Astro 20 Compton

2. In this chapter, you will discover…In this chapter, you will discover…
 why Earth is such an ideal environment for lifewhy Earth is such an ideal environment for life
 that Earth is constantly in motion inside and outthat Earth is constantly in motion inside and out
 how Earth’s magnetic field helps protect ushow Earth’s magnetic field helps protect us
 what made the craters on the Moonwhat made the craters on the Moon
 how the Sun and the Moon cause Earth’s tideshow the Sun and the Moon cause Earth’s tides
 that Earth and the Moon each have twothat Earth and the Moon each have two
(different) major types of surface features(different) major types of surface features
 that water ice has been found on the Moonthat water ice has been found on the Moon

4. Temperature Profile of Earth’s Atmosphere
The atmospheric temperature changes with altitude because
of the way sunlight and heat from Earth’s surface interact with
various gases at different heights.

5.  The atmosphere plays a key role in sustainingThe atmosphere plays a key role in sustaining
life on Earth.life on Earth.
 It is believed, Earth is on its third atmosphere.It is believed, Earth is on its third atmosphere.
 The first atmosphere was composed of traceThe first atmosphere was composed of trace
remnants of hydrogen and helium left over fromremnants of hydrogen and helium left over from
the formation of the solar system.the formation of the solar system.
 The second atmosphere came from inside theThe second atmosphere came from inside the
Earth, vented through volcanoes and cracks inEarth, vented through volcanoes and cracks in
its surface. It was composed primarily of carbonits surface. It was composed primarily of carbon
dioxide, water vapor, and nitrogen.dioxide, water vapor, and nitrogen.
 The third atmosphere is composed mostly ofThe third atmosphere is composed mostly of
nitrogen and oxygen.nitrogen and oxygen.
Earth’s Atmosphere

6. Earth’s Dynamic Oceans
This image shows the relative size of Earth compared to a sphere containing all
of Earth’s water. The sphere is about 860 miles in diameter and includes
freshwater, oceans, ice, and even water in the atmosphere. Nearly three-
quarters of Earth’s surface is covered with water, a substance that is essential
to the existence of life. In contrast, there is no liquid water at all on the surfaces
of Mercury, Venus, Mars, or the Moon.

7.  Liquid water is believed to have precipitated out ofLiquid water is believed to have precipitated out of
Earth’s second atmosphere and from impacts ofEarth’s second atmosphere and from impacts of
water-rich space debris forming the oceans.water-rich space debris forming the oceans.
 The oceans eventually soaked up about half theThe oceans eventually soaked up about half the
carbon dioxide in the second atmosphere, as raincarbon dioxide in the second atmosphere, as rain
absorbed it from the air and carried it earthward.absorbed it from the air and carried it earthward.
 Carbon dioxide combined with water in the earlyCarbon dioxide combined with water in the early
ocean forming clays, shales, and coral. As theocean forming clays, shales, and coral. As the
material settled to the ocean floor, it eventuallymaterial settled to the ocean floor, it eventually
formed limestones removing carbon dioxide fromformed limestones removing carbon dioxide from
the atmosphere.the atmosphere.
Ocean Formation and the Changing Atmosphere

8. The Greenhouse Effect in a Car
The glass windows in this car allow visible light to enter but prevent the
infrared radiation released by the car’s interior from escaping. The infrared,
therefore, heats the air in the car much higher than the outside air. This effect is
used to advantage in greenhouses and is why it is called the greenhouse effect.

9. The Greenhouse Effect
Sunlight and heat from Earth’s interior warm Earth’s surface, which in
turn radiates energy, mostly as infrared radiation. Much of this radiation
is absorbed by atmospheric carbon dioxide and water, heating the air,
which in turn increases Earth’s temperature even further. In equilibrium,
Earth radiates as much energy as it receives.

10. The Greenhouse Effect
The amount of carbon dioxide in our atmosphere since 1000 A.D. has been
determined. The increase in carbon dioxide since 1800 due to burning fossil fuels
and decreases in forestation have caused a dramatic temperature increase.

11. The surface of Earth, or
the crust, is made of
less-dense rock floating
on a layer of denser
material.
The theory of Plate
Tectonics indicates the
crust has three types of
boundaries, suggesting
each continent makes up
separate plates.
Shown here is an artist’s
rendition of one such
boundary, a mountain
range in the middle of
the ocean floor, called
the Mid-Atlantic Ridge.
The Mid-Atlantic Ridge

12. The present continents are pieces of what was once
a bigger, united body called Pangaea.
The Supercontinent Pangaea

13. The Supercontinent Pangaea
Geologists believe that Pangaea must have first split into two
smaller supercontinents, which they call Laurasia and Gondwana.

