C H A P T E R 7

Photographed by NASA's MarsReconnaissance Orbiter, this shaftdescends at least 78 m (255 ft),and probably much deeper, into theside of volcano Arsia Mons.Similar features form on Earth whendeep underground caves collapse.(NASA/JPL/University of Arizona)

The OtherTerrestrial Planets

WHAT DO YOU THINK?

1 Which terrestrial planet—Mercury,Venus, Earth, or Mars—has thecoolest surface temperature?

2 Which planet is most similar in sizeto Earth?

3 Which terrestrial planet—Mercury,Venus, Earth, or Mars—has thehottest surface temperature?

4 What is the composition of theclouds that surround Venus?

5 Does Mars have liquid water on itssurface today? Did it have liquidsurface water in the past?

6 Is life known to exist on Marstoday?

Answers to these questions appear in thetext beside the corresponding numbers inthe margins and at the end of the chapter.

Every human being isunique: Each personhas his or her own

genetic makeup and personalhistory. On the other hand,people have many similari-ties: we are either male or

female; we all breathe air; and, under normal circum-stances, we have common characteristics, such as twoeyes, two hands, and ten toes, among other traits. Tounderstand humans fully, biologists and physiciansstudy our common features and then our individualpeculiarities, just as psychiatrists and psychologistsstudy our common behaviors and then our differences.

The planets in our solar system also have similari-ties and differences that astronomers are learning tounderstand. For example, there are two basic groups ofplanets: terrestrial worlds (Mercury, Venus, and Mars),similar in size and chemistry to Earth, and the muchlarger giant or Jovian worlds (Jupiter, Saturn, Uranus,and Neptune).

In this first of two chapters on the rest of the plan-ets, we explore the terrestrial worlds both individuallyand in comparison to each other and to Earth. Theremaining four outer planets are presented in the nextchapter.

its proximity to the Sun. We can see it from Earth onlyfor a few hours before sunrise or a few hours after sun-set because its angle from the Sun (its elongation) isalways less than 28°.

7-1 Photographs from Mariner 10 revealMercury’s lunar-like surface

Until late last century, Mercury (Figure 7-1)was a complete mystery. Earth-based telescop-ic studies provided virtually no information

about the planet’s surface. Despite having been visitedthree times by the Mariner 10 spacecraft since 1974, westill haven’t seen 55% of its surface. The cratered surfacethat we have seen is reminiscent of our Moon.

Astronomers conclude that most of the craters onboth Mercury and the Moon were produced by impactsin the first 800 million years after these bodies con-densed from the solar nebula. Like the Moon, butunlike any other planet, the pounding that Mercuryreceived back then created what we see today. Ourstrongest evidence for believing this scenario and thetimescale of this cratering comes from analyzing anddating Moon rocks (see Chapter 6). Like the Moon,Mercury has an exceptionally low albedo of 0.12 (12%scattering of incoming light). It is very bright as seen

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EB LINK 7.1

In this chapter you will discover• Mercury, a Sun-scorched planet with a heavily

cratered surface and a substantial iron core

• Venus, perpetually shrouded in thick, poisonousclouds and mostly covered by gently rolling hills

• Mars, a red, dusty planet that once had runningwater on its surface and may still have liquidwater underground

MERCURYThe closest planet to the Sun, Mercury, is atruly inhospitable world of temperatureextremes. Its incredibly thin atmosphere

leaves it unprotected from countless impacts and a con-tinuous bath of deadly solar radiation. Despite having asurface nearly as dark as coal, Mercury sometimesappears as one of the brightest objects in our sky, due to

ANIMATION 7.1

Planet symbol:Mercury

FIGURE 7-1 Mercury Heavily cratered Mercurywas visited three times by the Mariner 10 spacecraftin 1974 and 1975. Nevertheless, we have images

of only about half of the planet’s surface. Although most of theseimages were recorded at distances of about 200,000 km(125,000 mi) from Mercury, their resolution is still vastly supe-rior to that of the best Earth-based images. (Astrogeology Team,U.S. Geological Survey)

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EB LINK 7.2

The Other Terrestrial Planets 183

from Earth only because the sunlight scattering fromMercury is so intense.

First impressions of Mercury evoke a lunar land-scape, and there are several similarities. Craters in theMoon’s highlands are densely packed, as are some ofthe craters on Mercury (Figure 7-2). The craters on bothworlds have similar features, such as ejecta blankets andcentral peaks, as discussed for the Moon in Section 6-5.

