From space, the Earth's appearance to the eye is that of a blue water-covered sphere with fairly large blocks of brownish or grayish land dividing the watery surface into a number of major oceans. Swirling and moving through the atmosphere, just barely above the surface, are white fluffy clouds. From any distance, say the Moon, the Earth appears to the eye to be quite spherical with an absence of any surface relief. It is not until one approaches much closer to the Earth that any height variations become apparent.
Our nearest cosmic neighbor, the Moon, has a different appearance from space than that of Earth. There is no watery surface, no white fluffy clouds floating over the surface. Like Earth though, the roughness of the lunar surface is not apparent to the eye until one is actually quite close to it. From close to the lunar surface, it is obvious that the cratered surface bears little resemblance to the active and ever-changing surface of Earth.
One of the most significant concepts in our view of the physical world, although it began in ancient Greece, did not become a dominant consideration in scientific thought until the eighteenth century; in a period of about 100 years it revolutionized scientific thinking. What was that concept? It is the concept of evolution. Although you may have encountered the concept in connection with biological evolution, it is a much broader one than just biology. Today our world view is one dominated by a knowledge that the Universe--galaxies, stars, even the Earth, and life on Earth's surface--is gradually, sometimes rapidly, evolving in directions shaped by natural processes governed by the laws of physics. Throughout this chapter and those which follow we shall try to sketch not only the results of evolution but also its processes.
Let us begin our study of the planets of the Solar System by considering the distinctive differences between the Earth and the Moon. First we need to describe what the two bodies are like and then consider in the next chapter what evolutionary process have occurred over their life times that has made these two bodies, that are so close to each other, so dramatically different.
7.1. Our Home, the Earth's Surface
Although from space the Earth may appear to be spherical, in point of fact it is not. Measurements at different points over the surface reveal that the number of kilometers in 1o of latitude increases slightly from the equator toward the poles. This means that the Earth's shape is actually that of an oblate spheroid with the longer diameter in the equatorial plane and the shorter one in the polar direction. Table 7.1 shows this fact numerically and provides some data about the Earth of which you should be aware as you read this and succeeding chapters.
[Table 7.1]
Earth's rotation is primarily responsible for causing the departure from the true shape of a sphere. Rotation causes the body of the Earth to flow from high latitudes toward lower latitudes, forming an equatorial bulge. Because Earth is not a perfect sphere, its gravitational field is not the same in all directions and such variations affect the motions of artificial satellites. From these unanticipated changes in satellite motions, called orbital perturbations, we can work the problem backward to find the Earth's shape, which we now know is slightly pear-shaped. The stem portion at the North Pole is about 19 m farther from the center, and the bottom portion at the South Pole about 26 m closer to the center.
Several phenomena were used historically to demonstrate the rotation of the Earth besides the rising and setting of the Sun and stars. One of the most vivid was the pendulum experiment devised in 1851 by the French physicist Jean Foucault (1819-1868). He hung an iron ball on a long wire from the dome of the Pantheon in Paris. Underneath it was a large circular table with a ridge of sand along its edge. As the pendulum swung, a pin attached to the bottom of the ball made a mark in the sand.
After the pendulum had been set into motion, it was apparent from the marks in the sand that it was deviating slowly in a clockwise direction. From Newton's laws of motion, we know that once a plane of oscillation has been established for a pendulum, only an external force can change the plane's orientation. No external forces were acting on Foucault's pendulum; in reality it was the spectators and building that were turning underneath the plane of oscillation of the pendulum because of the Earth's rotation. Figure 7.1 illustrates what would have happened if the experiment had been performed at either a geographic pole or the equator.
[Figure 7.1]
Consider an idealized Earth whose surface is entirely covered by water, as in Figure 7.2. Since the gravitational force exerted by the Moon decreases as one over distance squared, the Moon's attraction is greatest for the ocean closest to the Moon, and it decreases across the Earth, so that it is least on the ocean on the far side opposite the Moon. Relative to the Earth's center, this difference in force across the body of the Earth, called a tidal force, causes the ocean on the side nearest to the Moon to shift slightly closer to the Moon and the ocean on the opposite side to recede slightly. That is, the Moon's gravitational pull on the oceans produces two tidal bulges on opposite sides of the Earth in line with the Moon. Consequently, water piles up in the form of an ellipsoid whose long axis is directed toward the Moon. Midway between the high tides are the low tides.
