Chapter 1 Repair

8/11/2019 Chapter 1 Repair

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Chapter 1: The Earth Planets

1.1.

The Scientific Method

The principle objective of science is to discover the fundamental patterns of the

natural world. In trying to find the reasons underlying natural phenomena, scientists use

a distinctive strategy called the scientific method.

Step1. Recognizing an unsolved problem: Choosing a certain field to work in; in

many cases interpreting the data overlap.

Step 2. Making observation (collecting data) to try to solve the problems. They

gather all available information bearing on their subjects, such as measurements and

descriptions taken in the field and the results of laboratory experiments

Step 3. Formulating a hypothesis. After making observation, they develop a

hypothesis, a tentative (or untested) explanation that fit all of the data collected and is

expected to account for future observation as well. Often, a number of different

competing hypotheses are proposed to explain the same data.

Step 4. Testing the hypothesis: Hypotheses are tested over time as scientists

conduct further experiments and make further observation. If a hypothesis does not

explain subsequent findings, it must be modified or abandoned. A hypothesis that is

retained and may become a theory, an explanation that has remained consistent with all

the data and gained wide acceptance within the scientific community. So, the theory is a

scientific idea that has passed numerous tests and failed none. Even after a hypothesis

survives testing and becomes a theory sometimes new data become available perhaps

as a result of updated technologythat are not consistent with the theory. Scientists then

propose new hypotheses, modifying or completely replacing the established theory. A

theory that continuous to meet rigorous testing over a long period of time may be

declared a scientific law.

1.2.

The Probably Origin of the Sun and Its Planets

1.2.1.The Birth of the Solar System

Cosmologists (scientists who study the origin of the universe) have proposed that

the universe began as a very small, very hot volume of space containing an enormous

amount of energy. Many scientists believe that the birth of all the matter in universe

occurred when this space expanded rapidly with a Big Bang roughly 12 billion years

ago. Immediately after the Big Bang, they suggest, the universe began to expand and


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cool, which it continuous to do today. About a million years after Big Bang, when the

universe cooled sufficiently to allow the first atom to form, the universe consisted of

about 75% Hydrogen gas and 25% Helium gas, just it does today. As the universe

continued to expand, pockets of relatively high gas concentration began to form because

of gravitational attraction among the gas particle. Where enough gas gathered, the

resulting gas clouds collapsed inward from the force of gravity and created galaxies and

clusters of galaxies.

Within each galaxy, such as our own Milky Way, some gas clouds collapsed

further to form stars. Even today, stars continue to be born in this way in all galaxies,

including our own (Through a telescope, you can see a star nursery in the belt of the

constellation Orion, for example) As the gas within each star collapsed under gravity,

sufficient heat was generated to fuse together particles within a core, a phenomenonknown as nuclear fusion. Such nuclear fusion produces the light we see when we look at

stars, including the Sun.

The Sun is a star about 5 billion years old. The universe is at least twice and

possibly three times as old as the Sun, and so the Sun is a relatively young star. The birth

throes of the Sun and its planets were probably similar to those of billions of other stars,

but some of the details remain uncertain. Scientists hypothesize that the solar system

formed from a huge, rotating cloud of cosmic gas. One of the key questions that a

hypothesis needs to answer is why the Sun and the planets have different compositions.

Stars, includ ing the Sun, consist largely of the two lightest chemical elements, hydrogen

and helium. Rocky planets like the Earth, Mars, and Venus, on the other hand, consist

largely of heavier elements such as carbon, oxygen, silicon, and iron.

One clue concerning the origin of the solar system is provided by the discovery

that stars which formed during the earliest moments of the universe contained only the

lightest chemical element, hydrogen. From that observation scientists conclude that,

initially, hydrogen was the only chemical element in the universe. Stars generate lightand heat through nuclear fusion, a process by which hydrogen atoms combine to form

helium. As a star ages, hydrogen and helium atoms can combine through nuclear fusion

to form still heavier elements. Indeed, the only way elements heavier than helium can

form is by nuclear fusion inside stars, and the amounts so formed are tiny by comparison

with the amounts of hydrogen and helium present in the universe.


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In order for rocky planets to form, the heavy elements inside old stars must

somehow be separated from the remaining hydrogen and helium. One hypothesis about

the way separation occurs involves a massive star explosion called a supernova (Fig.

1.1). Astronomers have discovered and photographed the scattered remains of many

exploded stars, and what they observe is that all of the hydrogen, helium, and heavier

elements are scattered into space in a vast cos mic gas cloud. The next step in the

process is the formation of a new star and a planetary system from the debris of the

cosmic cloud.

