Questions
Review Questions
Mars passes closer to Earth than Jupiter does, but with an Earth-based telescope it is easier to see details on Jupiter than on Mars. Why is this?
Saturn is the most distant of the planets visible without a telescope. Is there any way we could infer this from naked-eye observations? Explain your answer. (Hint: Think about how Saturn’s position on the celestial sphere must change over the course of weeks or months.)
As seen from Earth, does Jupiter or Saturn undergo retrograde motion more frequently? Explain your answer.
In what ways are the motions of Jupiter’s atmosphere like the motion of water stirred in a pot (see Figure 12-3b)? In what ways are they different?Is the chemical composition of Jupiter as a whole the same as that of its atmosphere? Explain any differences.
Astronomers can detect the presence of hydrogen in stars by looking for the characteristic absorption lines of hydrogen in the star’s visible spectrum (Figure 5-21). They can also detect hydrogen in glowing gas clouds by looking for hydrogen’s characteristic emission lines (Figure 5-18). Explain why neither of these techniques helped Earth-based astronomers to detect hydrogen in Jupiter’s atmosphere.
On a warm, humid day, water vapor remains in the atmosphere. But if the temperature drops suddenly, the water vapor forms droplets, clouds appear, and it begins to rain. Relate this observation to why there is relatively little helium in Saturn’s atmosphere compared to the atmosphere of Jupiter.
What would happen if you tried to land a spacecraft on the surface of Jupiter?
What are the belts and zones in the atmospheres of Jupiter and Saturn? Is the Great Red Spot more like a belt or a zone? Explain your answer.
Give one possible explanation why weather systems on Jupiter are longer-lived than weather systems on Earth.
What are white ovals and brown ovals? What can we infer about them from infrared observations?
Compare and contrast the source of energy for motions in Earth’s atmosphere with the energy source for motions in the atmospheres of Jupiter and Saturn.
Both Jupiter and Saturn emit more energy than they receive from the Sun in the form of sunlight. Compare the internal energy sources of the two planets that produce this emission.
What observations from the Cassini spacecraft contradict the accepted picture of zone and belt convection in Jupiter’s atmosphere?
Compare the atmospheres of Jupiter and Saturn. Why does Saturn’s atmosphere look “washed out” in comparison to that of Jupiter?
Which data from the Galileo Probe were in agreement with astronomers’ predictions? Which data were surprising?
Fewer than one in every 105 atoms in Jupiter’s atmosphere is an argon atom, and fewer than one in 108 is an atom of krypton or xenon. If these atoms are so rare, why are scientists concerned about them? How do the abundances of these elements in Jupiter’s atmosphere compare to the abundances in the Sun? What hypothesis has been offered to explain these observations?
Why is Jupiter oblate? What do astronomers learn from the value of Jupiter’s oblateness?
What is liquid metallic hydrogen? What is its significance for Jupiter?
Describe the internal structures of Jupiter and Saturn, and compare them with the internal structure of Earth.
Explain why Saturn is more oblate than Jupiter, even though Saturn rotates more slowly.
Compare and contrast Jupiter’s magnetosphere with the magnetosphere of a terrestrial planet like Earth.
Why is Saturn’s magnetosphere less extensive than Jupiter’s?
What observations of Saturn’s rings proved that they are not solid?If Saturn’s rings are not solid, why do they look solid when viewed through a telescope?
Although the Voyager and Cassini spacecraft did not collect any samples of Saturn’s ring particles, measurements from these spacecraft allowed scientists to determine the sizes of the particles. Explain how this was done.
The Space Shuttle and other spacecraft orbit Earth well within Earth’s Roche limit. Explain why these spacecraft are not torn apart by tidal forces.
How do Jupiter’s rings differ from those of Saturn?
Describe the structure of Saturn’s rings. What evidence is there that ring particles do not migrate significantly between ringlets?
During the planning stages for the Pioneer 11 mission, when relatively little was known about Saturn’s rings, it was proposed to have the spacecraft fly through the Cassini division. Why would this have been a bad idea?
What is the relationship between Saturn’s satellite Mimas and the Cassini division?
Why is the term “shepherd satellite” appropriate for the objects so named? Explain how a shepherd satellite operates.
Advanced Questions
The question preceded by an asterisk (*) involves topics discussed in Box 7-2.