The most impressive feature discovered by Mariner10 is a huge circular region, called Caloris Basin, sonamed because caloris is Latin for “heat,” and this fea-ture is the closest part of Mercury to the Sun at the plan-et’s perihelion. Caloris basin measures 1300 km (810 mi)in diameter, meaning that it has twice the area of Texas.Lying along the terminator (the border between day andnight) in Figure 7-3a, you can see that it is surrounded bya 2-km-high ring of mountains, beyond which are rela-tively smooth plains. Like the lunar maria, Caloris Basinwas probably gouged out by the impact of a large mete-orite that penetrated the planet’s crust. Because relativelyfew craters pockmark the lava flows that filled the basin,the Caloris impact must have occurred toward the end ofthe major crater-making period.

The Caloris impact was a tumultuous event thatshook the entire planet. Indeed, the collision affectedthe side of Mercury directly opposite of Caloris Basin.That area (Figure 7-3b) has a jumbled, hilly surface cov-ering nearly half a million square kilometers, about

Mercury Our Moon

FIGURE 7-2 Mercury and Our Moon Mercury and ourMoon are shown here to the same scale. Mercury’s radius is2439 km and the Moon’s is 1738 km. For comparison, thedistance from New York to Los Angeles is 3944 km (2451mi). Mercury’s surface is more uniformly cratered than that ofthe Moon. Daytime temperatures at the equator on Mercuryreach 700 K (800°F), hot enough to melt lead or tin. (NASA,UCO/Lick Observatory)

Rings ofRings ofmountainsmountains

500 km500 km

HillsHills

Crater PetrarchCrater Petrarch

500 km500 km

c Mare Orientale on our Moon

This side showswhat the restof Caloris basinprobably lookslike.

This side is analogous toCaloris basinon Mercury.

Rings ofmountains

500 km

FIGURE 7-3 Major Impacts on Mercury and on Our Moon (a) CalorisBasin Mariner 10 sent back this view of a huge impact basin onMercury’s equator. Only about half the Caloris Basin appears becauseit happened to lie on the terminator when the spacecraft sped past theplanet. Although the center of the impact basin is hidden in the shad-ows (just beyond the left side of the picture), several semicircular ringsof mountains reveal its extent. (b) Unusual, Hilly Terrain What look liketiny, fine-grained wrinkles on this picture are actually closely spaced

hills, part of a jumbled terrain that covers nearly 500,000 square kilo-meters on the opposite side of Mercury from the Caloris Basin. Thelarge, smooth-floored crater, Petrarch, has a diameter of 170 km (106mi). This impact crater was produced more recently than Caloris Basin.(c) Mare Orientiale on the Moon This giant impact feature on our moonis 900 km (560 mi) in diameter and is very similar to Caloris Basin onMercury. There is also terrain similar to that shown in (b), on the oppo-site side of the Moon from Mare Orientale. (NASA)

a b

twice the size of Wyoming. The hills, which appear astiny wrinkles that cover most of the photograph, areabout 5 to 10 km wide and between 100 and 1800 mhigh. Geologists believe that energy from the Calorisimpact traveled through the planet and became focused,like light through a lens, as it passed through Mercury.As this concentrated energy reached the far surface ofthe planet, jumbled hills were pushed up. A similar pairof phenomena to Caloris and the jumbled terrain on theother side of Mercury can be seen in our Moon’s MareOrientale (Figure 7-3c) and chaotic hills on the oppositeside of that world. Finding such a similar pair of phe-nomena on a different world supports the connectionbetween the impact and the jumbled surface.

Looking at Mercury in more detail, we start to see avariety of differences from the features on the Moon.Unlike the Moon and its maria, Mercury lacks extensivecraterless regions. Instead of maria, Mercury has plainswith relatively small craters. Such plains are not foundon the Moon. Figure 7-4 shows a typical close-up viewof Mercury. As we learned in Section 6-7, the lunarmaria were produced by extensive lava flows thatoccurred between 3.1 and 3.8 billion years ago. Ancient

lava flows also probably formed theMercurian plains. As large meteoritespunctured the planet’s thin, newlyformed crust, lava welled up from themolten interior to flood low-lyingareas. The existence of craters pitting

Mercury’s plains suggests that these plains formed justover 3.8 billion years ago, near the end of the era ofheavy bombardment. Mercury’s plains are thereforeolder than most of the lunar maria, leaving more timefor cratering to eradicate maria-like features.