Figure 1.1 A supernova.

We don't know whether the hydrogen now in the Sun and the heavy atoms now in

the planets were formed in one ancient star or in several, but scientists have estimated

that the atoms now in the Sun and the Earth were part of a cosmic cloud about 6 billion

years ago. Though thinly spread, the scattered atoms formed a tenuous, turbulent,

swirling cloud of gas. Over a very long period of time, the gas thickened as a result of a

slow re-gathering of the thinly spread atoms. The gathering force of the gas was gravity,


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and as the atoms moved closer together, the gas became hotter and denser as a result of

compression. Near the center of the gathering cloud of gas, hydrogen atoms eventually

became so tightly pressed and the temperature so high that nuclear burning started again

and a new star was born. Surrounding the new sun was a flattened rotating disc of gas

and dust, named a solar nebula (Fig. 1.2).

Figure 1.2:

Formation of a

planetary nebula. The

gathering of atoms in

space created a

rotating cloud of dense

gas. The center of thegas cloud eventually

became the Sun; the

planets formed by

condensation of the

outer portions of the

gas cloud.

By the time the Sun started burning, about 5 billion years ago, the cooler outer

portions of the solar nebula had become compacted enough to allow solid objects to

condense in the same way that ice condenses from water vapor. The solid condensates

eventually became the planets, moons, and all the other objects of the solar system. The

planets and moons nearest the Sun, where temperatures are highest, consist mostly of


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compounds that can condense at high temperatures, mainly silicates, oxides, and iron-

nickel alloys. Farther away from the Sun, where the temperatures are lower, only more

volatile constituents like sulfur, water and methane were able to condense (Fig. 1.3).

Figure 1.3: Temperature gradient in the planetary nebula. Close to the Sun,

temperatures reached 2000 K and only oxides, silicates, and metallic iron and nickel

condensed to form planets. Farther away, in the region of Jupiter and Saturn,

temperatures were low enough for ices of water, ammonia, and methane to condense.

Condensation of a cosmic gas cloud is only one piece of the planetary birth puzzle.Condensation formed a cosmic snow of innumerable small rocky fragments, but the

fragments still had to be joined together somehow in order to form the cosmic snowballs

that we call planets. This apparently happened through impacts between fragments drawn

together by gravitational attraction. The growth process - a gathering of more and more

bits of solid matter from surrounding space - is called planetary accretion. Scientists

estimate that condensation of the solar nebula and planetary accretion was complete

about 4.6 billion years ago. The revolutions and rotations of the Sun, planets, and moons

are inherited from the rotation of the cosmic gas cloud. As the cloud thinned, the planets

and moons all formed within the same disk, so that their orbits are all coplanar, or in the

same plane. All the planets revolve around the Sun in the same direction. The planets

can be separated into two groups based on density and closeness to the Sun The

innermost planetsMercury, Venus, Earth, and Marsare small, rocky, and dense.


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Because they are all similar in composition to our Earth, they are called the terrestrial

planets. (Terra is the Latin for Earth.)

The asteroids are also rocky, dense bodies, but they are too small to be called

planets. The asteroids have orbits that fall in this gap, and astronomers hypothesize thatthey are rocky fragments that failed to accrete into a planet. The planets farther from the

Sun than Mars are much larger than the terrestrial planets, yet much less dense. These

jovian planets Jupiter, Saturn, Uranus, and Neptunetake their name from Jove, an

alternative designation for the Roman god Jupiter.

Table 1: The planets and their properties.

Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune

Diameter (km) 4880 12,104 12,756 6787 142,800 120,000 51,800 49,500

Mass(Earth=1) 0.055 0.815 1 0.108 317.8 95.2 14.4 17.2

Density, g/cm3 (water=1) 5.44 5.2 5.52 3.93 1.3 0.69 1.28 1.64

Number of moons 0 0 1 2 16 18 15 8

Length of day (in Earth hours) 1416 5832 24 24.6 9.8 10.2 17.2 16.1

Period of one revolution around Sun (inEarth years) 0.24 0.62 1.00 1.88 11.86 29.5 84.0 164.9

Average distance from Sun (millions of

kilometers)58 108 150 228 778 1427 2870 4497

Average distance from sun(astronomical units) 0.39 0.72 1.00 1.52 5.20 9.54 19.18 30.06

1.2.2. Evolution of the Planets

Space missions have provided abundant evidence that all the objects in the solar

system formed at the same time and from a single solar nebula. During the final phase of

planetary accretion, the Moon and the four terrestrial planets became so hot that they all

underwent a period of partial melting. As a result, they separated into layers of different

composition. The thick, cloud-encircling atmospheres of the jovian planets obscure

details of the evolutionary history of those planets, so the following remarks refer only to

the Moon and the terrestrial planets.

