In geoscience texts the Earth is often treated as the beginning and almost the end of what is known about planets and geology; the other planets in the solar system are treated as brief addenda to the Earth itself.The study of geology started with the Earth and therefore Earth is the standard against which other planets are compared. As tempting as it is to discuss the Earth from the point of view of an Earth-bound geologist, in The Earth and the Moon the Earth will be compared equally with the other planets, and the Earth will be described from the point of view of a planetary scientist.

Earth belongs to the group of small, inner solar-system planets that also includes Mercury, Venus, and Mars, as shown in the following figure of the solar system.These planets, called the terrestrial planets, all consist of a mantle made of silica-based minerals above an iron-rich core. Their crusts all show evidence for meteorite bombardment and volcanic lava flows. Beyond these basic similarities, and despite their similar formation processes in the inner solar system, these four planets are strangely different from each other. While the Earth's surface temperatures are relatively constant, Mercury goes through daily cycles of heat high enough to melt lead and cold deep enough to freeze carbon dioxide gas, and its thoroughly cratered surface shows that it has not been volcanically active for about the last half of its existence.

Though the Earth is the only known planet in the universe to house life, Mars is a good candidate to have developed life at some point in its history. Mars once had liquid surface water, and it may still be volcani-cally active, providing surface warmth from geothermal systems in the crust. The red planet was gradually transformed through the loss of its magnetic field and the changing luminosity of the Sun from a wet planet with a significant atmosphere into a dry, cold desert. Though it began more like the Earth, Mars evolved away from that state.

All Orbits: Asteroid Belt, Kuiper Belt, Oort Cloud

Asteroid belt Mercury Venus Earth Mars

Asteroid belt Mercury Venus Earth Mars

This book covers the Earth and the Moon. All the orbits are far closer to circular than shown in this oblique view, which was chosen to show the inclination of Pluto's orbit to the ecliptic.

Venus, however, should be the most similar of the four planets to Earth. The two planets are very close in size, density, composition, and distance from the Sun, but they have evolved in entirely different directions. Venus has a suffocatingly dense atmosphere consisting of carbon dioxide with clouds of sulfuric acid; its surface experiences such high atmospheric pressure, intense, unremitting heat, and toxic gases that no life similar to that on Earth could survive there. The Earth and the Moon will address the Earth in part 1. Unlike the other planets (with a slight exception for the Moon), there is some direct evidence about the Earth's interior, along with a great deal of indirect evidence from earthquake waves, heat flow measurements, and other physical observations. Chapter 1 covers fundamental aspects of the Earth as a planet: its size and mass, its orbit, and the causes of its seasons. Chapter 2 contains what is thought about the Earth's solid interior, including its probable composition and temperature, as well as its ability to flow like a liquid over very long periods of time. This ability to flow along with the action of liquid water on the surface and near-surface of the planet allows the process of plate tectonics to occur, with its attendant volcanoes, collisional mountain chains, earthquakes, and ocean basins.

A series of related hypotheses for the formation of the Earth and the other terrestrial planets has sprung from information on the composition and age of the Earth, as well as from studies of meteorites and the Moon. This fascinating subject, so remote in time as to be almost a branch of philosophy, is discussed in chapter 3. Finally, the most familiar aspects of the Earth, its surface appearance and atmosphere, are presented in chapter 4, followed in chapter 5 by a discussion of life, the most unique and important aspect of Earth. The Earth has several critical characteristics thought to have helped it stay hospitable to life. Earth is now the only terrestrial planet with

• Abundant liquid surface water, a requisite for life;

• An atmosphere rich in oxygen;

• Bimodal topography (low ocean basins and high continents);

• A strong magnetic field, which protects from solar radiation and preserves the atmosphere; and

• A large moon, keeping the Earth's orbit and therefore its climate stable.

Some of these attributes have helped Earth stay hospitable to life, but their efficacy may be limited in time. The Earth can sustain life now but its protective elements may not last forever, either because of natural physical or chemical processes, or because of the influence of humankind.

Biologists have trouble imagining a kind of life that does not depend on water; living creatures on Earth depend on water in many complex ways, and cannot have formed or survived without it.Water creates the unusual silica-rich rocks such as granites that appear to exist only on Earth, and water helps produce Earth's bimodal topography (on Mars, low and high topography may also be linked to water, but through different processes than occur on Earth). Earth's atmosphere is uniquely rich in oxygen, with just under 21 percent. Mars's atmosphere, by comparison, contains less than a half percent of oxygen, and Venus's atmosphere has none. Animals breathe oxygen, but the balance of oxygen and carbon dioxide in the atmosphere was created and is maintained by plants. Even the magnetic field is necessary for life:Without the magnetic field, the Sun's intense radiation would reach the surface with enough intensity to cause genetic mutations, cancers, and, eventually, extinctions. Without its magnetic field, the Earth's atmosphere would be blown away by the solar wind, and Earth would evolve toward the conditions of Mars.

The Moon, by contrast, is almost perfectly dry, has no currently active volcanism, no atmosphere, and no life. Its ancient surface is formed by impacts and by the constant flux of the solar wind, since it is unprotected by any significant magnetic field; it now stands virtually changeless on human timescales. Part 2 of The Earth and the Moon discusses the formation and evolution of the Moon, the body in the solar system about which the most is known outside the Earth. Chapter 6 describes the processes that link the Moon and Earth: tides, synchronous rotation, and eclipses.These physical links have been compelling to mankind for millennia, but the most exciting parts of the story of the Moon came about through space travel.

The Moon is the only body in the solar system outside Earth that a human has visited.There is over 840 pounds (382 kg) of lunar material brought by NASA and Soviet space missions to Earth for study. The information gleaned about the Moon from this relatively small pile of rocks is mind-boggling, and stands as the greatest proof that Martian planetary science would be greatly enhanced by returning samples to

The Lunar Orbiter 4 took this striking image of the lunar Orientale Basin, with portions of the mare basalt—filled Oceanus Procellarum visible on the upper right limb. (NASA/Lunar Orbiter 4)

Earth. Compositional studies of lunar rocks show that the Moon and Earth are made of similar material, and because lunar material has not been reworked through erosion and plate tectonics it also sheds light on the early formation of the Moon and its internal evolution. The Moon's light-colored highlands and dark mare basalts are shown in this image; the giant complex crater Orientale Basin is in the center.

Some moons in the solar system accreted from extra material orbiting around the main planet during planetary formation, and others are captured bodies that were orbiting past the main planet and were caught in its gravitational field.The Earth's Moon may have a unique provenance in all the solar system, intimately tied to the

The Lunar Orbiter 4 took this striking image of the lunar Orientale Basin, with portions of the mare basalt—filled Oceanus Procellarum visible on the upper right limb. (NASA/Lunar Orbiter 4)

formation of the Earth: It is thought that during accretion of the Earth, a body the size of Mars collided with the proto-Earth, smashing most of it into a swirl of particles that re-formed into the Earth and the Moon.Thus, these two bodies are thought to share the material that makes them. The reasoning for this theory of lunar formation, presented in chapter 7, is followed by discussions of the Moon's interior and surface in chapter 8.

The Earth and Moon are the closest planet and moon in size except for Pluto and Charon. Unlike Pluto and Charon, the Earth and Moon are startlingly different. Their differences form a natural laboratory for understanding the importance of size in retaining atmosphere, trapping solar energy for warmth, allowing the growth of liquid oceans—and thus, in allowing life. In this book the Earth and Moon take their rightful places among the rest of the bodies in the solar system, as a small planetary system with what appears to be a highly unusual set of fortuitous circumstances that allowed for the development of life.

Part One: The Earth

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