42246 ch23 ptg01 hr 466-487faculty.ccbcmd.edu/courses/ersc101c/unit1/ch23_the_solar_system.pdf ·...

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23 Pl

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S E C T I O N S 23-1 The Solar System:

A Brief Overview

23-2 The Terrestrial Planets

23-3 The Moon: Our Nearest

Neighbor

23-4 The Jovian Planets: Size,

Compositions, and Atmospheres

23-5 Moons of the Jovian Planets

23-6 Planetary Rings

23-7 Pluto and Other Dwarf Planets

23-8 Asteroids, Comets, and

Meteoroids

GO TO PAGE 487 FOR

STUDY TOOLS

While cruising around Saturn at a distance of about 6.3 million kilometers in early October 2004, Cassini orbiter captured a series of images that have been composed into the most detailed natural color view of Saturn and its rings ever made.

42246_ch23_ptg01_hr_466-487.indd 46642246_ch23_ptg01_hr_466-487.indd 466 19/12/13 3:51 PM19/12/13 3:51 PM

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Chapter 23 Planets and their Moons 467

Astronomy compels the soul

to look upward, and leads us

from this world to another.

Plato, The Republic

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23-1 THE SOLAR SYSTEM: A BRIEF OVERVIEW The Solar System formed about 4.6 billion years ago from a cold, diffuse cloud of dust and gas rotating slowly in space. The cloud was composed of about 92 percent hydrogen and 7.8 percent helium, about the same elemental composition as the Universe. 1 All of the other elements composed only 0.2 percent of the Solar System.

A portion of the cloud gravitated toward its cen-ter to form the Sun. Here the pressure became so intense that hydrogen fused, producing energy as some of the hydrogen converted to helium. Hydrogen fusion is still the source of the Sun’s energy and will be discussed in Chapter 24 . The remaining matter in the original cloud formed a disk-shaped, rotating nebula , or cloud of interstellar dust, that eventually coalesced into separate spheres to produce the plan-ets. The evolution of the planets is an example of a feedback mechanism occurring in space. Any object—including a rocket ship or a gas molecule—can escape from a planet’s gravity when it reaches a speed known as the escape velocity . The escape velocity is proportional to the mass, and hence the gravitational force, of the planet.

Let us now compare the evolution of Mercury, the closest planet to the Sun, with that of Jupiter, one of the more distant planets. In their primordial states, all planets were composed mainly of gases. Gases in a planetary atmosphere are in constant motion, and the higher the temperature, the higher the average speed. Mercury, being closer to the Sun, was originally hotter than Jupiter. Its gases were moving faster, so they were more likely to escape the planet’s gravity and fly off into space. As gases escaped, the planet lost mass, so the escape veloc-ity decreased, making it easier for gases to escape. In addition, the solar wind , a stream of electrons and positive ions radiating outward from the Sun at high speed, blew even more gases away from the primordial planets. Combining all processes, the

inner planets—Mercury, Venus, Earth, and Mars—lost most of their gases, leaving behind spheres composed mostly of nonvolatile metals and silicate rocks. These four are now called the terrestrial planets . In contrast, the protoplanets in the outer reaches of the Solar System were so far from the Sun that they were initially cool. As a result, the outer Jovian planets —Jupiter, Saturn, Uranus, and Neptune—retained large amounts of hydrogen, helium, and other light elements. In fact, they actually grew larger as they captured gases that escaped from the terrestrial planets. As the mass increased, the escape velocity also increased, and gas escape became very slow. Today, the Jovian planets all have relatively small, rocky or metal cores surrounded by swirling liquid and gaseous atmospheres.

Table 23.1 on page 470 provides an overview of the eight major planets. Due to the differences in composition, the terrestrial planets are much denser than the Jovian planets. However, the Jovian planets are much larger and more massive than Earth and its neighbors.

23-2 THE TERRESTRIAL PLANETS As the planets coalesced, the primordial Solar System was crowded with asteroids, meteoroids, comets, and other chunks of rock, gases, and ices. This space debris crashed into the planets, pock-marking their surfaces with millions of craters. Over geologic time, craters can be obliterated by tectonic events, weath-ering, and erosion. However, if these processes do not occur, then the craters remain for billions of years. Thus, scientists learn a lot about a planet’s history simply by observing crater density. With this back-ground, let us compare the geology and the atmo-spheres of the four terrestrial planets.

1. Composition is given here in percentage of number of atoms. Percentage by mass is significantly different.

escape velocity The speed that an object

must attain to escape the

gravitational field of a

planet or other object in

space.

solar wind A stream of

electrons and positive ions

radiating outward from the

Sun at high speed.

terrestrial planets The four Earth-like planets

closest to the Sun—

Mercury, Venus, Earth,

and Mars—which are

composed primarily of non-

volatile metals and silicate

rocks.

Jovian planets The

outer planets—Jupiter,

Saturn, Uranus, and

Neptune—all massive, with

relatively small rocky or

metal cores surrounded by

swirling liquid and gaseous

atmospheres.


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468 Unit 6 Astronomy

The Solar System

Jupiter

Mars

Earth

VenusMercury

SunSaturn

Uranus

Neptune

Pluto

A schematic view of the modern Solar System

A small portion of one of thesevast clouds began to rotate andcontract under the influenceof gravity.

As the cloud continued to rotateand contract, it flattened into athin disk. Matter concentrated tothe center, forming a protosun.

As the sun contracted andbecame denser, matter in theoutlying disk coalesced to formthe planets.

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About 5 billion years ago, exploding stars accelerated dust and gas

into the great void of space.


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JupiterJupiter

Mercury

Venus

Earth Moon

Mars

Jupiter

Saturn

Uranus

Neptune

Pluto

Saturn

Uranus

MercuryMercuryMoonMoon

Venus(radar image)

Venus(radar image)

MarsMars

Sun

NeptuneNeptune

Terrestrialplanets

The planets and the Sun drawn to scale. Note thatthe Earth–Moon orbital system would easily fit intoa portion of the Sun’s surface. The Earth is the mostmassive of the terrestrial planets, but is only 1/300the mass of Jupiter. Saturn’s rings would reach fromthe Earth to the Moon.

The planets drawn to scale. The terrestrialplanets are so small on this scale that theyare enlarged below. The Jovian planets(right) are predominantly composed ofgases and liquids with small rocky andmetallic cores, while the terrestrial planets(below) are composed primarily of rockand metal.

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year. Because the Sun is so low in the sky, regions inside meteorite craters are perpetually in the shade. With virtually no atmosphere to transport heat, the shaded regions have remained below the freezing point of water for billions of years.

Venus Recall from Chapter 1 that Venus closely resembles Earth in size, density, and distance from the Sun. As a result, Venus and Earth probably had similar atmo-spheres early in their histories. However, Venus is closer to the Sun than Earth is, and therefore it was initially hotter. One hypothesis suggests that because of the higher temperature, water never condensed—or if it did, it quickly evaporated again. Because there were no seas for carbon dioxide to dissolve into, most of the carbon dioxide also remained in the atmosphere. Water and carbon dioxide combined to produce a runaway greenhouse effect, and surface temperatures became torridly hot. The molecular weight of water is less than half that of carbon dioxide. Because water is so light, the hot surface temperature boiled the water into space and the solar wind swept most of the vapor toward the outer reaches of the Solar System. Today, the Venusian atmosphere is 90 times denser than that of Earth. Thus, atmospheric pressure at the surface of Venus is equal to the pressure 1,000 meters beneath the sea on our planet. The Venusian atmosphere is more than 97 percent carbon dioxide, with small amounts

23-2a Atmospheres and Climates of the Terrestrial Planets MercuryMercury , which is the closest planet to the Sun and therefore initially the hottest, has lost essentially all of its atmosphere.