During and after melting and compositional separation, the Moon and the four

terrestrial planets continued to be struck by rains of meteorites. Although meteorite

impacts still do happen, the period of nearly continuous massive impacts ended more than


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4 billion years ago. From about 4 billion years ago to the present, the terrestrial planets

and the Moon seem to have evolved along somewhat different paths.

Three key factors played the determining roles in the evolution of the terrestrial

planets. First, after partial melting, the planets remained hot inside because radioactiveelements were and still are present. All the terrestrial planets are cooling down, but the

rates of cooling are determined by the sizes of the planets and the rates vary greatly. The

largest planets, Venus and the Earth, are cooling very slowly and therefore are still

relatively hot today. One important indication of high internal temperature is volcanism,

which continues on the Earth and possibly on Venus. Volcanic activity has occurred on

Mars within the past billion years, but Mars is probably not active today. Both the Moon

and Mercury, the two smallest bodies, have been volcanically dead for billions of years.

The second factor that controlled the way the terrestrial planets evolved is their

distance from the Sun. The Sun-planet distance determines whether or not H2O can exist

as water and hence whether or not there can be oceans. The two planets closest to the

SunMercury and Venusare too hot for liquid water to occur. Venus does have H2O

in its atmosphere, but the temperature at the surface of Venus is close to 500C or

(932F). Mars, which is farther from the Sun than is the Earth, is too cold to have liquid

water but does have ice.

The third factor is the presence or absence of a biosphere. The hydrosphere andthe biosphere play essential roles in biogeochemical cycles that control the composition

of the atmosphere. If life had evolved on Venus, that planet might have developed an

atmosphere like the Earth's. On the Earth, plants and microorganisms have enabled

carbon dioxide and water to combine, through photosynthesis, to make organic matter

and oxygen. The burial of organic matter in sediment in effect removes carbon dioxide

and at the same time adds oxygen to the atmosphere. Because life did not develop on

Venus, all of the CO2 is still in the atmosphere, and as a result Venus suffers from a

horrendous greenhouse effect.

The Earth system and its many parts came into being a long time ago. What that

system is today, and how the many parts interact, are very much a product of the Earth's

long history and of the two great heat engines that drive it: the solar engine, which has


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warmed the Earth's surface for the last 4.6 billion years, and the internal heat engine,

which drives all the activities of the solid Earth.

1.2.3. The terrestrial planets

Each of the terrestrial planets and the Moon have the same gross structure,

consisting of three layers distinguished by differences in composition.

Layers of Different Composition

The structure common to all the planets is most clearly demonstrated in the Earth

(Fig. 1.4). At the center is the densest of the three layers, the core, a spherical mass

composed largely of metallic iron, with lesser amounts of nickel and other elements.

The thick shell of dense, rocky matter that surrounds the core is called the mantle. The

mantle is less dense than the core but denser than the outermost layer. Above the mantle

lies the thinnest and outermost layer, the crust, which consists of rocky matter that is less

dense than mantle rock.

Each of the terrestrial planets has a core, mantle, and crust, but there are

considerable differences in detail, particularly in the crust. For example, Figure 1.4 shows

that the core and the mantle of the Earth have nearly constant thicknesses, but the crust is

far from uniform and differs in thickness from place to place by a factor of nine. The

crust beneath the oceans, the oceanic crust, has an average thickness of about 8 km (5

mi), whereas the continental crust averages 45 km (28 mi) and ranges from 30 to 70 km

(19 to 44 mi) in thickness. The two different kinds of crust are the result of the special

internal processes that shape the Earth's surface, and in particular, plate tectonics. The

crusts of the other terrestrial planets are thicker than the Earth's crust and approximately

uniform in thickness. The uniformity of thickness is an indication that plate tectonics

does not, and probably never has, been active on any of the other terrestrial planets.

Because we cannot see and sample either the core or the mantle of a planet, it is

valid to ask how we know anything about their composition. The answer is that indirect

measurements are used, and again the Earth is used as an example. One way to determine

composition is to measure how the density of rock changes with depth below the Earth's

surface. We can do this by measuring the speeds with which earthquake waves pass

through the Earth because the speeds are influenced by rock density. At some depths,

abrupt changes in the speed of earthquake waves indicate sudden changes in density.


