CHAPTER 1 Meteorites Asteroids and Comets Astronomy The

CHAPTER 1 Meteorites, Asteroids, and Comets Astronomy: The Solar System and Beyond 5 th edition Michael Seeds

CHAPTER 1 Chapter 10 Meteorites, Asteroids, and Comets 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 1 Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Meteorites, Asteroids, and Comets – 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.

CHAPTER 1 Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Meteorites, Asteroids, and Comets • 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 over Earth, and their value is in what they can reveal about the origin of the planets.

CHAPTER 1 Inside Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Inside Meteorites, Asteroids, and Comets • Iron meteorites are dense and heavy. • They often have dark, rusted surfaces and fluted shapes caused by their passage through the atmosphere.

CHAPTER 1 Inside Meteorites, Asteroids, and Comets • When they are sliced open, polished, and etched with nitric acid, they reveal regular bands called Widmanstätten patterns.

CHAPTER 1 Inside Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Inside Meteorites, Asteroids, and Comets • Stony meteorites called chondrites have chemical compositions that resemble a cooled lump of matter from the sun with the volatile gases removed.

CHAPTER 1 Inside Meteorites, Asteroids, and Comets • Although there are many kinds of chondrites, most contain chondrules, rounded bits of glassy rock ranging from microscopic to pea-sized.

CHAPTER 1 Inside Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Inside Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Inside Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Inside Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Inside Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Inside Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Inside Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Origin of Meteors and Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Origin of Meteors and Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Origin of Meteors and Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Origin of Meteors and Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Origin of Meteors and Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Origin of Meteors and Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Origin of Meteors and Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Origin of Meteors and Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Origin of Meteors and Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Origin of Meteors and Meteorites, Asteroids, and Comets • Collisions could have broken such bodies up and produced different kinds of meteorites. – Iron meteorites appear to be fragments from the iron cores.

CHAPTER 1 Origin of Meteors and Meteorites, Asteroids, and Comets – 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.

CHAPTER 1 Origin of Meteors and Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Origin of Meteors and Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Building Scientific Arguments Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Building Scientific Arguments Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Building Scientific Arguments Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Building Scientific Arguments Meteorites, Asteroids, and Comets • 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?

CHAPTER 1 Asteroids Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Asteroids Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Properties of Asteroids Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Properties of Asteroids Meteorites, Asteroids, and Comets • Most asteroids are irregular in shape and battered by impact cratering. – In fact, some appear to be rubble piles of broken fragments.

CHAPTER 1 Properties of Asteroids Meteorites, Asteroids, and Comets • Some asteroids are double objects or have small moons in orbit around them. – This is further evidence of collisions among the asteroids.

CHAPTER 1 Properties of Asteroids Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Properties of Asteroids • Asteroids can be classified by their albedo and color, which reveal clues to their compositions. Meteorites, Asteroids, and Comets

CHAPTER 1 Properties of Asteroids Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Properties of Asteroids Meteorites, Asteroids, and Comets • 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.

CHAPTER 1 Properties of Asteroids Meteorites, Asteroids, and Comets • You have learned about the icy bodies of the Kuiper belt. – Roughly 70, 000 of these orbit in the outer solar system from the orbit of Neptune out to about 50 AU. • Some astronomers think of them as asteroids. – However, it is clear that they formed from the outermost parts of the solar nebula.

CHAPTER 1 Origin of the Asteroids Meteorites, Asteroids, and Comets • An old theory proposed that asteroids are the remains of a planet that exploded. • Planet-shattering death rays may make for exciting science-fiction movies. • However, in reality, planets do not explode.

CHAPTER 1 Origin of the Asteroids Meteorites, Asteroids, and Comets • The gravitational field of a planet holds the mass tightly, and disrupting the planet would take tremendous energy. – Shattering Earth would require all the energy generated by the sun over a period of two weeks. • In addition, the total mass of the asteroids is only about one-twentieth the mass of the moon—hardly enough to be the remains of a planet.

CHAPTER 1 Origin of the Asteroids Meteorites, Asteroids, and Comets • Astronomers believe that the asteroids are the remains of material that was unable to form a planet at 2. 8 AU from the sun—due to the gravitational influence of Jupiter, the next planet outward. – If this is true, then the asteroids are the remains of ancient planetesimals fragmented by collisions with one another.