14. The Supercontinent Pangaea
These bodies later separated into the continents of today. Gondwana
split into Africa, South America, Australia, and Antarctica, while
Laurasia divided to become North America and Eurasia.

15. Earth’s Major Tectonic Plates
Earth’s surface is divided into
a dozen or so rigid plates that
move relative to one another.
The boundaries of the plates
are the scenes of violent
seismic and geologic activity,
such as earthquakes,
volcanoes, rising mountain
ranges, and sinking seafloors.
The arrows indicate whether
plates are moving apart
(←→), together (→←), or
sliding past one another (↑↓).

16.  The boundaries between tectonic plates are the sites ofThe boundaries between tectonic plates are the sites of
some of the most impressive geological activity on oursome of the most impressive geological activity on our
planet.planet.
 Most of the earthquakes and volcanoes are found atMost of the earthquakes and volcanoes are found at
plate boundaries.plate boundaries.
 Divergent boundaries are located where plates moveDivergent boundaries are located where plates move
apart forming features like the Mid-Atlantic Ridge.apart forming features like the Mid-Atlantic Ridge.
 Convergent boundaries are located where plates comeConvergent boundaries are located where plates come
together forming features like the Himalayas.together forming features like the Himalayas.
 Transform boundaries are located where plates slideTransform boundaries are located where plates slide
grind past each others forming features like the Sangrind past each others forming features like the San
Andreas Fault Zone.Andreas Fault Zone.
Plate Boundaries

17. Planetary Differentiation as Earth FormedPlanetary Differentiation as Earth Formed
Early Earth was initially a homogeneous mixture of elements with no continents or
oceans. (a) Molten iron sank to the center and light material floated upward to form
a crust. (b) As a result, Earth has a dense iron core and a crust of light rock, with a
mantle of intermediate density between them.

18. Heat supplied by the heating coil warms the water at the bottom of the pot. The
heated water consequently expands, decreasing its density. This lower-density water
rises (like bubbles in soda) and transfers its heat to the cooler surroundings. When
the hot, rising water gets to the top of the pot, it loses a lot of heat into the room,
becomes denser, and sinks back to the bottom of the pot to repeat the process.
Convection

19. Mechanism of Plate Tectonics
Convection currents in Earth’s interior are responsible for pushing
around rigid plates on its crust. New crust forms in oceanic rifts, where
magma oozes upward between separating plates. Mountain ranges and
deep oceanic trenches are formed where plates collide and crust
sometimes sinks back into the interior. Note that not all tectonic plates
move together or apart—some scrape against each other.

20. Earth’s Magnetic Field
(a) The magnetic field of a bar magnet is revealed by the alignment of iron
filings on paper. (b) Although Earth does not contain a bar magnet, its rotation,
combined with moving electric charges in its core, creates an equivalent field.
Note that the field is not aligned with Earth’s rotation axis. By convention, the
magnetic pole near Earth’s north rotation axis is called the magnetic north pole
even though it is actually the south pole on a magnet! We will see similar
misalignments and flipped magnetic fields when we study other planets.

21. Earth’s Magnetosphere
A slice through Earth’s magnetic field, which surrounds the entire planet, carves
out a cavity in space that excludes charged particles ejected from the Sun, called
the solar wind. Most of the particles of the solar wind are deflected around Earth
by the fields in a turbulent region colored blue in this drawing. Because of the
strength of Earth’s magnetic field, our planet traps some charged particles in two
huge, doughnut-shaped rings called the Van Allen belts (in red).

22. The Northern Lights (Aurora Borealis)
A deluge of charged particles from the Sun can overload the Van Allen belts
and cascade toward Earth, producing auroras that can be seen over a wide
range of latitudes. View of an aurora from Polar spacecraft imposed on a
graphic image of Earth. Colors from blue to red indicate lowest to highest
auroral activity. Auroras typically occur 100 to 400 km above Earth’s surface.

23. The Northern Lights (Aurora Borealis)
View of aurora over the north central part of the United States
and southern Canada as seen from the International Space
Station when it was over south central Nebraska.

24. The Northern Lights (Aurora Borealis)
This is the Aurora borealis in Alaska. The gorgeous aurora seen
here is mostly glowing green due to emission by oxygen atoms in
our atmosphere. Some auroras remain stationary for hours, while
others shimmer, like curtains blowing in the wind.

