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Neil F. Comins • William J. Kaufmann III Discovering the Universe Ninth Edition CHAPTER 9 Vagabonds of the Solar System WHAT DO YOU THINK? 1. 2. 3. 4. 5. Are the asteroids a former planet that was somehow destroyed? Why or why not? How far apart are the asteroids on average? How are comet tails formed? Of what are they made? In which directions do a comet’s tails point? What is a shooting star? In this chapter you will discover… the properties of dwarf planets and small solar system bodies asteroids and meteoroids—pieces of interplanetary rock and metal comets—objects containing large amounts of ice and rocky debris space debris that falls through Earth’s atmosphere the asteroid belt and the Kuiper belt, both filled with a variety of debris, including orbiting pairs of objects the impacts from space 250 million and 65 million years ago that caused mass extinctions of life on Earth wayward asteroids that could again threaten life on Earth Different Classifications of Solar System Objects Some of the definitions of the different types of objects in the solar system overlap. For example, the largest asteroids are also being classified as dwarf planets; various transNeptunian objects (TNOs) are asteroids or comets; some comets are satellites of Jupiter; some Kuiper belt objects (KBOs) are satellites of other KBOs. Furthermore, TNOs exist in two groups: Kuiper belt objects and Oort comet cloud bodies. Pluto These three Hubble Space Telescope images of Pluto show little detail but indicate that the major features of Pluto’s surface each cover large amounts of its area. Comparing these observations to previous ones reveals that the surface changes in color and brightness seasonally. Discovery of Pluto Pluto was discovered in 1930 by searching for a dim, starlike object that slowly moved against the background stars. These two photographs were taken 1 day apart. Orbit of Pluto (a) The high-eccentricity orbit of dwarf planet (and KBO) Pluto stands out compared to the orbits of the outer three planets. Notice how many significant events occurred on Earth during Pluto’s present orbit of the Sun. (b) Details of Pluto’s passage inside the orbit of Neptune. The two bodies will never collide. Orbit of Pluto (c) A nearly edge-on view of the ecliptic and Pluto’s orbit compared to it. Discovery of Charon Long ignored as just a defect in the photographic emulsion, the bump on the upper left side of this image of Pluto led astronomer James Christy to discover the moon Charon. Pluto’s Moons Nix and Hydra Observations by the Hubble Space Telescope in 2005 revealed two small moons, each about 5000 times dimmer than Pluto. Named Nix and Hydra, they are between 2 and 3 times farther from Pluto than is its moon Charon. The lines radiating from Pluto and Charon are artifacts of the exposure. Comparison of Ceres with the Moon and Earth Ceres, the Moon, and Earth are shown here to scale. Dwarf planet Ceres, shown in this infrared photo (Earth and the Moon appear in visible light), is the largest asteroid but is so small that it is not considered a planet. Because it does not orbit a body other than the Sun, it is also not classified as a moon. This image of Ceres suggests it has regions of ice and rock on its surface. The asteroid will be visited by the Dawn spacecraft in 2015. Dwarf Planet Eris Three perpendicular views of the orbit of Eris and Dysnomia are compared to the planets and Pluto. Eris and Dysnomia’s orbit around the Sun ranges from 38 to 98 AU, with orbital eccentricity, e = 0.44, and an orbital inclination of 44°. Dwarf Planet Eris This is a Keck Telescope image of dwarf planet Eris and its moon Dysnomia. Asteroid Orbits (a) The orbits of belt asteroids Ceres, Pallas, and Juno are indicated to scale in this diagram. (b) These are the actual positions of all known asteroids at Jupiter’s orbit or closer. The locations of the belt asteroids are indicated by green dots. Objects passing closer than 1.3 AU to the Sun are shown by red circles. Objects observed at least twice are indicated by filled circles, and objects seen only once are indicated by outline circles. Jupiter’s Trojan asteroids are deep blue squares. Comets are filled and unfilled light-blue squares. Discovering Asteroids In 1998, the Hubble Space Telescope found this asteroid while observing objects in the constellation Centaurus. The exposure, tracking