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Transcript
Chapter 15 - Solar System Debris CHAPTER 15 SOLAR SYSTEM DEBRIS CHAPTER OUTLINE AND LECTURE NOTES 1. Meteors If you feel so inclined, this might be a good place to insert a discussion of UFOs. On April 10, 1972 a house-size chunk of rock entered the Earth’s atmosphere at a grazing angle and, after penetrating to an altitude of about 50 km, skipped back into space. The meteor it produced was bright enough to be visible in the daytime. It spent a few minutes in the atmosphere on a path that carried it northward over Utah, Idaho, Montana, and Alberta. The object was very extensively observed, even though it appeared without warning. There were many eyewitness reports, numerous still photographs (one of which was featured in Life magazine), and movie footage. It was also detected by an Air Force satellite designed to look for the infrared emission from enemy missiles. In all of the years that UFO sightings have been reported, there isn’t a single instance of a sighting anywhere near as well documented as the April 1972 meteor. I find it hard to believe that if UFOs are physical objects in the Earth’s atmosphere we have so little observational evidence of their existence. Incidentally, if the body that skimmed the Earth’s atmosphere had entered at a slightly steeper angle it would have penetrated to the ground rather than skipping back into space. It would have struck southern Alberta and produced an explosion comparable to a nuclear bomb. 2. Meteorites Some students may be curious how astronomers have been able to tell that the amino acids found in some carbonaceous chondrites are extraterrestrial rather than terrestrial contamination. After all, nearly all of the meteorites that have been recovered have been on Earth long enough for significant contamination to occur. The reason to reject the contamination hypothesis is that the amino acids in meteorites are an equal mixture of right- and left-handed stereoisomers. Those in terrestrial organisms are exclusively lefthanded. 3. Asteroids Figure 15.11, showing the location of 7000 asteroids on March 7, 1997, clearly demonstrates that the majority of asteroids are in the asteroid belt. This figure, however, may reinforce the impression students get from many movies, including the first Star Wars movie, that asteroid belts are filled with asteroids only a few miles from one another. We have now sent numerous spacecraft through the asteroid belt to get to the outer planets. While there was some concern about the safety of the spacecraft, the danger was from pinhead size bodies (which are also more numerous there than elsewhere) rather than large chunks of rock. 4. Comets It isn’t hard to predict that there will continue to be discoveries of additional bodies in the Kuiper belt. To get the current list of Kuiper belt bodies, look at the web site http://cfa-www.harvard.edu/iau/lists/TNOs.html. 15-1 Chapter 15 - Solar System Debris 5. Where Did Comets Form? 6. Collisions with Earth The announcement in spring 1998 that a newly discovered asteroid might impact the Earth in about 30 years created a news circus. The subsequent announcement that archival observations of the asteroid showed that it would clearly miss the Earth did nothing for the credibility of the argument that a collision with an Earth-crossing asteroid is inevitable and that we needed a much fuller inventory of such asteroids in order to assess the risks. That inventory, however, is being carried out. The list of Earth-crossing asteroids is considerably longer than it was a decade ago. The relationship between impacts and extinctions may not be completely settled, but the advocates of other extinction models such as volcanic activity seem to be less vocal than they were a decade or so ago. Certainly the public seems to have accepted the idea that extinctions are related to impacts. KEY TERMS achondrite — A stony meteorite lacking chondrules. Amor asteroid — A member of a class of asteroids having orbits that cross the orbital distance of the Earth. Apollo asteroid — A member of a class of asteroids having orbits that cross the orbital distance of the Earth. asteroid — A small, planetlike solar system body. Most asteroids are rocky in makeup and have orbits of low eccentricity and inclination. asteroid belt — The region of the solar system lying between 2.1 and 3.3 AU from the Sun. The great majority of asteroids are found in the asteroid belt. Aten asteroid — An asteroid having an orbit with semimajor axis smaller than 1 AU. C-type asteroid — One of a class of very dark asteroids whose reflectance spectra show no absorption features due to minerals. carbonaceous chondrite — A stony meteorite that contains carbon–rich material. Carbonaceous chondrites are thought to be primitive samples of material from the early solar system. chondrite — A meteorite containing chondrules. chondrule — A small, spherical body embedded in a meteorite. Chondrules are composed of iron, aluminum, and magnesium silicate rock. coma — A spherical gaseous region that surrounds the nucleus of a comet. The coma of a comet may be 100,000 km or more in diameter. comet — A small, icy body in orbit about the Sun. When a comet is near the Sun, it displays a coma and tail. cosmic ray — An extremely energetic ion or electron that travels through space almost at the speed of light. Most cosmic rays come from great distances and may be produced in supernovae and pulsars. cosmic ray exposure age — The length of time that has passed since a meteorite broke off from a larger body and became exposed to radiation damage from cosmic rays. dust tail — A comet tail that is luminous because it contains dust that reflects sunlight. The dust in a comet tail is expelled from the nucleus of the comet. 