Questions
Review Questions
Do all the planets orbit the Sun in the same direction? Are all of the orbits circular?
What are the characteristics of a terrestrial planet?
What are the characteristics of a Jovian planet?
In what ways are the largest satellites similar to the terrestrial planets? In what ways are they different? Which satellites are largest?
On March 16, 2007, Venus was 1.97 × 108 km from Earth and had an angular diameter of 12.7 arcsec. Using the small-angle formula from Box 1-1, calculate the diameter of Venus.What is meant by the average density of a planet? What does the average density of a planet tell us?
What are the differences in chemical composition between the terrestrial and Jovian planets?The absorption lines in the spectrum of a planet or satellite do not necessarily indicate the composition of the planet or satellite’s atmosphere. Why not?
Why are hydrogen and helium abundant in the atmospheres of the Jovian planets but present in only small amounts in Earth’s atmosphere?
What is an asteroid? What is a trans-Neptunian object? In what ways are these minor members of the solar system like or unlike the planets?
What are the asteroid belt, the Kuiper belt, and the Oort cloud? Where are they located? How do the objects found in these three regions compare?
In what ways is Pluto similar to a terrestrial planet? In what ways is it different?
What is the connection between comets and the Kuiper belt? Between comets and the Oort cloud?
What is one piece of evidence that impact craters are actually caused by impacts?
What is the relationship between the extent to which a planet or satellite is cratered and the amount of geologic activity on that planet or satellite?
How do we know that the surface of Venus is older than Earth’s surface but younger than the Moon’s surface?
Why do smaller worlds retain less of their internal heat?
How does the size of a terrestrial planet influence the amount of cratering on the planet’s surface?
How is the magnetic field of a planet different from that of a bar magnet? Why is a large planet more likely to have a magnetic field than a small planet?
Could you use a compass to find your way around Venus? Why or why not?
If Mars has no planetwide magnetic field, why does it have magnetized regions on its surface?
What is liquid metallic hydrogen? Why is it found only in the interiors of certain planets?
Advanced Questions
Questions preceded by an asterisk (*) involve topics discussed in the Boxes.
Problem-solving tips and tools
The volume of a sphere of radius r is 4πr3/3, and the surface area of a sphere of radius r is 4πr2. The surface area of a circle of radius r is πr2. The average density of an object is its mass divided by its volume. To calculate escape speeds, you will need to review Box 7-2. Be sure to use the same system of units (meters, seconds, kilograms) in all your calculations involving escape speeds, orbital speeds, and masses. Appendix 6 gives conversion factors between different sets of units, and Box 5-1 has formulas relating various temperature scales.
Mars has two small satellites, Phobos and Deimos. Phobos circles Mars once every 0.31891 day at an average altitude of 5980 km above the planet’s surface. The diameter of Mars is 6794 km. Using this information, calculate the mass and average density of Mars.
Figure 7-3 shows the spectrum of Saturn’s largest satellite, Titan. Can you think of a way that astronomers can tell which absorption lines are due to Titan’s atmosphere and which are due to the atmospheres of the Sun and Earth? Explain.
*(a) Find the mass of a hypothetical spherical asteroid 2 km in diameter and composed of rock with average density 2500 kg/m3. (b) Find the speed required to escape from the surface of this asteroid. (c) A typical jogging speed is 3 m/s. What would happen to an astronaut who decided to go for a jog on this asteroid?
*The hypothetical asteroid described in Question 25 strikes Earth with a speed of 25 km/s. (a) What is the kinetic energy of the asteroid at the moment of impact? (b) How does this energy compare with that released by a 20-kiloton nuclear weapon, like the device that destroyed Hiroshima, Japan, on August 6, 1945? (Hint: 1 kiloton of TNT releases 4.2 × 1012 joules of energy.)
*Suppose a spacecraft landed on Jupiter’s moon Europa (see Table 7-2), which moves around Jupiter in an orbit of radius 670,900 km. After collecting samples from the satellite’s surface, the spacecraft prepares to return to Earth. (a) Calculate the escape speed from Europa. (b) Calculate the escape speed from Jupiter at the distance of Europa’s orbit. (c) In order to begin its homeward journey, the spacecraft must leave Europa with a speed greater than either your answer to (a) or your answer to (b). Explain why.