stars, shows the asteroid as a 19-arcsec streak. This asteroid is about 2 km in diameter and was located about 140 million km (87 million mi) from Earth. The Kirkwood Gaps This graph displays the number of asteroids at various distances from the Sun. Note that few asteroids have orbital periods that correspond to such simple fractions as 1⁄3, 2⁄5, 3⁄7, and 1⁄2 of Jupiter’s orbital period. Resonant orbits with Jupiter have deflected asteroids away from these orbits. The Trojan asteroids accompany Jupiter as it orbits the Sun. Collision Between Two Asteroids Observed in 2010, this X-shaped “object” (inset) is believed to be the collision of two small asteroids. The event created dust that was pushed away from the Sun, which is to the left and below this image. The collision occurred 2 AU from the Sun and 1 AU from Earth. Ida and Its Satellite The 55-km-long rocky asteroid Ida, shown here with its satellite Dactyl, is about twice the size of the younger asteroid Gaspra (see Figure 5-7). Inset: Dactyl is also heavily cratered. Jupiter’s Trojan Asteroids Groups of asteroids orbit at the two stable Lagrange points along Jupiter’s orbit, trapped by the combined gravitational forces of Jupiter and the Sun. Asteroid 1994 XM1 This image was obtained on December 9, 1994, shortly before the near Earth asteroid arrived in Earth’s vicinity. When it passed by Earth just 12 h later, asteroid 1994 XM1 was less than half the distance from Earth to the Moon. Asteroids (a) Reflecting only half as much light as a charcoal briquette, Mathilde is half as dense as typical stony asteroids. Slightly larger than Ida, irregularly shaped Mathilde measures 66 km x 48 km x 46 km, rotates once every 17.4 days, and has a mass equivalent to 110 trillion tons. The part of the asteroid shown is about 59 km x 47 km. The large crater in shadow is about 20 km across. (b) The near-Earth asteroid Itokawa was visited by the Japanese space probe Hayabusa. Samples of dust particles from it are now being analyzed. Asteroid Eros The Near-Earth Asteroid Rendezvous (NEAR) Shoemaker spacecraft took these images of asteroid Eros in February 1999. (a) The top of the figure is the asteroid’s north polar region. Eros’s dimensions are 33 km x 13 km x 13 km (21 mi x 8 mi x 8 mi) and it rotates every 5¼ h. Its density is 2700 kg/m3, close to the average density of Earth’s crust and twice as dense as asteroid Mathilde. (b) This is an image taken while looking into the large crater near the top of (a), which is 5.3 km (3.3 mi) across. (c) This is the penultimate image taken by NEAR Shoemaker before it gently landed on Eros. Taken from an altitude of 250 m (820 ft), the image is only 12 m across. You can see rocks and boulders buried to different depths in the regolith. Current Positions of Known Dwarf Planets and SSBs in the Outer Solar System Objects with unusually high-eccentricity orbits are shown as cyan triangles. Objects roaming among the outer planets, called Centaur objects, are orange triangles. Plutinos are white circles. Miscellaneous objects are magenta circles, and classical KBOs are red circles. Objects observed only once are denoted by open symbols; objects with two separate observations are denoted by filled symbols. Comets are filled and unfilled light-blue squares. Kuiper Belt Objects (a, b) These 1993 images show the discovery (white lines) of 1 of more than 1524 known KBOs. These two images of KBO 1993 SC were taken 4.6 h apart, during which time the object moved against the background stars. (c) The KBO 1998 WW31 and its moon (lower left). Sedna’s Orbit (a) The farthest known body in the solar system is in a highly elliptical orbit (b) that ranges from the outer reaches of the Kuiper belt and possibly extends to the inner Oort comet cloud. Comet Nuclei (a) The nucleus of Comet Halley. This image, taken by the Giotto spacecraft, shows the potato-shaped nucleus of the comet. Its dark nucleus measures 15 km in its longest dimension and about 8 km in its shortest. The numerous bright areas on the nucleus are icy outcroppings that reflect more sunlight than surrounding areas of the comet. Two jets of gas can be seen emanating from the left side of the nucleus. (b) This is the nucleus of Comet Borrelly, in an image taken by Deep Space 1. The nucleus is 8 km (5 mi) long. Comet Wild 2 This picture shows two images combined. One is a highresolution photograph showing the surprisingly