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�6The Solar SystemAn Introduction to Comparative Planetology

In less than a single generation, we have learnedmore about the solar system—the Sun andeverything that orbits it—than in all the centuries

that went before. By studying the planets, theirmoons, and the countless fragments of material thatorbit in interplanetary space, astronomers have gaineda richer outlook on our own home in space.Instruments aboard unmanned robots have takenclose-up photographs of the planets and their moonsand in some cases have made on-site measurements.The discoveries of the past few decades haverevolutionized our understanding not only of ourpresent cosmic neighborhood, but also of its history,for our solar system is filled with clues to its own originand evolution.

LEARNING GOALS

Studying this chapter will enable you to

Discuss the importance of compara-tive planetology to solar systemstudies.

Describe the overall scale and struc-ture of the solar system.

Summarize the basic differences be-tween the terrestrial and the jovianplanets.

Identify and describe the major non-planetary components of the solarsystem.

Describe some of the spacecraft mis-sions that have contributed signifi-cantly to our knowledge of the solarsystem.

Outline the theory of solar-systemformation that accounts for theoverall properties of our planetarysystem.

Account for the differences be-tween the terrestrial and the jovianplanets.

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Visit astro.prenhall.com/chaisson for additional annotatedimages, animations, and links to related sites for this chapter.

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The engineering feats of the modern space age allow us to probe many of the di-verse worlds of the solar system in great detail. Here, we see a view of Mars’s land-scape from the Opportunity rover, a golf-cart-sized robot delivered to the(obviously) red planet by the Mars Exploration spacecraft that landed in a small,unnamed crater near the Martian equator in early 2004. The advantage of endingup in a crater is that geologists on Earth can explore what’s below the surfacewithout having to dig. (JPL)

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Section 6.1 | An Inventory of the Solar System 145144 CHAPTER 6 | The Solar System

6.1 An Inventory of the SolarSystem

The Greeks and other astronomers of old were aware ofthe Moon, the stars, and five planets—Mercury, Venus,Mars, Jupiter, and Saturn—in the night sky. • (Sec. 2.2)They also knew of two other types of heavenly objects thatwere clearly neither stars nor planets. Comets appear aslong, wispy strands of light in the night sky that remainvisible for periods of up to several weeks and then slowlyfade from view. Meteors, or “shooting stars,” are suddenbright streaks of light that flash across the sky, usually van-ishing less than a second after they first appear. Thesetransient phenomena must have been familiar to ancientastronomers, but their role in the “big picture” of the solarsystem was not understood until much later.

Human knowledge of the basic content of the solarsystem remained largely unchanged from ancient timesuntil the early 17th century, when the invention of the tel-escope made more detailed observations possible. GalileoGalilei was the first to capitalize on this new technology.(His simple telescope is shown in Figure 6.1.) Galileo’s dis-covery of the phases of Venus and of four moons orbitingJupiter early in the 17th century helped change forever hu-mankind’s vision of the universe. • (Sec. 2.4)

As technological advances continued, knowledge ofthe solar system improved rapidly. Astronomers began dis-covering objects invisible to the unaided human eye. Bythe end of the 19th century, astronomers had found Sat-

urn’s rings (1659), the planets Uranus (1758) and Neptune(1846), many planetary moons, and the first asteroids—“minor planets” orbiting the Sun, mostly in a broad band(called the asteroid belt) lying between Mars and Jupiter.Ceres, the largest asteroid and the first to be sighted, wasdiscovered in 1801. A large telescope of mid-19th-centuryvintage is shown in Figure 6.2.

The 20th century brought continued improvements inoptical telescopes. One more planet (Pluto) was discovered,along with three more planetary ring systems, dozens ofmoons, and thousands of asteroids. The century also sawthe rise of both nonoptical—especially radio and infrared—astronomy and spacecraft exploration, each of which hasmade vitally important contributions to the field of plane-tary science. Astronauts have carried out experiments onthe Moon (see Figure 6.3), and numerous unmannedprobes have left Earth and traveled to all but one (Pluto) ofthe other planets. Figure 6.4 shows a view from the Spiritrobot having just landed on the Martian surface in 2004, itsparachute and airbags still surrounding the vehicle.

As currently explored, our solar system is known tocontain one star (the Sun), nine planets, 135 moons (at lastcount) orbiting those planets, six asteroids larger than 300km in diameter, tens of thousands of smaller (but well-studied) asteroids, myriad comets a few kilometers in di-ameter, and countless meteoroids less than 100 m across.The list will undoubtedly grow as we continue to exploreour cosmic neighborhood.

As we proceed through the solar system in thenext few chapters, we will seek to understand

how each planet compares with our own and what eachcontributes to our knowledge of the solar system as a

1

� FIGURE 6.2 Nineteenth-Century Telescope By the mid-19th century,telescopes had improved enormously in both size and quality. Shown here is theNewtonian reflector built and used by Irish nobleman and amateur astronomerthe Earl of Rosse. For 75 years, this 72-inch-diameter instrument was the largesttelescope on Earth. (Birr Scientific & Heritage Foundation)

� FIGURE 6.3 Lunar Exploration An Apollo astronautdoes some lunar geology—prospecting near a hugeboulder in the Mare Serenitatis during the final mannedmission to the Moon in 1972. (NASA)

whole. We will use the powerful perspective ofcomparative planetology—comparing and contrastingthe properties of the diverse worlds we encounter—to un-derstand better the conditions under which planets formand evolve.

Our goal will be to develop (starting here, and con-cluding in Chapter 15) a comprehensive theory of the ori-gin and evolution of our planetary system—a theory thatexplains all, or at least most, of the solar system’s observedproperties. We will seek to answer basic questions such asWhy did planet X evolve in one way, while planet Y turnedout completely different and why are the planets’ orbits soorderly when their individual properties are not? In ad-dressing these issues, we will find many similarities andcommon features among planets. However, each planetwill also present new questions and afford unique insightsinto the ways planets work.

As we unravel the origin of our solar system, we mayhope to learn something about planetary systems beyondour own. Since the mid-1990s, astronomers have detectedmore than 100 extrasolar planets—planets orbiting starsother than our own Sun. Many new planets are discoveredeach year (see Chapter 15), and our observations of themprovide critical tests of modern theories of planet forma-tion. Before the discovery of extrasolar planets, those the-ories had necessarily been based on observations only ofour own solar system. Now astronomers have a whole newset of “proving grounds” in which to compare theory withreality.

Curiously, current data suggest that many of the newlydiscovered systems have properties rather different fromthose of our own, adding fuel to the long-standing debateamong astronomers on the prevalence of planets like Earthand the possible existence of life as we know it elsewhere in

the universe. It will be some time before astronomers canmake definitive statements about the existence (or nonex-istence) of planetary systems like our own.

CONCEPT CHECK

✔ In what ways might observations of extrasolarplanets help us understand our own solarsystem?

� FIGURE 6.1 Early TelescopeThe refracting telescope withwhich Galileo made his firstobservations was simple, but itsinfluence on astronomy wasimmeasurable. (Museo della Scienza;Scala/Art Resource, NY)

� FIGURE 6.4 Spirit on Mars The Mars rover Spirit took this group of images shortlyafter reaching Mars in 2004. The most prominent surface feature on the left, a circulartopographic depression dubbed Sleepy Hollow, may be a small impact crater orperhaps a dried up pond. (JPL)

Section 6.2 | Planetary Properties 147146 CHAPTER 6 | The Solar System

TABLE 6.1 Properties of Some Solar-System Objects

Orbital Orbital Number of Rotation Object Semimajor Axis Period Mass Radius Known Period* Average Density

(A.U.) (Earth Years) (Earth Masses) (Earth Radii) Satellites (days)

Mercury 0.39 0.24 0.055 0.38 0 59 5400 5.4

Venus 0.72 0.62 0.82 0.95 0 5200 5.2

Earth 1.0 1.0 1.0 1.0 1 1.0 5500 5.5

Moon — — 0.012 0.27 — 27.3 3300 3.3

Mars 1.52 1.9 0.11 0.53 2 1.0 3900 3.9

Ceres (asteroid) 2.8 4.7 0.00015 0.073 0 0.38 2700 2.7

Jupiter 5.2 11.9 318 11.2 61 0.41 1300 1.3

Saturn 9.5 29.4 95 9.5 31 0.44 700 0.7

Uranus 19.2 84 15 4.0 27 1300 1.3

Neptune 30.1 164 17 3.9 12 0.67 1600 1.6

Pluto 39.5 248 0.002 0.2 1 2100 2.1

Comet Hale–Bopp 180 2400 0.004 — 0.47 100 0.1

Sun — — 332,000 109 — 25.8 1400 1.4

*A negative rotation period indicates retrograde (backward) rotation relative to the sense in which all planets orbit the Sun.

1.0 * 10-9

-6.4

-0.72

-243

1g/cm321kg/m32

Finally, dividing Jupiter’s mass by the volume of a sphereof radius R, namely, we obtain a density of

This number differs from the figure listed inTable 6.1 because Jupiter is, in reality, not perfectly spherical.It is flattened somewhat at the poles, so our expression for thevolume is not quite correct. When we take the flattening intoaccount, the actual volume is a little lower than the numbergiven here, and the density is correspondingly higher.

We have determined several important physical proper-ties of Jupiter by combining observations made from Earthwith our knowledge of simple geometry and Newton’s laws ofmotion. In fact, Jupiter is perhaps the easiest planet to study inthis way, as it is big, is relatively close, and has several easy-to-see satellites. Nevertheless, these fundamental techniques areapplicable throughout the solar system—indeed, throughoutthe entire universe. Prior to the Space Age, they formed thebasis for virtually all astronomical measurements of planetaryproperties.

1240 kg/m3.

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MORE PRECISELY 6-1

Computing PlanetaryPropertiesLet’s look a little more closely at some of the methods used todetermine the physical properties of a planet. The accompa-nying figure shows the planet Jupiter and Europa, one of itsinner moons. The orbits of both Jupiter and Earth have beenmeasured very precisely so, at any instant, the distance be-tween the two planets is accurately known. However, sinceJupiter’s orbital semimajor axis is 5.2 A.U. and that of Earth is1 A.U., for definiteness in what follows we will simply assumea distance of 4.2 A.U., or about 630,000,000 km. • (Sec. 2.6)This distance corresponds roughly to the point of closest ap-proach between the two planets.