Today it is a rocky sphere with a radius of 2,400 kilo-meters, less than 0.4 that of Earth’s radius. Mercury makes a complete circuit around the Sun faster than any other planet; each Mercurial year is only about 88 Earth days long. Mercury rotates slowly on its axis, so there are only three Mercurial days every two Mercurial years. Because Mercury is so close to the Sun and its days are so long, the temperature on its sunny side reaches 427 ° C , hot enough to melt lead. In contrast, the temperature on its dark side drops to − 175 ° C , cold enough to freeze methane. The lack of an atmosphere is partly responsible for these extremes of temperature, because there is no wind to carry heat from the hot, sunlit regions to the dark, frigid shadows.

In 1991, radar images of Mercury revealed highly reflective regions at the planet’s poles. Data indicated that these regions were composed of ice. Mercury’s spin axis is almost perpendicular to its orbital plane around the Sun, so the Sun never rises or sets at the poles but remains low on the horizon throughout the

Mercury The closest

planet to the Sun.

Venus The second planet

from the Sun; resembles

Earth in size and density.

TABLE 23.1 Comparison of the Eight Major Planets

Planet

Distance from Sun ( millions of kilometers = 1 )

Radius (compared to radius of Earth = 1 )

Mass (comparedto mass of Earth = 1 )

Density ( comparedto density of water = 1 )

Composition of Planet

Density of Atmosphere ( compared to Earth’s atmosphere = 1 )

Number of Moons

Terrestrial Planets

Mercury 58 0.38 0.06 5.4

Rocky with metallic core

One-billionth 0

Venus 108 0.95 0.82 5.2 90 0

Earth 150 1 1 5.5 1 1

Mars 229 0.53 0.11 3.9 0.01 2

Jovian Planets

Jupiter 778 11.2 318 1.3 Liquid hydro-gen surface with liquid metallic mantle and solid core

Dense and turbulent

62

Saturn 1,420 9.4 94 0.7 62

Uranus 2,860 4.0 15 1.3 Hydrogen and helium outer layers with solid core

Similar to Jupiter except that some compounds that are gases on Jupiter are frozen on the outer planets

27

Neptune 4,490 3.9 17 1.7 13

© Cengage Learning


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abundance of iron-rich minerals that usually form in the presence of water. Opportunity also found sulfate salts that had clearly precipitated from a water-rich solution. Several weeks later, Opportunity took spectacular images of ripple marks preserved on the floor of a long-dead lake or ocean.

Meanwhile, Spirit climbed off the lava-covered Gusev plain and found precipitated sulfur salts on the crater rim, indicating that some water had existed here, too, although this environment was never as wet at the Meridiani site. Although water was definitely present in the Martian past, the lakes and oceans were either cold, short-lived, or covered with ice. This conclusion is derived mainly from the observation that carbonate rocks, such as limestone, precipitate from warm, carbon dioxide–rich oceans. It seems probable that carbon dioxide was pres-ent in the Martian atmosphere, but there are no carbonates.

Scientists now believe that water existed in isolated regions on Mars for relatively brief periods of time. Thus, the climate alternated from relative warmth to frigid cold, and back again. Today, Mars has abundant water frozen into soils and the polar ice caps. As evidenced so graphically by the pho-tos, mentioned earlier, of canyons and alluvial fans, liquid water ran across the surface as recently as a few million years ago. These cycles remind us, once again, that planetary climates are delicate and sub-ject to radical change.

of nitrogen, helium, neon, sulfur dioxide, and other gases. Corrosive sulfuric acid aerosols float in a dense cloud layer that perpetually obscures the surface. Due to greenhouse warming, the Venusian surface is hot-ter than that of Mercury, hot enough to destroy the complex organic molecules necessary for life.

Earth On Earth, outgassing from the mantle modified the primordial atmosphere. As life evolved, complex interactions among the geosphere, hydrosphere, and biosphere further altered the atmosphere, creating conditions favorable for life. This complex sequence of events is discussed in Chapter 17 .

Mars Today the surface of Mars , the fourth planet from the Sun, is frigid and dry. The surface temperature aver-ages − 56 ° C and never warms up enough to melt ice. At the poles, the temperature can dip to − 120 ° C , freezing carbon dioxide to form a dry ice. The atmo-sphere at the Martian surface is as thin as Earth’s atmosphere 43 kilometers high, which for us is the outer edge of space. Although water ice exists in the Martian polar ice caps and in the soil, there is cur-rently no liquid water on Mars.

However, abundant evidence indicates that the Martian climate was once much warmer and that water flowed across the surface. Photographs from Mariner and Viking spacecraft show eroded crater walls and extinct streambeds and lake beds. One giant canyon, Valles Marineris, is approximately 10 times longer and 6 times wider than Grand Canyon in the American Southwest ( Figure 23.1 ). Massive alluvial fans at the mouths of Martian canyons indicate that floods probably raced across the land at speeds up to 270 kilometers per hour.

In January 2004, NASA landed two mobile robots, called Spirit and Opportunity , on the Martian sur-face. Each robot was equipped with six wheels and a drive system, communications equipment to receive instructions from Earth and transmit data, and solar panels for power. Scientific research was conducted with a variety of sensitive instruments: a camera, a magnifying glass, a grinding wheel to burrow into a rock and expose fresh surfaces, a device to analyze the chemical elements in rock, two remote-sensing devices to identify minerals, and a magnet to collect magnetic dust particles. One of the main functions of these rovers was to look for evidence of the existence of water in the Martian past.

The Spirit rover landed on a dry crater and did not detect any evidence of water. However, the rover Opportunity discovered layered sedimentary rocks near its landing site in Meridiani Planum ( Figure 23.2 ). Chemical and mineralogical analysis indicated the

Mars The fourth planet

from the Sun.

Spirit One of two mobile

robots that landed on Mars

in 2004.

Opportunity One of two

mobile robots that landed

on Mars in 2004.

FIGURE 23.1 The giant Martian canyon, Valles Marineris, was

eroded by flowing water and is many times larger than the Grand

Canyon in the American Southwest.

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Venus Astronomers use spacecraft-based radar to penetrate the Venusian atmosphere and produce photolike imag-es of its surface. Gravity studies, also conducted by spacecraft, are used to infer the density of rocks near the Venusian surface. These density measurements provide information about the planet’s mineralogy and internal structure. The most spectacular data were obtained by the orbiting Magellan spacecraft, which was launched in May 1989. In October 1994, its mis-sion 99 percent accomplished and federal funding run-ning out, scientists sent Magellan on one final suicide mission into the Venusian atmosphere to provide addi-tional information about its density and composition. During the early history of the Solar System, thick swarms of meteorites bombarded all the planets. However, Magellan’s detailed maps show few mete-orite craters on Venus. This observation indicates that the Venusian surface was reshaped after the major meteorite bombardments. More detailed analysis shows that most of the landforms on Venus are 300 to 500 million years old.