[Figure 7.2]
The Earth's rotation underneath the tidal bulges results in alternating high and low tides in the oceans twice each day. Because there is a slight lag before the oceans fully adjust to the Moon's tidal force, the tidal bulges are dragged by the rotating Earth somewhat ahead of the line joining the centers of the Moon and the Earth (Figure 7.2).
The Sun also contributes to the tides, but only half as much as the Moon does because of its much greater distance, despite its larger mass. When the Sun and the Moon are roughly along a straight line, such as at new or full moon, their combined gravitational pull is greatest, producing the largest tides.
If the Sun and the Moon are pulling the Earth, why doesn't the land move too? It does, because the land is not absolutely rigid. The land has a greater internal strength than water, and therefore, land tides are much smaller than water tides. Approximately every 12 hours the ground on which you stand rises and falls a few centimeters at any given place. Tidal motions are also evident in the atmosphere, which is even less rigid than the oceans.
The constant friction generated by the lunisolar tides (mainly near the shores and in the shallow seas) has slowed the Earth's rotation. As a result, the day has lengthened over several billion years by an estimated several hours to the present 24 hours. The slowing down in the rotation is not uniform; a number of irregularities have been found. This conversion of Earth's rotational energy into heat by tidal friction will continue indefinitely.
Tidal forces and their resulting effects occur in many different situations involving astronomical bodies. We shall encounter them at several points later in the book.
The outer layers of the Earth consist of a crust and the uppermost part of what is called the mantle, and together they are known as the lithosphere. This is a fairly rigid zone of rocks that extends about 100 km below the surface in which the crust extends some 60 km or so beneath continents, but only about 10 km below the ocean floor. Rocks composing the continental crust have a lower density than those composing the oceanic crust, and they are primarily a light-colored granite which is rich in the silicates of aluminum, iron, and magnesium. In a simplified view, the continental crust possesses a layered structure: On the bottom is a layer of igneous rock (molten rock that has hardened, such as granite) over which lies a thin layer of sedimentary rocks (rocks formed by sediment and fragments that water deposited, such as limestone and sandstone). Over lying the sedimentary rocks is a layer of soil that has been deposited during past ages in those parts of continents that have not recently experienced volcanic or mountain building activity.
Sandwiched between the lithosphere and the lower mantle is a 150-km thick layer of partially molten material. This layer, called the asthenosphere, is readily deformed and can be made to flow when pressure is exerted. Its chemical composition is primarily iron and magnesium silicates.
In efforts to date various regions of continents, geochemists have shown with radioactive dating techniques (see box) that the oldest rock formations on continents have ages between 3.5 and 3.8 billion years. For North America, the oldest part is a crescent-shaped region bordering the west and south sides of Hudson's Bay. A younger crescent lying roughly to the west and south surrounds this oldest region, and the westernmost and southernmost parts of the continent are even younger. A somewhat similar pattern exists for other continents. The inference is that continents are not original with the Earth's formation 4.6 billion years ago but are a secondary aspect and they will continue to grow and evolve. We know that continental margins, particularly the western edge of North America, are new additions to continents. These coastal regions are growing due to the deposition of sediments washed down by rivers from the interior of the continent. In striking contrast, the oldest known parts of the oceanic crust are about 200 million years old or almost 20 times younger than the oldest parts of continents.
[Box - Dating the Earth, Table 7.2]
The concept that continents move or drift relative to each other was not one that was readily accepted when the German geologist Alfred Wegener (1880-1930) proposed it in 1912. Yet recent research has revealed a variety of evidence showing that the lithosphere is indeed segmented into about a dozen or so major plates of different sizes, as shown in Figure 7.3. Floating on the mantle, these plates move slowly, carrying the continents with them at a typical rate of several centimeters each year. This motion is known as plate tectonics or, more popularly, as continental drift.