As indicated in the figure, Jupiter’s angular diameter at adistance of 4.2 A. U. from Earth is 46.8 arc seconds. (Recallthat 1 arc second is 1/3600 of a degree.) From • More Pre-cisely 1-3, it follows that the planet’s actual diameter is

so Jupiter’s radius is Jupiter’s moon Europa is observed to orbit the planet

with an orbital period The orbit is circular,with an angular radius of 3.66 arc minutes, as seen from Earth.Converting this quantity to a distance in kilometers, as before,we find that the radius of Europa’s orbit around Jupiter is

The moon thus travels a distance equal tothe circumference of its orbit, million km, in atime The orbital speed of Europais Applying Newton’s laws, we thendetermine Jupiter’s mass to be • (More Precisely 2-3)

M =

rV2

G= 1.9 * 1027 kg.

V = 2pr/P = 13.7 km/s.P = 3.55 days = 307,000 s.

2pr = 4.22r = 671,000 km.

P = 3.55 days.

R = 71,500 km.

= 143,000 km,

= 14.2 * 150,000,000 km2 *

146.8/36002°57.3°

diameter 12R2 = distance *

angular diameter57.3°

R I V U X G

Europa

50"

6.2 Planetary PropertiesTable 6.1 lists some basic orbital and physical properties ofthe nine planets, with a few other solar-system objects (theSun, the Moon, an asteroid, and a comet) included forcomparison. Most of the quantities listed can be deter-mined using methods described in Chapters 1 and 2. Wepresent here some of the simpler techniques for makingthese measurements. Note that the Sun, with more than athousand times the mass of the next most massive object(the planet Jupiter), is clearly the dominant member of thesolar system. In fact, the Sun contains about 99.9 percentof all solar-system material. The planets—including ourown—are insignificant in comparison. More Precisely 6-1makes the methods presented a little more concrete by ap-plying them to observations of the planet Jupiter. Here is abrief summary of the properties listed in Table 6.1 and thetechniques used to measure them:

● The distance of each planet from the Sun is knownfrom Kepler’s laws once the scale of the solar system isset by radar ranging on Venus. • (Sec. 2.6)

● A planet’s (sidereal) orbital period can be measuredfrom repeated observations of its location on the sky,so long as Earth’s own motion around the Sun is prop-erly taken into account.

● A planet’s radius is found by measuring the angularsize of planet—the angle from one side to the other as

we view it on the sky—and then applying elementarygeometry. • (More Precisely 1-3)

● The masses of planets with moons may be calculated byapplying Newton’s laws of motion and gravity, just byobserving the moons’ orbits around the planets. •(More Precisely 2-3) The sizes of those orbits, like thesizes of the planets themselves, are determined bygeometry.

● The masses of Mercury and Venus (as well as those ofour own Moon and the asteroid Ceres) are a littleharder to determine accurately, because these bodieshave no natural satellites of their own. Nevertheless, itis possible to measure their masses by careful observa-tions of their gravitational influence on other planetsor nearby bodies. Mercury and Venus produce small,but measurable, effects on each other’s orbits, as wellas on that of Earth. The Moon also causes small “wob-bles” in Earth’s motion as the two bodies orbit theircommon center of mass.

● These techniques for determining mass were availableto astronomers well over a century ago. Today, themasses of most of the objects listed in Table 6.1 havebeen accurately measured through their gravitationalinteraction with artificial satellites and space probeslaunched from Earth. Only in the case of Ceres is themass still poorly known, mainly because that asteroid’sgravity is so weak.

● A planet’s rotation period may, in principle, be deter-mined simply by watching surface features alternatelyappear and disappear as the planet rotates. However,with most planets, this is difficult to do, as their surfacesare hard to see or may even be nonexistent. Mercury’ssurface features are hard to distinguish, the surface ofVenus is completely obscured by clouds, and Jupiter,Saturn, Uranus, and Neptune have no solid surfaces atall—their atmospheres simply thicken and eventuallybecome liquid as we descend deeper and deeper belowthe visible clouds. We will describe the methods used tomeasure these planets’ rotation periods in later chapters.

● The final two columns in Table 6.1 list the averagedensity of each object. Density is a measure of the

“compactness” of matter. Average density is computedby dividing an object’s mass (in kilograms, say) by itsvolume (in cubic meters, for instance). For example,we can easily compute Earth’s average density. Earth’smass, as determined from observations of the Moon’sorbit, is approximately • (More Pre-cisely 2-3) Earth’s radius R is roughly 6400 km, so its volume is or

• (Sec. 1.7) Dividing Earth’s mass byits volume, we obtain an average density of approxi-mately On average, then, there are about5500 kilograms of Earth matter in every cubic meterof Earth volume. For comparison, the density of ordi-nary water is rocks on Earth’s surfacehave densities in the range and2000–3000 kg/m3,

1000 kg/m3,

5500 kg/m3.

1.1 * 1021 m3.

4�3pR3L 1.1 * 1012 km3,

6.0 * 1024 kg.

(NASA)

Section 6.4 | Terrestrial and Jovian Planets 149148 CHAPTER 6 | The Solar System

iron has a density of some Earth’s atmo-sphere (at sea level) has a density of only a few kilogramsper cubic meter. Because many working astronomersare more familiar with the CGS (centimeter–gram–second) unit of density (grams per cubic cen-timeter, abbreviated where

), Table 6.1 lists density in both SI andCGS units.

CONCEPT CHECK

✔ How do astronomers go about determining thebulk properties (i.e., masses, radii, and densities)of distant planets?

6.3 The Overall Layout of theSolar SystemBy earthly standards, the solar system is immense.The distance from the Sun to Pluto is about 40

A.U., almost a million times Earth’s radius and roughly15,000 times the distance from Earth to the Moon. Yet,despite the solar system’s vast extent, the planets all lievery close to the Sun, astronomically speaking. Even thediameter of Pluto’s orbit is less than 1/1000 of a light-year, whereas the next nearest star is several light-yearsdistant.

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1000 g/cm31 kg/m3

=g/cm3,

8000 kg/m3. The planet closest to the Sun is Mercury. Moving out-ward, we encounter, in turn, Venus, Earth, Mars, Jupiter,Saturn, Uranus, Neptune, and Pluto. In Chapter 2, we sawthe basic properties of the planets’ orbits. Their paths areall ellipses, with the Sun at (or very near) one focus. • (Sec. 2.5) Most planetary orbits have low eccentricities.The exceptions are the innermost and the outermostworlds, Mercury and Pluto. Accordingly, we can reason-ably think of most planets’ orbits as circles centered on theSun. The orbits of the major bodies in the solar system areillustrated in Figure 6.5. Note that the planetary orbits arenot evenly spaced, becoming farther and farther apart aswe move outward from the Sun.

All the planets orbit the Sun counterclockwise as seenfrom above Earth’s North Pole, and in nearly the sameplane as Earth (i.e., the plane of the ecliptic). Mercury andPluto deviate somewhat from the latter condition: Theirorbital planes lie at 7° and 17° to the ecliptic, respectively.Still, we can think of the solar system as being quite flat. Its“thickness” perpendicular to the plane of the ecliptic is lessthan 1/50 the diameter of Pluto’s orbit. If we were to viewthe planets’ orbits from a vantage point in the plane of theecliptic about 50 A.U. from the Sun, only Pluto’s orbitwould be noticeably tilted. Figure 6.6 is a photograph ofthe planets Mercury, Venus, Mars, Jupiter, and Saturntaken during a chance planetary alignment in April 2002.These five planets can (occasionally) be found in the sameregion of the sky, in large part because their orbits lie near-ly in the same plane in space.

CONCEPT CHECK

✔ In what sense is the solar system “flat”?

6.4 Terrestrial and Jovian PlanetsOn large scales, the solar system presents us witha sense of orderly motion. The planets move

nearly in a plane, on almost concentric (and nearly circu-lar) elliptical paths, in the same direction around theSun, at steadily increasing orbital intervals. However,the individual properties of the planets are much lessregular.

Figure 6.7 compares the planets with one another andwith the Sun. A clear distinction can be drawn between theinner and the outer members of our planetary systembased on densities and other physical properties. Theinner planets—Mercury, Venus, Earth, and Mars—aresmall, dense, and rocky in composition. The outerworlds—Jupiter, Saturn, Uranus, and Neptune (but notPluto)—are large, of low density, and gaseous.

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Saturn

Sun

Mercury

Asteroid belt

Venus

Mars EarthJupiter

Uranus

Pluto

Neptune

� FIGURE 6.5 Solar System Might future space voyagers travel farenough from Earth to gain this perspective on our solar system? Exceptfor Mercury and Pluto, the orbits of the planets lie nearly in the sameplane. As we move out from the Sun, the distance between the orbits ofthe planets increases. The entire solar system spans nearly 80 A.U.

Mercury

Venus

Mars

Saturn

Jupiter

� FIGURE 6.6 Planetary Alignment This image shows sixplanets—Mercury, Venus, Mars, Jupiter, Saturn, and Earth—during a planetary alignment in April 2002. Because theplanets all orbit in nearly the same plane, it is possible forthem all to appear (by chance) in the same region of thesky, as seen from Earth. The Sun and the new Moon are justbelow the horizon. As usual, the popular press containedmany sensationalized predictions of catastrophes thatwould occur during this rare astronomical event. Also asusual, none came true. (J. Lodriguss)

Mercury

VenusEarth

MoonMars

Sun

Jupiter

Saturn

Uranus

Neptune

Pluto

� FIGURE 6.7 Sun and PlanetsDiagram, drawn to scale, of therelative sizes of the planets andour Sun. Notice how much largerthe Jovian planets are than Earthand the other terrestrials andhow much larger still is the Sun.

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Section 6.5 | Interplanetary Debris 151150 CHAPTER 6 | The Solar System

Because the physical and chemical properties of Mer-cury, Venus, and Mars are somewhat similar to Earth’s, thefour innermost planets are called the terrestrial planets.(The word terrestrial derives from the Latin word terra,meaning “land” or “earth.”) The larger outer planets—Jupiter, Saturn, Uranus, and Neptune—are all similar toone another chemically and physically (and very differentfrom the terrestrial worlds). They are labeled the jovianplanets, after Jupiter, the largest member of the group.(The word jovian comes from Jove, another name for theRoman god Jupiter.) The jovian worlds are all much largerthan the terrestrial planets and quite different from themin both composition and structure.

The four terrestrial planets all lie within about 1.5A.U. of the Sun. All are small and of relatively low mass,and all have a generally rocky composition and solid sur-faces. Beyond that, however, the similarities end:

● All four terrestrial planets have atmospheres, but theatmospheres are about as dissimilar as we could imag-ine, ranging from a near vacuum on Mercury to a hot,dense inferno on Venus.

● Earth alone has oxygen in its atmosphere and liquidwater on its surface.

● Surface conditions on the four planets are quite dis-tinct from one another, ranging from barren, heavilycratered terrain on Mercury to widespread volcanicactivity on Venus.