*A hydrogen atom has a mass of 1.673 × 10−27 kg, and the temperature of the Sun’s surface is 5800 K. What is the average speed of hydrogen atoms at the Sun’s surface?
*The Sun’s mass is 1.989 × 1030 kg, and its radius is 6.96 × 108 m. (a) Calculate the escape speed from the Sun’s surface (b) Using your answer to Question 28, explain why the Sun has lost very little hydrogen over its entire 4.56-billion-year history.
*Saturn’s satellite Titan has an appreciable atmosphere, yet Jupiter’s satellite Ganymede—which is about the same size and mass as Titan—has no atmosphere. Explain why there is a difference.
The distance from the asteroid 433 Eros (Figure 7-7) to the Sun varies between 1.13 and 1.78 AU. (a) Find the period of Eros’s orbit. (b) Does Eros lie in the asteroid belt? How can you tell?
Imagine a trans-Neptunian object with roughly the same mass as Earth but located 50 AU from the Sun. (a) What do you think this object would be made of? Explain your reasoning. (b) On the basis of this speculation, assume a reasonable density for this object and calculate its diameter. How many times bigger or smaller than Earth would it be?
Consider a hypothetical trans-Neptunian object located 100 AU from the Sun. (a) What would be the orbital period (in years) of this object? (b) There are 360 degrees in a circle, and 60 arcminutes in a degree. How long would it take this object to move 1 arcminute across the sky? (c) Trans-Neptunian objects are discovered by looking for “stars” that move on the celestial sphere. Use your answer from part (b) to explain why these discoveries require patience. (d) Discovering trans-Neptunian objects also requires large telescopes equipped with sensitive detectors. Explain why.
The surfaces of Mercury, the Moon, and Mars are riddled with craters formed by the impact of space debris. Many of these craters are billions of years old. By contrast, there are only a few conspicuous craters on Earth’s surface, and these are generally less than 500 million years old. What do you suppose explains the difference?
During the period of most intense bombardment by space debris, a new 1-km-radius crater formed somewhere on the Moon about once per century. During this same period, what was the probability that such a crater would be created within 1 km of a certain location on the Moon during a 100-year period? During a 106-year period? (Hint: If you drop a coin onto a checkerboard, the probability that the coin will land on any particular one of the board’s 64 squares is
.)When an impact crater is formed, material (called ejecta) is sprayed outward from the impact. (The accompanying photograph of the Moon shows light-colored ejecta extending outward from the crater Copernicus.) While ejecta are found surrounding the craters on Mercury, they do not extend as far from the craters as do ejecta on the Moon. Explain why, using the difference in surface gravity between the Moon (surface gravity = 0.17 that on Earth) and Mercury (surface gravity = 0.38 that on Earth).
R I V U X G(NASA)Mercury rotates once on its axis every 58.646 days, compared to 1 day for Earth. Use this information to argue why Mercury’s magnetic field should be much smaller than Earth’s.
As you can see in Figure 7-15, Mars Global Surveyor did not find a significant magnetic field in the northern region of Utopia Planitia, which is a large lava field. Based on this observation, would you expect the lava field to have formed before or after Mars ceased to have a molten core?
Liquid metallic hydrogen is the source of the magnetic fields of Jupiter and Saturn. Explain why liquid metallic hydrogen cannot be the source of Earth’s magnetic field.
Discussion Questions
*There are no asteroids with an atmosphere. Discuss why not.
The Galileo spacecraft that orbited Jupiter from 1995 to 2003 discovered that Ganymede (Table 7-2) has a magnetic field twice as strong as that of Mercury. Does this discovery surprise you? Why or why not?
Web/eBook Question
Search the World Wide Web for information about impact craters on Earth. Where is the largest crater located? How old is it estimated to be? Which crater is closest to where you live?Determining Terrestrial Planet Orbital Periods. Access the animation “Planetary Orbits” in Chapter 7 of the Universe Web site or eBook. Focus on the motions of the inner planets at the last half of the animation. Using the stop and start buttons, determine how many days it takes Mars, Venus, and Mercury to orbit the Sun once if Earth takes approximately 365 days.