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Homework Chapters 7-8 Due: January 7, 2014 ASTRON 311 Introduction to Astronomy Prof. Menningen p. 1/3 Name: __________________________________ 1. The correct sequence of planets in our solar system from the Sun outward is a. Mercury, Venus, Earth, Mars, Saturn, Uranus, Jupiter, Neptune, Pluto. b. Mercury, Earth, Venus, Mars, Jupiter, Saturn, Uranus, Pluto, Neptune. c. Mercury, Venus, Mars, Earth, Jupiter, Saturn, Uranus, Pluto, Neptune. d. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto. 2. Which of the following statements about the solar system is false? a. All planets revolve around the Sun in the same direction. b. All planets rotate on their axis in the same direction as they orbit about the Sun. c. All planets orbit in roughly the same plane. d. Most planetary orbits are not highly elliptical. 3. The average density of which of the following planetary groups is close to that of water (1000 kg/m3)? a. Mercury and Venus, because they are close to the Sun. b. Terrestrial planets, because they are of relatively low mass and have been compressed very little by gravitational forces. c. Asteroids, because they are very small objects. d. Jovian planets, because of their H and He composition. 4. What was the form of the material from which the solar system formed? a. A nebula made mostly of hydrogen and helium gas, but enriched in heavier elements from supernova explosions. b. A nebula made mostly of heavy elements, but enriched in hydrogen and helium from the supernova explosions. c. A nebula made entirely of hydrogen and helium gas. d. Debris from the explosion of a massive star. 5. What is meant by a substance's condensation temperature? What role did condensation temperatures play in the formation of the planets? The condensation temperature of a substance is the temperature at which the substance solidifies from gas. In the inner part of the solar system where the temperature was high, only substances with high-condensation temperatures could become planets. In the outer part of the solar system, substances with low-condensation temperatures could also become part of planets. Thus the inner planets are rocky while the outer planets contain more liquids and gases. 6. Why did the terrestrial planets form close to the Sun while the Jovian planets formed far from the Sun? Terrestrial planets are made of substances with high condensation temperature, substances that will not vaporize at the high temperatures that exist close to the Sun. Jovian planets are made of substances with low condensation temperatures, such as hydrogen and helium, and they can only form where temperatures are very cold, far from the Sun. 7. According to modern theories, the most significant difference between the formation of the terrestrial and Jovian planets is that a. both formed by accretion of planetesimals, but the Jovian planets became massive enough to attract gas onto them directly from the solar nebula. b. both formed by accretion of rocky and icy planetesimals, but the terrestrial planets were close enough to the Sun that almost all of ices escaped back to space after the planets formed. c. the terrestrial planets formed close to the Sun where there was lots of rock but no ice, whereas the Jovian planets formed far from the Sun where there was lots of hydrogen and ice but no rocky material. d. the terrestrial planets formed by accretion of planetesimals, whereas the Jovian planets formed from streamers of hot gas which shot out of the protostar. Homework Chapters 7-8 8. ASTRON 311 Introduction to Astronomy Prof. Menningen p. 2/3 The protoplanetary disk, or proplyd, in the figure below is seen edge-on. The diameter of the disk is about 700 AU. (a) Make measurements on this image to determine the thickness of the disk in AU. Explain why the disk will continue to flatten as time goes by. The ratio of the proplyd’s width to its thickness is about 4 to1. thickness 1 700 AU 175 AU 4 The disk will continue to flatten as gravity compresses the material along the axis of rotation, but the centrifugal force prevents the material from compressing perpendicular to the rotation axis. 9. Run the “Solar System Builder” from the chapter 7 section of the textbook companion site. Click “Reset” and then check the Pause checkbox. Move the slider (if necessary) to choose an Earth planet. Click once when the cursor position (see lower right) is at Y = 0, X = 1.0 