heavily cratered comet. The other image is a longer photograph showing gas and dust jetting from the comet. Its tails are millions of kilometers long. Comet Wild 2 A substance called aerogel was used to capture particles from Comet Wild 2’s dust tail. A piece of space debris pierced the aluminum foil holding the aerogel and embedded in it, along with pieces of the foil. Comet Wild 2 A 2-µm piece of comet dust, composed of a mineral called forsterite. On Earth this mineral is used to make gems called peridot. Comet Hale-Bopp In 1997, Comet Hale-Bopp had a hydrogen envelope 1 AU in diameter (blue ovals). This gas was observed in the ultraviolet. The visible light inset shows the scale of the visible tails (see also the image at the opening of this chapter). Comet West Astronomer Michael M. West first noticed this comet on a photograph taken with a telescope in 1975. After passing near the Sun, Comet West became one of the brightest comets of the 1970s. This photograph shows the comet in the predawn sky in March 1976. The Orbit and Tails of a Comet The sunlight and solar wind blow a comet’s dust particles and ionized atoms away from the Sun. Consequently, comets’ tails always point away from the Sun. The Two Tails of Comet Mrkos (a) Comet Mrkos dominated the evening sky in August 1957. These three views, taken at 2-day intervals, show dramatic changes in the comet’s gas tail. In contrast, the slightly curved dust tail remained fuzzy and featureless. The Two Tails of Comet Mrkos (b) Wind blowing smoke from this forest fire causes the smoke column to change shape and direction, just like the solar wind and sunlight cause the tails of comets to change shape and direction. The Structure of a Comet The solid part of a typical comet (the nucleus) is roughly 10 km in diameter. The coma can be as large as 105 to 106 km across, and the hydrogen envelope is typically 107 km in diameter. A comet’s tail can be enormous—even longer than 1 AU. The Head of Comet Brooks This comet had an exceptionally large, bright coma. Named after its discoverer, William R. Brooks, it dominated the night skies in October 1911. The Tail of Comet Ikeya-Seki Named after its codiscoverers in Japan, this comet dominated the predawn skies in late October 1965. The yellow in the tail comes from emission by sodium atoms in the dust that was released by the comet. Although its coma was tiny, its tail spanned over 1 AU. Comet Tempel 1 (a) This composite image of Comet Tempel 1 has higher resolution at the bottom, as the projectile from Deep Impact headed in that direction. The smooth regions on the comet have yet to be explained. (b) This image was taken 30 s before the projectile struck the comet. Comet Tempel 1 (c) Seconds after impact, hot debris explodes away from the comet nucleus. The white horizontal half-ellipses are areas where the CCDs were overloaded with light from the event. (d) Moments later, the gases and dust were expanding outward. (e) This is an image taken 67 s after impact. Within minutes, the cloud of debris eventually became much larger than the entire nucleus. Transformation and Evolution of a LongPeriod Comet (a) The gravitational force of a giant planet can change a comet’s orbit. Comets initially on highly elliptical orbits are sometimes deflected into more circular paths that keep them in the inner solar system. (b–d) These figures show the evolution of a comet into gas, dust, and rubble, and why debris from some of these comets strikes Earth. The Fragmentation of Comet Schwassmann-Wachmann-3 This comet, with a 5.4-year orbit, has been coming apart for decades. In 2006, it further fragmented after passing perihelion. One piece, Fragment B, shed at least 30 smaller pieces, shown here. Comet Hale-Bopp Discovered on July 23, 1995, this comet was at its breathtaking best in mid-1997. Inset: Jets of gas and debris were observed shooting out from Comet Hale-Bopp several times. This image shows the comet nucleus (lower bright region), an ejected piece of the comet’s surface (upper bright region), and a spiral tail. The ejected piece eventually disintegrated, following the same spiral pattern as the tail. Sungrazing Comet Comet SOHO LASCO C3 is shown in the smaller box and magnified in the larger one. Discovered in March 2004, it was the 750th sungrazing comet discovered from the SOHO data. It completely sublimated near perihelion. Meteor This brilliant meteor is seen