26. (a) This photograph, taken from
lunar orbit by astronauts, includes
the crater Aristillus. Note the crater’s
central peaks, the collapsed,
terraced crater wall, and the ejecta
blanket. Numerous smaller craters
resulting from the same impact
pockmark the surrounding lunar
surface. The following three
drawings show the crater formation
process. (b) An incoming meteoroid,
(c) upon impact, is pulverized and
the surface explodes outward and
downward. (d) After the impact, the
ground rebounds, creating the
central peak and causing the crater
walls to collapse. The lighter region
is the ejecta blanket.
Lunar Craters

27. Shallow Angle Impact Crater
This crater, in Mare Nubium, is the result of a very low-angle impact.
Despite the missing ejecta between the two lines, the impact crater is
still circular, indicating how powerful the impact was. The impacting
body came from the direction of the missing ejecta.

28. A Microscopic Lunar Crater
This photograph, taken through a microscope,
shows tiny microcraters less than 1 mm across
on a piece of Moon rock.

29. Mare Imbrium and the Surrounding Highlands
Mare Imbrium, the largest of the 14 dark plains that dominate the Earth-facing side
of the Moon, is ringed by lighter-colored highlands strewn with craters and towering
mountains. The highlands were created by asteroid impacts pushing land together.

30. Details of a Lunar Mare
Close-up views of the lunar surface reveal rilles and numerous small craters on
the maria. Astronauts in lunar orbit took this photograph of Mare Tranquillitatis
(Sea of Tranquility) in 1969 while searching for potential landing sites for the first
human landing. At 1100 km (700 mi) across, this mare is the same size as the
distance from London to Rome or Chicago to Philadelphia.

31. Details of a Lunar Mare
Astronaut David Scott on Hadley’s rille
during the Apollo 15 mission to the Moon.

32. Possible lava tube outlet on the Moon imaged
by NASA’s Lunar Reconnaissance Observer.
Details of a Lunar Mare

33. Bridge near King crater. We know this is a bridge, rather
than two depressions because sunlight, coming from the
lower right, is visible in the upper left depression.
Details of a Lunar Mare

34. The Far Side of the Moon
Unlike the side of the Moon facing Earth, the
lunar far side has only a few, small maria. It is
almost entirely covered with highlands.

35. An Apollo Astronaut on the Moon
Apollo 17 astronaut Harrison Schmitt enters the Taurus-Littow
Valley on the Moon. The enormous boulder seen here slid down a
mountain to the right of this image, fracturing on the way. This final
Apollo mission landed in the most rugged terrain of any Apollo flight.

36. The Moon’s surface is covered by a layer of (a) powdered rock and (b)
small pieces of rock. Called regolith, the powdered rock was created over
billions of years as a result of bombardment by space debris; it sticks
together like wet sand, as illustrated by this Apollo 11 astronaut bootprint.
The Regolith

37. Mare Basalt
This 1.53-kg (3.38-lb) specimen of mare basalt was brought back by
Apollo 15 astronauts in 1971. Small holes that cover about a third of
its surface suggest that gas was dissolved in the lava from which
this rock solidified. When the lava reached the airless lunar surface,
bubbles formed as the pressure dropped and the gas expanded.
Some of the bubbles were frozen in place as the rock cooled.

38. Anorthosite
The lunar highlands are covered with this ancient type of rock, which is believed
to be the material of the original lunar crust. This sample’s dimensions are 18 ×
16 × 7 cm. Although this sample is medium gray, other anorthosites retrieved
from the Moon have been white, while others are darker gray than this one.
This one was brought back by Apollo 16 astronauts.

39. Impact Breccias
These rocks are created from shattered debris fused together under
high temperature and pressure. Such conditions prevail immediately
following impacts of space debris on the Moon’s surface.

40. Apollo 11 Landing Site
On the Moon’s Sea of Tranquility, Astronaut Buzz Aldrin stands next to the package
of equipment containing the seismic detector. The corner reflectors are used, even
today, to determine the distance from Earth to the Moon. The stereo camera took
pairs of images of the Moon’s surface. Seeing them through special glasses gives a
3-D close-up view of the Moon’s surface. The bottom half of the lander is still on the
Moon’s surface. The top half brought astronauts Neil Armstrong and Buzz Aldrin
back into lunar orbit, where they transferred to the command module to fly home.

41. As the Moon’s interior
shrank, the surface
settled irregularly,
creating long lines of
cliffs called scarps.

42. Seismic experiments
revealed that the main
regions of the Moon’s
interior mimic those of
Earth, but in different
proportions.
Water ice may exist in
the polar craters,
where the energy
received from the Sun
is insufficient to melt it.