AU. Now move the planet slider to Jupiter. Click once at Y = 0, X = 5.0 AU. Now add 5 more Jupiters by clicking at different places along the line between Jupiter and the Earth, then move the planet slider to Earth and add 5 more Earths in that same region. Now uncheck the Pause checkbox and watch the system for 30 years. Describe the state of the system in comparison to how it began. Try the same simulation two more times, but vary the distribution of planets along the initial line a little bit. What common themes do you see in the outcomes of the three simulations? After 30 years some planets have collided and some have been flung off into space, so that only a few planets remain and the resulting orbits are noncircular and chaotic. The closer the planets are initially placed to one another, the stronger the interactions between them. 10. One theory about the origin of the Moon says that the Moon was formed from debris thrown out when a Mars-sized object collided with Earth. One fact that strongly supports this theory is that a. the Moon has several smooth plains formed by ancient lava flows. b. the Moon always turns the same side toward Earth. c. impact breccias (rock fragments cemented together by an impact) are common on the Moon. d. Moon rocks are very similar to those of Earth but are depleted in elements that melt at relatively low temperatures. 11. A planet [Mars] has two small satellites, [Phobos and Deimos]. Phobos circles the planet once every 0.31891 day at an average altitude of 5980 km above the planet's surface. The diameter of the planet is 6794 km. Using this information, calculate the mass and average density of the planet. First of all, you must calculate Phobos’ orbital radius: a 5980 km 12 6794 km 9377 km 9.377 106 m From the Extending Our Reach on page 82 we see that the appropriate form of Kepler’s third law is M 4 2 d 3 , where M is the mass of Mars. The orbital period of Phobos is G P2 P = 0.31891 days = 2.755 104 s. Applying the formula yields: 4π 2 M G 6 d3 9.377 10 m 4π 2 2 11 2 2 2 P 6.67 10 N·m /kg 2.755 104 s . M 6.43 1023 kg Now calculate the density: M M 6.43 1023 kg 4 3 3920 kg/m3 3 3 4 1 V 3r 3 2 6794 10 m 3 Homework Chapters 7-8 ASTRON 311 Introduction to Astronomy Prof. Menningen p. 3/3 12. Describe the differences between the maria and the lunar highlands. Which kind of terrain is more heavily cratered? Which kind of terrain was formed later in the Moon's history? How do we know? The maria are lava-flooded areas that are dark gray in color and are lower in elevation than the average. The highlands are light gray in color, rough in appearance, and higher than average in elevation. The highlands are much more heavily cratered. The maria were formed later in the Moon’s history. We know this because the crater density on the maria is lower than the the crater density on the highlands. 13. Rocks found on the Moon are between 3.1 and 4.6 billion years old. By contrast, the majority of the Earth's surface is made of oceanic crust that is less than 200 million years old, and the very oldest Earth rocks are about 3 billion years old. If the Earth and Moon are essentially the same age, why is there such a disparity in the ages of rocks on the two worlds? On the Earth, rocks that formed before about 3 billion years ago have been subducted into the mantle and melted. Old crust is continually pushed down into the mantle and replaced along the mid-oceanic ridges by new crust. These processes do not occur on the Moon, so ancient rocks are still found there on the surface. 14. In a whimsical moment during the Apollo 14 mission, astronaut Alan Shepard hit two golf balls over the lunar surface. Give two reasons why they traveled much farther than golf balls do on Earth. Golf balls hit on the Moon travel farther than on Earth because there is no air resistance on the Moon to slow the balls down, and the Moon's weaker gravity takes longer to pull the balls back down to the ground. 15. How much would an 80-kg person weigh on the Moon? How much does that person weigh on the Earth? Use Appendix Tables 3 and 5 together with the Universal Law of Gravity. The Universal Law of Gravity (p. 79) can be used to find these answers. Tables 3 and 5: On the Moon, Weight F G Mm d 2 6.67 1011 N m 2 /kg 2 or about 29.2 pounds. On Earth, F G Mm d 2 6.67 10 11 N m /kg 2 2 7.349 10 5.974 10 22 kg 80 kg 1738 km 100 m/km 2 24 kg 80 kg 6, 378 km 100 m/km 2 130 N 784 N 176 lb Using Appendix