CHAPTER 1 Origin of the Asteroids – This would explain why the C-type asteroids, which appear to be carbonaceous, are more common in the outer asteroid belt. – It is cooler there, and the condensation sequence predicts that carbonaceous material would form there more easily than in the inner belt. Meteorites, Asteroids, and Comets

CHAPTER 1 Origin of the Asteroids Meteorites, Asteroids, and Comets • As in the case of Vesta, a few asteroids may have been geologically active, with lava flowing on their surfaces when they were young. – Perhaps they incorporated short-lived radioactive elements such as aluminum-26. – Such elements could have been produced by a supernova explosion. – Such an explosion might have triggered the formation of the sun and planets.

CHAPTER 1 Origin of the Asteroids Meteorites, Asteroids, and Comets • However, not all asteroids have been active. – Ceres, 900 km in diameter, is almost twice as big as Vesta, but it shows no spectroscopic sign of past activity and is rich in water.

CHAPTER 1 Origin of the Asteroids Meteorites, Asteroids, and Comets • Although there are still mysteries to solve, you can now understand the compositions of the meteorites. • They are fragments of planetesimals, some of which developed molten cores, differentiated, and then cooled slowly. – The largest asteroids astronomers see today may be nearly unbroken planetesimals. – However, the rest are just the fragments produced by 4. 6 billion years of collisions.

CHAPTER 1 Building Scientific Arguments Meteorites, Asteroids, and Comets • What evidence makes you think that the asteroids have been fragmented? – Perhaps the best scientific arguments test the interpretation of evidence. – If you understand the evidence, you hold the key to the science.

CHAPTER 1 Building Scientific Arguments Meteorites, Asteroids, and Comets • To begin, you might note that the solar nebula theory of the formation of the solar system predicts that planetesimals collided and either stuck together or fragmented. – This is suggestive, but it is not evidence. – A theory can never be used as evidence to support some other theory or hypothesis. – Evidence refers to observations or the results of experiments. – So, you need to turn to observations of asteroids.

CHAPTER 1 Building Scientific Arguments Meteorites, Asteroids, and Comets • Spacecraft photographs of asteroids such as Ida, Gaspra, and Eros show irregularly shaped little worlds heavily scarred by impact craters. – Observations of some asteroids show what may be pairs of bodies in contact. – The Galileo image of Ida reveals its small satellite, Dactyl.

CHAPTER 1 Building Scientific Arguments Meteorites, Asteroids, and Comets • Furthermore, some meteorites appear to come from the asteroid belt, and a few have been linked to specific asteroids such as Vesta. • There are even families of asteroids that seem to be fragments from a single collision. • All this evidence suggests that the asteroids have been broken up by violent impacts.

CHAPTER 1 Building Scientific Arguments Meteorites, Asteroids, and Comets • The impact fragmentation of asteroids has been important, but it has not erased all traces of the original planetesimals from which the asteroids formed. • Build another argument based on evidence. – What evidence can you cite that reveals what those planetesimals were like?

CHAPTER 1 Comets Meteorites, Asteroids, and Comets • Of all the fossils left behind by the solar nebula, comets are the most beautiful. – Asteroids are dark, rocky worlds. – Meteors are flitting specks of fire. – However, comets move with the grace and beauty of a great ship at sea. – For example, Comet Hale. Bopp was visible for weeks in 1996 and 1997.

CHAPTER 1 Comets Meteorites, Asteroids, and Comets • Scientifically, comets are interesting because observations of the tail and head of a comet reveal facts about the small icy nucleus and the ancient solar nebula from which our solar system’s wealth of comets is inherited.

CHAPTER 1 Properties of Comets Meteorites, Asteroids, and Comets • As always, you should begin your study of a new kind of object by summarizing its observational properties. – What do comets look like? – How do they behave? • The observations are the evidence that reveal the secrets of the comets.

CHAPTER 1 Properties of Comets • Comets have three important properties. • One, they have two kinds of tails, shaped by the solar wind and solar radiation. Meteorites, Asteroids, and Comets

CHAPTER 1 Properties of Comets • The two kinds of tails show that the nucleus contains ices of water and other compounds plus rocky material most evident as dust. Meteorites, Asteroids, and Comets

CHAPTER 1 Properties of Comets Meteorites, Asteroids, and Comets • Two, comets contain dust. – This not only produces dust tails but spreads throughout the solar system.

CHAPTER 1 Properties of Comets Meteorites, Asteroids, and Comets • Finally, comet nuclei are fragile and can break into pieces.

