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5. The Planet Jupiter

Jupiter is the largest of the nine planets, more than 10 times the diameter of Earth and more than 300 times its mass. In fact, the mass of Jupiter is almost 2.5 times that of all the other planets combined. Being composed largely of the light elements hydrogen and helium, its mean density is only 1.314 times that of water. The mean density of Earth is 5.245 times that of water. The pull of gravity on Jupiter at the top of the clouds at the equator is 2.4 times as great as gravity's pull at the surface of Earth at the equator. The bulk of Jupiter rotates once in 9 hours, 55.5 minutes, although the period determined by watching cloud features differs by up to five minutes due to intrinsic cloud motions.

The visible surface of Jupiter is a deck of clouds of ammonia crystals, the tops of which occur at a level where the pressure is about half that at Earth's surface. The bulk of the atmosphere is made up of 89% molecular hydrogen (H2) and 11% helium (He). There are small amounts of gaseous ammonia (NH3), methane (CH4), water (H2O), ethane (C2H6), acetylene (C2H2), carbon monoxide (CO), hydrogen cyanide (HCN), and even more exotic compounds such as phosphine (PH3) and germane (GeH4). At levels below the deck of ammonia clouds, there are believed to be ammonium hydro-sulfide (NH4SH) clouds and water crystal (H2O) clouds, followed by clouds of liquid water. The visible clouds of Jupiter are very colorful. The cause of these colors is not yet known. Contamination by various polymers of sulfur (S3, S4, S5, and S8), which are yellow, red, and brown, has been suggested as a possible cause of the riot of color; but, in fact, sulfur has not yet been detected spectroscopically, and there are many other candidates as the source of the coloring.

The meteorology of Jupiter is very complex and not well understood. Even in small telescopes, a series of parallel light bands called zones and darker bands called belts is quite obvious. The polar regions of the planet are dark. Also present are light and dark ovals, the most famous of these being the Great Red Spot. The Great Red Spot is larger than Earth, and although its color has brightened and faded, the spot has persisted for at least 162.5 years, with the earliest definite drawing of it done by Schwabe on September 5, 1831. (There is less positive evidence that Hooke observed it as early as 1664.) It is thought that the brighter zones are cloud-covered regions of upward moving atmosphere, while the belts are the regions of descending gases, the circulation driven by interior heat. The spots are thought to be large-scale vortices, much larger and far more permanent than any terrestrial weather system.

The interior of Jupiter is totally unlike that of Earth. Earth has a solid crust floating on a denser mantle that is fluid on top and solid beneath, underlain by a fluid outer core that extends out to about half of Earth's radius and a solid inner core of about 1,220-kilometer (758-mile) radius. The core is probably 75 percent iron, with the remainder nickel, perhaps silicon, and many different metals in small amounts. Jupiter, on the other hand, may well be fluid throughout, although it could have a small solid core (say up to 15 times the mass of Earth!) of heavier elements such as iron and silicon extending out to perhaps 15% of its radius. The bulk of Jupiter is fluid hydrogen in two forms or phases, liquid molecular hydrogen on top and liquid metallic hydrogen below; the latter phase exists where the pressure is high enough, say 3-4 million atmospheres. There could be a small layer of liquid helium below the hydrogen, separated out gravitationally, and there is clearly some helium mixed in with the hydrogen. The hydrogen is convecting heat (transporting heat by mass motion) from the interior, and that heat is easily detected by infrared measurements, since Jupiter radiates twice as much heat as it receives from the Sun. The heat is generated largely by gravitational contraction and perhaps by gravitational separation of helium and other heavier elements from hydrogen, in other words, by the conversion of gravitational potential energy to thermal energy. The moving metallic hydrogen in the interior is believed to be the source of Jupiter's strong magnetic field.

Jupiter's magnetic field is much stronger than that of Earth. It is tipped about 11° to Jupiter's axis of rotation, similar to Earth's, but it is also offset from the center of Jupiter by about 10,000 kilometers (6,200 miles). The magnetosphere of charged particles which it affects extends from 3.5 million to 7 million kilometers (2.2 to 4.3 million miles) in the direction toward the Sun, depending upon solar wind conditions, and at least 10 times that far in the anti-Sun direction. The plasma trapped in this rotating, wobbling magnetosphere emits radio frequency radiation measurable from Earth at wavelengths from 1 meter (3 feet) or less to as much as 30 kilometers (19 miles). The shorter waves are more or less continuously emitted, while at longer wavelengths the radiation is quite sporadic. Scientists will carefully monitor the Jovian magnetosphere to note the effect of the intrusion of large amounts of cometary dust into the Jovian magnetosphere.

The two Voyager spacecraft discovered that Jupiter has faint dust rings extending out to about 53,000 kilometers (33,000 miles) above the atmosphere. The brightest ring is the outermost, having only about 800-kilometer (500-mile) width. Next inside comes a fainter ring about 5,000 kilometers (3,100 miles) wide, while very tenuous dust extends down to the atmosphere.

The innermost of the four large satellites of Jupiter, Io, has numerous large volcanoes that emit sulfur and sulfur dioxide. Most of the material emitted falls back onto the surface, but a small part of it escapes the satellite. In space, this material is rapidly dissociated (broken into its atomic constituents) and ionized (stripped of one or more electrons). Once it becomes charged, the material is trapped by Jupiter's magnetic field and forms a torus (donut shape) completely around Jupiter in Io's orbit. Accompanying the volcanic sulfur and oxygen are many sodium ions (and perhaps some of the sulfur and oxygen as well) that have been sputtered (knocked off the surface) from Io by high energy electrons in Jupiter's magnetosphere. The torus also contains protons (ionized hydrogen) and electrons. It will be fascinating to see what the effects are when large amounts of fine particulates collide with the torus.

Altogether, Jupiter has 16 known satellites. The two innermost, Metis and Adrastea, are tiny bodies, having radii near 20 and 10 kilometers (12 and 6 miles), respectively, that interact strongly with the rings and in fact may be the source of the rings. That is, the rings may be debris from impacts on the satellites. Amalthea and Thebe are still small, having mean radii of 86.2 and 50 kilometers (54 and 31 miles), respectively, but they are close to Jupiter. The Galilean satellites (the four moons discovered by Galileo in 1610), Io, Europa, Ganymede, and Callisto, have radii ranging from Europa's 1,565 kilometers (972 miles) to Ganymede's 2,634 kilometers (1,637 miles). (Earth's Moon has a radius of 1,738 kilometers or 1,080 miles) Of the Galilean satellites, Io is the closest to Jupiter at 421,700 kilometers (262,000 miles) while Callisto is the most distant at 1,883,000 kilometers (1,170,000 miles). The outer eight satellites are all tiny, having radii of less than 100 kilometers or 62 miles, and at large distances (greater than 11 million kilometers or 7 million miles) from Jupiter.

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