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From the sudden changes, we infer that the solid Earth consists of distinct layers with

different densities. Knowing these different densities, we can estimate what the

composition of the different layers must be.

Slight compositional variations probably exist within the mantle, but we knowlittle about them. We can see and sample the crust, however, and the sampling shows

that, even though the crust is quite varied in composition, its overall composition and

density are very different from those of the mantle, and the boundary between them is

distinct.

The composition of the core presents the most difficulty. The temperatures and

pressures in the core are so great that materials there probably have unusual properties.

Some of the best evidence concerning core composition comes from iron meteorites.

Such meteorites are believed to be fragments from the core of an asteroid, large enough

to be a planet, that was shattered by a gigantic impact early in the history of the solar

system. Scientists hypothesize that this now-shattered asteroid must have had

compositional layers similar to those of the Earth and the other terrestrial planets.

Layers of Different Rock Strength

In addition to compositional layering, the sphere that is our Earth can be divided

into three layers based on differences in the strength of the rock that makes up each layer:

the mesosphere, asthenosphere, and lithosphere (Fig. 1.4).

The strength of a solid is controlled by both temperature and pressure. When a

solid is heated, it loses strength; when it is compressed, it gains strength. Differences in

temperature and pressure divide the mantle and crust into three distinct strength regions.

In the lower part of the mantle, the rock is so highly compressed that it has considerable

strength, even though the temperature is very high. Thus, a solid region of high

temperature but also relatively high strength exists within the mantle from the core-

mantle boundary (at 2883 km, or 1791 mi depth) to a depth of about 350 km (218 mi) andis called the mesosphere ("intermediate, or middle, sphere") (Fig. 1.4).

Within the upper mantle, from 350 to about 100 km (218 mi to 62 mi) below the

Earth's surface, is a region called the asthenosphere ("weak sphere"), where the balance

between temperature and pressure is such that rocks have little strength. Instead of being

strong, like the rocks in the mesosphere, rocks in the asthenosphere are weak and easily


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deformed, like butter or warm tar. As far as geologists can tell, the compositions of the

mesosphere and the asthenosphere are the same. The difference between them is one of

physical properties; in this case, the property that changes is strength.

Figure 1.4 A sliced view of the Earth reveals layers of different composition and zones

of different rock strength. The compositional layers, starting from the inside, are the core,

the mantle, and the crust. Note that the crust is thicker under the continents than under the

oceans. Note, too, that boundaries between zones of different physical properties

lithosphere (outermost), asthenosphere, mesospheredo not coincide with compositional

boundaries.


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Above the asthenosphere, and corresponding approximately to the outmost 100

km (62 mi) of the Earth, is a region where rocks are cooler, stronger, and more rigid than

those in the plastic asthenosphere. This hard outer region, which includes the uppermost

mantle and all of the crust, is called the lithos phere ("rock sphere"). It is important to

remember that, even though the crust and mantle differ in composition, it is rock strength,

not rock composition, that differentiates the lithosphere from the asthenosphere.

The boundary between the lithosphere and the asthenosphere is caused by

differences in the balance between temperature and pressure. Rocks in the lithosphere are

strong and can be deformed or broken only with difficulty; rocks in the asthenosphere

below can be easily deformed. One analogy is a sheet of ice floating on a lake. The ice is

like the lithosphere, and the lake water is like the asthenosphere.

Layers of Different Physical State

Metallic iron in the Earth's core exists in two physical states. The solid center of

the Earth is the inner core. Pressures are so great in this region that iron is solid despite its

high temperature. Surrounding the inner core is a zone where temperature and pressure

are so balanced that the iron is molten and exists as a liquid. This is the outer core. The

difference between the inner and outer cores is not one of composition. (The composition

of the two is believed to be the same.) Instead, the difference lies in the physical states of

the two: one is a solid, and the other is a liquid.

