Astronomy: The Solar System and Beyond 5th edition

1. Astronomy:The Solar System and Beyond 5th edition Michael Seeds

2. When they shall cry “PEACE, PEACE” then cometh sudden destruction! COMET’S CHAOS?— What Terrible events will the Comet bring? - From a religious pamphlet predicting the end of the world because of the appearance of comet Kohoutek, 1973 Chapter 10

3. Of course, you are not afraid of comets. However, not long ago, people viewed them with terror. In 1910, Comet Halley was spectacular. On the night of May 19, Earth actually passed through the tail of the comet—and millions of people panicked.

4. The spectrographic discovery of cyanide gas in the tails of comets led many to believe that life on Earth would end. Householders in Chicago stuffed rags around doors and windows to keep out the gas, and bottled oxygen was sold out. Con artists in Texas sold comet pills and inhalers to ward off the noxious fumes. An Oklahoma newspaper reported, in what was apparently a hoax, that a religious sect tried to sacrifice a virgin to the comet.

5. Throughout history, bright comets have been seen as portents of doom. Even the more recent appearance of bright comets has generated predictions of the end of the world. Comet Kohoutek in 1973, Comet Halley in 1986, and Comet Hale-Bopp in 1997 all caused concern among the superstitious.

6. A bright comet moving slowly through the night sky is such an unusual sight that you should not be surprised if it generates some instinctive alarm.

7. Comets are not just graceful and beautiful visitors to our skies but they are also useful in the study of the solar system. Astronomers think of comets as messengers from the age of planet building. By studying comets, you can learn about the conditions in the solar nebula from which planets formed.

8. However, comets tell only part of the story. They are only the icy remains of the outer solar nebula. The asteroids are the rocky debris left over from terrestrial planet building.

9. However, you cannot easily visit comets and asteroids. Nevertheless, you can learn about them by discussing the fragments of those bodies that fall into our atmosphere—the meteorites.

10. You learned about meteorites when you studied the age of the solar system. You learned that the solar system is filled with small particles called meteoroids, which can fall into Earth’s atmosphere at speeds of 10 to 40 km/s. Friction with the air heats the meteoroids to glowing, and they vaporize as meteors streaking across the night sky. If a meteoroid is big and strong enough, it can survive its plunge through the atmosphere and reach Earth’s surface. Meteorites

11. Once the object strikes Earth’s surface, it is called a meteorite. The largest can blast out giant craters on Earth’s surface. However, such impacts are rare. The vast majority are too small to form craters. These meteorites fall all over Earth, and their value is in what they can reveal about the origin of the planets. Meteorites

12. Inside Meteorites • Meteorites can be divided into three broad categories. • Iron meteorites are solid chunks of iron and nickel. • Stony meteorites are silicate masses that resemble Earth rocks. • Stony-iron meteorites are mixtures of iron and stone.

13. Inside Meteorites • Iron meteorites are dense and heavy. • They often have dark, rusted surfaces and fluted shapes caused by their passage through the atmosphere.

14. Inside Meteorites • When they are sliced open, polished, and etched with nitric acid, they reveal regular bands called Widmanstätten patterns.

15. Inside Meteorites • The patterns arise from crystals of nickel-iron alloys that have grown very large over time. • This indicates that the meteorite cooled from a molten state no faster than a few degrees per million years. • Explaining how iron meteorites could have cooled so slowly will be a major step in analyzing their history.

16. Inside Meteorites • Stony meteorites called chondrites have chemical compositions that resemble a cooled lump of matter from the sun with the volatile gases removed.

17. Inside Meteorites • Although there are many kinds of chondrites, most contain chondrules, rounded bits of glassy rock ranging from microscopic to pea-sized.

18. The origin of chondrules is unknown. They appear to have formed in the young solar system as droplets of molten rock that cooled and hardened rapidly. As subsequent melting of the meteorite would have destroyed chondrules, their presence in meteorites indicates that the meteorites have not been melted since they formed. Inside Meteorites

19. Nevertheless, some kinds of chondrites show signs that they have been heated slightly. Those meteorites are poor in volatiles such as carbon compounds and water. Solids condensing out of the solar nebula should have incorporated volatiles. However, if they were heated slightly, they could lose them. Inside Meteorites

20. Yet, some chondrites are rich in water. This means that they formed in the presence of volatiles. The form and composition of chondrites dates back to the formation of the solar nebula. Only minor changes, if any, have occurred since. Inside Meteorites

21. Inside Meteorites • The carbonaceous chondrites generally contain both chondrules and volatile compounds, including significant amounts of carbon. • Heating would have modified and driven off these fragile compounds. • The carbonaceous chondrites are, along with certain kinds of chondrites, among the least modified bodies in our solar system.

22. Some stony meteorites contain no chondrules, and they are called achondrites. They also lack volatiles and appear to have been subjected to intense heat that melted chondrules and drove off volatiles, leaving behind rocks with compositions similar to Earth’s lavas. Inside Meteorites

23. Inside Meteorites • Stony-iron meteorites are a mixture of iron and stone. • They appear to have formed when a mixture of molten iron and rock cooled and solidified.

24. The meteorites carry hints about the origin of our solar system, as well as an older secret. Among the smallest grains in meteorites are specks of minerals whose abundance of isotopes brands them as star dust—grains of interstellar matter that predate our solar system. Inside Meteorites

25. Origin of Meteors and Meteorites • You can find evidence of the origin of meteors through one of the most pleasant observations in astronomy. • You can observe a meteor shower, a display of meteors that are clearly related in a common origin.

26. Origin of Meteors and Meteorites • On any clear, moonless night of the year, you could see 5 to 15 meteors an hour. • However, they are not related to each other and may streak in any direction across the sky.