● Earth and Mars spin at roughly the same rate—onerotation every 24 (Earth) hours—but Mercury andVenus both take months to rotate just once, and Venusrotates in the opposite sense from the others.

● Earth and Mars have moons, but Mercury and Venusdo not.

● Earth and Mercury have measurable magnetic fields,of very different strengths, whereas Venus and Marshave none.

Finding the common threads in the evolution of these fourdiverse worlds is no simple task! Comparative planetologywill be our indispensable guide as we proceed through thecoming chapters.

Comparing the average densities of the terrestrialplanets allows us to say something about their overallcompositions. However, before making the comparison, wemust take into account how the weight of overlying layerscompresses the interiors of the planets to different extents.When we do this, we find that the uncompressed densities ofthe terrestrial worlds—the densities they would have in theabsence of any compression due to their own gravity—de-crease as we move outward from the Sun: 5300, 4400,4400, and for Mercury, Venus, Earth, andMars, respectively. The amount of compression is greatestfor the most massive planets, Earth and Venus, and much

3800 kg/m3

less for Mercury and Mars. Partly on the basis of these fig-ures, planetary scientists conclude that Earth and Venusare quite similar in overall composition. Mercury’s higherdensity implies that it contains a higher proportion ofsome dense material—most likely nickel or iron. Thelower density of Mars probably means that it is deficient inthat same material.

Yet, for all their differences, the terrestrial worlds stillseem similar compared with the jovian planets. Perhapsthe simplest way to express the major differences betweenthe terrestrial and jovian worlds is to say that the jovianplanets are everything the terrestrial planets are not. Table6.2 compares and contrasts some key properties of thesetwo planetary classes.

The terrestrial worlds lie close together, near the Sun;the jovian worlds are widely spaced through the outer solarsystem. The terrestrial worlds are small, dense, and rocky;the jovian worlds are large and gaseous, made predomi-nantly of hydrogen and helium (the lightest elements),which are rare on the inner planets. The terrestrial worldshave solid surfaces; the jovian worlds have none (theirdense atmospheres thicken with depth, eventually mergingwith their liquid interiors). The terrestrial worlds haveweak magnetic fields, if any; the jovian worlds all havestrong magnetic fields. The terrestrial worlds have onlythree moons among them; the jovian worlds have manymoons each, no two of them alike and none of them likeour own. Furthermore, all the jovian planets have rings, afeature unknown on the terrestrial planets. Finally, all fourjovian worlds are thought to contain large, dense “terres-trial” cores some 10 to 15 times the mass of Earth. Thesecores account for an increasing fraction of each planet’stotal mass as we move outward from the Sun.

(b)

(a)R I V U X GR I V U X G

R I V U X GR I V U X G

� FIGURE 6.8 Asteroid andComet (a) Asteroids, likemeteoroids, are generallycomposed of rocky material. Thisasteroid, Eros, is about 34 kmlong. It was photographed bythe NEAR (Near Earth AsteroidRendezvous) spacecraft thatactually landed on the asteroidin 2001. (b) Comet Hale–Bopp,seen as it approached the Sun in1997. Most comets arecomposed largely of ice and sotend to be relatively fragile. Thecomet’s vaporized gas and dustform the tail, here extendingaway from the Sun for nearly aquarter of the way across thesky. (JHU/APL; J. Lodriguss)

Beyond the outermost jovian planet, Neptune, lies onemore small world, frozen and mysterious. Pluto doesn’t fitwell into either planetary category. Indeed, there is ongo-ing debate among planetary scientists as to whether itshould be classified as a planet at all. In both mass andcomposition, Pluto has much more in common with theicy jovian moons than with any terrestrial or jovian planet.Many astronomers suspect that it may in fact be the largestmember of a recently recognized class of solar system ob-jects that reside beyond the jovian worlds (see Chapter 14).In 1999, the International Astronomical Union, whichoversees the rules for classifications in astronomy, decidedthat Pluto should, for now at least, still be called a planet.However, that status may well change as the makeup of theouter solar system becomes better understood.

CONCEPT CHECK

✔ Why do astronomers draw such a cleardistinction between the inner and the outerplanets?

6.5 Interplanetary DebrisIn the vast space among the nine known majorplanets move countless chunks of rock and ice, all

orbiting the Sun, many on highly eccentric paths. Thisfinal component of the solar system is the collection ofinterplanetary matter—cosmic “debris” ranging in sizefrom the relatively large asteroids, through the smallercomets and even smaller meteoroids, down to the small-est grains of interplanetary dust that litter our cosmicenvironment.

4

The dust arises when larger bodies collide and breakapart into smaller pieces that, in turn, collide again and areslowly ground into microscopic fragments, which eventu-ally settle into the Sun or are swept away by the solar wind,a stream of energetic charged particles that continuallyflows outward from the Sun and pervades the entire solarsystem. The dust is quite difficult to detect in visible light,but infrared studies reveal that interplanetary space con-tains surprisingly large amounts of it. Our solar system isan extremely good vacuum by terrestrial standards, butpositively dirty by the standards of interstellar or inter-galactic space.

Asteroids (Figure 6.8a) and meteoroids are generallyrocky in composition, somewhat like the outer layers ofthe terrestrial planets. The distinction between the two issimply a matter of size: Anything larger than 100 m in di-ameter (corresponding to a mass of about 10,000 tons) is

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TABLE 6.2 Comparison of the Terrestrial andJovian Planets

Terrestrial Planets Jovian Planets

close to the Sun far from the Sun

closely spaced orbits widely spaced orbits

small masses large masses

small radii large radii

predominantly rocky predominantly gaseous

solid surface no solid surface

high density low density

slower rotation faster rotation

weak magnetic fields strong magnetic fields

few moons many moons

no rings many rings

Section 6.6 | Spacecraft Exploration of the Solar System 153152 CHAPTER 6 | The Solar System

conventionally termed an asteroid; anything smaller is ameteoroid. Their total mass is much less than that ofEarth’s Moon, so these objects play no important role inthe present-day workings of the planets or their moons. Yetthey are of crucial importance to our studies, for they pro-vide the keys to answering some very fundamental ques-tions about our planetary environment and what the solarsystem was like soon after its birth. Many of these bodiesare made of material that has hardly evolved since the earlydays of the solar system. In addition, they often convenient-ly deliver themselves right to our doorstep, in the form ofmeteorites (the name we give them if they happen to sur-vive the plunge through Earth’s atmosphere and find theirway to the ground—see Section 14.3), allowing us to studythem in detail without having to fetch them from space.

Comets are quite distinct from the other small bodies inthe solar system. They are generally icy rather than rocky incomposition (although they do contain some rocky materi-al) and typically have diameters in the 1–10-km range. Theyare quite similar in chemical makeup to some of the icymoons of the outer planets. Even more so than the asteroidsand meteoroids, comets represent truly ancient material—the vast majority probably have not changed in any signifi-cant way since their formation long ago along with the restof the solar system (see Chapter 15). Comets striking Earth’satmosphere do not reach the surface intact, so we do nothave actual samples of cometary material. However, they dovaporize and emit radiation as their highly elongated orbitstake them near the Sun. (See Figure 6.8b.) Astronomers candetermine a comet’s makeup by spectroscopic study of theradiation it gives off as it is destroyed. • (Sec. 4.2)

CONCEPT CHECK

✔ Why are astronomers so interested ininterplanetary matter?

6.6 Spacecraft Exploration of theSolar SystemSince the 1960s, dozens of unmanned space mis-sions have traveled throughout the solar system. All

of the planets but Pluto have been visited and probed atclose range, and spacecraft have visited numerous cometsand asteroids. The first landing on an asteroid (by theNEAR spacecraft) occurred in February 2001. The impactof these missions on our understanding of our planetarysystem has been nothing short of revolutionary.

In the next eight chapters, we will see many examplesof the marvelous images radioed back to Earth, and we willdiscover how they fit into our modern picture of the solarsystem. Here, we focus on just a few of these remarkabletechnological achievements; Table 6.3 summarizes the var-ious successful missions highlighted in this section.

5

The Mariner 10 Flybys of MercuryIn 1974, the U.S. spacecraft Mariner 10 came within10,000 km of the surface of Mercury, sending back high-resolution images of the planet. (A “flyby,” in NASA parl-ance, is any space mission in which a probe passesrelatively close to a planet—within a few planetary radii,say—but does not go into orbit around it.) The photo-graphs, which showed surface features as small as 150 macross, dramatically increased our knowledge of the planet.For the first time, we saw Mercury as a heavily crateredworld, in many ways reminiscent of our own Moon.

Mariner 10 was launched from Earth in November1973 and was placed in an eccentric 176-day orbit aboutthe Sun, aided by a gravitational assist (see Discovery 6-1)from the planet Venus (Figure 6.9). In that orbit, Mariner10’s nearest point to the Sun (perihelion) is close to Mer-cury’s path, and its farthest point away (aphelion) lies be-tween the orbits of Venus and Earth (More Precisely 2-1).The 176-day period is exactly two Mercury years, so thespacecraft revisits Mercury roughly every six months.However, only on the first three encounters—in March1974, September 1974, and March 1975—did the space-craft return data. After that, the craft’s supply of maneu-vering fuel was exhausted; the craft still orbits the Suntoday, but out of control and silent. In total, over 4000photographs, covering about 45 percent of the planet’ssurface, were radioed back to Earth during the mission’sactive lifetime. The remaining 55 percent of Mercury isstill unexplored.

No new missions have been sent to Mercury sinceMariner 10. NASA plans a return in 2009, when theMessenger probe will be placed in orbit around the planetto map its entire surface at much higher resolution thanwas possible with Mariner. Both the European and theJapanese Space Agencies also have plans to place spacecraftin orbit around Mercury at roughly the same time as theMessenger mission.

Exploration of VenusIn all, some 20 spacecraft have visited Venus since the1970s, far more than have spied on any other planet in thesolar system. The Soviet space program took the lead inexploring Venus’s atmosphere and surface, while Americanspacecraft have performed extensive radar mapping of theplanet from orbit.