[Figure 7.3]
One type of evidence for lithospheric plates comes from extensive exploration of the ocean floor which has revealed the existence of a number of midocean ridges rising several kilometers above the ocean floor and extending thousands of kilometers in length. We now know that these ridges mark one type of plate boundary. Lithospheric plates are internally quite rigid so that their principal interactions with each other are on their boundaries. As a consequence, the boundaries are the locations of large-scale geological activity. An example of a midocean ridge is the Mid-Atlantic Ridge (Figure 7.4) that separates the North and South American plates from the Eurasian and African plates. Midocean ridges and the other two types of plate boundary are illustrated in Figure 7.5. For the midocean ridge-type of boundary, it appears that lava is first being forced upward from the asthenosphere into the ridges from which the lava pushes out laterally from the ridge. This new material gradually cools, thickens, and solidifies at the trailing edge of the plate (Figure 7.6). Rock samples from as far down as 8 km below sea level verify that the Earth's youngest volcanic rocks are those found near these midocean ridges.
[Figure 7.4]
[Figure 7.5]
[Figure 7.6]
We have further confirmation that the plates move from the shape, geologic structure, and fossil record of continents. Evidence also comes from igneous rocks with similar magnetic fields that were frozen into the rocks at the time they solidified. Such rocks have been found at continental margins that are now widely separated from each other.
Another line of evidence comes from heat flow out of the interior. Compared to the energy falling on the Earth from the Sun, the interior flow is scarcely a trickle: The heat conducted through an area the size of a football field is roughly equivalent to the energy given off by three 100-watt light bulbs. Yet over Earth's 4.6-billion year history this trickle of energy has contributed to the work of making continents drift, opening and closing ocean basins, building mountains, and causing volcanos and earthquakes. The geographic variation in the heat flow from the interior is not great, but the global variation shows that the major oceanic ridges are high-heat-flow zones, while the older continental shields and sedimentary regions are low-heat-flow zones.
How are lithospheric plates transported across the top of the mantle? It appears that they are driven by a horizontal flow of convective currents in the upper, softer portion of the mantle, as illustrated in Figure 7.6. Convection is a process that transports energy from one place to another. The closest example to our everyday experience is the rising of heated air in a room followed by cooler air sinking to the floor to be reheated and cycle upward again. If new plate material is being added at a plate's trailing edge near a midocean ridge, then other material must be taken off the plate at some other location. This process is occurring at the leading edge of one plate that is being pushed downward underneath an overriding plate to create a deep ocean trench where the two plates meet. Such a process forces old plate material into the mantle to melt and be recycled over hundreds of millions of years as part of the convective currents that drive the plate motion. This process can form a coastal mountain belt, like the Andes, on the overriding plate. Over millions of years as the other plate descends, it heats up and becomes part of the general circulation in the asthenosphere. Plates separate along midocean ridges. Most of the great geologic processes--volcanic activity, mountain building, formation of ocean trenches, earthquakes--are concentrated near plate boundaries.
About 200 million years ago, the last mass movement of continents began. There was at that time but one single land mass, today called Pangaea. This supercontinent probably accumulated from migrations produced by previous drifting. Some 20 million years later, sea-floor spreading had separated the supercontinent into two segments. About 45 million years later, the North Atlantic and Indian Oceans widened and South America began to separate from Africa. During the next 70 million years, the South Atlantic Ocean widened into a major ocean, the Mediterranean Sea began to open up, and North America just began to separate from Eurasia. A computer-generated projection for the next 50 million years suggests that the Atlantic and Indian Oceans will enlarge and the Pacific will contract. Africa's northward movement will eventually doom the Mediterranean.