Comparison of the Terrestrial Planets

The terrestrial planets, and possibly the Moon, seem to have had similar early

histories. Where ancient surfaces exist, as on the Moon, Mercury, and the southern haff

of Mars, evidence of a violent period of planetary accretion remains. Each body seems to

have experienced a period of heating during which a core formed. The striking feature

about the various cores, the sizes of which are calculated from the densities of the

planets, is how greatly they differ in relative size (Fig. 1.5A).The most remarkable bodyis Mercury, for on this planet the core is 42 percent of the volume and an estimated 80

percent of the mass. At present, we cannot assert with any certainty whether any of the

terrestrial planets besides the Earth have molten or partially molten cores. The molten

outer core and the relatively rapid rotation of the Earth give rise to the Earth's strong


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magnetic field. Magnetic fields do exist on the other planets, but they are much weaker

than the Earth's field.

Spacecraft have landed on the Moon, Mars, and Venus, and on those bodies

we have been able to make direct measurements of the crust. Fly by missions toMercury reveal that a crust is present there too. The existence of a core and a crust

suggests a mantle, and the necessary measurements have been made on the Moon,

Venus, and Mars to establish that indeed mantles are present. We can be reasonably

sure, therefore, that the structures of all the terrestrial planets are similar.

Figure 1.5: The internal

structures of the Moon and

the terrestrial planets. A.

Comparative sizes of the

cores. Mercury, nearest the

Sun, where only the

highest temperature

materials could condense,

has a huge core. Mars,

farthest away from the

Sun, has a small core. B.

Structure of the Moon.

Crusl composition is

known with certainty- only

in the vicinity of the

astronauts' landing sites.

Whether or not each terrestrial planet has a lithosphere, asthenosphere, and

mesosphere is a more difficult question to answer. Simple observation reveals that rocks


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on the surface of each planet fracture and deform as they do on the Earth, and this

indicates that a lithosphere is present. Astronauts left instruments on the Moon to

measure the properties of moonquakes, and from those measurements the presence of an

asthenosphere can be inferred (Fig.1.5B), but the presence of a mesosphere seems

unlikely. Measurements made on Mars have determined that an asthenosphere exists

there, too, but for Venus and Mercury it is possible only to infer the existence of an

asthenosphere. What little evidence we have suggests that asthenospheres and

lithospheres probably are present in each terrestrial planet but that the asthenosphere of

the Earth is unusually close to the surface and hence that the lithosphere is unusually thin.

It is probable that the Earth is such a dynamic planet because its lithosphere is thin. The

other terrestrial planets seem to have much thicker lithospheres and to be much less

dynamic than the Earth.

Venus, the Earth, and Mars are large enough that their gravitational fields have

been able to retain the atmospheres formed as a result of melting and out- gassing:

release of gases from rocks or other nongaseuos materials, especially through

volcanoes. Mercury and the Moon are too small to have held on to the gases given off,

and so they lack atmospheres.

1.2.4. The Jovian Planets

We cannot see anything that lies below the thick blankets of atmosphere that coverthe jovian planets. Therefore, we can only hypothesize about the irinternal structure,

based on remote-sensing measurements of various kinds. For example, we can calculate

that the masses of Jupiter and Saturn are so great that none of their atmospheric gases has

been able to escape their gravitational pulls. This is true even for the two lightest gases,

hydrogen and helium, which made up the bulk of the planetary nebula. This means,

therefore, that the bulk composition of the two largest jovian planets must be about the

same as that of the solar nebula from which they formed. For example, the composition

of Jupiter is estimated to be 74 percent hydrogen, 24 percent helium, and 2 percent heavyelements.

Because the moons of the jovian planets are rocky with thick sheaths of ice (Fig. 1.6), it

is presumed that a rocky mass resides at the center of each planet. The rocky cores of


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Jupiter and Saturn may be as large as 20 or more earth masses. Surrounding the rocky

cores is possibly a layer of ice, analogous to the ice sheaths seen on the moons (Fig. 1.7).

Figure 1.6 Europa, smallest of the four

large moons of Jupiter. Europa has alow density, indicating it contains a

substantial amount of ice. The surface is

mantled by ice to a depth of 100 km.

The fractures indicate that some internal

process must be disturbing and

renewing the surface of Europa. The

dark material (here appearing red) in the

fractures apparently rises up frombelow. The cause of the fracturing is not

known. The image was taken by

Voyager 2 in July 1979.

Figure 1.7 Comparison of the probable interior structures of Jupiter and Saturn.

Pressures inside the jovian planets must be enormous; we may therefore

hypothesize that deep in the interiors hydrogen may be so tightly squeezed that it is


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condensed to a liquid. Proceeding inward from the outer atmosphere, which consists

mostly of hydrogen gas, we hypothesize that a point is soon reached where a thick layer

of liquid hydrogen is present. Still deeper inside Jupiter and Saturn, pressures equivalent

to 3 million times the pressure at the surface of the Earth are reached. Under such

conditions, the electrons and protons of hydrogen become less closely linked and

hydrogen becomes metallic; a layer of molten metallic hydrogen is the result. In Jupiter

pressures may even reach values high enough for solid metallic hydrogen to form a

sheath around the ice core.