43. • Based on data gathered
from Moon rocks and other
sources, the collision-
ejection theory seems to
be the best explanation for
the Moon’s formation.
• This computer simulation
models the creation of the
Moon from material
ejected by the impact of a
large planetlike body with
the young Earth.
Collision-Ejection
Theory

44. The collision that created the Moon could
have also knocked Earth’s rotation axis
over so that today it has a 23½° tilt, thereby
creating the seasons.

45. Motion of Earth-Moon System
(b) Analogously, this time-lapse image shows a pair of different-mass disks
connected by a piece of wood sliding across a table. Note that the center of mass
of this collection of objects moves in a straight line, while its other parts follow
curved paths. As with the Earth–Moon system, the center of mass is closer to the
more massive disk than to the less massive one.
(a) The paths of Earth and the Moon as their barycenter follows an elliptical orbit
around the Sun.

46. Synchronous Rotation of the Moon
This is the motion of the Moon around Earth as seen from above Earth’s north
polar region (ignoring Earth’s orbit around the Earth-Moon barycenter). For the
Moon to keep the same side facing Earth as it orbits our planet, the Moon must
rotate on its axis at precisely the same rate that it revolves around Earth.

47. Tidal Forces
(a) The Moon induces tidal forces on Earth. At each point, this force is the
difference between the force, Fout, created by the orbital motion of the two
bodies around their barycenter, and the Moon’s gravitational force, Fgrav, at
that point. The magnitude and direction of each arrow represent the
strength and direction of each force. (b) Water slides along Earth to create
the tides. Ignoring Earth’s rotation and the effects of the continents, this
figure shows how two high tides are created on Earth by the Moon’s
gravitational pull. The Sun has a weaker, but otherwise identical, effect.

48. During new and full moon
phases, the Sun’s gravitation
boosts the tidal bulges in the
same direction as the Moon,
creating larger spring tides.
During the quarter moon phases, the
Sun pulls the tidal bulges in a different
direction from the Moon, diminishing
the tides. These are called neap tides.
Tides on Earth

49. Lunar Ranging
Beams of laser light are fired through three telescopes at the Observatoire
de la Côte d’Azur, France. The light is then reflected back by the corner
reflectors placed on the Moon by Apollo astronauts. From the time it takes
the light to reach the Moon and return to Earth, astronomers can determine
the distance to the Moon to within a few millimeters.

50. Summary of Key IdeasSummary of Key Ideas

51. Earth: A Dynamic, Vital World
 Earth’s atmosphere is about four-fifths nitrogen and one-fifthEarth’s atmosphere is about four-fifths nitrogen and one-fifth
oxygen. This abundance of oxygen is due to the biologicaloxygen. This abundance of oxygen is due to the biological
processes of life-forms on the planet.processes of life-forms on the planet.
 Earth’s atmosphere is divided into layers named the troposphere,Earth’s atmosphere is divided into layers named the troposphere,
stratosphere, mesosphere, and ionosphere.stratosphere, mesosphere, and ionosphere.
 Ozone molecules in the stratosphere absorb ultraviolet light rays.Ozone molecules in the stratosphere absorb ultraviolet light rays.
 The outermost layer, or crust, of Earth offers clues to the historyThe outermost layer, or crust, of Earth offers clues to the history
of our planet.of our planet.
 Earth’s surface is divided into huge plates that move over theEarth’s surface is divided into huge plates that move over the
upper mantle. Movement of these plates, a process called plateupper mantle. Movement of these plates, a process called plate
tectonics, is caused by convection in the mantle. Also, upwellingtectonics, is caused by convection in the mantle. Also, upwelling
of molten material along cracks in the ocean floor occurs duringof molten material along cracks in the ocean floor occurs during
seafloor spreading. Plate tectonics is responsible for most of theseafloor spreading. Plate tectonics is responsible for most of the
major features of Earth’s surface, including mountain ranges,major features of Earth’s surface, including mountain ranges,
volcanoes, and the shapes of the continents and oceans.volcanoes, and the shapes of the continents and oceans.