Although typical plate motions are a few centimeters per year, this reshaping of the Earth's face is actually quite dramatic when one considers the age of the Earth. It is estimated that in about 2 billion years the gradual cooling of Earth from heat loss will mean that the asthenosphere will flow less readily and the plate-motion phase of the Earth's evolution will probably come to an end. Thus the Earth will enter a new phase, in which plate motions of the lithosphere are not responsible for most of the large-scale terrain features. Large mountain ranges, like the Himalayas, will no longer be uplifted, and they will erode away over millions of years.
As far as surface geography is concerned, there appear to have been two major terrain-shaping mechanisms at work on the Earth (and, for that matter, presumably on the Moon, Mars, Venus, and Mercury, the other Terrestrial planets). These are impact cratering, whose most intense period of bombardment some 3 billion years ago is now long past, and thermal-tectonic activity due to an outflow of thermal energy from the deep interior. The thermal-tectonic mechanism (plate motions and deformations of the crust with accompanying volcanic and earthquake activity), aided by erosion by wind, water, and life processes, is now the dominant one, but only on Earth. It has all but erased the results of that long-past impact cratering phase in Earth's history, except that remnants of the last of that phase can still be seen in nearly a hundred ancient impact structures, some of which are as large as the largest visible ones on the Moon (Figure 7.7).
It is estimated that on Earth thermal-tectonic activity is responsible for better than 90 percent of the present terrain, with not more than 10 percent of the cratered terrain remaining. Present evidence suggests that surface evolution on the other terrestrial planets, as revealed by various space missions, has not been so heavily dominated by thermal-tectonic activity as that for Earth.
[Figure 7.7]
A revolution is in the making in the study of the geography of the surface. Satellites now orbit the Earth photographing the surface in narrow-wavelength regions that are in either the visible or the near infrared. Coverage of each photograph is about 30,000 km2 with resolutions varying from about 100 m down to 10 m. These satellite images coupled with the power of the modern supercomputer to analyze data from them promises to enhance geographical and ecological studies beyond the boldest expectations of just ten years ago. Earth surveillance will dramatically change agriculture, prospecting for natural resources, fishing, economic development, commodities trading, and a host of areas concerned with human activities. In the next century little will happen on the surface, such as a developing crop failure in Africa, or an oil pipeline break in the Middle East, or the movement of cargo vessels at sea, about which the information will not be almost immediately available worldwide. It is even hard to visualize all of the uses to which this constant surveillance data will be put.
Although the surface is far from perfectly understood, its study at least holds out the hope that we may go where necessary to either directly observe or perform some type of experiments that will improve our understanding. The interior of the Earth, however, is a far different situation in that at no time will we venture to the center to observe its physical processes. The internal structure of the Earth has and will continue to be a problem that we must attack from afar. Knowing what happens on the outer boundary of the region we wish to investigate, such as measuring the heat outflow from the interior, is a start. But what we really need is a probe that reaches into the deepest parts of the body of the Earth to reveal its secrets. And conveniently enough, Earth itself provides us with that probe.
From what we know about the rotation of the Earth, it is apparent that the Earth is not absolutely rigid. The body of the Earth will deform when subjected to various forces, such as those occurring in the propagation of waves. In such waves, known as seismic waves, geophysicists have a natural probe of the planet's deep interior. Seismic waves are generated by earthquakes and spread out in all directions from the site of the quake. There are also artificial means of producing seismic waves, such as explosives. From the manner in which these waves propagate throughout the body of the Earth, i.e., their periods and amplitudes of vibration, and their arrival time at various observing stations, we can deduce much about Earth's internal structure.
There are two kinds of seismic waves, pressure (P) and shear (S) waves, that propagate inside the Earth. The speed with which these waves travel through the Earth (between 5 and 15 km/s) depends on the material's density, compressibility, and rigidity. The particles of the Earth that transmit the P waves vibrate back and forth in the direction in which the wave propagates, similar to the way sound waves are propagated through air. The S waves cause the particles that transport the disturbance to vibrate perpendicular to the direction of the waves' propagation, as waves on a string do. S waves move at about half the speed of the P waves. Also unlike P waves, S waves cannot propagate through liquids, which damp their vibrations. As the P and S waves move downward through the Earth, their speed increases with the increasing density of the material they are traversing. Since seismic waves are refracted or reflected on reaching a boundary between two distinctly different layers, a picture of the Earth's interior can be produce by tracking their path through the body of the Earth. Such pictures show that the Earth possesses a layered structure like that of an onion.