Mariner 10 also revealed numerous, long cliffs,called scarps, meandering across Mercury’s surface(Figure 7-5). The scarps are believed to have developedas the planet cooled. Almost everything that cools, con-tracts. Therefore, as Mercury’s mantle and molten ironcore cooled and contracted, its surface moved inward.Because it was solid, Mercury’s crust could not collapseuniformly. Instead, it wrinkled as it contracted, formingthe scarps. These features and the lack of recent vol-canic activity suggest that the planet’s interior is solid toa significant depth. Otherwise, lava would have leakedout as the scarps formed.

7-2 Mercury has a higher percentage ofiron than does Earth

Mercury’s average density of 5430 kg/m3 is quite simi-lar to Earth’s (5520 kg/m3). As we saw in Section 6-3,typical rocks on Earth’s surface have a density of onlyabout 3000 kg/m3 because they are composed primari-ly of lightweight elements. The high average densities ofboth Mercury and our planet are caused by their denseinteriors.

Because Mercury is less dense than Earth, you mightconclude that Mercury has a lower percentage of iron, a

184 CHAPTER 7

Intercraterplains

100 km

FIGURE 7-4 Mercury’s Craters and Plains Thisview of Mercury’s northern hemisphere was takenby Mariner 10 as it sped past the planet in 1974.

Numerous craters on the bottom half of the image and broadintercrater plains on the top half cover an area 480 km (300mi) wide. (NASA)

50 km3. This crater was distorted when the scarp formed.

2. Some time after the lava cooled, Mercury’s crust contracted to form this scarp.

1. The floors of these craters were flooded by lava from Mercury’s interior.

FIGURE 7-5 Scarps on Mercury A long, mean-dering cliff, called Santa Maria Rupes, runs fromnorth to south across this Mariner 10 image of a

region near Mercury’s equator. This cliff, called scarp, bygeologists, is more than 1 km high and runs for several hun-dred kilometers. Note how the crater in the center of theimage was distorted vertically when the scarp formed. (NASA)

VIDEO 7.1

ANIMATION 7.2

The rings of mountainsfrozen into Mercury’s sur-face have what transientanalog on Earth?

The Other Terrestrial Planets 185

heavy element, than does our planet. However, in per-cent of its total mass, Mercury is the most iron-rich plan-et in the solar system. Only Earth’s greater mass, press-ing inward and thereby compressing our planet’s innerparts, makes it denser than Mercury.

Figure 7-6 shows a scale drawing of Mercury’s inte-rior, where an iron core fills 42% of the planet’s vol-ume. Surrounding the core is a 600-km-thick rockymantle. For comparison, Earth’s iron core occupies only17% of the planet’s volume. How much of Mercury’score is molten is still unknown.

Events early in Mercury’s history must somehowaccount for its high iron content. We know that the inner

regions of the primordial solar nebula were incrediblyhot. Perhaps only iron-rich minerals were able to with-stand the solar heat there, and these subsequentlyformed iron-rich Mercury. According to another theory,an especially intense outflow of particles from the youngSun stripped Mercury of its low-density mantle shortlyafter the Sun formed. A third possibility is that, duringthe final stages of planet formation, Mercury was struckby a large planetesimal (debris formed as the solar sys-tem came into being), just as Earthwas struck by a Mars-sized body thatled to the formation of our Moon.Computer simulations show that thiscataclysmic collision would haveejected much of Mercury’s lightermantle (Figure 7-7).

7-3 Mercury’s rotation and revolution are coupled

Mercury has one of the most unusual orbits in thesolar system. Recall from Section 6-8 that when Earthwas young, its gravitational force created tides on theMoon, thereby forcing the Moon into synchronousrotation. Something similar (but not identical) hap-pened with the Sun playing the role of Earth, andMercury acting in the place of our Moon. To beginwith, the Sun created a significant tidal bulge onyoung, molten Mercury that locked into place whenthe planet solidified. Mercury is so close to the Sun(average separation: 0.387 AU) that the Sun’s gravita-tional force on Mercury’s tidal bulges changed theplanet’s rotation rate. However, Mercury’s highlyeccentric orbit (e ! 0.21) prevented the planet frombeing locked into synchronous orbit like our Moon.