Neptune and Uranus are thought to be similar to Jupiter and Saturn, although neither is

large enough for pressures to be sufficiently high to form metallic hydrogen.

1.3.The moons in solar system

A moon is defined to be a celestial body that makes an orbit around a planet,

including the eight major planets, dwarf planets, and minor planets. A moon may also be

referred to as a natural satellite. Astronomers have found at least 146 moons orbiting

planets in our solar system. Another 27 moons are awaiting official confirmation of their

discovery. This number does not include the six moons of the dwarf planets, nor does this

tally include the tiny satellites that orbit some asteroids and other celestial objects. Of the

terrestrial (rocky) planets of the inner solar system, neither Mercury nor Venus have any

moons at all, Earth has one Moon and Mars has its two small moons.

Usually the termmoon brings to mind a spherical object, like Earth's Moon. The two moons of Mars,

Phobos and Deimos, are different. While both have nearly circular orbits and travel close

to the plane of the planet's equator, they are lumpy and dark. Phobos is slowly drawing

closer to Mars and could crash into the planet in 40 or 50 million years. Or the planet's

gravity might break Phobos apart, creating a thin ring around Mars. In the outer solar

system, the gas giants Jupiter and Saturn and the ice giants Uranus and Neptune have

numerous moons. As these planets grew in the early solar system, they were able to

capture objects with their large gravitational fields.

Jupiter has 50 known moons (plus 17 awaiting official confirmation), including the

largest moon in the solar system, Ganymede. Many of Jupiter's outer moons have highly

elliptical orbits and orbit backwards (opposite to the spin of the planet). Saturn, Uranus


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and Neptune also have some irregular moons, which orbit far from their respective

planets.

Saturn has 53 known moons (plus 9 awaiting official confirmation). The chunks of ice

and rock in Saturn's rings (and the particles in the rings of the other outer planets) are notconsidered moons, yet embedded in Saturn's rings are distinct moons or moonlets. These

shepherd moons help keep the rings in line. Saturn's moon Titan, the second largest in the

solar system, is the only moon with a thick atmosphere.

In the realm of the ice giants, Uranus has 27 known moons. The inner moons appear

to be about half water ice and half rock. Miranda is the most unusual; its chopped-up

appearance shows the scars of impacts of large rocky bodies.

Neptune has 13 known moons. And Neptune's moon Triton is as big as the dwarf

planet Pluto and orbits backwards compared with Neptune's direction of rotation.

Earth's Moon probably formed when a large body about the size of Mars collided with

Earth, ejecting a lot of material from our planet into orbit. Debris from the early Earth

and the impacting body accumulated to form the Moon approximately 4.5 billion years

ago (the age of the oldest collected lunar rocks). Twelve American astronauts landed on

the Moon during NASA's Apollo program from 1969 to 1972, studying the Moon and

bringing back rock samples. The birth of our Moon has sparked likely debate for

centuries. Did it form as a companion planet coalescing independently from the solar

nebula at the same time as Earth? Did it form elsewhere, only to be drawn into Earths

orbit by our planets relatively strong gravity? Or was the Moon once part of the Earth?

The answer may lie in the Moons composition. It is 36% less dense than the Earth and

apparently contains much less iron. This difference rules out independent accretion from

the solar nebula, for if the Moon did form in the same way as the Earth, its composition

would be similar. The Moons composition, confirmed in part by the rock-collecting

efforts of U.S.Apollo astronauts, is actually quite similar to that of the Earths mantle, a

fact that has led many scientists to suggest that the Moon was formed in a cataclysmic

collision between the Earth and another planetestimal. By roughly 4.55 billion years ago,

the Earth has probably attained much of its current size and had become layered, with

most of its iron having migrated toward the center to form the core. With the Earth s

relatively large gravitational pull, it may have attracted a Mars-sized palnetestimal. With


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the planetastimal traveling toward the Earth perhaps as fast as 14 kilometers per second,

its impact would be quite literally Earth shattering (Figure 1.8). At the moment of impact,

the Earths young atmosphere would have been blown away, replaced by a rain of molten

iron blobs, remnants of the planetestimals iron core. Such a collision would have

vaporized much of the crust and mantle of both the Earth and the planetestimal. Jets of

the vaporized crust and mantle would be shot into orbit around the Earth, where the

material could eventually to form the Moon.