James Head, Magellan investigator, suggests that a catastrophic series of volcanic eruptions occurred 300 to 500 million years ago, creating volcanic moun-tains and covering much of the surface with basalt flows ( Figure 23.4 ). According to this model, rising mantle plumes generated magma that repaved the planet in a short time with a rapid series of cataclys-mic volcanic eruptions. Head concludes: “The planet’s entire crust and lithosphere turned itself over. It cer-tainly makes a strong case that catastrophic events

23-2b Geology and Tectonics of the Terrestrial Planets Mercury Little was known about the surface of Mercury before the spring of 1974, when the spacecraft Mariner 10 passed within a few hundred kilometers of the planet. Images relayed to Earth revealed a cratered surface remarkably similar to that of our Moon ( Figure 23.3 ).

Recall that craters formed on all planets and their moons during intense meteorite bombardment early in the history of the Solar System. However, tectonic activity and erosion have erased Earth’s early meteorite craters. Yet, Mercury is so close to the Sun that its water and atmosphere boiled off into space, so no wind, rain, or rivers have eroded its surface. Today, 4 -billion-year-old craters look as fresh as if they formed yesterday.

Flat plains on Mercury are lava flows that formed early in its history, when the planet’s interior was hot enough to produce magma. However, Mercury is so small that its interior cooled quickly and igneous activity ceased when the planet was still young. Yet, curiously, Mercury has a magnetic field. On Earth, the magnetic field is generated by the flow of molten metals in the core. But the Mercurial surface shows no evidence of tectonic activity or of a hot interior. Therefore, astronomers cannot explain how the Mercurial magnetic field is generated.

FIGURE 23.3 Mercury has a cratered surface. The craters are

remarkably well preserved because there is no erosion or tectonic

activity on Mercury. The photograph shows an area 580 km from side

to side.

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FIGURE 23.2 On March 8, 2006, the Opportunity rover

photographed these cross-bedded sands on Mars that show

sedimentary structures similar to those commonly found on Earth.

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are in the geological records of planets and it ought to make us think about the possibilities of such cata-strophic events in Earth’s past.” An alternative hypoth-esis contends that the resurfacing occurred more slowly.

Crater abundances indicate that most tectonic activity on Venus stopped after the intense volcanic activity 300 to 500 million years ago. Some evidence indicates that the volcanic activity has ceased perma-nently because the planet’s interior cooled. This cool-ing may have occurred because radioactive elements floated toward the surface during the volcanic events, removing the mantle heat source. However, other data imply that Venus remains volcanically active and that another repaving event may occur in the future.

Most Earth volcanoes form at tectonic plate boundaries or over mantle plumes, so the discovery of volcanoes on Venus led planetary geologists to look for evidence of plate tectonic activity there. Radar images from spacecraft show that 60 percent of Venus’s surface consists of a flat plain. Two large and several smaller mountain chains rise from the plain ( Figure 23.5 ). The tallest mountain is 11 kilometers high— 2 kilometers higher than Mount Everest. The images also show large, crustal fractures and deep canyons. If Earth-like horizontal motion of tectonic

FIGURE 23.4 The volcano Maat Mons, on Venus, has produced

large lava flows, shown in the foreground. According to one hypothesis,

a catastrophic series of volcanic eruptions altered the surface of Venus

300 to 500 million years ago. This image was produced from radar data

recorded by the Magellan spacecraft. Simulated color is based on color

images supplied by Soviet spacecraft.

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FIGURE 23.5 Scientists used radar images from the Pioneer Venus orbiter to produce this map of Venus. The lowland plains are shown in blue,

and the highlands are shown in yellow and red-brown. The colors are assigned arbitrarily.

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One hypothesis suggests that the geology of Mars is similar to that of Venus and is dominated by blob tectonics. Supporters of this concept point out that the largest volcanic mountain on Earth, Mauna Kea in the Hawaiian Islands, is limited in size because the mountain is riding on a tectonic plate. Therefore, it will drift away from the underlying hot spot before it can grow much larger. In contrast, because horizon-tal movement on Mars is nonexistent or very slow, Olympus Mons has remained stationary over its hot spot. As a result, it has grown bigger and bigger.

plates caused these features, then spreading centers and subduction zones should exist on Venus. The images show no features like a mid-oceanic ridge system, transform faults, or other evidence of lithospheric spreading. However, geophysi-cists have located 10,000 kilo-

meters of trenchlike structures that they believe to be subduction zones.

Despite the apparent existence of subduction zones, the most popular current model suggests that Venusian tectonics have been dominated by mantle plumes. In some regions the rock has melted and erupted from volcanoes by processes similar to those that formed the Hawaiian Islands. In other regions the hot, Venusian mantle plumes have lifted the crust to form nonvolcanic mountain ranges. Some geologists have suggested the term blob tectonics to describe Venusian tectonics because Venus is dominated by rising and sinking of the mantle and crust. In contrast, tectonic activity on Earth causes significant horizontal movement of its plates.

Mantle plumes on Earth may initiate rifting of the lithosphere and formation of a spreading center. Why have spreading centers not developed over mantle plumes on Venus? Perhaps surface temperature on Venus is so high that the surface rocks are more plas-tic than those on Earth. Therefore, rock flows plasti-cally rather than fracturing into lithospheric plates. It is also possible that Venus has a thicker lithosphere, which can move vertically but does not fracture and slide horizontally.

Earth Earth is large enough to have retained considerable internal heat. This heat drives convection within the mantle that produces tectonic motion. In turn, Earth tectonics continuously reshapes the surface of our planet and is partially responsible for the atmo-sphere that sustains us. This topic was discussed in detail in Unit 2 .

Mars The Martian surface consists of old, heavily cratered plains and younger regions that have been altered by tectonic activity ( Figure 23.6 ). Lava flows much like those on Venus and the Moon cover the plains. The Tharsis bulge is the largest plain, crowned by Olympus Mons, the largest volcano in the Solar System ( Figure 23.7 ). Olympus Mons is nearly 3 times higher than Mount Everest, with a height of 25 kilometers and a diameter of 500 kilometers. Its cen-tral crater is so big that Manhattan Island would easily fit inside.

blob tectonics Tectonic

activity dominated by rising

and sinking of the mantle and

crust, believed to predomi-

nate on Venus, as opposed

to the horizontal movement

of plates associated with

tectonic activity on Earth.

FIGURE 23.6 The cratered plain on the left in this photo of the

Mars surface is a geologically old surface, whereas the mountains on

the lower right are evidence of more recent tectonic activity.

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FIGURE 23.7 Olympus Mons is the largest volcano on Mars and

also the largest in the Solar System.

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scientists suggest that a rising mantle plume formed the Tharsis bulge and its volcanoes. The parallel cracks may be the result of stretching as the crust uplifted.

An alternative hypothesis suggests that Earth-like, horizontal tectonics is occurring on Mars. As evi-dence, researchers have identified what they believe to be strike-slip faults. To support this hypothesis, sci-entists note that a linear distribution of volcanoes on and near the Tharsis bulge is similar to linear chains of volcanoes along terrestrial subduction zones.