A model of the Earth's internal structure derived from seismic studies is shown in Figure 7.8. At the center is a hot, high-density inner core, presumably solid and composed of iron and nickel primarily. We infer from the inability of S waves to penetrate the core to any extent that surrounding the inner core is a liquid outer core. This outer shell of molten material is also composed of iron and nickel primarily with whatever lighter materials are present being able to rise to its top.
Surrounding the core is an envelope known as the mantle. The upper portion of the mantle is mostly solid rock composed of olivine, an iron-magnesium silicate, while the lower portion is chiefly iron and magnesium oxides. Overlying the mantle is a thin crust of metal silicates and oxides, such as basalt (largely oxygen, silicon, aluminum, magnesium, and iron) and granite (oxygen, silicon, aluminum, sodium, and potassium). Obviously, since we live on top of the crust, we have the most immediate knowledge about its structure and composition. The degree of certainty of our knowledge of the interior diminishes deeper down toward the Earth's center. A summary of the physical characteristics of each of these zones is given in Table 7.3.
[Table 7.3]
[Figure 7.8]
Earth's internal structure gives all of the indications that it is a product of a completely molten body having undergone chemical differentiation, which is the process of heavier materials settling under gravitational attraction, while lighter materials migrate outward. This molten period when chemical differentiation took place seems to have been shortly after the Earth's formation. Why would the Earth have become molten followed by this process of gravitational settling? Let us defer consideration of that question to Chapter 11 after we have looked at the possibility that the other Terrestrial planets have also gone through a molten phase followed by chemical differentiation.
If the Earth had no atmosphere, life would not and could not exist here. The insulating blanket of air surrounding us maintains a temperature range favorable for life in part because the incoming solar radiant energy, which is primarily in the visible wavelengths, is trapped by atmospheric carbon dioxide and water vapor molecules. What they do is to absorb energy coming from the surface of the Earth and reradiate much of it back to the surface. This process is known as the greenhouse effect and is discussed in more detail in Section 8.2. Without its atmosphere the Earth's average temperature would be about 20 to 30 degrees lower than its present value of 15o C (288 K). Since water freezes at this temperature, it could neither have facilitated the development nor could it maintain life as it does in its liquid form. Worldwide circulation of the atmosphere also transports thermal energy and helps to moderate extremes in temperature that would otherwise exist.
Moreover, even the upper atmosphere is important for our survival. It protects us from harmful ultraviolet and X-ray radiation from the Sun, vaporizes meteoroids entering the atmosphere, and absorbs most of the incoming highly energetic subatomic particles called cosmic rays. Finally, the atmosphere creates the soft blue appearance of the sky. This is because atmospheric gases scatter photons in the blue region of incoming sunlight much more efficiently than they do photons of longer wavelengths. This is why the rising or setting Sun is redder than when highest in the sky. There being more atmosphere along the line of sight toward the horizon than along the line of sight towards the zenith, more blue photons have been deviated in directions away from our line of sight toward the horizon than toward the zenith.
In all, the Earth's atmosphere plays a very vital role beyond the obvious one of providing the oxygen we breath. One of humanity's most important challenges is to understand and to preserve the atmosphere, for our continued existence depends on that knowledge.
The mass of the atmosphere is about one-millionth the total mass of the Earth. It has several layers, shown in Figure 7.9, each with distinctive thermal, physical, chemical, and electrical properties. Approximately half the atmosphere is contained in the first 5.6 km, and 99 percent of it lies below 35 km.