Mantle

Mantle

Mercury

Earth

Earth’s iron coreis 55% of thediameter of theentire planet, or17% of itsvolume...

...whereas Mercury’siron core is about75% of the planet’sdiameter, or 42% ofits volume

FIGURE 7-6 The Interiors of Earth and Mercury Mercury hasthe highest percentage of iron of any planet in the solar sys-tem. Consequently, its iron core occupies an exceptionallylarge fraction of its interior.

Mantle oforiginal mercury

Core oforiginal mercury

Planetesimal

Time = 2.24 min Time = 8.22 min

Remaining core

Accumulatingthin crust

Time = 3 h 14 min

FIGURE 7-7 The Stripping of Mercury’s Mantle To account forMercury’s high iron content, one theory proposes that a collision with amassive planetesimal stripped Mercury of most of its rocky mantle. Thesethree images show a computer simulation of a nearly head-on collisionbetween proto-Mercury and a body one-sixth its mass. Both worlds are

shattered by the impact, which vaporizes much of their rocky mantles.Mercury eventually re-forms from the remaining iron-rich debris. The restof the original Mercury and the impactor left this area of the solar system.(a, b, and c: Royal Astronomical Society)

If Mercury were struck by alarge planetesimal, whymight that planet not havehad a moon form, as Earthdid when it was struck?

Instead, Mercury developed what is called a 3-to-2spin-orbit coupling. This means that Mercury under-goes three sidereal rotations (rotations measured withrespect to the distant stars, not the Sun), while under-going two revolutions around the Sun (Figure 7-8).

The major consequence of spin-orbit coupling isthat one or the other of Mercury’s regions of high tideis facing the Sun whenever the planet is at perihelion(see Figure 7-8). There is some slight variation in eachorbit so that the exact high tide doesn’t always pointtoward the Sun.

A Day on Mercury Is Two Years Long The motion ofthe Sun across Mercury’s sky is unique in the solar sys-tem. First, a solar day there (noon to noon) is 176 Earthdays long, twice the length of a year! Furthermore, if youwere to set up a camera at the location of high tide onMercury to take a series of pictures when the planet ispassing through perihelion, you would see the Sun startto rise in the east, stop high in the sky, move back towardthe east, stop again, and then resume its westward jour-ney. This is analogous to the retrograde motion of theplanets that we observe from Earth (see Section 2-2).

Mariner 10 discovered that Mercury has a weakmagnetic field. Explaining it presents a challenge

to astronomers because the dynamomodel used to explain Earth’smagnetic field doesn’t work here. Aswe saw in Section 6-4, electric cur-rents in a planet’s liquid iron core

create a planetwide magnetic field. Earth creates its elec-tric current by rotating once a day. Mercury rotates 59times more slowly, which is hardly fast enough to gener-ate the observed magnetic field.

Mercury is the planet from which we have the leastobservational data. That is in the process of changing.In August 2004, the Messenger spacecraft was launchedtoward Mercury and passed by that planet on January14, 2008. Swinging around, Messenger will settle intoorbit around Mercury in 2011.

7-4 Mercury’s atmosphere is the thinnest ofall terrestrial planets

Mercury’s mass is only 5.5% that of Earth. Like ourMoon, the force of gravity is too weak on Mercury tohold a permanent atmosphere, but trace amounts of fivedifferent gases have been detected around Mercury.Scientists think the Sun is the source of the hydrogenand helium gas near it, while sodium and potassium gasescape from rocks inside the planet (a process, calledoutgassing, which also occurs on Earth). Oxygenobserved in Mercury’s atmosphere may come frompolar ice that is slowly evaporating. All of these gasesdrift into space and are continually being replenished inthe atmosphere from their respective sources. The aver-age density of this atmosphere is at least 1017 times lessdense than the air we breathe.