Problem-solving tips and tools
Box 4-4 describes how to use Newton’s form of Kepler’s third law. Newton’s law of universal gravitation, discussed in Section 4-6, is the basic equation from which you can calculate a planet’s surface gravity. Box 7-2 discusses escape speed, and Section 5-3 and Section 5-4 discuss the properties of thermal radiation (including the Stefan-Boltzmann law, which relates the temperature of a body to the amount of thermal radiation that it emits).
The angular diameter of Jupiter at opposition varies little from one opposition to the next. By contrast, the angular diameter of Mars at opposition is quite variable. Explain why there is a difference between these two planets.
Jupiter was at opposition on June 5, 2007. On that date Jupiter appeared to be in the constellation Ophiuchus. Approximately when will Jupiter next be at opposition in this same region of the celestial sphere? Explain your answer.
Jupiter’s equatorial diameter and the rotation period at Jupiter’s equator are both given in Table 12-1. Use these data to calculate the speed at which an object at the cloudtops along Jupiter’s equator moves around the center of the planet.
Using orbital data for a Jovian satellite of your choice (see Appendix 3), calculate the mass of Jupiter. How does your answer compare with the mass quoted in Table 12-1?
The density of water is 1,000 kg/m3. It has been claimed that Saturn would float if one had a large enough bathtub. Using the mass and size of Saturn given in Table 12-2, confirm that the planet’s average density is about 690 kg/m3, and comment on this somewhat fanciful claim.
Roughly speaking, Jupiter’s composition (by mass) is three-quarters hydrogen and one-quarter helium. The mass of a single hydrogen atom is given in Appendix 7; the mass of a single helium atom is about 4 times greater. Use these numbers to calculate how many hydrogen atoms and how many helium atoms there are in Jupiter.
Use the information given in Section 12-3 to estimate the wind velocities in the Great Red Spot, which rotates with a period of about 6 days.
An astronaut floating above Saturn’s cloudtops would see a blue sky, even though Saturn’s atmosphere has a very different chemical composition than Earth’s. Explain why. (Hint: See Box 5-4.)
If Jupiter emitted just as much energy per second (as infrared radiation) as it receives from the Sun, the average temperature of the planet’s cloudtops would be about 107 K. Given that Jupiter actually emits twice this much energy per second, calculate what the average temperature must actually be.
(a) From Figure 12-10a, by how much does the temperature increase as you descend from the 100-millibar level in Jupiter’s atmosphere to an altitude 100 km below that level? (b) Use Figure 12-10b to answer the same question for Saturn’s atmosphere. (c) In Earth’s troposphere (see Section 9-5), the air temperature increases by 6.4°C for each kilometer that you descend. In which planet’s atmosphere—Earth, Jupiter, or Saturn—does the temperature increase most rapidly with decreasing altitude?
*Consider a hypothetical future spacecraft that would float, suspended from a balloon, for extended periods in Jupiter’s upper atmosphere. If we want this spacecraft to return to Earth after completing its mission, calculate the speed at which the spacecraft’s rocket motor would have to accelerate it in order to escape Jupiter’s gravitational pull. Compare with the escape speed from Earth, equal to 11.2 km/s.
The Galileo Probe had a mass of 339 kg. On Earth, its weight (the gravitational force exerted on it by Earth) was 3320 newtons, or 747 lb. What was the gravitational force that Jupiter exerted on the Galileo Probe when it entered Jupiter’s clouds?
From the information given in Section 12-6, calculate the average density of Jupiter’s rocky core. How does this compare with the average density of Earth? With the average density of Earth’s solid inner core? (See Table 9-1 and Table 9-2 for data about Earth.)
In the outermost part of Jupiter’s outer layer (shown in yellow in the upper part of Figure 12-13), hydrogen is principally in the form of molecules (H2). Deep within the liquid metallic hydrogen layer (shown in orange in Figure 12-13), hydrogen is in the form of single atoms. Recent laboratory experiments suggest that there is a gradual transition between these two states, and that the transition layer overlaps the boundary between the ordinary hydrogen and liquid metallic hydrogen layers. Use this information to redraw the upper part of Figure 12-12 and to label the following regions in Jupiter’s interior: (i) ordinary (nonmetallic) hydrogen molecules; (ii) nonmetallic hydrogen with a mixture of atoms and molecules; (iii) liquid metallic hydrogen with a mixture of atoms and molecules; (iv) liquid metallic hydrogen atoms.