Observations reported in 2005 show very few meteor craters in some lava flows, indicating that eruptions have occurred as recently as a few million years ago ( Figure 23.9 ). Thus, the planet has remained hot and active until relatively recent times.

23-3 THE MOON: OUR NEAREST NEIGHBOR Most planets have small, orbiting satellites called moons . The Earth’s Moon is close enough so that we can see some of its surface features with the naked eye. In the early 1600s, Galileo studied the Moon with a telescope and mapped its mountain ranges, cra-ters, and plains. Galileo thought that the plains were oceans and called them seas, or maria ( Chapter 22 ). The word maria is still used today, although we now know that these regions are dry, barren, flat expanses of volcanic rock ( Figure 23.10 ). Much of the lunar

Another line of evidence for Venusian-type tecton-ics on Mars is that tremendous parallel cracks split the crust adjacent to the Tharsis bulge ( Figure 23.8 ). If this bulge lay near a tectonic plate boundary, there would be folding or offsetting of the cracks. However, the cracks are neither folded nor offset. Therefore,

FIGURE 23.8 The huge parallel cracks near the Tharsis bulge on Mars are neither folded nor

offset. Scientists speculate that the bulge and the cracks were formed by a rising mantle plume.

However, the absence of folding or offset movement provides evidence that horizontal tectonics is not

active in this region. Two large volcanoes appear at right.

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maria Dry, barren, flat

expanses of volcanic rock

on the Moon, first thought

to be seas.

FIGURE 23.9 Elysium Mons is one of three large Martian

volcanoes that occur on the Elysium Rise. The volcano rises about 12.5

kilometers above the surrounding plain.

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Earth? If so, does it still have a molten core, and is it tectonically active?

23-3a Formation of the Moon According to the most popular current hypothesis, the Moon was created when a huge object—the size of Mars or even larger—smashed into Earth shortly after our planet formed. This massive bolide plowed through Earth’s mantle, and silica-rich rocks from the mantles of both bodies vaporized and created a cloud around Earth. The vaporized rock condensed and aggregated to form the Moon ( Figure 23.13A ).

Perhaps the single most significant discovery of the Apollo program was that much of the Moon’s sur-face consists of igneous rocks. The maria are mainly basalt flows. The highland rocks are predominantly anorthosite, a feldspar-rich igneous rock not common on Earth. Additionally, the rock of both the maria and the highlands has been crushed by meteorite impacts and then welded together by lava. Since igneous rocks form from magma, it is clear that portions of the Moon were once hot and liquid.

How did the Moon become hot enough to melt? Earth was heated initially by energy released by col-lisions among particles as they collapsed under the influence of gravity. Later, radioactive decay and intense meteorite bombardment heated Earth fur-ther. But what about the Moon? Radiometric dating shows that the oldest lunar igneous rocks formed before there was enough time for radioactive decay to have melted a significant amount of lunar rock. Thus, gravitational coalescence and meteorite bom-bardment must have been the main causes of early melting of the Moon.

surface is heavily cratered, similar to that of Mercury ( Figure 23.11 ).

A Soviet orbiter took the first close-up photo-graphs of the Moon in 1959. A decade later the United States landed the first of six manned Apollo spacecraft on the lunar surface ( Figure 23.12 ). The Apollo program was designed to answer several questions about the Moon: How did it form? What is its geologic history? Was it once hot and molten like

FIGURE 23.10 The Moon as photographed from the Apollo

spacecraft from a distance of 18,000 kilometers. Even from this

distance, we see cratered regions and the maria, which are smooth

lava flows.

NA

SA

FIGURE 23.11 Most of the lunar surface is heavily cratered. In

places where smaller craters lie within the larger ones, scientists

deduce that the larger craters formed first.

NA

SA

FIGURE 23.12 The six Apollo manned Moon landings answered

many scientific questions about the origin, structure, and history of

the Moon.

NA

SA


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FIGURE 23.13 (A) According to the most widely held hypothesis, the Moon was formed about 4.5 billion years ago when a Mars-sized object

struck Earth, blasting a cloud of vaporized rock into orbit. The vaporized rock rapidly coalesced to form the Moon. (B) During its first 0.6 billion years,

intense meteorite bombardment cratered the Moon’s surface. (C) The dark, flat maria formed about 3.8 billion years ago as lava flows spread across

portions of the surface. Today, all volcanic activity has ceased.

A

B

C

NA

SA


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energy released by moonquakes is only one-billionth to one-trillionth that released by earthquakes on our own planet. However, in 2005, 25 years after the first reports were released, scientists realized that the computers used in the study had only 64 kilobytes of memory, insufficient to properly record and analyze the data. Under reexamination, Yosio Nakamura from the University of Texas detected numerous deep moonquakes, indicating the Moon has a hot, possibly molten, core.

Data from the lunar Prospector spacecraft, released in March 1998, indicated that water may be present in shadowed craters at the lunar poles. In 2009, NASA instruments confirmed the presence of water molecules in the Moon’s polar regions, although in relatively small amounts. This discovery complements the earlier conclusion that ice exists on Mercury. The water on the Moon could be used to support human colonies or to synthesize hydrogen fuel from a Moon base for exploration of more distant planets. 2

23-4 THE JOVIAN PLANETS: SIZE, COMPOSITIONS, AND ATMOSPHERES 23-4a JupiterJupiter is the largest planet in the Solar System, 71,000 kilometers in radius. It is composed mainly of hydrogen and helium, similar to the composition of the Sun. However, Jupiter is not quite massive enough to generate fusion temperatures, so it never became a star.

Jupiter has no hard, solid, rocky crust on which an astronaut could land or walk. Instead, its surface is a vast sea 12,000 kilometers deep, made of cold, liquid molecular hydrogen ( H 2 ) and atomic helium ( He ) . Beneath the hydrogen/helium sea is a layer where temperatures are as high as 30 , 000 ° C and pressures are as great as 100 million times the Earth’s atmospheric pressure at sea level ( Figure 23.14 ). Under these extreme conditions, hydrogen molecules dis-sociate to form atoms. Pressure forces the atoms together so tightly that the electrons move freely throughout the packed nuclei, much as electrons travel freely among metal atoms. As a result, the hydrogen conducts electricity and is called liquid metallic hydrogen . Flow patterns in this fluid con-ductor generate a magnetic field 10 times stronger than that of Earth.

23-3b History of the Moon The Moon formed about 4.45 to 4.5 billion years ago, shortly after Earth. So much energy was released during the Moon’s rapid accretion that, as the lunar sphere grew, it melted to a depth of a few hundred kilome-ters, forming a magma ocean. Meteorite bombardment kept the Moon’s outermost layer mol-ten. Eventually meteorite bom-bardment diminished enough for the Moon’s surface to cool. The

igneous rocks of the lunar highlands are about 4.4 bil-lion years old, indicating that the highlands were solid by that time.