[Figure 7.9]
Our weather takes place in the bottom layer, called the troposphere. At an altitude of 11 km, the temperature drops to -55o C. Above this region lies a 40 km thick layer, the stratosphere, where the temperature slowly rises, reaching a maximum of about 0o C at 50 km. Somewhat below this altitude an absorbing layer of ozone screens out most of the incoming ultraviolet radiation. Within the next layer up, the mesosphere, the temperature rapidly drops to a minimum of -85o C at its upper limit, which is 90 km.
Above the mesosphere is the thermosphere. Here the still more dangerous X-rays and gamma rays are effectively filtered out by molecular oxygen and nitrogen and by their dissociated atoms at even higher altitudes. The temperature climbs steadily throughout the thermosphere and into the exosphere, the atmospheric fringe several hundred kilometers above sea level.
Within the Earth's atmosphere are layers in which the concentration of free electrons is above the average atmospheric value. These layers constitute the ionosphere. The electrons are due to the ionization of atmospheric molecules and atoms by solar ultraviolet and X-ray photons. Radio waves of certain wavelengths, for example, the AM band of conventional broadcasting, transmitted by ground stations are reflected between the ionosphere and the Earth's surface. This makes possible long-distance communication between stations that are not along a direct line of sight because of the Earth's curvature. Radio wavelengths greater than about 10 m are turned back by the ionospheric layers, whereas shorter wavelengths pass through the ionosphere into space with little or no bending.
Up to about 90 km, gravitational settling causes no significant separation of atmospheric gases by atomic weight. No separation occurs because the atomic and molecular constituents are mixed by air currents and random thermal motion. The chemical composition of the atmosphere therefore remains nearly uniform, with 77 percent nitrogen, 21 percent oxygen, nearly 1 percent argon, 0.03 percent carbon dioxide, and almost 1 percent water vapor (which varies up to several percent in the troposphere). The atmosphere has minute traces of other gases, including neon, krypton, xenon, methane, ammonia, nitrous oxide, carbon monoxide, and ozone.
Above about 90 km or so, the constituents are not well mixed; the heavier molecules and atoms settle toward the bottom, the lighter ones diffusing to the top. At extreme heights a rarefied layer of helium extends from about 600 to 1000 km, and this is topped by a very tenuous hydrogen layer that merges into interplanetary space.
The chemical composition of the atmosphere is not static. The present composition results from a balance between those processes which introduce a particular molecule into the atmosphere and those which remove it, as shown in Figure 7.10. Probably the most significant example is that of oxygen, since it is essential for our existence. Atmospheric oxygen is almost entirely produced in photosynthesis, primarily by green plants in shallow seas and to a lesser extent by plant life on land. A little oxygen comes from the direct dissociation of atmospheric water molecules by ultraviolet photons from the Sun. Chemically, oxygen is quite an active molecule, combining readily with a number of different atoms, such as in the formation of oxides in rocks removing oxygen from the atmosphere. Breathing by animal life also depletes atmospheric oxygen. If the supply of oxygen were shut off, it would take only a few tens of thousands of years to remove the major portion of oxygen now existing in the atmosphere.
[Figure 7.10]
The abundance of the other molecules in the atmosphere is also controlled by various "production and destruction" processes. And as in the evolution of the Earth's surface, the atmosphere has also changed over time. Clearly, if atmospheric oxygen is due to the existence of life, then oxygen would not have been present prior to the emergence of life. The origin of the primitive Earth's atmosphere is probably the result of outgassing by volcanos and the escape of gases from the crust. The gaseous emission from present-day volcanos includes water vapor, carbon dioxide, nitrogen, inert gases, and small amounts of methane, ammonia, and sulfur compounds. It is probable that on the very young and lifeless Earth, with no significant amounts of liquid water, the dominant atmospheric constituent was carbon dioxide in a very dense atmosphere. This estimate is based on the fact that a large amount of carbon dioxide is trapped in carbonate rocks on the Earth's surface. Carbon dioxide is the main component of the atmospheres of Venus and Mars, with the Venusian atmosphere being some hundred times denser than ours. About 2 billion years ago the transition began to an oxygen-nitrogen atmosphere. The amount of oxygen grew from a trace to the present 21 percent as a result of the development of photosynthesis by green plants, while carbon dioxide diminished as shown in Figure 7.11.