186 CHAPTER 7

Mercury

Sun

One sidereal day since timestep 1

18

14

159

13

10

12

Perihelion(closest to the Sun)

2 7

3 6

4 5

Year 1a

One year since timestep 1

11

Mercury

Sun

Year 2

Two sidereal days since timestep 1

b

15 22

28

23

27

24

2625

16 21

1720

18 19

Perihelionclosest toSun

Three siderealdays = two yearssince timestep 1 = one solar day

29

FIGURE 7-8 3-to-2 Spin-Orbit Coupling Mercuryundergoes three sidereal rotations every twoyears. You can see this by following an observer

on Mercury (a) from time 1 to time 15 and then (b) from 15 to29. The observer points to the right 4 times during this interval(at time 1, between times 10 and 11, 19 and 20, and, finally,

at 29). This means that Mercury has rotated three times in exact-ly two sidereal Mercurian years. Because a sidereal year is 88Earth days long, a sidereal day on Mercury is 58.7 Earth dayslong. During the same interval, however, the Sun is at noon asseen from the observer’s location only twice: at time 1 and time29. Therefore, a solar day is 176 Earth days long.

ANIMATION 7.3

Why isn’t Mercury in syn-chronous rotation withrespect to the Sun?

The Other Terrestrial Planets 187

Mercury’s Temperature Range Is the Most Extreme inthe Solar System Earth’s thick atmosphere stores heat,which helps explain why it feels warm on a cloudy day ornight. Nevertheless, Earth continually loses heat intospace. Because of Earth’s rapid daily rotation, this lostheat is quickly replaced by the Sun during daylight hours,so the average temperature change between day andnight on Earth is only about 11 K (20°F). Because ofMercury’s slow rotation and minimal atmosphere, thedifferences in temperature between day and night thereare far more noticeable than on Earth. At noon onMercury, the surface temperature is 700 K (800°F). Atthe terminator, where day meets night, the temperature isabout 425 K (305°F), and, on the night side, the temper-ature falls as low as 100 K ("280°F), the coldest temper-ature on any terrestrial planet! The resulting daily rangeof temperature is therefore 600 K (1080°F) on Mercury.

Just as astronomers use scattered radio waves tosearch for water on the Moon (see Section 6-6), they havealso sent radio waves to Mercury. In 1992 they made anextraordinary discovery—evidence for ice near Mercury’spoles, in craters that are permanently in shadow. Dozensof circular regions, presumably craters, sent back signalswith characteristics distinct to ice. Confirmation of thepresence of this ice will have to wait until the Messengerspacecraft begins orbiting it. If the ice is there, its origin—whether from cometary impacts, from gases rising frominside the planet and then freezing, or from bothsources—also remains to be determined.

between the two planets leads to a host of others, mak-ing Venus inhospitable to life.

7-5 The surface of Venus is completelyhidden beneath a permanentcloud cover

At nearly twice the distance from the Sun as Mercury,Venus is often easy to view without interference from theSun’s glare. At its greatest western elongation, Venus isseen high above the western horizon after sunset, where,like Mercury, it is often called the “evening star.”Conversely, high in the eastern sky before sunrise, it iscalled the “morning star.”

Venus is easy to identify because it is usually one ofthe brightest objects in the night sky. Only the Sun andthe Moon outshine Venus at its greatest brilliance.Venus is often mistaken for a UFO (Figure 7-9), becausewhen it is low on the horizon, its bright light is strong-ly refracted by Earth’s atmosphere, making it appear torapidly change color and position.You can see the appearance of theinner planets using Starry NightEnthusiast™ (see also GuidedDiscovery: The Inner Solar System).

INSIGHT INTO SCIENCEModel-Building In modeling real situations, scientistsmust consider several different effects simultaneously.Omit any crucial property and you get inaccurateresults. For example, consider how long ice canremain at Mercury’s poles. Astronomers must takeinto account the planet’s distance from the Sun, the tiltof its axis of rotation, its rotation rate, surface fea-tures, whether the ice is exposed or mixed with othermaterial, and the chemical composition of Mercury.

VENUSOne of the few things that Mercury and Venus have incommon is that neither has a moon. Venus and Earth,on the other hand, have much in common. They havealmost the same mass, the same diameter, and the sameaverage density. Indeed, if Venus were located at thesame distance from the Sun as is Earth, then it, too,might well have evolved life. However, Venus is 30%closer to the Sun than Earth, and this one difference

FIGURE 7-9 UFO? Just Venus Venus is bright and is oftenseen near the horizon where rising and sinking gases inEarth’s atmosphere make it appear to move and change color,like alleged UFOs. (Joe Orman)

1

2

What technology would giveus direct information aboutMercury’s interior?