15-2 Chapter 15 - Solar System Debris fireball — An especially bright streak of light in the sky produced when an interplanetary dust particle enters the Earth’s atmosphere, vaporizing the particle and heating the atmosphere. iron meteorite — A meteorite composed primarily of iron and nickel. long–period comet — A comet with an orbital period of 200 years or longer. M-type asteroid — One of a class of asteroids that have reflectance spectra like those of metallic iron and nickel. meteor — A streak of light produced by a meteoroid moving rapidly through the Earth’s atmosphere. Friction vaporizes the meteoroid and heats atmospheric gases along the path of the meteoroid. meteor shower — A temporary increase in the normal rate at which meteors occur. Meteor showers last for a few hours or days and occur on about the same date each year. meteorite — The portion of a meteoroid that reaches the Earth’s surface. meteoroid — A solid interplanetary particle passing through the Earth’s atmosphere. micrometeorite — A meteoritic particle less than 50 millionths of a meter in diameter. Micrometeorites are slowed by atmospheric gas before they can be vaporized, so they drift slowly to the ground. minor planet — Another name for asteroid. new comet — A comet that has entered the inner solar system for the first time. nucleus — An irregularly shaped, loosely packed lump of dirty ice several kilometers across that is the permanent part of a comet. Oort cloud — The region beyond the planetary system, extending to 100,000 AU or more, within which a vast number of comets orbit the Sun. When comets from the Oort cloud enter the inner solar system, they become new comets. plasma tail — A narrow, ionized comet tail pointing directly away from the Sun. radiant — The point in the sky from which the meteors in a meteor shower seem to come. reflectance spectrum — The reflectivity of a body as a function of wavelength. S-type asteroid — One of a class of asteroids whose reflectance spectra show an absorption feature due to the mineral olivine. short–period comet — A comet with an orbital period shorter than 200 years. solidification age — The amount of time since a meteorite solidified from the molten state. stony meteorite — A meteorite made of silicate rock. stony–iron meteorite — A meteorite made partially of stone and partially of iron and other metals. terminal velocity — The speed with which a body falls through the atmosphere of a planet when the force of gravity pulling it downward is balanced by the force of air resistance. Trojan asteroid — One of a group of asteroids that orbit the Sun at Jupiter’s distance and lie 60° ahead of or behind Jupiter in its orbit. V-type asteroid — The asteroid Vesta, which is unique in having a reflectance spectra resembling those of basaltic lava flows. zodiacal light — The faint glow extending away from the Sun caused by the scattering of sunlight by interplanetary dust particles lying in and near the ecliptic. 15-3 Chapter 15 - Solar System Debris ANSWERS TO QUESTIONS AND PROBLEMS Conceptual Questions 1. A meteoroid is a small body in interplanetary space. When it enters the Earth’s atmosphere it produces a meteor. The parts of it that reach the Earth’s surface become meteorites. 2. (a) Slowed by gases high in the atmosphere and gently fall to the ground. (b) Burns up completely in the atmosphere. (c) Burns away the outer few cm of material, the rest is slowed so that it falls to the ground at terminal velocity. (d) Strikes the ground before it can be slowed to terminal velocity. Makes a crater when it hits the ground. 3. They are traveling along parallel paths from the direction of the radiant. 4. They recur whenever the Earth recrosses the orbit of the meteor swarm, which occurs on the same date each year. 5. They appear to be unaltered samples of material from the early solar system. 6. They show that the iron in the meteorite cooled very slowly from a molten state. 7. It tells how long it has been since the meteorite broke off from a larger parent body. 8. They must have been at least 100 km in diameter in order to have cooled slowly enough to account for the sizes of crystals in iron meteorites. 9. It is too faint to be seen with the unaided eye. 10. Meteorites are fragments of asteroids. 11. When the nucleus comes within about 3 AU of the Sun it is warmed by sunlight to the point where water and other ices evaporate and flow outward carrying dust particles with them. 12. From the nucleus 13. Dust is pushed away from the Sun by solar radiation. The solar wind pushes the ionized gas in the plasma tail away from the Sun. 14. The new comets have aphelia between 10,000 and 100,000 AU, showing that the new comets come from a swarm orbiting the Sun at enormous distances. 15. Their orbits are altered by the gravity of passing stars. 16. From a disk of comets that orbit between Neptune’s orbit and about 1000 AU 17. They strike planets, are eroded by many passages near the Sun, or are fragmented by passing too near the Sun or a planet. 18. Meteor showers are produced by swarms of dust particles left behind when the icy material in a comet is used up. 19. No comet has ever been found to have a hyperbolic path when it first entered the planetary system. Thus none of them have speeds as great as escape velocity. 20. The flash and heat would ignite vegetation triggering large fires. Ejected molten and solid material would cover the region near the impact crater. Vaporized material would solidify in the upper atmosphere and block sunlight. There would also be strong acid rain and temporary heating of the atmosphere. 21. An iridium-rich layer of sediment was laid down at the time that widespread extinction took place at the end of the Cretaceous era. Iridium is abundant in meteorites but rare in the Earth’s surface rocks. 15-4 Chapter 15 - Solar System Debris Problems 1. 3.0 years, 6.0 years 2. 11.9 years, the same as Jupiter’s orbital period 3. Twice each 1.12 year-long orbital period 4. 29,000 AU 15-5