ASTR 2020, Spring 2018

Transcription

1 ASTR 2020, Spring 2018 Last Name First Name Professor Jack Burns Exam #3 April 27, 2018 Student ID Number A INSTRUCTIONS: Closed books, one page (2 sides) of notes allowed, calculators may be used, strictly individual effort. WRITE your name and student ID number on this page BEFORE you begin the exam. The exam consists of 10 multiple choice questions worth 4 points each, 4 short answer questions worth 15 points each, and an optional extra credit short answer question worth 5 points on the last page. Please allocate your time accordingly among these parts of the exam. Multiple Choice. In questions 1-10, choose the best answer (2 pts). Then explain your reasoning in 1-2 complete sentences, including why these statements are correct or incorrect (2 pts). So, a correct answer and correct explanation is worth a total of 4 pts. 1. Which of the following is not a major difference between the terrestrial and jovian planets in our solar system? a. Terrestrial planets contain large quantities of ice and jovian planets do not. b. Terrestrial planets orbit much closer to the Sun than jovian planets. c. Terrestrial planets are higher in average density than jovian planets. d. Jovian planets have rings and terrestrial planets do not. e. Jupiter and the Jovian planets have stronger magnetospheres than does Earth and the terrestrial planets. a Ice makes up a very small percentage of a terrestrial planet s mass. What ices do exist on terrestrial planets was brought by comets. On the other hand, jovian planets, especially Uranus and Neptune, have large amounts of ice. Being beyond the frost line, where ices form, is what allowed jovian planets to form. 2. In what way is Venus most similar to Earth? a. Both planets have very similar atmospheres. b. Both planets have similar surface geology. c. Both planets have warm days and cool nights. d. Both planets are nearly the same size. e. Both planets have large surface oceans. d Venus is only slightly smaller than Earth. Venus has a much thicker atmosphere than Earth, as its oceans evaporated through a runaway greenhouse effect. The thick atmosphere, along with winds, makes both day and night on Venus very hot. Finally, their surface geologies are not particularly similar, with Earth having much more erosion and plate tectonics, for example. 1

2 3. Which of the following statements is NOT true about the planets so far discovered around other stars? a. Most of them are much more massive than Earth. b. Photographs reveal that most of them have atmospheres much like that of Jupiter. c. Many of them orbit closer to their star than Jupiter orbits the Sun. d. Many of them have been discovered by observing Doppler shifts in the spectra of the stars they orbit. b Most exoplanets which have been detected to date have not been directly imaged, so b) cannot be true. They are generally too far away to be resolved by even the largest telescope on the ground and in space. Instead, they have been indirectly detected with methods such as the Doppler and transit methods, which allow for calculations of their densities but not of their atmospheric compositions. 4. What does Pluto most resemble? a. Venus b. Mars c. Earth s Moon d. Halley s comet e. The asteroid Ceres d Pluto is most similar to a comet like Halley s comet because of a few things: 1) its density indicates that it is composed mostly of ice, 2) it has a seasonal atmosphere which forms through sublimation (ice to gas) when it is closest to the Sun and condenses onto its surface when it is furthest from the Sun, 3) it has a highly elliptical orbit (relative to the planets). 5. In general, which type of exoplanet would you expect to cause the largest Doppler shift in the spectrum of its star? a. a massive planet that is close to its star b. a massive planet that is far from its star c. an Earth-like planet at a distance similar to that of Jupiter from the Sun d. a low-mass planet that is close to its star e. a low-mass planet that is far from its star a Since of the Doppler shift in the spectrum of a star is caused by the force of gravity from the orbiting planet on the star, it is strongest when the force of gravity acting on the star from the planet is the strongest: when the planet is most massive and it is closest to the star. 2

3 6. Which of the following most likely explains why Jupiter's interior releases so much heat? a. heat from radioactive decay b. Jupiter is contracting very gradually c. a slow rate of nuclear fusion in Jupiter's core d. tidal heating e. the metallic hydrogen core generates electric currents and convection which slowly heats the planet b As mass in Jupiter s interior fall toward the center, they are losing gravitational potential energy. This energy is transformed into thermal energy, i.e. heat. 7. All the following statements are true. Which one is most important in explaining the tremendous tidal heating that occurs on Io? a. Io is the furthest away from Jupiter of Jupiter's large moons. b. Io orbits Jupiter on an elliptical orbit which induces tidal stresses. c. Jupiter has the largest magnetosphere of any planet in the solar system. d. Io exhibits synchronous rotation, meaning that its rotation period and orbital period are the same. e. Io orbits Jupiter in the Io torus, and therefore has a surface that is bombarded by many charged particles. b Io s elliptical orbit means that when it is closer to Jupiter the tidal forces are stronger than when it is further away. This leads to Io getting squished and stretched during its orbit, which results in frictional heating of its interior. Since Io exhibits synchronous rotation (tidally locked with one side always facing Jupiter), this would not be the case if Io had a circular orbit. 8. Which of the following is not a piece of evidence supporting the idea that Europa may have a subsurface ocean? a. Photos of Europa's surface show regions that appear to consist of jumbled icebergs frozen in place. b. Europa's surface shows very few impact craters. c. The Hubble Space Telescope has tentatively detected water jets emanating from Europa. d. Astronomers have detected small lakes of liquid water on Europa's surface. d The surface of Europa is too cold to maintain liquid water lakes. The rest of the evidence suggests that occasionally liquid water makes its way to the surface, where it freezes. However, we have yet to observe liquid water. 3