CHAPTER 1 The Geology of Comet Nuclei Meteorites, Asteroids, and Comets • Astronomers can put these and other observations together to discuss the structure of comet nuclei. – The nuclei of comets are quite small and cannot be studied in detail from Earth-based telescopes. – Nevertheless, astronomers are beginning to understand the geology of these peculiar worlds.

CHAPTER 1 The Geology of Comet Nuclei Meteorites, Asteroids, and Comets • Comet nuclei contain ices of water and other volatile compounds such as carbon dioxide, carbon monoxide, methane, and ammonia. – These ices are the kinds of compounds that should have condensed from the outer solar nebula. – That makes astronomers think that comets are ancient samples of the gases and dust from which the outer planets formed.

CHAPTER 1 The Geology of Comet Nuclei Meteorites, Asteroids, and Comets • When the nuclei of comets approach the sun, the ices absorb energy from sunlight and sublime—change from a solid directly into a gas—to produce the observed tails. – As the gases break down and combine chemically, they release many compounds found in comet tails. – Vast clouds of hydrogen gas observed around the heads of comets are derived from the breakup of molecules from the ices.

CHAPTER 1 The Geology of Comet Nuclei Meteorites, Asteroids, and Comets • Five spacecraft flew past the nucleus of Comet Halley when it visited the inner solar system in 1985 and 1986. • The Deep Space 1 spacecraft flew past the nucleus of Comet Borrelly in 2001.

CHAPTER 1 The Geology of Comet Nuclei Meteorites, Asteroids, and Comets • The Stardust spacecraft flew past Comet Wild 2 in 2004.

CHAPTER 1 The Geology of Comet Nuclei Meteorites, Asteroids, and Comets • Photos show that these comet nuclei are irregular in shape and very dark.

CHAPTER 1 The Geology of Comet Nuclei Meteorites, Asteroids, and Comets • In general, the nuclei are darker than a lump of coal, suggesting the composition of the carbon-rich meteorites called carbonaceous chondrites.

CHAPTER 1 The Geology of Comet Nuclei Meteorites, Asteroids, and Comets • From the gravitational influence of a nucleus on a passing spacecraft, astronomers can find the mass and density of the nucleus. – Comet nuclei appear to have densities of 0. 1 to 0. 25 g/cm 3, much less than the density of ice. – From these observations, astronomers can conclude that comet nuclei are not solid balls of ice but must be fluffy mixtures of ices and dust with significant amounts of empty space.

CHAPTER 1 The Geology of Comet Nuclei Meteorites, Asteroids, and Comets • Photos of the comas of comets often show jets springing from the nucleus and being swept back by the pressure of sunlight and by the solar wind to form the tail.

CHAPTER 1 The Geology of Comet Nuclei Meteorites, Asteroids, and Comets • Studies of the movements of these jets as the nucleus rotates and the photographs of the nucleus of Comet Halley reveal that the jets originate from active regions that may be faults or vents.

CHAPTER 1 The Geology of Comet Nuclei Meteorites, Asteroids, and Comets • As the rotation of a cometary nucleus carries an active region into sunlight, it begins venting gas and dust. • As it rotates into darkness, it shuts down.

CHAPTER 1 The Geology of Comet Nuclei Meteorites, Asteroids, and Comets • In 2005, the Deep Impact spacecraft released an instrumented impactor, weighing 816 lb on Earth, into the path of comet Tempel 1.

CHAPTER 1 The Geology of Comet Nuclei Meteorites, Asteroids, and Comets • The nucleus of the comet, traveling at almost 10 km/s (23, 000 mph), collided with the impactor exactly as planned. – The impact penetrated the crust of the nucleus and blasted material out into space, where the mothership could analyze it as it flew past. – The burst of vapor and dust was detected by the mothership and also by Earth-based telescopes.

CHAPTER 1 The Geology of Comet Nuclei Meteorites, Asteroids, and Comets • Analysis of the data from the Deep Impact mission and images of other comet nuclei show that they are not solid objects. • To call them dirty snowballs or icy mud balls is misleading. • Rather, they appear to be irregular bodies with large voids.

CHAPTER 1 The Geology of Comet Nuclei Meteorites, Asteroids, and Comets • Although comet nuclei seem to have porous crusts of dark material, the ice and rock are not uniformly mixed through the interior. – Breaks in the crust can expose pockets of highly volatile ices and cause sudden bursts of gas production.

