Jupiter is the largest planet in the solar system. Its mass is more than twice the mass of all the other planets combined, although it is still about 1/1000 the mass of the Sun. It is composed primarily of hydrogen and helium.
Jupiter rotates very rapidly, producing a pronounced equatorial flattening. The amount of flattening allows astronomers to infer the presence of a large rocky core in its interior. Jupiter displays differential rotation—because the planet has no solid surface, the rotation rate varies from place to place in the atmosphere. Measurements of radio emission from Jupiter's magnetosphere provide a measure of the planet's interior rotation rate.
Jupiter's atmosphere consists of three main cloud layers. The colors we see are the result of chemical reactions, fueled by the planet's interior heat, solar ultraviolet radiation, auroral phenomena, and lightning, at varying depths below the cloud tops, seen through "holes" in the overlying clouds. The cloud layers on Jupiter, as on all the jovian worlds, are arranged into bands of bright zones and darker belts crossing the planets parallel to the equator. The bands are the result of convection in the planet's interior and the planet's rapid rotation. The lighter zones are the tops of upwelling, warm currents, and the darker bands are cooler regions where gas is sinking. Underlying them is a stable pattern of eastward or westward wind flow called the zonal flow. The wind direction alternates as we move north or south away from the equator.
The main weather pattern on Jupiter is the Great Red Spot, an Earth-sized hurricane that has been raging for at least three centuries. Other, smaller, weather systems—the white and brown ovals—are also observed. They can persist for decades.
Jupiter's atmosphere becomes hotter and denser with depth, eventually becoming liquid. Interior pressures are so high that the hydrogen is "metallic" in nature near the center. The planet has a large "terrestrial" core 10—20 times the mass of Earth. Jupiter radiates about twice as much energy into space as it receives from the Sun. The source of this energy is most likely heat left over from the planet's formation 4.6 billion years ago.
The magnetosphere of Jupiter is about a million times more voluminous than Earth's magnetosphere, and the planet has a long magnetic "tail" extending away from the Sun to at least the distance of Saturn's orbit. Energetic particles spiral around magnetic field lines, accelerated by Jupiter's rotating magnetic field, producing intense radio radiation.
Jupiter and its system of moons resemble a small solar system. Sixteen moons have been discovered so far. The outermost eight moons resemble asteroids and have retrograde orbits, suggesting that they may have been captured by Jupiter's gravity long after the planets and largest moons formed. Jupiter's four major moons are called the Galilean satellites, after their discoverer, Galileo Galilei. Their densities decrease with increasing distance from the planet.
The innermost Galilean moon, Io, has active volcanoes powered by the constant flexing of the moon by Jupiter's tidal forces. As Io orbits Jupiter the moon "wobbles" because of the gravitational pull of Europa. The ever-changing distortion of its interior energizes Io, and geyserlike volcanoes keep its surface smooth with constant eruptions. The material ejected by these volcanoes forms the Io plasma torus in Jupiter's inner magnetosphere.
Europa has a cracked, icy surface that may conceal an ocean of liquid water. Its fields of ice are nearly devoid of craters but have extensive fractures, which are most likely due to the tidal influence of Jupiter and the gravitational effects of the other Galilean satellites. Ganymede and Callisto have ancient, heavily cratered surfaces. Ganymede, the largest moon in the solar system, shows some evidence of past geological activity, but it is now unmoving rock and ice. Callisto apparently froze before tectonic activity could start there.
Jupiter has a faint, dark ring extending down to the planet's cloud tops. It was discovered in 1979 by Voyager 1.