4 9. Which of the following is most unlikely to be found on Titan? a. lakes of liquid methane or ethane b. rain or snow consisting of methane or ethane droplets or ice crystals c. lakes of liquid water in the warmer equatorial regions d. volcanic outgassing of methane and other gases e. channels or riverbeds cut by flowing liquid c Nowhere on Titan s surface is warm enough for liquid water. Titan is simply too far away from the Sun for this to be possible. Lakes of liquid methane exist on Titan because unlike water, methane freezes at temperatures similar to those on its surface. 10. In science fiction movies, spaceships are often shown dodging through large numbers of closely spaced, boulder-size objects. Which of the following real things in our solar system would look most like such science fiction dangers? a. the rings of Saturn b. the asteroid belt c. the atmosphere of Jupiter d. the moons of Jupiter e. the magnetosphere of Uranus a Despite popular depictions, if you were flying through the asteroid belt, you probably wouldn t notice any asteroids at all at least not any sizable asteroids because they are too far apart. On the other hand, Saturn s rings are relatively densely populated. For illustration, every particle in the densest part of the rings collides with another particle approximately every 5 hours or so. 4

5 Short Answer Questions 11-14: Please answer the following questions in a few sentences. Be sure to write legibly. Also, use sketches, if helpful, in addition to the text. Please be brief. Literacy and clarity count! Each short answer is worth 15 points. 11. Pick one of the planets or moons in the Solar System. Discuss how we have explored this planet or moon and what we have learned. Discuss the exploration tools for this object which might include telescopes, robotic spacecraft, and/or human missions. Example (Mars): While we have observed Mars with Hubble, Mars is close enough that the main part of our exploration of Mars has been done with orbiting satellites and rovers. Orbiters such as the Mars Global Surveyor and the Mars Reconnaissance Orbiter have mapped out the surface of Mars. This led to the discovery of systems of dry riverbeds and evidence for a former northern ocean. Surface rovers such as Spirit, Opportunity, and Curiosity have studied the geology of Mars and the chemical composition of Martian rocks. Recently, the orbiter named MAVEN has been studying how Mars lost its atmosphere. Example (Moon): Our Moon is the only object in our solar system to which we have sent human missions. The Apollo program led to the return of surface samples from the Moon. These samples contributed the giant impact theory of the formation of the Earth-Moon system. More recently, the Lunar Reconnaissance Orbiter has created a 3D map of the lunar surface. This map will be used to plan future landing missions. Example (Titan): The Cassini mission to Saturn sent the Huygens probe down to its surface, where it took images and transmitted it back to Earth (Images were also taken during its descent). The image of Titan was the first human-made picture taken on a planetary surface beyond Venus and Mars. It measured a surface temperature of 95 K. While Huygens did not see lakes of liquid methane directly, further data from the Cassini mission confirmed that they do exist. 12. Why is the discovery of water fountains on Enceladus so important to make this moon of Saturn an attractive place to search for life? All life that we know of (i.e. life on Earth) relies on liquid water to survive. The fountains on Enceladus have been confirmed through their spectra to be water, implying the existence of subsurface oceans. This is a potential ecosystem we ve never been able to explore with a non-solar energy source (tidal heating). 5

6 13. Why did Earth retain most of its liquid water while Venus and Mars lost theirs? Earth was formed with the correct size and at the correct distance from the Sun to have liquid water at this point in time. Venus was too close to the Sun. While it may have had liquid oceans earlier in its life, at some point the Sun got too hot for Venus to handle. The higher temperatures increase evaporation, which started a runaway greenhouse effect, evaporating all of the water. The downfall of Mars, on the other hand, was mostly due to its small size. This meant that it cooled faster than Earth and lost its magnetic field. Without the protection of a magnetosphere, the solar wind stripped the atmosphere of Mars. Without an atmosphere, there is no greenhouse effect to keep temperatures on Mars warm enough to keep water from freezing. In addition, without an atmosphere, there is not enough pressure to maintain liquid water, even if the temperature permitted. 14. Describe how the greenhouse effect makes a planetary surface warmer than it would be otherwise. Why is the result of this effect so different on Venus, Earth, and Mars? Visible light is absorbed by the surface of the planet. The planet then re-radiates that light in the infrared (IR). While greenhouse gases are transparent to visible light, they absorb IR. Therefore, instead of going directly back into space, the IR light given off by the planet bounces around the atmosphere. It is absorbed and reradiated over and over again by the atmosphere. The main difference in the greenhouse effects of Venus, Earth, and Mars, are the thicknesses of their atmospheres. Earth s atmosphere is thick enough such that it keeps the surface warm enough for liquid water. Venus has a much thicker atmosphere, and therefore a much stronger greenhouse effect warming it to higher temperatures. Mars, on the other hand, has a very thin atmosphere, which doesn t warm it much at all. Extra Credit (5 pts). The star Rho Cancri B has about the same mass as our Sun, and the planet discovered around it orbits somewhat closer than Mercury orbits our Sun. The mass of the planet is estimated to be 1.1 times the mass of Jupiter. Why, according to our theory of solar system formation, is it surprising to find a planet the size of Rho Cancri B's planet orbiting at this distance? In the nebular theory for our solar system formation, jovian planets form beyond the frost line. In our solar system, this was beyond the orbit of Mars, so it is surprising to find a jovian planet orbiting so close to its star. 6