CHAPTER 1 The Geology of Comet Nuclei Meteorites, Asteroids, and Comets • The nuclei of comets range from a few kilometers to a few tens of kilometers in diameter. • Each passage near the sun costs a nucleus many millions of tons of ices. – So, the nucleus slowly wastes away until there is nothing left but dust and rock falling along an orbit around the sun.

CHAPTER 1 Origin of Comets Meteorites, Asteroids, and Comets • The fate of a comet is clear. • The mystery is its origin. – Family relationships among the comets provide clues to their origin.

CHAPTER 1 Origin of Comets Meteorites, Asteroids, and Comets • Most comets have long, elliptical orbits with periods greater than 200 years. • These are known as long-period comets. – Their orbits are randomly inclined, with comets falling into the inner solar system from all directions. – As many circle the sun clockwise as counterclockwise.

CHAPTER 1 Origin of Comets Meteorites, Asteroids, and Comets • In contrast, about 100 of the 600 wellstudied comets have orbits with periods less than 200 years. • These short-period comets follow orbits that lie within 30° of the plane of the solar system. – Most revolve around the sun counterclockwise—the same direction the planets orbit. – Comet Halley, with a period of 76 years, is a shortperiod comet.

CHAPTER 1 Origin of Comets Meteorites, Asteroids, and Comets • Comets cannot survive long before the heat of the sun drives away their ices and reduces them to inactive bodies of rock and dust. – A comet may last only 100 to 1, 000 orbits around the sun.

CHAPTER 1 Origin of Comets Meteorites, Asteroids, and Comets • The comets seen in our skies can’t have survived 4. 6 billion years since the formation of the solar system. • So, there must be a continuous supply of new comets. – Where do they come from?

CHAPTER 1 Origin of Comets Meteorites, Asteroids, and Comets • In the 1950 s, Dutch astronomer Jan Oort proposed that the long-period comets are objects that fall in from the Oort cloud. – This is a spherical cloud of icy bodies believed to extend from 10, 000 to 100, 000 AU from the sun.

CHAPTER 1 Origin of Comets Meteorites, Asteroids, and Comets • Astronomers estimate that the cloud contains several trillion icy bodies. • Far from the sun, they are very cold, lack comas and tails, and are invisible. – The gravitational influence of occasional passing stars could perturb a few of these objects to fall into the inner solar system.

CHAPTER 1 Origin of Comets Meteorites, Asteroids, and Comets – In the inner solar system, the sun’s heat warms their ices and transforms them into the brilliant display you see in the night sky. – As the Oort cloud is spherical, these longperiod comets fall inward from random directions.

CHAPTER 1 Origin of Comets Meteorites, Asteroids, and Comets • Some of the short-period comets, including Comet Halley, appear to have originated in the Oort cloud and probably had their orbits altered by a close encounter with Jupiter. • However, many of the short-period comets cannot have begun in the Oort cloud.

CHAPTER 1 Origin of Comets Meteorites, Asteroids, and Comets • Interactions with a planet can’t put objects from the Oort cloud into the orbits that some short-period comets occupy. • There must be another source of icy bodies in our solar system.

CHAPTER 1 Origin of Comets Meteorites, Asteroids, and Comets • In 1951, Dutch-American astronomer Gerard P. Kuiper proposed that the formation of the solar system should have left behind a belt of small, icy planetesimals beyond the Jovian planets and in the plane of the solar system. – Those objects could not be detected with the telescopes and instruments available in those days. – Nevertheless, the proposed band of icy bodies became known as the Kuiper belt.

CHAPTER 1 Origin of Comets Meteorites, Asteroids, and Comets • Modern astronomers have found hundreds of Kuiper-belt objects in orbits extending from Neptune at 30 AU out to about 50 AU from the sun. • Few are seen further away. – Those that are found there were probably scattered outward by interactions with other belt objects.

CHAPTER 1 Origin of Comets Meteorites, Asteroids, and Comets • The entire Kuiper belt would be hidden behind the yellow dot.

CHAPTER 1 Origin of Comets Meteorites, Asteroids, and Comets • Astronomers can detect similar belts around other stars. – A number of nearby stars such as Beta Pictoris are surrounded by disks of dust believed to be released by icy bodies in the equivalent of Kuiper belts around these stars. • Thus, the detection of the dust implies the presence of Kuiper belts.