[Figure 7.11]
That Earth possesses a magnetic field is not a new discovery for the compass, which is responding to the magnetic field, has been in use for some time. When it became known that the Earth's interior is hot, however, it was obvious that the Earth's magnetic field could not be like a permanent magnet. This is because heating disorients various parts of a magnet destroying its ability to produce a coherent magnetic field. Thus arose a puzzle as to where the Earth's magnetic field comes from. Scientists now believe it to be caused by circulation of liquid metal in the outer core: If friction can ionize metal atoms, then the flow of ionized material becomes an electric current, which produces the magnetic field. Such a mechanism is known as a dynamo, a device that converts mechanical energy of motion into electrical energy. Thus the Earth is more of an electromagnet than a permanent magnet (Figure 7.12).
[Figure 7.12]
In appearance the Earth's magnetic field resembles that of a bar magnet inclined slightly to the Earth's axis of rotation. The magnetic field lines run between northern and southern polar regions, much as the pattern formed by iron filings sprinkled around a bar magnet. The intensity of the magnetic field decreases away from the Earth's surface, but the magnetic field can still be measured many tens of thousands of kilometers out in space.
However it began, Earth's magnetic field has changed polarity (the north magnetic pole becomes the south magnetic pole and vice versa) many times over geologic history. Scientists trace the record of these changes through the magnetism frozen into rocks of different ages: Iron particles in molten lava beds align themselves along the lines of the existing magnetic field lines, and after the rocks solidify, they retain that orientation indefinitely. Such rocks show that magnetic reversals have come at intervals as short as 35,000 years. Why the reversals? One suggestion is that they are related to changes in the Earth's rotation or in the fluid state of its outer core. But in fact we do not know for sure.
The magnetosphere is that part of the magnetic field surrounding the Earth that exerts a force strong enough to control the motions of charged subatomic particles entering the field. Exerting a strong force even 50,000 km away from the Earth's surface, the magnetic field protects us from continuous bombardment by cosmic rays; subatomic particles traveling through space at speeds that are a significant fraction of the velocity of light.
From satellites monitoring the magnetic field, we have learned much about the magnetosphere's strength, direction, and composition. A cutaway section of its structure is shown in Figure 7.13. Within the magnetosphere are several concentric belts; the principal ones are zones of high particle density known as the Van Allen radiation belts (named after the American physicist James Van Allen (b. 1914) who discovered them in 1958 from Explorer 1 satellite data). The Van Allen belts encircle the planet in two doughnut-shaped regions about 3000 and 17,000 km from Earth's surface.
[Figure 7.13]
Charged particles, mainly protons and electrons, populate the magnetosphere's radiation belts. Most of these particles are ejected from the Sun as a reasonably steady flow of matter in the plane of the ecliptic known as the solar wind. When the solar wind particles reach the Earth, they are either diverted away from it or trapped by its magnetic field. The collision of solar wind particles with the Earth's magnetosphere creates a shock wave that distorts and compresses the magnetic field on the sunlit side and stretches it into a long tail on the night side. (A shock wave is a compression, such as the sonic boom made by a jet plane.)
[Box - Cosmic Rays]
The trapped particles in the outer Van Allen belt can on occasion spill out and fall into the Earth's atmosphere at high geographic latitudes. There they collide with atoms of oxygen and nitrogen and stimulate these gases to radiate pale greens and occasional bright reds in patches or across the whole sky. These are the auroras, called the northern lights in our hemisphere (Figure 7.14). They are most often seen in zones between 65o and 70o north and south magnetic latitudes. Because these particles enter the atmosphere most easily when the solar wind is more intense, more auroras color our night skies during the height of the 11-year sunspot cycle (Section 15.1).