52.  Study of seismic waves (vibrations produced byStudy of seismic waves (vibrations produced by
earthquakes) shows that Earth has a small, solid inner coreearthquakes) shows that Earth has a small, solid inner core
surrounded by a liquid outer core. The outer core issurrounded by a liquid outer core. The outer core is
surrounded by the dense mantle, which in turn issurrounded by the dense mantle, which in turn is
surrounded by the thin, low-density crust on which we live.surrounded by the thin, low-density crust on which we live.
Earth’s inner and outer cores are composed primarily ofEarth’s inner and outer cores are composed primarily of
iron. The mantle is composed of iron-rich minerals.iron. The mantle is composed of iron-rich minerals.
 Earth’s magnetic field produces a magnetosphere thatEarth’s magnetic field produces a magnetosphere that
surrounds the planet and deflects the solar wind.surrounds the planet and deflects the solar wind.
 Some charged particles from the solar wind are trapped inSome charged particles from the solar wind are trapped in
two huge, doughnut-shaped rings called the Van Allentwo huge, doughnut-shaped rings called the Van Allen
radiation belts. An Earthward deluge of particles from aradiation belts. An Earthward deluge of particles from a
coronal mass ejection on the Sun can pierce the belts andcoronal mass ejection on the Sun can pierce the belts and
produce exceptional auroras.produce exceptional auroras.
Earth: A Dynamic, Vital World

53. The Moon and Tides
 The Moon has heavily cratered highlands and relatively smooth-The Moon has heavily cratered highlands and relatively smooth-
surfaced maria.surfaced maria.
 Impacts have been the only significantImpacts have been the only significant “weathering” agent on the“weathering” agent on the
Moon; the MoonMoon; the Moon’’s regolith (pulverized rock layer) was formed bys regolith (pulverized rock layer) was formed by
meteoritic action.meteoritic action.
 Lunar rocks brought back to Earth contain no water and areLunar rocks brought back to Earth contain no water and are
depleted of volatile elements.depleted of volatile elements.
 Powdered into regolith, the anorthosite rock of the highland isPowdered into regolith, the anorthosite rock of the highland is
brighter than the powdered basalt of the maria.brighter than the powdered basalt of the maria.
 Many lunar rock samples are solidified lava formed largely ofMany lunar rock samples are solidified lava formed largely of
minerals also found in Earth rocks.minerals also found in Earth rocks.
 Anorthositic rock in the lunar highlands was formed between 4.0Anorthositic rock in the lunar highlands was formed between 4.0
and 4.3 billion years ago, whereas the mare basalts solidifiedand 4.3 billion years ago, whereas the mare basalts solidified
between 3.1 and 3.8 billion years ago. The Moon’s surface hasbetween 3.1 and 3.8 billion years ago. The Moon’s surface has
undergone very little geologic change over the past 3 billion years.undergone very little geologic change over the past 3 billion years.

54. The Moon and Tides
 Frozen water has been discovered in numerous places justFrozen water has been discovered in numerous places just
below the Moon’s surface.below the Moon’s surface.
 The collision-ejection theory of the Moon’s origin, acceptedThe collision-ejection theory of the Moon’s origin, accepted
by most astronomers, holds that the young Earth was struckby most astronomers, holds that the young Earth was struck
by a huge planetesimal, and debris from this collisionby a huge planetesimal, and debris from this collision
coalesced to form the Moon.coalesced to form the Moon.
 The Moon was molten in its early stages, and theThe Moon was molten in its early stages, and the
anorthositic crust solidified from low-density magma thatanorthositic crust solidified from low-density magma that
floated to the lunar surface. The mare basins were createdfloated to the lunar surface. The mare basins were created
later by the impact of planetesimals and were then filled withlater by the impact of planetesimals and were then filled with
lava from the lunar interior.lava from the lunar interior.
 Gravitational interactions between Earth and the MoonGravitational interactions between Earth and the Moon
produce tides in the oceans of Earth and set the Moon intoproduce tides in the oceans of Earth and set the Moon into
synchronous rotation. The Moon is moving away from Earth,synchronous rotation. The Moon is moving away from Earth,
and consequently, Earth’s rotation rate is decreasing.and consequently, Earth’s rotation rate is decreasing.

55. Key TermsKey Terms
anorthosite
capture theory
cocreation theory
collision-ejection theory
continental drift
convection
core (of Earth)
crust
dynamo theory
ejecta blanket
fission theory
highlands
impact breccias
ionosphere (thermosphere)
mantle
mare (plural maria)
mare basalt
mascons
mesosphere
neap tide
northern lights (aurora
borealis)
ozone layer
planetary differentiation
plate tectonics
regolith
rille
scarps
seafloor spreading
seismic waves
seismograph
southern lights (aurora
australis)
spring tide
stratosphere
synchronous rotation
troposphere
Van Allen radiation belts