Figure 1.8: The origin of the Moon. Many scientists now believe that a catastrophic

impact between the proto-Earth and a Mars sized planetesimal spawned the Earth's

Moon


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Where are the wounds of great collision? Unfortunately, the Earths dynamic

internal processes related to volcanism, earthquakes, and mountain building would

have eradicated much of the evidence, and erosion would have eliminated the rest. A

search for the evidence of the greatest collision in the Earths history is unlikely to yield a

clue, and the story of the formation of the Earths Moon must remain only a hypothesis.

1.4.

The Earth in the Solar System

Earth is the third planet from the sun at a distance of about 150 million km (93

million miles) or one AU and it is the it is largest in the solar system and the densest .

One year equals 365.25 days in orbit around the sun. Earth is a rocky planet, also known

as a terrestrial planet, with a solid and dynamic surface of mountains, valleys, canyons,

plains and so much more. What makes Earth different from the other terrestrial planets is

that it is also an ocean planet: 70 percent of the Earth's surface is covered in oceans.

Earth, our home planet, is the only planet in our solar system known to harbor life.

All the things we need to survive exist under a thin layer of atmosphere that separates us

from the cold, airless void of space.

The Earth, the largest of the four inner planets, began as a mostly solid,

homogeneous body of rock and metal. This temporary state was later changed by the

extreme violenace and chaos that characterized the Earths first 20 million years. Proto-

Earths collisions with other planetestimals converted the enormous energy of motion to

thermal energy upon impact. Some of this energy was retained Compressional heating

also resulted from the accumulation of the mass of overlying rocks. In addion,

radiogenic heating occurred as the atoms of radioative substances, such as uranium,

released heat as their nuclei split apart, a process known as fission. The heat from these

two sources caused the palnets internal temperature to rise tremendously and set in

motion the process that created a layered Earth.

During the Earths first 10 to 20 million years, the planets internal temperaturerose to the melting point of iron. As a result, much of the iron liquefued. Because it was

more dense than the surrouding materials, the iron sack to the proto-Earths center by the

pull of gravity. As it sank, less dense materials rose and became concentrated closer to

the planets surface. Thus the metter that had originally made up a homogenous Earth

became separated into three major concentric zones of differing densities. (This


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separation is called differentiation). The denest materials, probably iron and nickel,

formed a core at the planets center. Lighter materials, composed largely of silicon and

oxygen as well as other relatively light elements, formed the Earths ou ter layers (the

mantle and crust). The arrange ment of these three layers is somewhat like that of a hard-

boiled egg with its thin shell, extensive white and center yolk. The egg model, however,

does not show a number of important sublayers that are fundamental to our

understanding of our dynamic Earth. Even lighter materials gases that had been

trapped in the interior escaped, combining to form the Earths first atmosphere and

ocean.

The area near the surface of the earth can be divided up into four spheres:" the

lithosphere, hydrosphere, biosphere, and atmosphere. Scientists can classify life and

material on or near the surface of the earth to be in any of these four spheres.

1. The atmosphere , which is the mixture of gasespredominantly

nitrogen, oxygen, carbon dioxide, and water vaporthat surrounds the Earth.

2. The hydrosphere , which is the totality of the Earth's water, including

oceans, lakes, streams, underground water, and all the snow and ice, but exclusive of

the water vapor in the atmosphere.

3. The biosphere , which is all of the Earth's organ isms as well as any organic

matter not yet decomposed.

4. The geosphere (solid Earth), which is composed principally of rock (by which

we mean any naturally formed, nonliving, firm coherent aggregate mass of solid matter

that constitutes part of a planet) and regolith (the irregular blanket of loose,

uncemented rock particles that covers the solid Earth).

Using this concept of four reservoirs and the interflow of materials and energy,

we can represent the Earth system as shown in Figure 1.9.