When Saturn is at different points in its orbit, we see different aspects of its rings because the planet has a 27° tilt. If the tilt angle were different, would it be possible to see the upper and lower sides of the rings at all points in Saturn’s orbit? If so, what would the tilt angle have to be? Explain your answers.
As seen from Earth, the intervals between successive edge-on presentations of Saturn’s rings alternate between about 13 years, 9 months, and about 15 years, 9 months. Why do you think these two intervals are not equal?
(a) Use Newton’s form of Kepler’s third law to calculate the orbital periods of particles at the outer edge of Saturn’s A ring and at the inner edge of the B ring. (b) Saturn’s rings orbit in the same direction as Saturn’s rotation. If you were floating along with the cloudtops at Saturn’s equator, would the outer edge of the A ring and the inner edge of the B ring appear to move in the same or opposite directions? Explain.
This Cassini close-up image of Saturn’s rings shows a number of bright, straight features elongated outward called spokes. As these features orbit around Saturn, they tend to retain their shape like the rigid spokes on a rotating bicycle wheel. The spokes rotate at the same rate as Saturn’s magnetic field and are thought to be clouds of tiny, electrically charged particles kept in orbit by magnetic forces. Explain why the spokes could not maintain their shape if they were kept in orbit by gravitational forces alone.
R I V U X G(NASA/JPL/Space Science Institute)The Cassini division involves a 2-to-1 resonance with Mimas. Does the location of the Encke gap—133,500 km from Saturn’s center—correspond to a resonance with one of the other satellites? If so, which one? (See Appendix 3.)
Discussion Questions
Describe some of the semipermanent features in Jupiter’s atmosphere. Compare and contrast these long-lived features with some of the transient phenomena seen in Jupiter’s clouds.
Suppose you were asked to design a mission to Jupiter involving an unmanned airplanelike vehicle that would spend many days (months?) flying through the Jovian clouds. What observations, measurements, and analyses should this aircraft be prepared to make? What dangers might the aircraft encounter, and what design problems would you have to overcome?
What sort of experiment or space mission would you design in order to establish definitively whether Jupiter has a rocky core?
The classic science fiction films 2001: A Space Odyssey and 2010: The Year We Make Contact both involve manned spacecraft in orbit around Jupiter. What kinds of observations could humans make on such a mission that cannot be made by robotic spacecraft? What would be the risks associated with such a mission? Do you think that a manned Jupiter mission would be as worthwhile as a manned mission to Mars? Explain your answers.
Suppose that Saturn were somehow moved to an orbit around the Sun with semimajor axis 1 AU, the same as Earth’s. Discuss what long-term effects this would have on the planet and its rings.
Web/eBook Questions
On Jupiter, the noble gases argon, krypton, and xenon provide important clues about Jupiter’s past. On Earth, xenon is used in electronic strobe lamps because it emits a very white light when excited by an electric current. Argon, by contrast, is one of the gases used to fill ordinary incandescent lightbulbs. Search the World Wide Web for information about why argon is used in this way. Why do premium, long-life lightbulbs use krypton rather than argon?
Search the World Wide Web, especially the Web sites for NASA’s Jet Propulsion Laboratory and the European Space Agency, for information about the current status of the Cassini mission. What recent discoveries has Cassini made?
The two Voyager spacecraft were launched from Earth along a trajectory that took them directly to Jupiter. The force of Jupiter’s gravity then gave the two spacecraft a “kick” that helped push them onward to Saturn. The much larger Cassini spacecraft, by contrast, was first launched on a trajectory that took it past Venus. Search the World Wide Web, especially the Web sites for NASA’s Jet Propulsion Laboratory and the European Space Agency, for information about the trajectory that Cassini took through the solar system to reach Saturn. Explain why this trajectory was so different from that of the Voyagers.
The Rotation Rate of Saturn. Access and view the video “Saturn from the Hubble Space Telescope” in Chapter 12 of the Universe Web site or eBook. The total amount of time that actually elapses in this video is 42.6 hours. Using this information, identify and follow an atmospheric feature and determine the rotation period of Saturn. How does your answer compare with the value given in Table 12-2?