The Soviet Venera (derived from the Russian word forVenus) program began in the mid-1960s, and the SovietVenera 4 through Venera 12 probes parachuted into theplanet’s atmosphere between 1967 and 1978. The earlyspacecraft were destroyed by enormous atmospheric pres-sures before reaching the surface, but in 1970, Venera 7(Figure 6.10) became the first spacecraft to soft-land onthe planet. During the 23 minutes it survived on the sur-face, it radioed back information on the planet’s atmos-pheric pressure and temperature. Since then, a number of

Earth’sorbit

Venus flybyFeb. 1974

Mariner’s orbit

Earth

Sun

Mercury’s orbit

Mercury

LaunchNov. 1973

Venus

Venus’sorbit

Mercury flybysMar. 1974Sept. 1974Mar. 1975

� FIGURE 6.9 Mission to MercuryThe path of the Mariner 10 probe to theplanet Mercury included a gravitationalboost from Venus. The spacecraft (inset)returned data from March 1974 untilMarch 1975, providing astronomers witha wealth of information on Mercury.(NASA)

Venera landers have transmitted photographs of the surfaceback to Earth and have analyzed the atmosphere and thesoil. None of them survived for more than an hour in theplanet’s hot, dense atmosphere. The data they sent backmake up the entirety of our direct knowledge of Venus’ssurface. In 1983, the Venera 15 and Venera 16 orbiters sentback detailed radar maps (at about 2-km resolution) oflarge portions of Venus’s northern hemisphere.

In 1978, the U.S. Pioneer Venus mission placed an or-biter at an altitude of some 150 km above Venus’s surfaceand dispatched a “multiprobe” consisting of five separateinstrument packages into the planet’s atmosphere. Duringits hour-long descent to the surface, the probe returned in-formation on the variation in density, temperature, andchemical composition with altitude in the atmosphere.The orbiter’s radar produced images of most of the planet’ssurface.

The most recent U.S. mission was the Magellan probe(shown in Figure 6.11), which entered orbit around Venusin August 1990. Its radar imaging system could distinguishobjects as small as 120 m across. Between 1991 and 1994,the probe mapped 98 percent of the surface of Venus with

� FIGURE 6.10 Venus Lander One of the Soviet Veneralanders that reached the surface of Venus. The design wasessentially similar for all the surface missions. Note theheavily armored construction, necessary to withstand theharsh conditions of high temperature and crushingatmospheric pressure on the planet’s surface.(Sovfoto/Eastfoto)

Section 6.6 | Spacecraft Exploration of the Solar System 155154 CHAPTER 6 | The Solar System

unprecedented clarity and made detailed measurements ofthe planet’s gravity, rendering all previous data virtuallyobsolete. The mission ended in October 1994 with a(planned) plunge into the planet’s dense atmosphere, send-ing back one final stream of high-quality data. Many theo-ries of the processes shaping the planet’s surface have hadto be radically altered or abandoned completely because ofMagellan’s data.

Exploration of MarsBoth NASA and the Soviet (now Russian) Space Agencyhave Mars exploration programs that began in the 1960s.However, the Soviet effort was plagued by a string of tech-nical problems, along with a liberal measure of plain badluck. As a result, almost all of the detailed planetary datawe have on Mars has come from unmanned U.S. probes.

The first spacecraft to reach the Red Planet wasMariner 4, which flew by Mars in July 1965. The imagessent back by the craft showed large numbers of craterscaused by impacts of meteoroids with the planet’s surface,instead of the Earthlike terrain some scientists had expect-ed to find. Flybys in 1969 by Mariner 6 and Mariner 7 con-firmed these findings, leading to the conclusion that Marswas a geologically dead planet having a heavily cratered,

old surface. This conclusion was soon reversed by the ar-rival in 1971 of the Mariner 9 orbiter. The craft mappedthe entire Martian surface at a resolution of about 1 km,and it rapidly became clear that here was a world far morecomplex than the dead planet imagined only a year or twopreviously. Mariner 9’s maps revealed vast plains, volca-noes, drainage channels, and canyons. All these featureswere completely unexpected, given the data provided bythe earlier missions. These new findings paved the way forthe next step: actual landings on the planet’s surface.

The two spacecraft of the U.S. Viking mission arrivedat Mars in mid-1976. Viking 1 and Viking 2 each consistedof two parts. An orbiter mapped the surface at a resolutionof about 100 m (about the same as the resolution achievedby Magellan for Venus), and a lander (see Figure 6.12) de-scended to the surface and performed a wide array of geo-logical and biological experiments. Viking 1 touched downon Mars on July 20, 1976. Viking 2 arrived in September ofthe same year. By any standards, the Viking mission was acomplete success: The orbiters and landers returned awealth of long-term data on the Martian surface and at-mosphere. Viking 2 stopped transmitting data in April1980. Viking 1 continued to operate until November 1982.

The relative positions of Mars and Earth in their re-spective orbits are such that it is most favorable to launch aspacecraft from Earth roughly every 26 months. It thentakes roughly eight to nine months for the craft to arrive atMars (Figure 6.13). In August 1993, the first U.S. probesince Viking—Mars Observer—exploded just before enter-ing Mars orbit, most probably due to a fuel leak as its en-gine fired to slow the craft. Mars Observer had beendesigned to radio back detailed images of the planet’s sur-face and provide data on the Martian atmosphere, gravity,and magnetic field. Its replacement, called Mars GlobalSurveyor, was launched from Earth in 1996 and arrived at

TABLE 6.3 Some Missions to the Other Planets

Target Launched Type of Scientific Achievements or Goals, and Planet Year Project by Mission other Comments

Mercury 1974–1975 Mariner 10 U.S. flyby Photographed 45 percent of the planet’s surface; still orbiting the Sun

Venus 1967 Venera 4 U.S.S.R. atmospheric First probe to enter the planet’s atmosphereprobe

1970 Venera 7 U.S.S.R. lander First landing on the planet’s surface1978–1992 Pioneer Venus U.S. orbiter Radar mapping of the entire planet1983 Venera 15, 16 U.S.S.R. orbiter Radar mapping of northern hemisphere1990–1994 Magellan U.S. atmospheric High-resolution radar mapping of the entire

probe, orbiter planet

Mars 1965 Mariner 4 U.S. flyby Photographs showed cratered surface1969 Mariner 6, 7 U.S. flyby More photos of surface1971 Mariner 9 U.S. orbiter First complete survey of the surface; revealed

complex terrain, past geological activity1976–1982 Viking 1, 2 U.S. orbiter, lander Detailed surface maps, implying geological and

climatic change; first surface landings, first atmospheric and soil measurements; search for life

1997 Mars Pathfinder U.S. lander First Mars rover, local geological survey1997– Mars Global U.S. orbiter High-resolution surface mapping, remote analysis

Surveyor of surface composition2001– Mars Odyssey U.S. orbiter Remote surface chemical analysis, search for

subsurface water, measurement of radiation levels2003–2004 Mars Express European atmospheric Study of Martian atmosphere and geology; search

Space Agency probe, orbiter for water and evidence of life2004 Mars Exploration U.S. two landers Assessment of likelihood that life arose on Mars,

Rover measurement of Martian climate and geology

Eros1 2001–2002 NEAR U.S. orbiter First mission to an asteroid; detailed geological study

Jupiter 1973 Pioneer 10 U.S. flyby First mission to the outer planets1974 Pioneer 11 U.S. flyby Detailed close-up images; gravity assist from

Jupiter to reach Saturn1979 Voyager 1 U.S. flyby Detailed observations of planet and moons1979 Voyager 2 U.S. flyby Continued reconaissance of Jovian system1995–2003 Galileo U.S. atmospheric Atmospheric studies; long-term precision

probe, orbiter measurements of the planet’s moon system

Saturn 1979 Pioneer 11 U.S. flyby First close-up observations of Saturn; now leaving the solar system

1981 Voyager 1 U.S. flyby Observations of planet, rings and moons; close-up measurements of the moon Titan

1982 Voyager 2 U.S. flyby Observations of planet and moons2004– Cassini U.S., European atmospheric Atmospheric studies; repeated and detailed

Space Agency probe, orbiter measurements of the planet’s moons

Uranus 1986 Voyager 2 U.S. flyby Observations of planet and moons; “Grand Tour” of the outer planets

Neptune 1989 Voyager 2 U.S. flyby Observations of planet and moons; now leaving the solar system

1An asteroid.

P

Antenna to Earth

Cameras

Biology Inlets

Weather Instrument

Power Supply

Fuel

Descent Engine

Surface Sampler

� FIGURE 6.12 Viking Lander A Viking lander, herebeing tested in the Mojave Desert prior to launch. For anidea of the scale, the reach of its extended arm at left isabout 1 m. (NASA)

� FIGURE 6.11 Magellan Orbiter The U.S. Magellanspacecraft was launched from the space shuttle Atlantis(at bottom) in May 1989 on a mission to explore theplanet Venus. The large radio antenna at the top wasused both for mapping the surface of Venus and forcommunicating with Earth. Contrast the relatively delicatestructure of this craft, designed to operate in space, withthe much more bulky design of the Venus lander shown inFigure 6.10. (NASA)

Section 6.6 | Spacecraft Exploration of the Solar System 157156 CHAPTER 6 | The Solar System

Mars’sorbit

Earth atlaunch

Sun

Earth atarrival

Earth’sorbit

Mars atarrival

Mars atlaunch

Spacecraftorbit

� FIGURE 6.13 Mission to Mars A typical orbital pathfrom Earth to Mars. A spacecraft orbiting in the pathshown can take anywhere from eight to nine months tomake the trip, depending on precisely where Marshappens to be in its orbit, which is illustrated here ascircular, but which, in reality, is somewhat eccentric.

Mars in late 1997. This spacecraft is currently orbiting theplanet, scanning it with cameras and other sensors andcharting the Martian landscape. Originally scheduled toend in 2002, the mission (as of early 2004) continues to re-turn high-quality images and data.

Mars Global Surveyor was followed (and in fact over-taken) by Mars Pathfinder, which arrived at Mars in lateJune 1997. On July 4, Pathfinder parachuted an instrumentpackage to the Martian surface. Near the ground, the para-chute fell away and huge air bags deployed, enabling therobot to bounce softly to a safe landing. Side panelsopened, and out came a small six-wheeled minirover,

while NASA launched Mars Exploration Rover, comprisingtwo Sojourner-like landers, named Spirit and Opportunity.Mars Express reached the planet in December 2003; its or-biter worked fine and began transmitting data back toEarth, but its surface probe apparently crash landed as nosignals were heard from it. A month later, the two NASAprobes successfully soft-landed by bouncing across the sur-face until they rolled to a stop. One ended up, as planned, ina huge crater that might be a dried-up lake; the other mis-takenly rolled into a equatorial depression that surprisinglydisplayed evidence for subsurface layering. Both craft begana two-month odyssey searching for water on Mars.

In 2007, if all goes according to plan, a NASA landingcraft will return samples of Martian surface material toEarth (or at least Earth orbit—the issues of possible con-tamination of the sample by Earth bacteria, or vice versa,are still being debated). NASA also has plans for mannedmissions to Mars. Indeed, parts of the Mars Odyssey andMars Exploration Rover missions are devoted specifically todetermining the feasibility of human exploration of theplanet. However, the enormous expense (and danger) ofsuch an undertaking, coupled with the belief of many as-tronomers that unmanned missions are economically andscientifically preferable to manned missions, makes the fu-ture of these projects uncertain at best.