lighting up the dark skies of the California desert area of Joshua Tree National Park. Just to the right of the meteor trail are the Pleiades. Meteorite Impact, Poughkeepsie, NY, 1992 Meteor Crater An iron meteor measuring 50 m across struck the ground in Arizona 50,000 years ago. The result was this beautifully symmetric impact crater. The Origin of Meteor Showers As comets dissipate, they leave debris behind that spreads out along their orbits. When Earth plows through such material, many meteors can be seen emanating from the same place within a very short time—a meteor shower. As shown in this diagram, many comets have high orbital inclinations. Meteor Shower This time exposure, taken in 1998, shows meteors streaking away from the constellation Leo Major. They are part of the Leonid meteor shower. This shower occurs because Earth is moving through debris left by comet Temple-Tuttle. Recent Impacts on the Moon The locations A–F are places on the Moon where impacts were observed from Earth in 1999 during the Leonid meteor shower. The impacting bodies hit the Moon at around 260,000 km/hr (160,000 mph) and had masses of between 1 and 10 kg. Each impact created a shortlived cloud that momentarily heated to between 5 x 104 and 10 x 104 K, much hotter than the surface of the Sun. The Mass of Impacts on Earth The Vatican Obelisk is about 300 tons, the amount of mass that strikes Earth daily. As a result, Earth’s mass increases by this amount every day. Stony Meteorites (a) Most meteorites that fall to Earth are stones. Many freshly discovered specimens, like the one shown here, are coated with thin, dark crusts. This stony meteorite fell in Morocco. (b) Some stony meteorites contain tiny specks of iron, which can be seen when the stones are cut and polished. This specimen was discovered in Ohio. Iron Meteorites (a) Irons are composed almost entirely of iron-nickel minerals. The surface of a typical iron is covered with thumbprint-like depressions created as the meteorite’s outer layers vaporized during its high-speed descent through the atmosphere. This specimen was found in Argentina. (b) When cut, polished, and etched with a weak acid solution, most iron meteorites exhibit interlocking crystals in designs, called Widmanstätten patterns. This meteorite was found in Australia. Stony-Iron Meteorite Stony-irons account for about 1% of all meteorites that fall to Earth. This specimen, a variety of stony-iron called a pallasite, was found in Antarctica. This specimen is thinly cut and appears to glow because of a light located behind it. Pieces of the Allende Meteorite (a) This carbonaceous chondrite fell near Chihuahua, Mexico, in February 1969. Note the meteorite’s dark color, caused by a high abundance of carbon. Geologists believe that this meteorite is a specimen of primitive planetary material. The ruler is 15 cm long. (b) Sliced open, the Allende meteorite shows round, rocky inclusions called chondrules in a matrix of dark rock. Finding a Meteorite in Antarctica Good places to find meteorites include deserts and icecovered regions, such as Antarctica. By surveying such areas, astronomers and geologists can accurately determine the correct percentage of each of the different types of meteorites. Aftermath of the Tunguska Event In 1908, a stony asteroid traveling at supersonic speed struck Earth’s atmosphere and exploded over the Tunguska region of Siberia. Trees were blown down for many kilometers in all directions from the impact site. Iridium-Rich Layer of Clay This photograph of strata in the Apennine Mountains of Italy shows a dark-colored layer of iridium-rich clay sandwiched between white limestone (bottom) from the late Mesozoic era and grayish limestone (top) from the early Cenozoic era. The coin is the size of a U.S. quarter. Confirming an Extinction-Level Impact Site By measuring slight variations in the gravitational attraction of different materials under Earth’s surface, geologists create images of underground features. Concentric rings of the underground Chicxulub Crater (right inset) lie under a portion of the Yucatán Peninsula. This crater has been dated to 65 million years ago and is believed to be the site of the impact that led to the extinction of the dinosaurs. A piece of 65-million-year-old meteorite discovered in the middle of the Pacific Ocean in 1998 is believed to be a fragment of that meteorite. The fragment, about 0.3 cm (0.1 in.) long, was cut into two pieces for study (left inset). Summary of Key Ideas Asteroids Pieces of solar system debris larger than 10 m and composed primarily of rock and metal are called asteroids. Tens of thousands of belt asteroids with diameters larger than a kilometer are known to orbit the Sun between the orbits of Mars and Jupiter. The gravitational attraction of Jupiter depletes certain orbits within the asteroid belt. The resulting Kirkwood gaps occur at simple fractions of Jupiter’s orbital period. Jupiter’s and the Sun’s gravity combine to capture Trojan asteroids in two locations, called stable Lagrange points, along Jupiter’s orbit. Asteroids The Apollo asteroids move in highly elliptical orbits that cross the orbit of Earth. Many of these asteroids will eventually strike the inner planets. A belt asteroid, Ceres, along with four KBOs (Pluto, Eris, Haumea, and Makemake) are classified as dwarf planets. Pluto, a KBO and dwarf planet, is an icy world that may well resemble the moon Triton. Comets Comet nuclei are fragments of ice and rock often orbiting at a great inclination to the plane of the ecliptic. In the Kuiper belt and Oort cloud, comets have fairly circular orbits. When close to the Sun, they generally move in highly elliptical orbits. Many comet nuclei orbit the Sun in the Kuiper belt, a doughnut-shaped region beyond Pluto. Billions of cometary nuclei are also believed to exist in the spherical Oort cloud located far beyond the Kuiper belt. As an icy comet nucleus approaches the Sun, it develops a luminous coma surrounded by a vast hydrogen envelope. A gas (or ion) tail and a dust tail extend from the comet, pushed away from the Sun by the solar wind and radiation pressure. Meteoroids, Meteors, and Meteorites Boulder-sized and smaller pieces of rock and metal in space are called meteoroids. When a meteoroid enters Earth’s atmosphere, it produces a fiery trail, and it is then called a meteor. If part of the object survives the fall, the fragment that reaches Earth’s surface is called a meteorite. Meteorites are grouped in three major classes according to their composition: iron, stony-iron, and stony meteorites. Rare stony meteorites, called carbonaceous chondrites, may be relatively unmodified material from the primordial solar nebula. These meteorites often contain organic hydrocarbon compounds, including amino acids. Fragments of rock from “burned-out” comets produce meteor showers. Meteoroids, Meteors, and Meteorites An analysis of the Allende meteorite suggests that a nearby supernova explosion may have been involved in the formation of the solar system some 4.6 billion years ago. An asteroid that struck Earth 65 million years ago probably contributed to the extinction of the dinosaurs and many other species. Another impact may have caused the “Great Dying” of life 250 million years ago. Such devastating impacts occur on average every 100 million years. Key Terms amino acid Apollo asteroid asteroid belt belt asteroid carbonaceous chondrite chondrites coma (of a comet) dust tail dwarf planet gas (ion) tail hydrogen envelope impact crater iron meteorite Kirkwood gaps long-period comet meteor meteor shower meteorite meteoroid nucleus (of a comet) Oort cloud planet radiation (photon) pressure short-period comet small solar-system bodies (SSSBs) stable Lagrange points stony meteorite stony-iron meteorite Trojan asteroid Widmanstätten Patterns WHAT DID YOU THINK? Are the asteroids a former planet that was somehow destroyed? Why or why not? No. The gravitational pull from Jupiter prevented a planet from ever forming in the asteroid belt. Also, the total mass of the asteroids is much less than even the mass of tiny Pluto, a dwarf planet. WHAT DID YOU THINK? How far apart are the asteroids on average? The distance between asteroids averages 10 million km. WHAT DID YOU THINK? How are comet tails formed? Of what are they made? Ices in comet nuclei are turned into gas by absorbing energy from the Sun. Debris is released in this process. Sunlight and the solar wind push on the gas and dust, creating the tails. WHAT DID YOU THINK? In what directions do a comet’s tails point? Comets’ gas tails point directly away from the Sun; their dust tails make arcs pointing away from the Sun. WHAT DID YOU THINK? What is a shooting star? A shooting star is a piece of space debris plunging through Earth’s atmosphere—a meteor. It is not a star.