Swarms of meteorites, some as large as Rhode Island, bombarded the Moon again between 4.2 and 3.9 billion years ago ( Figure 23.13B ). In the meantime, radioactive decay was also heating the lunar interior. As a result, by 3.8 billion years ago, most of the Moon’s interior was molten and magma erupted onto the lunar surface. The maria formed when lava filled circular meteorite craters ( Figure 23.13C ). This episode of volcanic activity lasted approximately 700 million years. The Moon and Earth shared a similar history until this time, but the Moon is so much smaller that it soon cooled and has remained geologically inactive for the past 3.1 billion years.

Apollo astronauts left seismographs on the lunar surface. Initial analysis of the data indicated that the

Red spot

Atmosphere

Earth

Liquidmetallichydrogen

Rockymetalliccore

Liquidmolecularhydrogenand atomichelium

FIGURE 23.14 Jupiter consists of four main layers: a turbulent

atmosphere, a sea consisting mainly of liquid molecular hydrogen with

smaller amounts of atomic helium, a layer of liquid metallic hydrogen,

and a rocky metallic core. Earth is drawn to scale on the left.

Jupiter The largest

planet in the Solar System

and fifth from the Sun.

liquid metallic hydrogen A form of

hydrogen under extreme

temperature and pressure,

which forces the atoms

together so tightly that

the electrons move freely

throughout the packed

nuclei, and as a result

the hydrogen conducts

electricity.

2. Keep in mind that “water on the Moon” does not refer to seas or even puddles. It means molecules of water and hydroxyl that interact with molecules of rock and dust in the top millimeters of the Moon’s surface.

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outer atmosphere, where the pressure was 0.4 bar, the Galileo probe measured a wind speed of 360 kilo-meters per hour with a temperature of − 140 ° C . After falling 130 kilometers, the probe reported that the wind speed had increased to 650 kilometers per hour, the pressure was 22 bar, and the temperature was about

Beneath the layer of metallic hydrogen, Jupiter’s core is a sphere about 10 to 20 times as massive as Earth’s. It is probably composed of metals and rock surrounded by lighter elements such as carbon, nitro-gen, and oxygen.

The Galileo spacecraft was launched in October 1989 and rendezvoused with Jupiter in December 1995. Once in orbit around the gas giant, the space-craft launched a suicide probe to parachute through the outer atmosphere and collect data until it heated up and eventually vaporized. As expected, the atmosphere was primarily hydrogen and helium, with smaller con-centrations of ammonia, water, and methane.

More than 300 years ago, two European astrono-mers reported seeing what came to be called the Great Red Spot on the surface of Jupiter. Although its size, shape, and color have changed from year to year, the spot remains intact to this day ( Figure 23.15 ). If the Earth’s crust were peeled off like an orange rind and laid flat on the Jovian surface, it would fit entirely within the Great Red Spot. Measurements show that the Great Red Spot is a giant hurricane-like storm. Hurricanes on Earth dissipate after a week, yet this storm on Jupiter has existed for centuries. Other wind systems, rotating in linear bands around the planet, have also persisted for centuries ( Figure 23.16 ).

One important mission of the Galileo suicide probe was to measure atmospheric pressures, temperatures, and wind speeds below the visible outer layers. In the

FIGURE 23.15 Jupiter’s Great Red Spot dwarfs the superimposed image of the Earth (to scale) shown on the

bottom right. The vivid colors were generated by computer enhancement.

NA

SA

Great Red Spot A giant hurricane-like

storm on the surface of

Jupiter that has existed for

centuries.

FIGURE 23.16 This colorful, turbulent complex cloud system of

Jupiter was photographed by the Voyager spacecraft. The sphere on

the left, in front of the Great Red Spot, is Io; Europa lies to the right

against a white oval.

NA

SA/J

PL


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( 1 10 16 ) . It took 38 radio antennas on four continents to absorb enough radio energy to interpret the signals.

Both Uranus and Neptune are enveloped by thick atmospheres composed primarily of hydrogen and helium, with smaller amounts of carbon, nitrogen, and oxygen compounds. Beneath the atmosphere, their outer layers are molecular hydrogen, but neither body is massive enough to generate liquid metallic hydro-gen. Their interiors are composed of methane, ammo-nia, and water, and the cores are probably a mixture of rock and metals. Uranus and Neptune are denser than Jupiter and Saturn, because these outermost giants contain relatively larger, solid cores.

Spacecraft and the Hubble Space Telescope have revealed rapidly changing weather on Uranus and Neptune. On Neptune, winds of at least 1,100 kilome-ters per hour rip through the atmosphere, clouds rise and fall, and one region is marked by a cyclonic storm system called the Great Dark Spot, similar to Jupiter’s Great Red Spot. According to one controversial hypothesis, under the intense pressure near the core, methane decomposes into carbon and hydrogen and the carbon then crystallizes into diamond. Convection currents carry the heat released during the formation of diamond to the planet’s surface to power the winds.

Voyager II recorded that the magnetic field of Uranus is tilted 58 ° from its axis. This was unexpect-ed, as current explanations suggest that the magnetic fields of all planets should be roughly aligned with the spin axis. At first, scientists thought that Voyager II just happened to pass Uranus during a magnetic field reversal. However, Voyager II later recorded that the magnetic field on Neptune is tilted 50 ° from its axis. Because the probability of catching two planets during magnetic reversals is extremely low, there must be another explanation. However, at present no satisfac-tory hypothesis has been developed.

23-5 MOONS OF THE JOVIAN PLANETS 23-5a The Moons of Jupiter In 1610, Galileo discovered four tiny specks of light orbiting Jupiter. He reasoned that they must be satel-lites of the giant planet. By 1999, astronomers had identified 16 moons orbiting Jupiter. By 2009, 62 were known, although at least 52 are relatively small, with irregular orbits. The four discovered by Galileo, referred to as the Galilean moons , are the largest and most widely studied: Io, Europa, Ganymede, and Callisto.

Io The innermost moon of Jupiter, Io , is about the size of Earth’s Moon and is slightly denser. Because it is too small to have retained heat generated during its

+ 150 ° C . Buffeted by winds, squeezed by intense pressure, and heated beyond the tolerance of its electronics, the spacecraft stopped transmitting. Scientists calculate that 40 minutes later, when the spacecraft had sunk to deeper levels of the Jovian atmosphere, the temperature had increased to 650 ° C , the pressure increased to 260 bar, and the aluminum shell liquefied. As the probe continued to fall, pressure and temperature rose until the titanium hull melted; streaming metallic droplets flashed into vapor as the probe vanished.

On Earth, winds are driven by the Sun’s heat. But Jupiter receives only about 4 percent of the solar energy that Earth receives. Moreover, strong winds rip through the Jovian atmo-sphere far below the deepest penetration of solar light and heat. For these reasons, scientists deduce that the Jovian weather is not driven by solar heat but by heat from deep within the planet itself. This heat accumulated when Jupiter first formed. As the heat slowly rises, it warms the lower atmosphere, which then transmits heat by convection to higher levels. The weather systems are stable because the planet’s interior heat flux changes only over hundreds to thou-sands of years.

23-4b SaturnSaturn , the second-largest planet, is similar to Jupiter. It has the lowest density of all the planets, so low that the entire planet would float on water if there were a basin of water large enough to hold it. Such a low density implies that, like Jupiter, it must be composed primarily of hydrogen and helium, with a relatively small core of rock and metal. In fact, in many ways Saturn and Jupiter are alike. For example, Saturn’s atmosphere is similar to that of Jupiter. Dense clouds and great storm systems envelop the planet.