1. Jupiter has over 300 times the mass of Earth, and twice the mass of all the other planets combined. HINT
2. The solid surface of Jupiter lies just below the cloud layers visible from Earth. HINT
3. There is no evidence to suggest that Jupiter has a rocky core. HINT
4. Jupiter has only one large storm system. HINT
5. In general, a storm system in Jupiter's atmosphere is much longer lived than storms in Earth's atmosphere. HINT
6. The element helium plays an important role in producing the colors in Jupiter's atmosphere. HINT
7. The magnetosphere of Jupiter is similar in intensity to Earth's magnetosphere. HINT
8. Most of Jupiter's moons rotate synchronously with their orbits. HINT
9. The densities of the Galilean moons increase with increasing distance from Jupiter. HINT
10. Io has a noticeable lack of impact craters on its surface. HINT
11. The surface of Europa is completely covered by water ice. HINT
12. Ganymede shows evidence of ancient plate tectonics. HINT
13. Jupiter's ring is made up of icy particles a few meters across. HINT
14. Most of the small moons of Jupiter have diameters of several hundred kilometers. HINT
15. The thickness of Jupiter's cloud layer is less than 1 percent of the planet's radius. HINT
1. The _____ of Jupiter indicates that its overall composition differs greatly from that of the terrestrial planets. HINT
2. The main constituents of Jupiter are _____ and _____. HINT
3. Jupiter's rapid _____ produces a significant equatorial bulge. HINT
4. Jupiter's Great Red Spot has similarities to _____ on Earth. HINT
5. The diameter of the Great Red Spot is about _____ that of Earth. HINT
6. Jupiter's clouds consist of a series of bright _____ and dark _____. HINT
7. Jupiter emits about _____ more radiation than it receives from the Sun. HINT
8. Although often referred to as a gaseous planet, Jupiter is mostly _____ in its interior. HINT
9. Jupiter's magnetic field is generated by its rapid rotation and the element _____, which becomes metallic at the high pressure found in the planet's deep interior. HINT
10. Because of Jupiter's strong tidal field, most of Jupiter's satellites rotate _____ with their orbits around the planet. HINT
11. The Galilean moons make up _____ of the _____ moons of Jupiter. (Give the numbers.) HINT
12. The Galilean moon _____ is larger than the planet Mercury. HINT
13. In contrast with the inner Galilean moons, the outer two Galilean moons have compositions that include significant amounts of _____. HINT
14. Io is the only moon in the solar system with active _____. HINT
15. Scientists speculate that _____ may have liquid water below its frozen surface. HINT
1. In what sense does our solar system consist of only two important objects? HINT
2. What is differential rotation, and how is it observed on Jupiter? HINT
3. Describe some of the ways in which the Voyager mission changed our perception of Jupiter.
4. What is the Great Red Spot? What is known about the source of its energy? HINT
5. What is the cause of the colors in Jupiter's atmosphere? HINT
6. Why has Jupiter retained most of its original atmosphere? HINT
7. Explain the theory that accounts for Jupiter's internal heat source. HINT
8. What is Jupiter thought to be like beneath its clouds? Why do we think this? HINT
9. What is responsible for Jupiter's enormous magnetic field? HINT
10. In what sense are Jupiter and its moons like a miniature solar system? HINT
11. How does the density of the Galilean moons vary with increasing distance from Jupiter? Is there a trend to this variation? If so, why? HINT
12. What is the source of Io's volcanic activity? HINT
13. How does the amount of cratering vary among the Galilean moons? Does it depend on their location? If so, why? HINT
14. Why is there speculation that the Galilean moon Europa might be an abode for life? HINT
15. Water is relatively uncommon among the terrestrial planets. Is it common among the moons of Jupiter? HINT
1. How does the force of gravity at Jupiter's cloudtops compare with the force of gravity at Earth's surface? HINT
2. How long does it take Jupiter's equatorial winds to circle the planet, relative to the interior? HINT
3. Given Jupiter's current atmospheric temperature, what is the smallest possible mass the planet could have and still retain its hydrogen atmosphere? (See More Precisely 8-1.)
4. If Jupiter had been just massive enough to fuse hydrogen (see Interlude 11-1), calculate what the planet's gravitational force on Earth would have been at closest approach, relative to the gravitational pull of the Sun. Also estimate what the magnitude of the planet's tidal effect on our planet would have been, again relative to that of the Sun. HINT
5. Calculate the ratio of Jupiter's mass to the total mass of the Galilean moons. Compare this with the ratio of Earth's mass to that of the Moon. HINT
6. Io orbits Jupiter at a distance of six planetary radii in 42 hours. At what distance would a satellite orbit Jupiter in the time taken for Jupiter to rotate exactly once (10 hours, say) so that the satellite would appear "stationary" above the planet? (Use Kepler's third law; see Chapter 2.) HINT
7. Estimate the strength of Jupiter's gravitational tidal force on Io, relative to the moon's own surface gravity. HINT
8. Calculate the synodic period of Europa, as seen from Io, and the strength of Europa's gravitational pull on Io at closest approach, relative to Jupiter's gravitational attraction. HINT
9. What is the speed needed to escape from Io? HINT
10. Compare the apparent sizes of the Galilean moons, as seen from Jupiter's cloudtops, with the angular diameter of the Sun at Jupiter's distance. Would you expect ever to see a total solar eclipse from Jupiter's cloudtops? HINT
1. Are there any stars in the night sky that look as bright as Jupiter? What other difference do you notice between Jupiter and the stars?
2. Use binoculars to peer at Jupiter. Be sure to hold them steady (try propping your arms up on the hood of a car, or sitting down and bracing them against your knees). Can you see any of Jupiter's four largest moons? If you come back the following evening, the moons' relative positions will have changed. Have some changed more than others?
3. Through a telescope, you should be able to see the red-and-tan cloud bands of Jupiter, and you can clearly see some moons. Do the moons orbit in the equatorial plane? Before observing, look up the positions of the Galilean moons in a current magazine such as Astronomy or Sky & Telescope. Identify each of the moons. Watch Io over a period of at least an hour or more. Can you see its motion? Do the same for Europa.