Missions to the Outer PlanetsTwo pairs of U.S. spacecraft launched in the 1970s—Pioneerand Voyager—revolutionized our knowledge of Jupiter andthe Jovian planets. Pioneer 10 and Pioneer 11 were launchedin March 1972 and April 1973, respectively, and arrived atJupiter in December 1973 and December 1974.

The Pioneer spacecraft took many photographs andmade numerous scientific discoveries. Their orbital trajec-tories also allowed them to observe the polar regions ofJupiter in much greater detail than later missions wouldachieve. In addition to their many scientific accomplish-ments, the Pioneer craft also played an important role as“scouts” for the later Voyager missions. The Pioneer seriesdemonstrated that spacecraft could travel the long routefrom Earth to Jupiter without colliding with debris in thesolar system. They also discovered—and survived—theperils of Jupiter’s extensive radiation belts (somewhat likethe Van Allen radiation belts that surround Earth, but on amuch larger scale). In addition, Pioneer 11 used Jupiter’sgravity to propel it along the same trajectory to Saturn thatthe Voyager controllers planned for Voyager 2’s visit to Sat-urn’s rings.

The two Voyager spacecraft (see Figure 6.15) left Earthin 1977 and reached Jupiter in March (Voyager 1) and July(Voyager 2) of 1979 to study the planet and its major satel-lites in detail. Each craft carried sophisticated equipmentto investigate the planet’s magnetic field, as well as radio,visible-light, and infrared sensors to analyze its reflectedand emitted radiation. Both Voyager 1 and Voyager 2 usedJupiter’s gravity to send them on to Saturn. Voyager 1 was

programmed to visit Titan, Saturn’s largest moon, and sodid not come close enough to the planet to receive a grav-ity-assisted boost to Uranus. However, Voyager 2 went onto visit both Uranus and Neptune in a spectacularly suc-cessful “Grand Tour” of the outer planets. The data re-turned by the two craft are still being analyzed today. LikePioneer 11, the two Voyager craft are now headed out of thesolar system, still sending data as they race toward inter-stellar space. The figure in Discovery 6-1 shows the pastand present trajectories of the Voyager spacecraft.

The most recent mission to Jupiter is the U.S. Galileoprobe, launched by NASA in 1989. The craft arrived at itstarget in 1995 after a rather roundabout route involving agravity assist from Venus and two from Earth itself. Themission consisted of an orbiter and an atmospheric probe.The probe descended into Jupiter’s atmosphere in Decem-ber 1995, slowed by a heat shield and a parachute, takingmeasurements and performing chemical analyses as itwent. The orbiter executed a complex series of gravity-assisted maneuvers through Jupiter’s system of moons, returning to some already studied by Voyager and visitingothers for the first time. Some of Galileo’s main findingsare described in Chapter 11.

The Galileo program was originally scheduled to lastuntil December 1997, but NASA extended its lifetime forsix more years to obtain even more detailed data onJupiter’s inner moons. The mission finally ended in Sep-tember 2003. With the fuel supply dwindling, Galileo’scontrollers elected to steer the craft directly into Jupiter,rather than run any risk that it might collide with, and pos-sibly contaminate, Jupiter’s moon Europa, which (thankslargely to data returned by Galileo) is now a leading candi-date in the search for extraterrestrial life.

In October 1997, NASA launched the Cassini missionto Saturn. The launch (from Cape Canaveral) sparked

Rover Tracks

Yogi

1.7 meters

Booboo

Lamb

Solar Panel

Hard Stop

North Knob

Shaggy

Scooby-Doo

Lumpy

AirbagsAirbags

SojournerRover

� FIGURE 6.14 Martian LanderThe Mars rover Sojourner was asmall self-propelled vehicle usedto explore the Martian surface. It is seen at left center using adevice called an alpha proton X-ray spectrometer to determinethe chemical composition of alarge Martian rock, nicknamedYogi by mission scientists. Forscale, the rock is about 1 m talland lies some 10 m fromSojourner’s mother ship,Pathfinder (foreground). (NASA)

called Sojourner (Figure 6.14). During its three-month life-time, the lander took measurements of the Martian atmos-phere and atmospheric dust, while Sojourner roamed theMartian countryside at a rate of a few meters per day, car-rying out chemical analyses of the soil and rocks withinabout 50 m of the parent craft. In addition, over 16,000images of the region were returned to Earth.

NASA’s next two Mars exploration missions met withfailure. In September 1999, Mars Climate Orbiter missed itsproper orbit insertion point and instead burned up in theMartian atmosphere, apparently as a result of navigationalcommands being sent to the spacecraft’s onboard computerin English, rather than in metric, units. In December of thesame year, the sister mission, Mars Polar Lander, failed toreestablish contact with Earth after entering the Martianatmosphere en route to a planned soft landing on the plan-et’s south polar ice cap. The reason is still unknown.

Despite these setbacks, the U.S. Mars program hasnow moved into high gear, with an ambitious program ofmissions spanning the first decade of the 21st century. Anew orbiter, called Mars Odyssey, reached Mars in October2001. Its sensors are designed to probe the chemical make-up of the Martian surface and look for possible water icebelow. In the summer of 2003, no fewer than three Mars-bound missions left Earth. The European Space Agencylaunched Mars Express, consisting of an orbiter and a lander,

Science instrument boom High gainantenna

Magnetometer boom

Radioisotope generators

Star trackers

Cosmic raydetector

Cameras andspectrometer

Low gain antenna

� FIGURE 6.15 Voyager The two Voyager spacecraftthat swung by several of the outer planets wereidentically constructed. Their main features are shownhere. (NASA)

Section 6.7 | How Did the Solar System Form? 159158 CHAPTER 6 | The Solar System

Gravitational “Slingshots”Celestial mechanics is the study of the motions of gravitational-ly interacting objects, such as planets and stars, applying New-ton’s laws of motion to understand the intricate movements ofastronomical bodies. • (Sec. 2.7) Computerized celestialmechanics lets astronomers calculate planetary orbits to highprecision, taking the planets’ small gravitational influences onone another into account. Even before the computer age, thediscovery of one of the outermost planets, Neptune, cameabout almost entirely through studies of the distortions ofUranus’s orbit that were caused by Neptune’s gravity.

Celestial mechanics is also an essential tool for scientistsand engineers who wish to navigate manned and unmannedspacecraft throughout the solar system. Robot probes can nowbe sent on stunningly accurate trajectories, expressed in thetrade with such slang phrases as “sinking a corner shot on abillion-kilometer pool table.” Near-flawless rocket launches,aided by occasional midcourse changes in flight paths, nowenable interplanetary navigators to steer remotely controlledspacecraft through an imaginary “window” of space just a fewkilometers wide and a billion kilometers away.

However, sending a spacecraft to another planet requiresa lot of energy—often more than can be conveniently provid-ed by a rocket launched from Earth or safely transported in ashuttle for launch from orbit. Faced with these limitations,mission scientists often use their knowledge of celestial me-chanics to carry out “slingshot” maneuvers, which can boostan interplanetary probe into a more energetic orbit and alsoaid navigation toward the target, all at no additional cost!

The accompanying figure illustrates a gravitationalslingshot, or gravity assist, in action. A spacecraft approaches aplanet, passes close by, and then escapes along a new trajecto-ry. Obviously, the spacecraft’s direction of motion is changed

by the encounter. Less obviously, the spacecraft’s speed is alsoaltered as the planet’s gravity propels the spacecraft in the di-rection of the planet’s motion. By a careful choice of incom-ing trajectory, the craft can speed up (by passing “behind” theplanet, as shown) or slow down (by passing in front), by asmuch as twice the planet’s orbital speed. Of course, there isno “free lunch”—the spacecraft gains energy from, or loses itto, the planet’s motion, causing the planet’s own orbit tochange ever so slightly. However, since planets are so muchmore massive than spacecraft, the effect on the planet is in-significant.

Such a slingshot maneuver has been used many times inmissions to both the inner and the outer planets, as illustratedin the second figure, which shows the trajectories of theVoyager spacecraft through the outer solar system. The gravi-tational pulls of these giant worlds whipped the craft aroundat each visitation, enabling flight controllers to get consider-

DISCOVERY 6-1

Orbit

Incominglow–speedspacecraft

Planet motion

Outgoinghigher–speed

spacecraft

Voyager 1

Voyager 2

1: Sept 5, 19172: Aug 20, 1977

1: Mar 3, 19792: Aug 9, 1979

1: Nov 13, 1980

2: Jan 30, 1986

2: Aug 27, 1981

2: Aug 15, 1989

Jupiter at launch

Uranus atlaunch

Neptune atlaunch

Saturn at launch

Asteroidbelt

able extra “mileage” out of the probes. Voyager 1 is now highabove the plane of the solar system, having been deflected upand out following its encounter with Saturn. Voyager 2 contin-ued on for a “Grand Tour” of the four Jovian planets. It is nowoutside the orbit of Pluto.

More recently, the Galileo mission to Jupiter, which waslaunched in 1989 and arrived at its target in 1995, receivedthree gravitational assists en route—one from Venus and two

from Earth. Once in the Jupiter system, Galileo used the grav-ity of Jupiter and its moons to propel it through a complex se-ries of maneuvers designed to bring it close to all the majormoons, as well as to the planet itself. Every encounter with amoon had a slingshot effect—sometimes accelerating andsometimes slowing the probe, but each time moving it into adifferent orbit—and every one of these effects was carefullycalculated long before Galileo ever left Earth.

controversy because of fears that the craft’s plutoniumpower source might contaminate parts of our planet fol-lowing an accident either during launch or during a subse-quent gravity assist from Earth in 1999—one of four assistsneeded for the craft to reach Saturn’s distant orbit (Figure6.16). When the craft reaches its destination in 2004, itwill dispatch a probe built by the European Space Agencyinto the atmosphere of Titan, Saturn’s largest moon, andorbit among the planet’s moons for four years, much asGalileo did at Jupiter. If the experience with Galileo is anyguide, Cassini will likely resolve many outstanding ques-tions about the Saturn system, but it is sure to pose manynew ones, too.