23-4c Uranus and NeptuneUranus and Neptune are so distant and faint that they were unknown to ancient astronomers. The Voyager II spacecraft, launched in 1977, flew by Jupiter in 1979 and by Saturn in 1981. It encountered Uranus by 1986 and Neptune in 1989. The journey from Earth to Neptune covered 7.1 billion kilometers and took 12 years. The craft passed within 4,800 kilometers of Neptune’s cloud tops, only 33 kilometers from the planned path. The strength of the radio signals received from Voyager measured 1 ten-quadrillionth of a watt

Saturn The

second-largest planet and

sixth from the Sun; marked

by its distinctiverings.

Uranus The seventh

planet from the Sun; similar

to Neptune in size, compo-

sition, and atmosphere.

Neptune The eighth

planet from the Sun; similar

to Uranus in size, composi-

tion, and atmosphere.

Io The innermost moon of

Jupiter and the most active

volcanic body in the Solar

System.


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Galileo spacecraft transmitted images showing a fractured, jum-bled, chaotic terrain resembling patterns created by Arctic ice on Earth ( Figure 23.18 ).

Figure 23.19 shows a smooth region overlying an older, wrinkled surface. Data suggest that this smooth region is young ice formed when liquid water erupted to the surface and froze. Thus, pools or oceans of liquid water or a water/ice slurry prob-ably lies beneath the surface ice. Astronomers estimate that this surface crust is 10 kilometers thick in many regions. Calculations show that the subterranean oceans are warmed by tidal effects similar to, though weaker than, those that cause Io’s volcanism. Scientists spec-ulate that the chemical and physical environment in these subterranean oceans is favorable for life. There is no evidence whatsoever that life actually exists there, but the possibility is tantalizing.

Ganymede and Callisto The Galileo spacecraft measured a magnetic field on Ganymede , indicating that this moon has a convect-ing, metallic core. Other measurements imply that the core is surrounded by a silicate mantle covered by a water/ice crust ( Figure 23.20 ). The surface ice is so cold that it is brittle and behaves much like rock. Photographs show two terrains on Ganymede: one is densely cratered, and the other contains fewer craters but many linear grooves. The cratered regions were formed by ancient meteorite storms. The grooved

formation or by radioactive decay, many astronomers expected that it would have a cold, lifeless, cratered, Moon-like surface. However, images beamed to Earth from the Voyager spacecraft showed huge masses of gas and rock erupting to a height of 200 kilometers above the satellite’s surface. This was the first evidence of active, extraterrestrial volcanism ( Figure 23.17 ). Images from the later Galileo spacecraft showed 100 volcanoes erupting simultaneously, making Io the most active volcanic body in the Solar System.

Recall that the gravitational field of our Moon causes the rise and fall of ocean tides on Earth. At the same time, Earth’s gravity distorts lunar rock. Thus, Earth’s gravitation is responsible for deep-focus moonquakes. Jupiter is 300 times more massive than Earth, so its gravitational effects on Io are cor-respondingly greater. In addition, the three nearby satellites—Europa, Ganymede, and Callisto—are large enough to exert significant gravitational forces on Io, but these forces pull in directions different from that of Jupiter. This combination of oscillating and oppos-ing gravitational forces causes so much rock distortion and frictional heating that volcanic activity is nearly continuous on Io.

Astronomers infer that meteorites bombarded Io and the other moons of Jupiter, as they did all other bodies in the Solar System. Yet, the frequent lava flows on Io have obliterated all ancient landforms, giv-ing it a smooth and nearly crater-free surface.

Europa The second-closest of Jupiter’s moons, Europa , is similar to Earth in that much of its interior is com-posed of rock and much of its surface is covered with water. One major difference is that, on Europa, the water is frozen into a vast, planetary ice crust. The

FIGURE 23.17 Voyager I captured an image of a volcanic

explosion on Io (shown on the horizon). The eruption is ejecting solid

material to an altitude of about 200 kilometers.

NA

SA

Europa The second-

closest of Jupiter’s moons;

similar to Earth in that

much of its interior is

composed of rock and much

of its surface is covered

with water, although the

water is frozen into a vast

planetary ice crust.

Ganymede A moon of

Jupiter marked by a con-

vecting, metallic core and a

brittle water/ice crust that

behaves much like rock.

FIGURE 23.18 This jumbled terrain on Europa resembles Arctic

pack ice as it breaks up in the spring. Scientists estimate that in

this region, the ice crust is a few kilometers thick and is floating on

subsurface water.

NA

SA/J

PL


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Ganymede or the Earth’s Moon. Perhaps they have been modified by ice flowing slowly across its sur-face. Recent measurements indicate that a subter-ranean ocean may exist on Callisto, but the jury is still out on a similar feature on Ganymede.

23-5b Saturn’s Moons At least 62 moons orbit Saturn. (Two of these were discovered in 2009, and more may be detected by the time you read this book.) Saturn’s largest moon, Titan , is larger than the planet Mercury. Titan is unique because it is the only moon in the Solar System with an appreciable atmosphere. This atmosphere has been retained because Titan is relatively massive and extremely cold. The major constituents are nitrogen, mixed with methane (CH 4 ) and smaller concentrations of trace gases. The average temperature on the surface of Titan is − 178 ° C , and the atmospheric pressure is 1.5 times greater than that on Earth’s surface. These conditions are close to the temperatures and pressures at which methane can exist as a solid, liquid, or vapor.

After a seven-year journey, the Cassini space-craft reached Titan in December 2004, and released a probe named Huygens that parachuted through the outer clouds and landed on the surface. Images showed a surprisingly Earth-like landscape, with steep-sided hills and features that looked like

regions probably developed when the crust cracked and water from the warm interior flowed over the surface and froze, much as lava flowed over the surfaces of the terrestrial planets and the Moon. Lateral displacements of the grooves and ridges are indicative of Earth-like, horizontal, plate tec-tonic activity ( Figure 23.21 ).

Callisto , the outermost Galilean moon, is heav-ily cratered, indicating that its surface is very old. Its craters are shaped differently from those on either

Callisto Jupiter’s

outermost Galilean moon;

marked by a heavily

cratered surface.

Titan Saturn’s largest

moon; the only moon in

the Solar System with an

appreciable atmosphere.

FIGURE 23.19 The smooth, circular region in the center-left

of this photograph was formed when subsurface water rose to the

surface of Europa and froze, covering older wrinkles and fractures in

the crust.

NA

SA/J

PL

2,640 km

~1,800 km

~800 km

Ice

Possiblewaterocean

Liquid outercore (iron sulfide)

Solid iron innercore

High-pressurewater/ice phases

Silicatemantle

FIGURE 23.20 Recent data suggest that Ganymede has a

conducting, convecting core; a silicate mantle; and surface layers

consisting of ice and water.

FIGURE 23.21 A close-up of a young terrain on Ganymede shows

numerous grooves less than a kilometer wide. One likely explanation

is that these grooves were formed by recent tectonic activity.

NA

SA/J

PL

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to form more complex organic molecules. These organic com-pounds do not decompose at low temperature, so the satellite’s surface is likely to be covered by a tarlike organic goo. It is possible that a similar layer collected on early Earth and later underwent chemical reactions to form life. However, Titan is so cold that life probably has not formed there.

