 1979 by Voyager 1, and were subsequently photographed by Voyager 2. Most ring particles do not lie on the plane of Jupiter's equator. When these particles interact with Jupiter's magnetosphere, they attain an electrical charge and repel each other; the repulsive force lifts them out of the plane of the equator.

A silhouette of Jupiter and its rings. NASA Galileo spacecraft.

Jupiter has a Main Ring which extends 6,400 km, from a distance of 122,800 to 129,200 km above Jupiter, or 1.72 to 1.81 Jupiter's radii. Its thickness is less than 30 km. The Inner Ring extends 51,400 km, starting near Jupiter's cloud tops and going all the way to the beginning of the Main Ring. The thickness of the Inner Ring is some 20,000 km.

Moons and Planets, W. Hartmann, 1993, 3rd edition, Wadsworth Publishing Co.
The Planetary System, D. Morrison and T. Owen, 1996, 2nd edition, Addison-Wesley Publishing Co., Inc.
Astrophysical Data: Planets and Stars, K. R. Lang, 1991, Springer-Verlag

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