Discovering the Universe Eighth Edition

1. Neil F. Comins • William J. Kaufmann III Discovering the Universe Eighth Edition CHAPTER 6 Earth and Moon

2. WHAT DO YOU THINK? • Can Earth’s ozone layer, which is now being depleted, be replenished? • Who was the first person to walk on the Moon, and when did this event occur? • Do we see all parts of the Moon’s surface at some time throughout the lunar cycle? • Does the Moon rotate and, if so, how fast? • What causes the ocean tides? • When does the spring tide occur?

3. In this chapter you will discover… • why Earth is such an ideal environment for life • that Earth is constantly in motion inside and out • how Earth’s magnetic field helps protect us • what made the craters on the Moon • how the Sun and the Moon cause Earth’s tides • that both Earth and the Moon have two (different) major types of surface features • that water ice may have been found at the Moon’s poles

4. Views of Earth’s Surface An oasis in the forbidding void of space, Earth is a world of unsurpassed beauty and variety. Ever changing cloud patterns drift through its skies. More than two-thirds of its surface is covered with oceans. This liquid water, in combination with a huge variety of chemicals from its lands, led to the formation and evolution of life over most of the planet’s surface. The symbol is astronomers’ shorthand for “Earth.”

5. Temperature Profile of Earth’s Atmosphere The atmospheric temperature changes with altitude because of the way sunlight interacts with various gases at different heights.

6. 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 action also occurs in greenhouses and is called the greenhouse effect.

7. 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 raises Earth’s temperature even further. In equilibrium, Earth radiates as much energy as it receives.

8. The Greenhouse Effect The amount of carbon dioxide in our atmosphere since 1000 AD 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.

9. The surface of the Earth, or crust, is made of less dense rock floating on a layer of denser material. The crust has distinct boundaries, indicating that the continents are separate bodies. Shown here is an artist conception of one such boundary, a mountain range in the middle of the ocean floor called the Mid-Atlantic Ridge.

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

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

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

13. The Supercontinent Pangaea

14. Earth’s Major Tectonic Plates

15. Differentiation as the 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.

16. Convection Heat supplied by the heating coil warms the water at the bottom of the pot. The heated water consequently expands and thereby decreases 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.

17. 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.

18. Earth’s Magnetic Field The magnetic field of a bar magnet is revealed by the alignment of iron filings on paper. Generated in Earth’s molten, metallic core, Earth’s magnetic field extends far into space. 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.

19. 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).

20. 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 the deep space satellite Dynamics Explorer 1. The green lines show the locations of land masses under the aurora. Auroras typically occur 100 to 400 km above Earth’s surface.

21. The Northern Lights (Aurora Borealis) View of aurora from space. Part of a Space Shuttle is visible at the bottom left of the picture.

22. The Northern Lights (Aurora Borealis) 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.

23. The Moon Our Moon is one of seven large satellites in the solar system. The Moon’s diameter of 3476 km (2160 mi) is slightly less than the distance from New York to San Francisco. This photograph is a composite of first-quarter and last-quarter views, in which long shadows enhance the surface features.

24. This photograph, taken from lunar orbit by astronauts, includes the crater Aristillus. Note the crater’s central peaks; collapsed, terraced crater wall; and ejecta blanket. Numerous smaller craters resulting from the 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.

25. A Microscopic Lunar Crater This photograph made with a microscope shows tiny microcraters less than 1 mm across on a piece of moon rock.

26. Mare Imbrium and the Surrounding Highlands Mare Imbrium, the largest of the 14 dark plains that dominate 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.

27. 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.

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

29. The Far Side of the Moon A composite image from the Galileo spacecraft. Mare Orientale lies on the boundary between the near and far sides.

30. 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.

31. The Moon’s surface is covered with a layer of powdered rock called regolith.

32. Mare Basalt This 1.53-kg (3.38-lb) specimen of mare basalt was brought back by Apollo 15astronauts 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.

33. 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 x 16 x 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.

34. 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.

35. 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.

36. Seismic experiments revealed that the main regions of the Moon’s interior mimic those of the 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.

37. This computer simulation shows how the Moon could have been formed in a collision between the Earth and a large planetesimal.

38. The collision that created the Moon could have also knocked Earth’s rotation axis over so that today it has a 231/2° tilt, thereby creating the seasons.

39. Motion of Earth-Moon System (a) The paths of Earth and the Moon as their barycenter follows an elliptical orbit around the Sun. (b) Analogously, this time-lapse image shows a spinning wrench sliding across a table. Note that its center of mass (the red plus) moves in a straight line, while its other parts follow curved paths.

40. Synchronous Rotation of the Moon The motion of the Moon around Earth as seen from above Earth’s north polar region (ignoring Earth’s orbit around 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.

41. 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.

42. 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.

43. 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.

44. Summary of Key Ideas

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

46. Earth: A Dynamic, Vital World • Study of seismic waves (vibrations produced by earthquakes) shows that Earth has a small, solid inner core surrounded by a liquid outer core. The outer core is surrounded by the dense mantle, which in turn is surrounded by the thin, low-density crust. Earth’s inner and outer cores are composed primarily of iron. The mantle is composed of iron-rich minerals. • Earth’s magnetic field produces a magnetosphere that surrounds the planet and blocks the solar wind. • Some charged particles from the solar wind are trapped in two huge, doughnut-shaped rings called the Van Allen radiation belts. A deluge of particles from a coronal mass ejection by the Sun can initiate an auroral display.

47. The Moon and Tides • The Moon has light-colored, heavily cratered highlands and dark-colored, smooth-surfaced maria. • Many lunar rock samples are solidified lava formed largely of minerals also found in Earth rocks. • Anorthositic rock in the lunar highlands was formed between 4.0 and 4.3 billion years ago, whereas the mare basalts solidified between 3.1 and 3.8 billion years ago. The Moon’s surface has undergone very little geologic change over the past 3 billion years. • Impacts have been the only significant “weathering” agent on the Moon; the Moon’s regolith (pulverized rock layer) was formed by meteoritic action. Lunar rocks brought back to Earth contain no water and are depleted of volatile elements.

48. The Moon and Tides • Frozen water may have been discovered at the Moon’s poles. • The collision-ejection theory of the Moon’s origin, accepted by most astronomers, holds that the young Earth was struck by a huge asteroid, and debris from this collision coalesced to form the Moon. • The Moon was molten in its early stages, and the anorthositic crust solidified from low-density magma that floated to the lunar surface. The mare basins were created later by the impact of planetesimals and were then filled with lava from the lunar interior. • Gravitational interactions between Earth and the Moon produce tides in the oceans of Earth and set the Moon in synchronous rotation. The Moon is moving away from Earth, and, consequently, Earth’s rotation rate is decreasing.

49. Key Terms anorthosite capture theory cocreation theory collision-ejection theory continental drift convection core coronal mass ejection 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 seafloor spreading seismic waves seismograph solar wind southern lights (aurora australis) spring tide stratosphere synchronous rotation troposphere Van Allen radiation belts

50. WHAT DID YOU THINK? • Can Earth’s ozone layer, which is now being depleted, be naturally replenished? • Yes. Ozone is created continuously from normal oxygen molecules by their interaction with the Sun’s ultraviolet radiation.