23-5c The Moons of Uranus and Neptune Twenty-seven known moons orbit Uranus. Several of the moons are small and irregularly shaped, indicating that they may be debris from a collision with a smaller planet or moon.

Neptune has at least 13 moons. The largest is Triton , which is about 75 percent rock and 25 percent ice. Like many other planets and moons, its surface is covered by impact craters, mountains, and flat, crater-free plains. While the maria on Earth’s Moon are blanketed by lava, those on Triton are filled with ice or frozen methane.

23-6 PLANETARY RINGS Although all the Jovian planets have one or more rings, by far the most spectacular are those of Saturn, which are visible from Earth even through a small tele-scope. Photographs from space probes show seven major rings, each containing thousands of smaller ringlets ( Figures 23.23 and 23.24 ). The entire ring

riverbeds, eroded hillsides, coastlines, and sandbars. The best evidence indicates that these topographic features were formed by wind, tectonic activity, and flowing liquids ( Figure 23.22 ). During the extreme cold on Titan, water is permanently locked up as ice, but the temperature and pressure are such that methane in the Titan atmosphere could exist in the liquid, vapor, or solid states. Thus in the past, methane rain has fallen from the clouds and methane rivers flowed across Titan’s surface. Liquid methane may remain on the planetary surface today. This situation is analogous to Earth’s environment, where water can exist as liquid, gas, or solid and frequently changes among those three states.

Methane, the simplest organic compound, reacts with nitrogen and other materials in Titan’s environment

FIGURE 23.22 Radar images of Titan obtained in February 2005

show a well-developed drainage pattern in the lower-right of the

image, and apparent sand dunes. Previous images show features that

may have been formed by tectonic processes. Titan appears to have a

young and dynamic surface that is modified by volcanism, tectonism,

erosion, and impact cratering.

NA

SA/J

PL

FIGURE 23.23 An image of Saturn shows its spectacular ring

system.

NA

SA/J

PL

Triton The largest of

Neptune’s moons; marked

by craters filled with ice or

frozen methane.

FIGURE 23.24 Voyager I took this color-enhanced close-up view

of Saturn’s rings and ringlets.

NA

SA/J

PL


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of Pluto’s diameter. By measuring the orbits of these two bodies, astronomers determined the densities of Pluto and Charon, deducing that each contain about 35 percent ice and 65 percent rock.

Infrared measurements show that Pluto’s sur-face temperature is about − 220 ° C . Spectral analysis of Pluto’s bright surface shows that it contains fro-zen methane. Its atmosphere is extremely thin and composed mainly of carbon monoxide, nitrogen, and some methane.

Pluto’s orbit is highly elliptical, at times bring-ing it closer to the Sun than Neptune is. It may be related to numerous similar icy bodies that are part of the Kuiper Belt. These Kuiper Belt objects are residual planetesimals left over from the formation of the Solar System. One Kuiper Belt object, named Eris, is 5 percent larger than Pluto. Three other Kuiper Belt objects found so far are at least half the size of Pluto and may have moons of their own. These bod-ies, while having enough gravitational force to pull themselves into spherical shapes, cannot be consid-ered planets because they do not meet one of the International Astronomical Union’s criteria: they are not large enough to dominate and gravitationally clear their orbital regions of all or most other objects. The International Astronomical Union has classified them as prototypes of new objects called dwarf planets .

23-8 ASTEROIDS, COMETS, AND METEOROIDS 23-8a Asteroids Astronomers have discovered a wide ring between the orbits of Mars and Jupiter that contains tens of thousands of small orbiting bodies, called asteroids. The largest asteroid, Ceres, has a diameter of 930 kilometers. Three others are about half that size, and most are far smaller. The orbit of an asteroid is not permanent like that of a planet. If an asteroid passes near a planet without getting too close, the planet’s

system is only 10 to 25 meters thick, less than the length of a football field. However, the ring system is extremely wide. The innermost ring is only 7,000 kilo-meters from Saturn’s surface, whereas the outer edge of the most distant ring is 432,000 kilometers from the planet, a dis-tance greater than that between Earth and our Moon. Thus, the ring system measures 425,000 kilometers from its inner to its outer edge. A scale model of the ring system with the thickness of a compact disk would be 30 kilometers in diameter.

Saturn’s rings are composed of dust, rock, and ice. The par-ticles in the outer rings are only a few ten-thousandths of a cen-timeter in diameter (about the size of a clay particle), but the innermost rings contain chunks as large as a flying barn. Each

piece orbits the planet independently. Saturn’s rings may be fragments of a moon that

never coalesced. Alternatively, they may be the rem-nants of a moon that formed and was then ripped apart by Saturn’s gravitational field. If a moon were close enough to its planet, the tidal effects would be greater than the gravitational attraction holding the moon together, and it would break up. Thus, a solid moon cannot exist too close to a planet. Images from Cassini spacecraft show that gravitational forces from Saturn’s moons have herded the ring particles into intricate spirals and twists.

23-7 PLUTO AND OTHER DWARF PLANETS In recent years, Pluto has been a controversial figure in our Solar System. Although NASA’s New Horizons spacecraft will reach Pluto in 2015, no spacecraft has yet visited Pluto, and our highest-resolution photo-graphs are of poor quality compared with those of other planets ( Figure 23.25 ). Until 2006, Pluto was considered the ninth planet in our Solar System. However, in 2006 the International Astronomical Union voted to remove Pluto from the list of planets. It has been grouped with similar space objects as a dwarf planet —a new classification to be described at the end of this section.

Pluto has three moons: Nix, Hydra, and Charon. Nix and Hydra are very small but Charon is nearly half

Pluto Once considered to

be the ninth planet from the

Sun in our Solar System,

reclassified in 2006 as a

dwarf planet.

Kuiper Belt objects Tiny

ice dwarfs, similar to Pluto,

orbiting in a disk-shaped

region at the outer reaches of

the Solar System.

dwarf planets A body

(for example, Pluto or Eris)

that orbits the Sun, is not a

satellite of a planet, and is

massive enough to pull itself

into a spherical shape, but is

not massive enough to clear

out other bodies in and near

its orbit.

asteroids Small celestial

bodies in orbit around the

Sun, primarily in the region

between Mars and

Jupiter.

Ground-based telescope Hubble Space Telescope

FIGURE 23.25 (A) Ground-based image of Pluto and its moon

Charon and (B) a similar view from the Hubble Space Telescope.

SPA

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of methane, ammonia, carbon dioxide, and other compounds. Smaller concentrations of dust particles, composed of silicate rock and metals, are mixed with the lighter ices.

When a comet is millions of kilometers from the Sun, it is a ball without a tail. As the comet approaches the Sun and is heat-ed, some of its surface vapor-izes. Solar wind blows some of the lighter particles away from the comet’s head to form a long tail. At this time, the comet con-sists of a dense, solid nucleus , a bright outer sheath called a coma , and a long tail ( Figure 23.27 ). Some comet tails are more than 140 million kilometers long, almost as long as the dis-tance from Earth to the Sun. As a comet orbits the Sun, the solar wind constantly blows the tail so that it always extends away from the Sun. By terrestrial standards, a comet tail would represent a good, cold laboratory vacuum—yet viewed from a celestial perspective it looks like a hot, dense, fiery arrow.