6.7 How Did the Solar SystemForm?During the past four decades, interplanetaryprobes have vastly increased our knowledge of the

solar system, and their data form the foundation for muchof the discussion in the next eight chapters. However, wecan understand the overall organization of our planetarysystem—the basic properties presented in Sections 6.3 and

6

Earth gravity assist flybyAug. 18, 1999

Earth at launchOct.15, 1997

Jupiter

Jupiter gravityassist flybyDec. 30, 2000

Arrival at SaturnJuly 1, 2004

Orbit of Jupiter

Orbit of Mars

Venus gravity assist flybyJune 24, 1999Mars

Venus gravity assist flyby April 26, 1998

Cassini spacecraft’s interplanetary trajectoryOrbit of Saturn� FIGURE 6.16 Cassini Mission to Saturn To reachSaturn by July 2004, the Cassini spacecraft was launched in1997, increased its velocity progressively by flying byVenus in 1998 and 1999 and also by Earth in 1999, and,finally, flew by Jupiter in the year 2000, picking upanother gravity boost. The inset shows the Cassinispacecraft under construction. (NASA)

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160 CHAPTER 6 | The Solar System

6.4—without dwelling on these details. Indeed, some keyelements of the modern theory of planetary formation pre-date the Space Age by many years.

We present here (in simplified form) the “standard”view of how the solar system came into being. This picturewill underlie much of our upcoming discussion of the plan-ets, their moons, and the contents of the vast spaces be-tween them. Later (in Chapter 15) we will assess how wellour theory holds up in the face of detailed observationaldata—including evidence for the existence of well over 100extrasolar planets orbiting neighboring stars—and exam-ine what it means for the prospects of finding Earth-likeplanets, and maybe even life, elsewhere in the Galaxy.

Nebular ContractionOne of the earliest heliocentric models of solar-system for-mation may be traced back to the 17th-century Frenchphilosopher René Descartes. Imagine a large cloud of in-terstellar dust and gas (called a nebula) a light-year or soacross. Now suppose that, due to some external influence,such as a collision with another interstellar cloud or per-haps the explosion of a nearby star, the nebula starts tocontract under the influence of its own gravity. As it con-tracts, it becomes denser and hotter, eventually forming astar—the Sun—at its center (see Chapter 19). Descartessuggested that, while the Sun was forming in the cloud’shot core, the planets and their moons formed in the cloud’scooler outer regions. In other words, planets are by-prod-ucts of the process of star formation.

In 1796, the French mathematician–astronomerPierre Simon de Laplace developed Descartes’s ideas in amore quantitative way. He showed mathematically thatconservation of angular momentum (see More Precisely 6-2)demands that our hypothetical nebula spin faster as it con-tracts. A decrease in the size of a rotating mass must bebalanced by an increase in its rotational speed. The latter,in turn, causes the nebula’s shape to change as it collapses.Centrifugal forces (due to rotation) tend to oppose thecontraction in directions perpendicular to the rotationaxis, with the result that the nebula collapses most rapidlyalong that axis. As shown in Figure 6.17, the fragmenteventually flattens into a pancake-shaped primitive solarsystem. This swirling mass destined to become our solarsystem is usually referred to as the solar nebula.

If we now simply suppose that the planets formed outof this spinning material, then we can already understandthe origin of much of the large-scale architecture observedin our planetary system today, such as the circularity of theplanets’ orbits and the fact that they move in the samesense in nearly the same plane. The planets inherited allthese properties from the rotating disk in which they wereborn. The idea that the planets formed from such a disk iscalled the nebular theory.

Astronomers are fairly confident that the solar nebulaformed a disk, because similar disks have been observed (orinferred) around other stars. Figure 6.18(a) shows visible-

light images of the region around a star called Beta Pic-toris, lying about 60 light-years from the Sun. When thelight from Beta Pictoris itself is suppressed and the result-ing image enhanced by a computer, a faint disk of warmmatter (viewed almost edge-on here) can be seen. Thisparticular disk is roughly 1000 A.U. across—about 10times the diameter of Pluto’s orbit. Astronomers believethat Beta Pictoris is a very young star, perhaps only 20 mil-lion years old, and that we are witnessing it pass throughan evolutionary stage similar to the one our own Sun un-derwent long ago. Figure 6.18(b) shows an artist’s concep-tion of the disk.

The Condensation TheoryScientific theories must continually be tested and refined asnew data become available. • (Sec. 1.2) Unfortunately forthe original nebular theory, while Laplace’s description ofthe collapse and flattening of the solar nebula was basicallycorrect, we now know that a disk of warm gas would notform clumps of matter that would subsequently evolve into

Contractingcloud

Spin up/disk formation

(a)

(b)

� FIGURE 6.17 Nebular Contraction (a) Conservation ofangular momentum demands that a contracting, rotatingcloud spin faster as its size decreases. (b) Eventually, theprimitive solar system comes to resemble a giant pancake.In the case of our solar system, the large blob at thecenter ultimately became the Sun.

(a) (b)

Solar System to Scale

Size of Pluto’s Orbit

R I V U X G

� FIGURE 6.18 Beta Pictoris (a) A computer-enhanced view of a disk of warm matter surroundingthe star Beta Pictoris. Both images in (a) display data taken at visible wavelengths, but are presentedhere in false color to accentuate the details; the bottom image is a close-up of the inner part of thedisk, indicating the presence of a warp, possibly caused by the gravitational pull of unseencompanions. In both images, the overwhelmingly bright central star has been covered to let us see themuch fainter disk surrounding it. The disk is nearly edge-on to our line of sight. For scale, thedimension of Pluto’s orbit (78 A.U.) has been drawn adjacent to the images. (b) An artist’s conceptionof the disk of clumped matter, showing the warm disk with a young star at the center and severalcomet-sized or larger bodies already forming. The colors are thought to be accurate: At the outeredges of the disk, the temperature is low and the color is dull red. Progressing inward, the colorsbrighten and shift to a more yellowish tint as the temperature increases. Mottled dust is seenthroughout—such protoplanetary regions are probably very “dirty.” (NASA; D. Berry)

planets. In fact, modern computer calculations predict justthe opposite: Clumps in the gas would tend to disperse, notcontract further. However, the model currently favored bymost astronomers, known as the condensation theory,rests squarely on the old nebular theory, combining its basicphysical reasoning with new information about interstellarchemistry to avoid most of the original theory’s problems.

The key new ingredient is the presence of interstellardust in the solar nebula. Astronomers now recognize thatthe space between the stars is strewn with microscopic dustgrains, an accumulation of the ejected matter of manylong-dead stars (see Chapter 22). These dust particlesprobably formed in the cool atmospheres of old stars andthen grew by accumulating more atoms and molecules

from the interstellar gas within the Milky Way galaxy. Theend result is that our entire Galaxy is littered with minia-ture chunks of icy and rocky matter having typical sizes ofabout Figure 6.19 shows one of many such dustyregions found in the vicinity of the Sun.

Dust grains play two important roles in the evolu-tion of a gas cloud. First, dust helps to cool warm matterby efficiently radiating its heat away in the form of in-frared radiation. • (Sec. 3.4) As the cloud cools, itsmolecules move around more slowly, reducing the inter-nal pressure and allowing the nebula to collapse moreeasily under the influence of gravity. • (More Precisely3-1) Second, acting as condensation nuclei—micro-scopic platforms to which other atoms can attach, form-

10-5 m.

R I V U X G

� FIGURE 6.19 Dark Cloud Interstellar gas anddark dust lanes mark this region of starformation. The dark cloud known as Barnard 86(left) flanks a cluster of young blue stars calledNGC 6520 (right). Barnard 86 may be part of alarger interstellar cloud that gave rise to thesestars. (D. Malin/Anglo-Australian Telescope)

AN

IMA

TION

Beta Picto

ris Warp

peared at this stage, while the grains in the outermost partsprobably remained largely intact.

As the dusty nebula radiated away its heat, its temper-ature decreased everywhere except in the very core, wherethe Sun was forming. As the gas cooled, new dust grainsbegan to condense (or crystallize) out, much as raindrops,snowflakes, and hailstones condense from moist, coolingair here on Earth. It may seem strange that although therewas plenty of interstellar dust early on, it was partly de-stroyed, only to form again later. However, a criticalchange had occurred. Initially, the nebular gas was uni-formly peppered with dust grains of all compositions;when the dust re-formed later, the distribution of grainswas very different.

Figure 6.21 plots the temperature in various parts ofthe primitive solar system just before accretion began. Atany given location, the only materials to condense outwere those able to survive the temperature there. Asmarked on the figure, in the innermost regions, aroundMercury’s present orbit, only metallic grains could form; itwas simply too hot for anything else to exist. A little far-ther out, at about 1 A.U., it was possible for rocky, silicategrains to form, too. Beyond about 3 or 4 A.U., water icecould exist, and so on, with the condensation of more andmore material possible at greater and greater distancesfrom the Sun. The composition of the material that could

condense out at any given radius would determine the typeof planet that formed there.

Terrestrial and Jovian PlanetsIn the inner regions of the primitive solar system,condensation from gas to solid began when the av-

erage temperature was about 1000 K. The environmentthere was too hot for ice to survive. Many of the moreabundant heavier elements, such as silicon, iron, magne-sium, and aluminum, combined with oxygen to produce avariety of rocky materials. The dust grains in the innersolar system were therefore predominantly rocky or metal-lic in nature, as were the protoplanets and planets theyeventually became.

In the middle and outer regions of the primitive plan-etary system, beyond about 5 A.U. from the center, thetemperature was low enough for the condensation of sev-eral abundant gases—water ammonia andmethane —into solid form. After hydrogen (H) andhelium (He), the elements carbon (C), nitrogen (N), andoxygen (O) are the most common materials in the uni-verse. As a result, wherever icy grains could form, theygreatly outnumbered rocky and metallic particles. Conse-quently, the objects that formed at these distances wereformed under cold conditions out of predominantly low-density, icy material. These ancestral fragments were des-tined to form the cores of the jovian planets.

Because more material could condense out of the solarnebula at large radii than in the inner regions near the Sun,accretion began sooner, with more resources to draw on.The outer solar system thus had a “head start” in the accre-tion process, and the outer planets grew rapidly, eventuallybecoming massive enough to enter a new phase of planetformation, in which their strong gravitational fields sweptup large amounts of gas directly from the solar nebula.The smaller, inner protoplanets never reached this stage,and, as a result, their masses remained relatively low. Thisis the chain of events depicted in Figures 6.20(c) and (d).

Formation StopsThe events just described did not take long, astronomical-ly speaking. The giant planets formed within a few millionyears of the appearance of the flattened solar nebula—the

1CH421NH32,1H2 O2,

7

Section 6.7 | How Did the Solar System Form? 163

ing larger and larger balls of matter—the dust grainsgreatly speed up the process of collecting enough atomsto form a planet. This mechanism is similar to the wayraindrops form in Earth’s atmosphere: Dust and soot inthe air act as condensation nuclei around which watermolecules cluster.