CHAPTER 1 Origin of Comets Meteorites, Asteroids, and Comets • The Kuiper-belt objects found so far are mostly too big to becomets. • However, there must be lots of smaller objects that can’t be seen. • When a small Kuiper-belt object is perturbed into the inner solar system, it can interact with planets and be captured into orbit as a short-period comet.

CHAPTER 1 Origin of Comets Meteorites, Asteroids, and Comets • If comets are icy planetesimals from the Oort cloud and the Kuiper belt, how did those planetesimals form? – The Kuiper-belt objects appear to have formed as icy planetesimals in the outer solar nebula, not much farther from the sun than the outer planets.

CHAPTER 1 Origin of Comets Meteorites, Asteroids, and Comets – However, the objects in the Oort cloud lie much farther from the sun, and they can’t have formed there. – The solar nebula would have been too tenuous at such great distances. – Also, you would expect objects that formed from the nebula to be confined to a disk and not distributed in a sphere.

CHAPTER 1 Origin of Comets Meteorites, Asteroids, and Comets • Astronomers think the objects now in the Oort cloud formed in the outer solar system among the present orbits of the Jovian planets. – As the Jovian planets grew more massive, they swept up some of these planetesimals and ejected others to form the Oort cloud. • If this idea is true, then the long-period comets are the same kind of icy planetesimals that make up the shortperiod comets.

CHAPTER 1 Impacts on Earth Meteorites, Asteroids, and Comets • For centuries, superstitious people have associated comets with doom, which seems silly. • Of course, comets and asteroids must hit planets now and then. • So, you might wonder just how dangerous such impacts would be.

CHAPTER 1 Impacts on Earth Meteorites, Asteroids, and Comets • Earthlings watched in awe, during the summer of 1994, as the fragmented head of a comet slammed into Jupiter, producing impacts equaling millions of megatons of TNT.

CHAPTER 1 Impacts on Earth Meteorites, Asteroids, and Comets • Such impacts on Jupiter probably occur every century or so, and you might expect similar impacts on Earth—smaller and with less gravitational power—to occur much less often.

CHAPTER 1 Impacts on Earth Meteorites, Asteroids, and Comets • Nevertheless, these impacts do occur. – On some moons, you can find chains of craters that seem to have been formed by fragmented comets.

CHAPTER 1 Impacts on Earth Meteorites, Asteroids, and Comets • Small meteorite impacts occur quite often. – A building is damaged by a falling meteorite every few years. • Larger impacts are less common. • Truly large impacts are rare. • A large impact could have devastating consequences.

CHAPTER 1 Impacts on Earth • Earth is marked by about 150 meteorite craters that illustrate the power of an impacting object. Meteorites, Asteroids, and Comets

CHAPTER 1 Impacts on Earth Meteorites, Asteroids, and Comets • Studies of sediments laid down all over the world 65 million years ago at the time of the extinction of the dinosaurs have found overabundances of the element iridium— common in meteorites but rare in Earth’s crust.

CHAPTER 1 Impacts on Earth Meteorites, Asteroids, and Comets • This finding, along with other evidence, suggests that the impact of a large meteorite may have altered the atmosphere and climate on Earth so dramatically that the dinosaurs and over 75 percent of the species then on Earth became extinct.

CHAPTER 1 Impacts on Earth Meteorites, Asteroids, and Comets • Mathematical models and observations of the impact of the comet fragments on Jupiter in 1994 have combined to create a plausible scenario of the events following a major impact on Earth. – Creatures living near the site of the impact would probably die in the initial shock. – An impact at sea would create tsunamis (tidal waves) many hundreds of meters high that would devastate coastal regions halfway around the world for many kilometers inland.

CHAPTER 1 Impacts on Earth Meteorites, Asteroids, and Comets • However, the worst effects would begin after the initial explosion. – On land or sea, a major impact would excavate large amounts of pulverized rock, heat it to high temperatures, and loft it high above the atmosphere. – As this material fell back, Earth’s atmosphere would be turned into a glowing oven of falling red-hot meteors.

CHAPTER 1 Impacts on Earth Meteorites, Asteroids, and Comets • The heat would trigger massive forest fires around the world. – Soot from such fires has been detected in the layers of clay laid down at the end of the Cretaceous period. • Once the firestorms cooled, the remaining dust in the atmosphere would block sunlight and produce deep darkness for a year or more, killing most plant life.

CHAPTER 1 Impacts on Earth Meteorites, Asteroids, and Comets • At the same time, large amounts of carbon dioxide locked in limestone deposits and released into the atmosphere by the impact would produce intense acid rain. • All these consequences make it surprising that any life could have survived such an impact.