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Figure 1.9:

Diagrammatic

representation of the

Earth as a system of

interacting parts. Each

character represents a

reservoir, and each

arrow a flow of

energy or materials

Seawater provides an example of the way we can think about the Earth as a

system of reservoirs and flows (Fig. 1.10). Water leaves the ocean by evaporation and

forms water vapor, which then mixes with the other gases of the atmosphere. Thus,

water vapor moves from the hydrosphere reservoir to the atmosphere reservoir. As

water vapor in the atmosphere rises, it cools and condenses to form clouds andeventually rain or snow, which falls on either the land or the sea. Thus, water flows from

the atmosphere reservoir to the hydrosphere reservoir and from the atmosphere reservoir

to the solid Earth reservoir. The water that falls on the land can either evaporate again,

be taken up by plants in the biosphere reservoir (in both cases, water vapor is added to

the atmosphere and eventually forms clouds and rain), run back to the sea, or seep into

the ground. Transpiration is the name given to the passage of water vapor from a living

body through a membrane or pore. This means water flows from land to atmosphere

and from land to ocean. Snow that falls on the sea melts and mixes back into the

ocean. Snow that falls on the land will also eventually melt, but most of the snow that

happens to fall in Greenland, Antarctica, or high mountains may become part of an ice

sheet or mountain glaciers. It could be hundreds or even thousands of years before

melting occurs and the water flows back into the sea again.


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The seawater flows depicted by Figure 1.10 are not isolated events. For example,

if rain didn't fall, trees could not grow and there would be no streams in which fish

and frogs could live. Much of the biosphere therefore depends on the flow of water

from the atmosphere to the land and the ocean. Consider, too, what happens when

rainwater falls on the land; the water dissolves small amounts of various salts from

the regolith and carries them, via streams and rivers, to the sea. (It is these salts that

maintain the saltiness of seawater.) In this way, material in the regolith moves from the

solid earth reservoir to the hydrosphere reservoir. The movement represented by the

arrows in Figures 1.9 and 1.10 may be fast or slow, and so an essential part of Earth

system science is the measurement of rates of movement. Flows between the reservoirs,

and even between parts of the same reservoir, never cease, but the rates of flow may

change, and when this happens, volumes must change too. One of the keys to

understanding the Earth is therefore an appreciation of why and ho w reservoir volumes

change.

We can observe that rivers flow continuously to the sea, that rain falls with some

regularity, and that clouds are always forming in the atmosphere, which means that

evaporation and transpiration never stop. If the rates of any of the flows in Figure 1.10

changed markedly for a long period, the reservoirs would change in volume. In fact,

world sea level is essentially constant on a time scale of several decades. Therefore, we

conclude that the volume of the ocean reservoir is nearly constant and that thedifferent flows must be very nearly in balance. But a short-term balance does not mean

that changes never happen; changes do indeed occur. During glacial ages, for example,

glaciers around the world grow larger. Because water to make the ice comes from

the ocean, the ocean volume shrinks, leading to a fall in sea level. At the end of an ice

age, the opposite happens. Ice in the glaciers melts quickly, the melt water flows back to

the ocean, and sea level rises


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Figure 1.10 The flows

influencing ocean volume.

Water flowing to and from

the atmosphere and the

solid Earth keeps the

ocean volumeapproximately con stant

on a time scale of a few

decades. Major flows are

shown as solid arrows,

minor ones as dashed

arrows

Different time scales are involved in the examples just described. Evaporation of

water from the ocean, the formation of clouds, and the falling of rain or snow all

take place in a few days or weeks. In contrast, the buildup or meltdown of glaciers is a

much slower process that may require hundreds or thousands of years. An approximate

balance may therefore be maintained on a short time scale, even though changes are

slowly occurring on a long time scale. A scientific investigation of the Earth, then, is

concerned with both fast and slow rates (that is, with events that happen on both

short and long time scales). Rates in the atmosphere and biosphere tend to be rapid and

to occur on short time scales. Rates in the solid Earth tend to be slow and to operate on

time scales of thousands or millions of years, and rates in the hydrosphere vary from

rapid (as in flowing streams) to slow (as in the flow of water deep beneath the ground

surface) (see Table 1.2).

Table 1.2: Examples of Fast and Slow Rates

Event Time

ATMOSPHERE Formation of clouds (fast)

Tornado (fast) Hurricanes (fast)

Duration of an ice age (slow)

Minutes

Hours Days

Thousands of years


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HYDROSPHERE Flash flood in a desert stream (fast)

Flood in a great river system (fast)Circulation of deep ocean water (slow)

Minutes to hours Days to

months Years

BIOSPHERE Lifetime of a grass (fast)

Lifetime of a redwood (slow)

Growth of a forest (slow)

Months

Hundreds of years

Hundreds to thousands ofyears

SOLID EARTH Landslide (fast)

Volcanic eruption (fast)

Elevation of a mountain range (slow)

Minutes

HoursTens of millions of years

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