Thus, according to the modern condensation theory,once the solar nebula had formed and begun to cool(Figure 6.20a), dust grains formed condensation nucleiaround which matter began to accumulate (Figure 6.20b).This vital step greatly hastened the critical process offorming the first small clumps of matter. Once theseclumps formed, they grew rapidly by sticking to otherclumps. (Imagine a snowball thrown through a fiercesnowstorm, growing bigger as it encounters moresnowflakes.) As the clumps grew larger, their surface areasincreased and consequently, the rate at which they sweptup new material accelerated. Gradually, the accreted mat-ter grew into objects of pebble size, baseball size, basket-ball size, and larger.

Eventually, this process of accretion—the gradualgrowth of small objects by collision and sticking—createdobjects a few hundred kilometers across (Figure 6.20c). Bythat time, their gravity was strong enough to sweep up

material that would otherwise not have collided withthem, and their rate of growth accelerated, allowing themto form still larger objects. Because larger bodies havestronger gravity, eventually (Figure 6.20d) almost all theoriginal material was swept up into a few largeprotoplanets—the accumulations of matter that would intime evolve into the planets we know today (Figure 6.20e).

The Role of HeatThe condensation theory just described can account—inbroad terms, at least—for the formation of the planets andthe large-scale architecture of the solar system. What doesit say about the differences between the terrestrial and thejovian planets? Specifically, why are smaller, rocky planetsfound close to the Sun, while the larger gas giants orbit atmuch greater distances? To understand why a planet’scomposition depends on its location in the solar system,we must consider the temperature profile of the solar nebu-la. Indeed, it is in this context that the term condensation de-rives its true meaning.

As the primitive solar system contracted under the in-fluence of gravity, it heated up as it flattened into a disk.The density and temperature were greatest near the centerand much lower in the outlying regions. Detailed calcula-tions indicate that the gas temperature near the core of thecontracting system was several thousand kelvins. At a dis-tance of 10 A.U., out where Saturn now resides, the tem-perature was only about 100 K.

The high temperatures in the warmer regions of thecloud caused dust grains to break apart into molecules,which in turn split into excited atoms. Because the extentto which the dust was destroyed depended on temperature,it therefore also depended on location in the solar nebula.Most of the original dust in the inner solar system disap-

162 CHAPTER 6 | The Solar System

(a) initially

few million years

100 million years

(b)

(d)

(e)

(c)

� FIGURE 6.20 Solar-System FormationThe condensation theory of planetary formation (notdrawn to scale, nor is Pluto shown in part e). (a) The solarnebula after it has contracted and flattened to form aspinning disk (Figure 6.17b). The large blob in the centerwill become the Sun. (b) Dust grains act as condensationnuclei, forming clumps of matter that collide, sticktogether, and grow into moon-sized bodies. Thecomposition of the grains—and hence the composition ofthe bodies that form—depends on location within thenebula. (c) After a few million years, strong winds fromthe still-forming Sun begin to expel the nebular gas. Bythis time, some particularly massive bodies in the outersolar system have already captured large amounts of gasfrom the nebula. (d) With the gas ejected, objects in theinner solar system continue to collide and grow. The outergas giant planets are already formed. (e) Over the nexthundred million years or so, most of the remaining smallbodies are accreted or ejected, leaving a few large planetsthat travel in roughly circular orbits.

Ammonia ice

Water ice

Distance from Sun (A.U.)(a)

(b)

Tem

per

atur

e (K

)

Metals

Silicates,rocky material

5 15 20100

2000

1000

UranusSaturnJupiterEarth

� FIGURE 6.21 Temperature in the Early Solar Nebula(a) Theoretically computed variation of temperatureacross the primitive solar nebula, illustrated in (b), whichshows half of the disk in Figure 6.20(b). In the hot centralregions, only metals could condense out of the gaseousstate to form grains. At greater distances from the centralproto-Sun, the temperature was lower, so rocky and icygrains could also form. The labels indicate the minimumradii at which grains of various types could condense outof the nebula.

164

Chapter Review | 165

blink of an eye compared with the 4.6-billion-year age ofthe solar system. At that point, intense radiation andstrong winds from violent activity on the surface of thenewborn Sun ejected the nebular gas, effectively haltingfurther growth of the outer worlds. Accretion in the innersolar system proceeded more slowly, taking perhaps 100million years to form the planets we know today (Figure6.20e). The rocky asteriods and icy comets are all that re-main of the matter that originally condensed out of the

The Concept of AngularMomentumMost celestial objects rotate. Planets, moons, stars, and galax-ies all have some angular momentum, which we can define asthe tendency of a body to keep spinning or moving in a circle.Angular momentum is as important a property of an object asits mass or its energy.

Consider first a simpler motion—linear momentum, whichis defined as the product of an object’s mass and its velocity:

Linear momentum is the tendency of an object to keep mov-ing in a straight line in the absence of external forces. Picturea truck and a bicycle rolling equally fast down a street. Eachhas some linear momentum, but you would obviously find iteasier to stop the less massive bicycle. Although the two vehi-cles have the same speed, the truck has more momentum. Wesee, then, that the linear momentum of an object depends onthe mass of the object. It also depends on the speed of the ob-ject: If two bicycles were rolling down the street at differentspeeds, the slower one could be stopped more easily.

Angular momentum is an analogous property of objectsthat are rotating or revolving. It is a measure of the object’stendency to keep spinning, or, equivalently, of how much ef-fort must be expended to stop the object from spinning. How-ever, in addition to mass and (angular) speed, angularmomentum also depends on the way in which an object’s massis distributed. Intuitively, we know that the more massive anobject, or the larger it is, or the faster it spins, the harder it isto stop. In fact, angular momentum depends on the object’smass, rotation rate (measured in, say, revolutions per second),and radius, in a very specific way:

(Recall that the symbol means “is proportional to”; theconstant of proportionality depends on the details of how theobject’s mass is distributed.)

According to Newton’s laws of motion, both types of mo-mentum—linear and angular—must be conserved at all times.• (Sec. 2.7) In other words, both linear and angular momen-tum must remain constant before, during, and after a physicalchange in any object (so long as no external forces act on theobject). For example, as illustrated in the first figure, if aspherical object having some spin begins to contract, the pre-vious relationship demands that it spin faster, so that the prod-uct remains constant. Thesphere’s mass does not change during the contraction, yet the

mass * angular speed * radius2

“r”

angular momentum r mass * rotation rate * radius2.

linear momentum = mass * velocity.

size of the object clearly decreases. Its rotation speed musttherefore increase in order to keep the total angular momen-tum unchanged.

Figure skaters use the principle of conservation of angu-lar momentum, too. They spin faster by drawing in their arms(as shown in the second pair of figures) and slow down by ex-tending them. Here, the mass of the human body remains thesame, but its lateral size changes, causing the body’s rotationspeed to increase or decrease, as the case may be, to keep itsangular momentum unchanged.

EXAMPLE 1: Suppose the sphere has radius 1 m andstarts off rotating at 1 revolution per minute. It then con-tracts to one-tenth its initial size. Conservation of angularmomentum entails that the sphere’s final angular speed Amust satisfy the relationship

The mass is the same on either side of the equation andtherefore cancels, so we find that

or about 1.7 rev/s.

EXAMPLE 2: Now suppose that the “sphere” is a largeinterstellar gas cloud that is about to collapse and form thesolar nebula. Initially, let’s imagine that it has a diameter of1 light-year and that it rotates very slowly—once every 10million years. Assuming that the cloud’s mass stays con-stant, its rotation rate must increase to conserve angular mo-mentum as the radius decreases. By the time it hascollapsed to a diameter of 100 A.U., the cloud has shrunkby a factor of Conservationof angular momentum then implies that the cloud’s (aver-age) spin rate has increased by a factor of to roughly 1 revolution every 25 years—about the orbitalperiod of Saturn.

Incidentally, the law of conservation of angular momen-tum also applies to planetary orbits (where the radius is nowthe distance from the planet to the Sun). In fact, Kepler’s sec-ond law is just conservation of angular momentum, expressedanother way. • (Sec. 2.5)

6302L 400,000,

11 light-year/100 A.U.2 L 630.

11 m22/10.1 m22 = 100 rev/min,11 rev/min2 *A =

mass * A * 10.1 m22 = mass * 11 rev/min2 * 11 m22.

MORE PRECISELY 6-2

(Orban/Corbis/Sygma)

Large radius

Slow rotation

Small radius

Rapid rotation

Chapter Review

SUMMARY

The solar system (p. 144) consists of the Sun and everythingthat orbits it, including the nine major planets, the moons thatorbit them, and the many small bodies found in interplanetaryspace. The science of comparative planetology (p. 145) com-pares and contrasts the properties of the diverse bodies found inthe solar system and elsewhere in order to understand better theconditions under which planets form and develop. The asteroids,or “minor planets,” are small bodies, none of them larger thanEarth’s Moon and most of which orbit in a broad band called theasteroid belt between the orbits of Mars and Jupiter. Comets arechunks of ice found chiefly in the outer solar system. Their im-portance to planetary astronomy lies in the fact that they arethought to be “leftover” material from the formation of the solarsystem and therefore contain clues to the very earliest stages of itsdevelopment.

The major planets orbit the Sun in the same sense—coun-terclockwise as viewed from above Earth’s North Pole—onroughly circular orbits that lie close to the plane of the ecliptic.The orbits of the innermost planet, Mercury, and the outermost,Pluto, are the most eccentric and have the greatest orbital incli-nation. The spacing between planetary orbits increases as wemove outward from the Sun.

Density (p. 147) is a convenient measure of the compact-ness of any object. The average density of a planet is obtained bydividing the planet’s total mass by its volume. The innermost fourplanets in the solar system have average densities comparable toEarth’s and are generally rocky in composition. The outermostplanets have much lower densities than the terrestrial worlds and,with the exception of Pluto, are made up mostly of gaseous or liq-uid hydrogen and helium.

Planetary scientists divide the eight large planets (excludingPluto) in the solar system, on the basis of their densities and com-position, into the rocky terrestrial planets (p. 150)—Mercury,Venus, Earth, and Mars—which lie closest to the Sun, and thegaseous Jovian planets (p. 150)—Jupiter, Saturn, Uranus, andNeptune—which lie at greater distances. Compared with the ter-

restrial worlds, the Jovian planets are larger and more massive,rotate more rapidly, and have stronger magnetic fields. In addi-tion, the Jovian planets all have ring systems and many moons or-biting them. All the major planets, with the exception of Pluto,have been visited by unmanned space probes. Spacecraft havelanded on Venus and Mars. In many cases, to reach their destina-tions, the spacecraft were set on trajectories that included “gravi-tational assists” from one or more planets.

