Astronomy

Jupiter is the largest planet in the solar system and contains two‐thirds of the mass found outside the Sun in the solar system. It appears reddish, with well‐defined belts (darker zones) and bands (brighter zones) encircling the planet. These features do not represent surface area, but are clouds. The bands are tops of an upwelling of light colored, warm gas from the planet's interior. In the upper atmosphere, these gases react with solar ultraviolet radiation, forming dark hydrocarbons or photochemical smog. At the same time, these gases cool, become more dense, and sink back into the interior. The planet's rapid rotation (its period is about 10 hours) not only produces a significant equatorial bulge, but also a strong Coriollis effect, giving gases moving north or south additional westward or eastward motion, thus distorting the convective regions into latitudinal zones that wrap around the planet. The Coriolis effect also produces the cyclonic storms in the planet's atmosphere, including the immense Great Red Spot, which has survived for at least 400 years. (The Spot is 35,000 km diameter, has a top that is 8 km higher than surrounding cloud layers, and rotates at 500 km/hr; it also drifts somewhat in position.)

Chemically, the outer atmosphere (see Figure ) is composed of molecular hydrogen (60 percent by mass), helium (36 percent), neon (2 percent), water vapor (0.9 percent), and lesser amounts of ammonia, argon, and methane. The reddish color is due to acetylene formation and the release of phosphorous from phosphine. In the upper atmosphere, distinct cloud layers form from ammonia, water, and ammonium hydrosulfide.

Figure 1 The interior of Jupiter.

The outer few hundred kilometers of the planet were explored by the Galileo probe in 1995 and its physical characteristics deduced from the impacts of the fragments of Comet Shoemaker‐Levy in the previous year. The internal structure of the planet, however, cannot be directly observed and must be deduced from its mean density of 1.3 g/cm ³ and applicable physical laws. In the outer part, the planet is gaseous, and the balance between gas pressure and gravity is the major factor in setting its structure. At high enough pressure, molecular hydrogen is dissociated into individual atoms. But this same high pressure also means that the electrons are no longer tightly bound to individual hydrogen atoms, but can easily move from one atom to another allowing the generation of electrical currents. The ability to permit an electrical current is a property of metals, hence this state of hydrogen is termed metallic atomic hydrogen. The electrical currents in turn produce a strong magnetic field inclined 10 degrees to the planet's rotational axis. Exterior to the planet there are super versions of Earth's van Allen belts in which charged solar particles are trapped. As these move along the magnetic field lines and enter the upper atmosphere in the regions of the magnetic poles, strong aurorae are produced.

Much closer to the center of the planet, under higher temperatures and pressures, the possible existence of ices, as well as a solid core, must be considered. The interior of Jupiter is not as well known as astronomers would like, for they must make various assumptions in the effort to calculate interior conditions; slightly different assumptions lead to some variance in the computed interior conditions.

The temperature of the upper atmosphere of Jupiter (125 K) is set by the balance between its absorption of solar energy and its thermal radiation. Jupiter also emits a large amount of radiation in the radio region, produced by lightning in the atmosphere and from Jupiter's magnetic interaction with its moon Io. Considering all forms of energy emission, Jupiter radiates about two times as much energy as absorbed from sunlight; thus it must have an internal energy source. Powerful convection keeps its outer composition well mixed, thus the only possible energy source is gravitational contraction. A slow shrinking of the planet, about three centimeters per century, is adequate to produce the excess energy. (Alternatively, it can be considered that the planet is still losing its primordial heat. As it cools, gas pressure slowly decreases relative to gravity and thus the planet continues to shrink slowly in size.) See Table 1 for Jupiter's physical and orbital data.