27. Origin of Meteors and Meteorites • During a meteor shower, you could see as many as 50 meteors an hour. • These meteors would all seem to come from the same part of the sky. • For example, the Perseid meteor shower occurs each year in August and seems to come from a spot in the constellation Perseus.

28. Origin of Meteors and Meteorites • That is because all the meteors in a shower are traveling in the same direction as they meet Earth’s atmosphere. • Like railroad tracks extending from a point on the horizon, the meteors appear to approach from a point in space.

29. Origin of Meteors and Meteorites • Meteor showers occur when Earth passes near the orbit of a comet. • So, the meteors must be dust and debris left behind by the head of the comet.

30. Origin of Meteors and Meteorites • The orbits of comets are filled with such debris. • The telescope aboard the Infrared Astronomy Satellite detected the dusty orbits of a number of comets glowing in the far-infrared because of sun-warmed dust scattered along the orbits.

31. Like any natural phenomenon, a meteor shower is more enjoyable when you know more about it. Actually, most of the meteors you see on any given night are cometary material that has been scattered out of the original orbits of the comets and spread throughout the solar system. Origin of Meteors and Meteorites

32. Comet debris is small, delicate, and weak. No comet material has ever been found on Earth’s surface. It all burns up in Earth’s atmosphere. Meteorites that reach Earth’s surface are structurally much stronger than cometary material. They are iron and stone, and appear to be fragments of larger solid bodies. Origin of Meteors and Meteorites

33. Origin of Meteors and Meteorites • Some meteorites appear to be fragments of planetesimals that were large enough to grow hot from radioactive decay, melt, and differentiate to form iron-nickel cores and rocky mantles. • The molten iron cores would have been well insulated by the thick rocky mantles. • The iron would have cooled slowly to produce Widmanstätten patterns.

34. Origin of Meteors and Meteorites • Collisions could have broken such bodies up and produced different kinds of meteorites. • Iron meteorites appear to be fragments from the iron cores.

35. Origin of Meteors and Meteorites • Some stony meteorites that have been strongly heated appear to have come from the mantles and surfaces of such bodies. • The stony-iron meteorites apparently come from the boundary where the stony mantle meets the iron core. • Chondrites are probably fragments of smaller bodies that never melted. • The carbonaceous chondrites may have formed in small, colder bodies further from the sun.

36. These theories trace the origin of meteorites to planetesimallike parent bodies. However, the small meteorites in the solar system cannot be fragments of the planetesimals that formed the planets. They would have been swept up by the planets in only a billion years or less. They could not have survived for 4.6 billion years. Origin of Meteors and Meteorites

37. When astronomers study the orbits of objects seen to fall to Earth as meteorites, the orbits lead back into the asteroid belt. Thus, astronomers have good evidence to believe that the meteorites now in museums all over the world must have been broken off asteroids within the last billion years. Although nearly all meteors are pieces of comets, the meteorites are pieces of asteroids. Origin of Meteors and Meteorites

38. How can meteors come from comets but meteorites come from asteroids? This is a revealing argument, because it contains a warning that seeing is not enough in science. Thinking about seeing is critical. A selection effect can determine what you notice when you observe nature. A very strong selection effect prevents people from finding meteorites that originated in comets. Building Scientific Arguments

39. Cometary particles are physically weak. They vaporize in Earth’s atmosphere easily. Very few ever reach the ground. People are unlikely to find them. Even if a particle reached the ground, it would be so fragile that it would weather away rapidly. Again, people would be unlikely to find it. Building Scientific Arguments

40. Asteroidal particles are made from rock and metal, and so are stronger. They are more likely to survive their plunge through the atmosphere and to survive erosion on the ground. Meteors from the asteroid belt are rare. Almost all the meteors you see come from comets. However, not a single meteorite is known to be cometary. Building Scientific Arguments

41. The meteorites are valuable because they provide hints about the process of planet building in the solar nebula. Build a new argument but, as always, think carefully about what you see. Why do meteors in showers seem to come from a point? Building Scientific Arguments

42. According to old-time pulp-fiction, space pirates lurk in the asteroid belt. However, astronomers have found that there isn’t much in the asteroid belt for a pirate to stand on. Most are quite small. Given its vast size, the asteroid belt between Mars and Jupiter is mostly empty. Asteroids

43. Asteroids • Nevertheless, you must consider the nature of the asteroids. • You have learned that they were the last remains of material that was unable to form a planet between Mars and Jupiter. • You will now examine these small worlds in more detail.

44. Asteroids are distant objects too small to study in detail with Earth-based telescopes. Yet astronomers have learned a surprising amount about these little worlds. Spacecraft have provided a few close-ups too. Properties of Asteroids

45. Properties of Asteroids • Most asteroids are irregular in shape and battered by impact cratering. • In fact, some appear to be rubble piles of broken fragments.

46. Properties of Asteroids • Some asteroids are double objects or have small moons in orbit around them. • This is further evidence of collisions among the asteroids.

47. Properties of Asteroids • A few larger asteroids show signs of geological activity on their surfaces that may have been caused by volcanic activity when the asteroid was young.

48. Properties of Asteroids • Asteroids can be classified by their albedo and color, which reveal clues to their compositions.

49. Not all asteroids lie in the asteroid belt. A few thousand objects larger than 1 km follow orbits that cross Earth’s orbit. A number of searches are under way to locate these near-Earth objects (NEOs). For example, Lowell Observatory Near Earth Object Search (LONEOS) is searching the entire sky once a month and should be able to locate a thousand NEOs over the next 10 years. Properties of Asteroids

50. Astronomers are searching for these asteroids not only because they want to understand the asteroids better but because these collide with Earth occasionally. Although such collisions occur very rarely, a single impact could cause planetwide devastation. Properties of Asteroids