CHAPTER 1 Impacts on Earth Meteorites, Asteroids, and Comets • Geologists have located a crater at least 150 km in diameter centered near the village of Chicxulub in the northern Yucatán.

CHAPTER 1 Impacts on Earth Meteorites, Asteroids, and Comets • Although the crater is totally covered by sediments, mineral samples show that it contains shocked quartz typical of impact sites and that it is the right age.

CHAPTER 1 Impacts on Earth Meteorites, Asteroids, and Comets • The impact of an object 10 to 14 km in diameter formed the crater about 65 million years ago, just when the dinosaurs and many other species died out. • Many Earth scientists now believe that this is the scar of the impact that ended the Cretaceous period.

CHAPTER 1 Impacts on Earth Meteorites, Asteroids, and Comets • Chicxulub may not be the only impact that triggered an extinction. • The biggest extinction on record occurred 250 million years ago at the end of the Permian period, when 95 percent of life in the oceans and 80 percent of life on land died out. – The Permian extinction is also called the Great Dying.

CHAPTER 1 Impacts on Earth Meteorites, Asteroids, and Comets • Core samples from the ocean floor northwest of Australia reveal shattered rock, meteoric fragments, and shocked quartz. • Seismic and gravity data support the idea that a major impact occurred there at about the right time to cause the Great Dying.

CHAPTER 1 Impacts on Earth Meteorites, Asteroids, and Comets • Could such impacts happen again? – Earth gets hit by small meteorites every day and by larger objects less often. – Impacts by large asteroids may happen many millions of years apart, but they continue to happen.

CHAPTER 1 Impacts on Earth Meteorites, Asteroids, and Comets • In mid-March 1998, newspaper headlines announced ‘Mile-Wide Asteroid to Hit Earth in October 2028. ’ • However, the news media did not report that there was uncertainty in the orbit. – Within days, astronomers found the asteroid on old photographic plates, recalculated the orbit adding the new data, and concluded that the asteroid—known as 1997 XF 11—would miss Earth by 600, 000 miles.

CHAPTER 1 Impacts on Earth Meteorites, Asteroids, and Comets • There will be no impact by this asteroid in 2028. • However, there are plenty more asteroids that haven’t been discovered. – It is just a matter of time.

CHAPTER 1 Impacts on Earth Meteorites, Asteroids, and Comets • A solar system is a dangerous place to put an inhabited planet. • Comets, asteroids, and meteoroids constantly rain down on the planets, and Earth gets hit relatively often. – Chicxulub isn’t the only large impact scar on Earth. – About 150 are known, including giant craters buried under sediment in Iowa and another underlying most of Chesapeake Bay.

CHAPTER 1 Building Scientific Arguments Meteorites, Asteroids, and Comets • How do comets help explain the formation of the planets? – This scientific argument pulls together several ideas. • According to the solar nebula hypothesis, the planets formed from planetesimals that accreted in a disk-shaped nebula around the forming sun.

CHAPTER 1 Building Scientific Arguments Meteorites, Asteroids, and Comets • In the outer solar nebula, it was cold, and the planetesimals would have contained large amounts of ices. • Many of these planetesimals were destroyed when they fell together to make the planets.

CHAPTER 1 Building Scientific Arguments Meteorites, Asteroids, and Comets • However, some of the planetesimals survived. • The icy bodies of the Oort cloud and the Kuiper Belt may be the last surviving icy planetesimals in our solar system.

CHAPTER 1 Building Scientific Arguments Meteorites, Asteroids, and Comets • When these bodies fall into the inner solar system, they becomets. • The gases they release reveal that they are rich in volatile materials such as water, carbon dioxide, carbon monoxide, methane, and ammonia. – These are the ices you would expect to find in the icy planetesimals.

CHAPTER 1 Building Scientific Arguments Meteorites, Asteroids, and Comets • Furthermore, comets are rich in dust, and the planetesimals must have included large amounts of dust frozen into the ices when they formed. • The nuclei of comets seem to be frozen samples of the ancient solar nebula.

CHAPTER 1 Building Scientific Arguments Meteorites, Asteroids, and Comets • Nearly all the mass of a comet is in the nucleus, but the light you see comes from the coma and the tail. • Build an argument pulling together ideas about cometary nuclei. – What kind of spectra do comets produce? – What does that tell you about their nuclei?