Answers

ConceptChecks

ConceptCheck 7-1: Our solar system contains only one star, the Sun. The other stars are very far away.

ConceptCheck 7-2: Mars is classified as an inner planet, and all four inner planets are also terrestrial planets with hard surfaces (these are Mercury, Venus, Earth, and Mars). The inner planets orbit within 2 AU of the Sun, while the outer planets (Jupiter, Saturn, Uranus, and Neptune) all orbit farther out, more than 5 AU from the Sun.

ConceptCheck 7-3: If the rocks on the surface have a density lower than the planet’s average density, then the planet’s core has a density greater than the planet’s average. This is not surprising, since gravity causes material of greater density to sink towards the center of a planet. (The concept of average density is discussed in Box 7-1.)

ConceptCheck 7-4: Table 7-2 lists four having diameters greater than our Moon’s 3476 km: Io, Ganymede, Callisto, and Titan.

ConceptCheck 7-5: As sunlight passes through a planet’s atmosphere (just before and after reflection off of the planet’s surface), the atoms and molecules in the atmosphere absorb specific wavelengths of light unique to these atoms and molecules. By looking at this absorption spectrum in the reflected sunlight, astronomers can infer the composition of the atmosphere as illustrated in Figure 7-3.

ConceptCheck 7-6: The reflected spectrum from a solid surface shows broad absorption features, whereas the spectrum observed from light passing through a gaseous atmosphere shows sharper spectral lines.

ConceptCheck 7-7: Hydrogen molecules in a gaseous atmosphere have a range of speeds based on the temperature of the gas, with some moving about six times the average speed. These faster molecules exceed Earth’s escape speed and leave Earth entirely as described in Box 7-2. Continuously, some of the remaining slower hydrogen molecules speed up through molecular collisions and steadily escape.

ConceptCheck 7-8: Ceres has a diameter of about 900 km. This is about the same size as a large U.S. state, such as the length of California.

ConceptCheck 7-9: Pluto orbits beyond Neptune. All such objects are called trans-Neptunian objects. One group of trans-Neptunian objects—including Pluto—orbits in the Kuiper belt, so Pluto is both a trans-Neptunian object and a Kuiper belt object. Pluto is neither a planet nor an asteroid.

ConceptCheck 7-10: Whereas the Kuiper belt lies in the same plane as Earth’s orbit around the Sun, the Oort cloud is a spherical distribution of comets that completely surrounds the solar system. If a comet has an orbit that is considerably different from that of the flat plane of the solar system, it most likely came from the spherical Oort cloud that exists in all directions around our solar system.

ConceptCheck 7-11: No. Io’s heat comes from tidal forces exerted by a very massive Jupiter. A planet, on the other hand, is not likely to experience strong interior-melting tidal forces from its smaller, orbiting moons.

ConceptCheck 7-12: No. Figure 7-13a shows the field created by a bar magnet. Figure 7-13b shows that these fields are similar in structure, but the source of the magnetic field is not a bar magnet. Instead, Earth’s magnetic field is due to electric currents flowing in a molten iron interior.

ConceptCheck 7-13: The answer to both questions is no. Based on its similar size to Earth, Venus probably has a molten interior. However, it exhibits no magnetic field, probably due to its very slow rotation: Earth rotates about 243 times for each single rotation of Venus. As in the cases of Jupiter, Saturn, Uranus, and Neptune, some electrically conducting material other than iron is needed to explain the observed magnetic fields.

CalculationChecks

CalculationCheck 7-1: The shape of a planet’s orbit is given by the value of its eccentricity. The closer this value is to zero, the closer the orbit’s shape is to that of a perfect circle. According to the table, the orbit of Venus has the eccentricity closest to zero (0.007), making it the most circlelike of all planetary orbits.

CalculationCheck 7-2: If we divide Saturn’s 120,536-km diameter by Earth’s 12,756-km diameter—we find that 120,536 km ÷ 12,756 km = 9.449, so about 9½ Earth’s would fit across Saturn’s diameter.

CalculationCheck 7-3: The asteroid belt is located between Mars and Jupiter at about 3 AU from the Sun, whereas the much larger Kuiper belt is beyond the orbit of Neptune and is located between about 30 and 50 AU from the Sun.

CalculationCheck 7-4: The masses in this figure are all given as multiples of Earth’s mass, so it would take just over 317 Earth masses to equal Jupiter’s mass. The number of Saturn masses that would equal Jupiter’s mass is 317.8/95.16 = 3.33.