According to the nebular theory (p. 160) of the formation ofthe solar system, a large cloud of dust and gas—the solar nebula(p. 160)—began to collapse under its own gravity. As it did so, itbegan to spin faster, to conserve angular momentum, eventuallyforming a disk out of which the planets arose. The condensationtheory (p. 161) builds on the nebular theory by incorporating theeffects of particles of interstellar dust, which helped cool the nebu-la and acted as condensation nuclei (p. 162), allowing the planet-building process to begin. Small clumps of matter grew by accre-tion (p. 162), gradually sticking together and growing intomoon-sized bodies whose gravitational fields were strong enoughto accelerate the accretion process, subsequently causing thebodies to grow into protoplanets (p. 162). Eventually, only afew planet-sized objects remained. In the outer solar system, pro-toplanet cores became so large that they could capture hydrogenand helium gas directly from the solar nebula.

At any given location, the temperature in the solar nebuladetermined which materials could condense out and hence alsodetermined the composition of any planets forming there. Theterrestrial planets are rocky because they formed in the hot innerregions of the solar nebula, near the Sun, where only rocky andmetallic materials condensed out. Farther out, the nebula wascooler, and ices of water and ammonia could also form, ultimate-ly leading to the observed differences in composition between theinner and outer solar system.

When the Sun became a star, its strong winds blew away anyremaining gas in the solar nebula. Leftover small bodies that neverbecame part of a planet are seen today as the asteroids and comets.

solar nebula—the last surviving witnesses to the birth ofour planetary system.

CONCEPT CHECK

✔ Why was interstellar dust so important to theformation of our solar system?

166 CHAPTER 6 | The Solar System Chapter Review | 167

REVIEW AND DISCUSSION

1. Name and describe all the different types of objects found inthe solar system. Give one distinguishing characteristic ofeach. Include a mention of interplanetary space.

2. What is comparative planetology? Why is it useful? What isits ultimate goal?

3. Why is it necessary to know the distance to a planet in orderto determine the planet’s mass?

4. List some ways in which the solar system is an “orderly”place.

5. What are some “disorderly” characteristics of the solarsystem?

6. Which are the terrestrial planets? Why are they given thatname?

7. Which are the Jovian planets? Why are they given thatname?

8. Name three important differences between the terrestrialplanets and the Jovian planets.

9. Compare the properties of Pluto given in Table 6.1 with theproperties of the terrestrial and Jovian planets presented inTable 6.2. What do you conclude regarding the classificationof Pluto as either a terrestrial or Jovian planet?

10. Why are asteroids and meteoroids important to planetaryscientists?

11. Comets generally vaporize upon striking Earth’s atmos-phere. How, then, do we know their composition?

12. Why has our knowledge of the solar system increased great-ly in recent years?

13. How and why do scientists use gravity assists to propelspacecraft through the solar system?

14. Which planets have been visited by spacecraft from Earth?On which ones have spacecraft actually landed?

15. Why do you think Galileo and Cassini took such circuitousroutes to Jupiter and Saturn, while Pioneer and Voyager didnot?

16. How do you think NASA’s new policy of building less com-plex, smaller, and cheaper spacecraft—with shorter times be-tween design and launch—will affect the future explorationof the outer planets? Will missions like Galileo and Cassini bepossible in the future?

17. What is the key ingredient in the modern condensation the-ory of the solar system’s origin that was missing or unknownin the nebular theory?

18. Give three examples of how the condensation theory ex-plains the observed features of the present-day solar system.

19. Why are the Jovian planets so much more massive than theterrestrial planets?

20. How did the temperature structure of the solar nebula deter-mine planetary composition?

CONCEPTUAL SELF-TEST: TRUE OR FALSE/MULTIPLE CHOICE

1. Most planets orbit the Sun in nearly the same plane as Earthdoes.

2. The largest planets also have the largest densities.3. The total mass of all the planets is much less than the mass of

the Sun.4. The jovian planets all rotate more rapidly than Earth.5. Saturn is the largest and most massive planet in the solar

system.6. Asteroids are similar in overall composition to the terrestrial

planets.7. Comets have compositions similar to the icy moons of the

jovian planets.8. All planets have moons.9. Both Voyager missions used gravity assists to visit all four jov-

ian planets.10. Interstellar dust plays a key role in the formation of a plane-

tary system.11. A planet’s mass can most easily be determined by measuring

the planet’s (a) moon’s orbits; (b) angular diameter; (c) posi-tion in the sky; (d) orbital speed around the Sun.

12. If we were to construct an accurate scale model of the solarsystem on a football field with the Sun at one end and Pluto

at the other, the planet closest to the center of the fieldwould be (a) Earth; (b) Jupiter; (c) Saturn; (d) Uranus.

13. The inner planets tend to have (a) fewer moons; (b) fasterrotation rates; (c) stronger magnetic fields; (d) higher gravi-ty than the outer planets have.

14. The planets that have rings also tend to have (a) solid sur-faces; (b) many moons; (c) slow rotation rates; (d) weakgravitational fields.

15. A solar system object of rocky composition and comparablein size to a small city is most likely (a) a meteoroid; (b) acomet; (c) an asteroid; (d) a planet.

16. The asteroids are mostly (a) found between Mars andJupiter; (b) just like other planets, only younger; (c) just likeother planets, only smaller; (d) found at the very edge of oursolar system.

17. The Sojourner Mars rover, part of the Mars Pathfinder mis-sion in 1997, was able to travel over an area about the size of(a) a soccer field; (b) a small U.S. city; (c) a very large U.S.city; (d) a small U.S. state.

18. To travel from Earth to the planet Neptune at more than30,000 mph, the Voyager 2 spacecraft took nearly (a) a year;(b) a decade; (c) three decades; (d) a century.

19. In the leading theory of solar-system formation, the plan-ets (a) were ejected from the Sun following a close en-counter with another star; (b) formed from the sameflattened, swirling gas cloud that formed the Sun; (c) aremuch younger than the Sun; (d) are much older than theSun.

20. The solar system is differentiated because (a) all the heavyelements in the outer solar system have sunk to the center;(b) all the light elements in the inner solar system becamepart of the Sun; (c) all the light elements in the inner solarsystem were carried off in the form of comets; (d) only rockyand metallic particles could form close to the Sun.

PROBLEMS

Algorithmic versions of these questions are available in the Practice Problems module of the Companion Website atastro.prenhall.com/chaisson.

The number of squares preceding each problem indicates its approximatelevel of difficulty.

1. ■ Use Newton’s law of gravity to compute your weight (a) onEarth, (b) on Mars, (c) on the asteroid Ceres, and (d) onJupiter (neglecting temporarily the absence of a solid surfaceon this planet!). • (Sec. 2.7)

2. ■ Only Mercury, Mars, and Pluto have orbits that deviatesignificantly from circles. Calculate the perihelion and aphe-lion distances of these planets from the Sun. • (More Pre-cisely 2-1)

3. ■■■ At closest approach, the planet Neptune lies roughly29.1 A.U. from Earth. At that distance, Neptune’s angulardiameter is Its moon Triton moves in a circular orbitwith an angular diameter of and a period of 5.9 days.Use these data to compute the radius, mass, and density ofNeptune, and compare your results with the figures given inTable 6.1.

4. ■■■ Use the data given in Table 6.1 to calculate the angulardiameter of Saturn when it lies 9 A.U. from Earth. Saturn’smoon Titan is observed to orbit from the planet. Whatis Titan’s orbital period?

5. ■ Suppose the average mass of each of the 7000 asteroids inthe solar system is about Compare the total mass ofall asteroids with the mass of Earth.

6. ■ Assuming a roughly spherical shape and a density ofestimate the diameter of an asteroid having the

average mass given in the previous question.7. ■■ A short-period comet is conventionally defined as a comet

having an orbital period of less than 200 years. What is themaximum possible aphelion distance for a short-periodcomet with a perihelion of 0.5 A.U.? Where does this placethe comet relative to the outer planets?

8. ■ How many times has Mariner 10 now orbited the Sun?9. ■■ The asteroid Icarus has a perihelion distance of 0.2 A.U.

and an orbital eccentricity of 0.7. What is Icarus’s apheliondistance from the Sun?

10. ■■■ A spacecraft has an orbit that just grazes Earth’s orbit atperihelion and the orbit of Mars at aphelion. What are theorbital eccentricity and semimajor axis of the orbit? How

3000 kg/m3,

1017 kg.

3.1¿

33.6–

2.3–.

long does it take to go from Earth to Mars? (The orbit givenis the so-called minimum-energy orbit for a craft leavingEarth and reaching Mars. Assume circular planetary orbitsfor simplicity.)

11. ■■■ Earth and Mars were at closest approach in Septem-ber 2003. The first Mars Exploration Rover was launched inJune 2003 and arrived at Mars in January 2004. Sketch theorbits of the two planets and the trajectory of the space-craft. Be sure to indicate the location of Mars at launchand of Earth when the spacecraft reached Mars. (For sim-plicity, neglect the eccentricity of Mars’s orbit in yoursketch.)

12. ■■ How long would it take for a radio signal to complete theround-trip between Earth and Saturn? Assume that Saturn isat its closest point to Earth. How far would a spacecraft or-biting the planet in a circular orbit of radius 100,000 kmtravel in that time? Do you think that mission control couldmaneuver the spacecraft in real time—that is, control all itsfunctions directly from Earth?

13. ■■ An interstellar cloud fragment 0.2 light-year in diameteris rotating at a rate of 1 revolution per million years. It nowbegins to collapse. Assuming that the mass remains constant,estimate the cloud’s rotation period when it has shrunk to (a)the size of the solar nebula, 100 A.U. across, and (b) the sizeof Earth’s orbit, 2 A.U. across.

14. ■■ By what factor would Earth’s rotational angular momen-tum change if the planet’s spin rate were to double? By whatfactor would Earth’s orbital angular momentum change if theplanet’s distance from the Sun were to double (assuming thatthe orbit remained circular)? The orbital angular momentumof a planet in a circular orbit is simply the product of theplanet’s mass, orbital speed, and distance from the Sun.

15. ■■ Consider a planet growing by the accretion of materialfrom the solar nebula. As the planet grows, its density re-mains roughly constant. Does the force of gravity at the sur-face of the planet increase, decrease, or stay the same?Specifically, what would happen to the surface gravity andescape speed as the radius of the planet doubled? Give rea-sons for your answer.

In addition to the Practice Problems module, the Companion Website at astro.prenhall.com/chaisson provides for eachchapter a study guide module with multiple choice questions as well as additional annotated images, animations, andlinks to related Websites.