Halley’s comet passed so close to Earth in 1910 that its visit was a momentous event. When the comet returned to the inner Solar System in 1986, it was studied by six spacecraft as well as by several ground-based observatories. Its nucleus is a peanut-shaped mass approximately 16 -by - 8 -by - 8 kilometers, about the same size and shape as Manhattan Island. The cold, relatively dense coma of Halley’s comet had a radius of about 4,500 kilometers when it passed by.

In one spectacular experiment conducted in 2005, astronomers fired Deep Impact, a 372 -kilogram metal probe, into comet Tempel 1 , while a mother ship recorded the effects of the impact. The probe triggered two quick flashes, the first when the col-lision heated the surface to thousands of degrees, and the second, a few milliseconds later, when the now-molten probe penetrated deeper into the nucleus and ejected a layer of volatile material ( Figure 23.28 ). Photographs taken just prior to the collision reveal a landscape sculpted by outgassing, melting, and natural impacts—all evidence of a complex geologi-cal history. Analysis of the impact ejecta suggest that comet Tempel 1 was composed of fine dust, more like talcum powder than beach sand. There were no large chunks, so the comet did not have a solid ice crust.

gravity pulls the asteroid out of its current orbit and deflects it into a new orbit around the Sun. Thus, an asteroid may change its orbit frequently and errati-cally. Many asteroids orbit near Earth, and some even cross Earth’s orbit. If an asteroid passes too close to a planet, it will crash into its surface. As discussed previously, ancient asteroid impacts may have caused mass extinctions on Earth. Statistical analysis shows that there is a 1 percent chance that an asteroid with a diameter greater than 10 kilometers will strike Earth in the next 1,000 years. Such an impact would kill bil-lions and could possibly cause the extinction of the human race.

One series of images of the asteroid Mathilde show an impact crater larger than the asteroid’s mean radius. How could an object sustain such an impact without breaking apart? According to one hypothesis, Mathilde is not solid rock, as Earth is. Instead it is a compressed mass of fractured rock and rubble. Thus, the bolide impact scattered the fractured rock with-out transmitting force through a solid, brittle body. However, craters, grooves, and surface rocks on the nearby asteroid Eros indicate that it is a solid body.

23-8b Comets Occasionally, a glowing object appears in the sky, travels slowly around the Sun in an elongated elliptical orbit, and then disappears into space ( Figure 23.26 ). Such an object is called a comet , after the Greek word for “long-haired.” Despite their fiery appearance, com-ets are cold, and their light is reflected sunlight.

Comets originate in the outer reaches of the Solar System, and much of the time they travel through the cold void beyond Pluto’s orbit. A comet is com-posed mainly of water/ice mixed with frozen crystals

FIGURE 23.26 Hale–Bopp was the brightest comet seen from

Earth in decades. It was brightest in March and April of 1997.

GE

OFF

CH

EST

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/ comet An interplanetary

orbiting body composed

of loosely bound rock and

ice, which forms a bright

head and extended fuzzy

tail when it approaches the

Sun. It appears to be fiery

hot, but it is actually a cold

object and its “flame” is

reflected light.

nucleus The dense, solid

core of a comet.

coma The bright outer

sheath of a comet, sur-

rounding the nucleus.

tail The long trailing por-

tion of a comet, always

pointing away from the

Sun, formed when solar

winds blow away lighter

particles from the comet’s

head. What appears to be

a fiery arrow is actually

reflected light from the Sun.


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FIGURE 23.28 The eight images depict the development of the ejecta plume formed when Deep Impact’s probe collided with

comet Tempel 1 on July 3, 2005. The red arrows in images 3 and 7 highlight shadows cast by the ejecta. The yellow arrow on

image 8 indicates the zone of avoidance in the up range direction. The eight images were spaced 0.84 second apart.

Direction of

travel of comet

TO SUN

Magnetic field lines

Magnetic

barrier

Nucleus

Coma

Tail

FIGURE 23.27 The nucleus of this comet has been enlarged several thousand times to show detail. When the comet interacts with the solar

wind, magnetic field lines are generated as shown. Ions produced from gases streaming away from the nucleus are trapped within the field,

creating the characteristically shaped tail.

NA

SA/J

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in Earth. Therefore, geologists think that meteorites reflect the primordial composition of the Solar System and are windows into our Solar System’s past. Most stony meteorites contain small grains about 1 millimeter in diameter called chondrules , which contain organic molecules, including amino acids, the build-ing blocks of proteins.

Some meteorites are metal-lic and consist mainly of iron and nickel, the elements that make up Earth’s core, while the remainder are stony-iron, containing roughly equal quanti-ties of silicates and iron-nickel. Some of our knowledge of Earth’s mantle and core comes from studying meteorites, which may be similar to the mantles and cores of other planetary bodies. With the exception of rocks returned from the Apollo Moon missions, meteorites are the only physical samples we have from space.

B

In the near-surface interior, the space probe detected organic compounds, which could form the basis for living organisms.

23-8c Meteoroids As tens of thousands of asteroids race through the Solar System in changing paths, many collide and break apart, forming smaller fragments and pieces of dust. A meteoroid is an asteroid or a fragment of a comet that orbits through the inner Solar System. If a meteoroid travels too close to Earth’s gravita-tional field, it falls. Friction with the atmosphere heats it until it glows. To our eyes it is a fiery streak in the sky, which we call a meteor or, colloquially, a shooting star. Most meteors are barely larger than a grain of sand when they enter the atmosphere and vaporize completely during their descent. Larger ones, however, may reach Earth’s surface. A meteor that strikes Earth’s surface is called a meteorite ( Figure 23.29 ).

Most meteorites are stony meteorite and are composed of 90 percent silicate rock and 10 percent iron and nickel. The 90 : 10 mass ratio of rock to metal is similar to the mass ratio of the mantle to the core

FIGURE 23.29 Scientists believe that this meteorite is a fragment

from the asteroid Vesta.

NA

SA

meteoroid A small

interplanetary body, most

often an asteroid or comet

fragment, traveling in an

irregular orbit through the

inner Solar System.

meteor A falling mete-

oroid that enters Earth’s

atmosphere and glows as

it vaporizes; colloquially

called a shooting star .

meteorite A meteor that

does not completely vapor-

ize and that strikes Earth’s

surface.

stony meteorite A meteorite with a mass

ratio of rock to metal similar

to the mass ratio of Earth‘s

mantle to Earth‘s core, thus

reflecting the primordial com-

position of the Solar System

and representing a window

into its past. Most meteorites

are stony meteorites.

chondrules A small

grain about 1 millimeter in

diameter embedded in a

meteorite, often contain-

ing amino acids or other

organic molecules.

STUDY TOOLS

In the BookTear Out the Review Card on

Planets and their Moons

OnlineVisit www.cengagebrain.com for additional resources, including:

◻ Interactive Quizzing

◻ Flashcards

◻ Videos


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42246 ch23 ptg01 hr 466-487faculty.ccbcmd.edu/courses/ersc101c/unit1/ch23_the_solar_system.pdf ·...

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