[Figure 7.14]
The place of the Moon in human culture, appearing as it does in our literature, music, and art, needs no elaboration. The raising of questions as to its nature, its relationship to Earth, and its origin can be traced all the way back to ancient myths and forward to our own accounts. Although the space age has given us new incite to this nearest cosmic neighbor, many of the old questions are still have not been answered to everyone's general satisfaction. And thus the study of the Moon continues including the desire to return, but this time to establish permanent outposts. Let us begin with a discussion of the dynamical relationship between the Earth and Moon.
The mean distance between the Moon's center and the Earth's center is about 400,000 km. This distance has been measured to within several centimeters by timing the round trip of a laser beam bounced off reflectors left on the Moon by the Apollo astronauts. The Moon's orbit is a small-eccentricity ellipse with the Earth at one focus. Some of the physical parameters of the lunar orbit are given in Table 7.4.
[Table 7.4]
The point that orbits the Sun annually according to Kepler's laws is not the geographic center of the Earth. It is a point on the line joining the Earth and the Moon known as the center of mass. One can think of the center of mass as the center of balance of an imaginary rod supporting the Earth at one end and the Moon at the other, as shown in Figure 7.15. The center of mass for the Earth-Moon system lies inside the Earth, since the mass of the Moon is only about 1 percent that of Earth. Thus in actuality the geometrical center of the Earth orbits the center of mass of the Earth-Moon system with the same sidereal period as that of the Moon, namely 27.3 days. Although such a motion is not readily apparent to us on Earth's surface, it nevertheless has been measured.
[Figure 7.15]
The Moon turns once on its axis in the same time that it completes one orbit around the Earth, so that the same hemisphere is always toward us. This is relatively easy to demonstrate to yourself. Walk around a stool, continually facing it; next walk around the stool, keeping your head and body pointed in the same direction. In the first instance, you rotated once while you revolved once, just as the Moon does; in the second, you did not rotate about your axis. If the Moon did not rotate, we could see all its sides during the month. That the Moon's rotation period is equal to the period of its orbital revolution (27.3 days) is not accidental. Tidal forces between the Earth and the Moon over eons have equalized the rotation and revolution periods.
Originally, both bodies were probably much closer, perhaps only 5 to 10 percent of their present distance, and were rotating more rapidly. The Earth's day was a few hours shorter than at present and its month, or the Moon's orbital period, much shorter than now. Because of the Earth's greater tidal force, the Moon's rotation has slowed more rapidly than the Earth's has.
Some of the kinetic energy associated with the Earth's rotation is gradually being transferred by lunar tides to the orbiting Moon so that the Moon is receding from the Earth by several centimeters every year. The reason for this is that the Moon's tidal force has a braking effect on the Earth, which decreases its quantity of rotational motion, or angular momentum. To conserve the total angular momentum of the Earth-Moon system, the angular momentum in the Moon's orbital motion must be increasing. Hence it is accelerated ever so slightly in its orbit, spiraling outward from Earth.
As the Moon recedes, the month must lengthen, according to Kepler's third law. Eventually, the Earth and the Moon will face each other with equal periods of rotation and revolution (about 47 days) at a distance of about 560,000 km. But the calculated time for this event to happen, several tens of billions of years, far exceeds our estimates of the Earth-Moon system's probable life span.
7.6. The Surface of Our Nearest Cosmic Neighbor
The ratio of the mass of the Moon, a satellite, to that of the Earth, a planet, is exceed only by that of Pluto and its satellite. As a consequence of this exceptional situation, we can think of the Earth-Moon system as being more a double planet than a true planet-satellite system. In spite of this, the Moon would not quite span the width of the United States, and its mass is roughly 1 percent that of the Earth. Table 7.5 summarizes the Moon's physical properties.
[Table 7.5]
The program of lunar exploration that began in 1964 with unmanned craft and culminated in six manned Apollo landings between 1969 and 1972 (Table 7.6) has provided us with a priceless legacy of lunar materials and data. Lunar rocks have been collected from nine different locations, six by the United States and three by the Soviet Union (the most recent being August of 1976). The samples returned amount to more than 2000 individual specimens, weighing about 382 kg (843 pounds).
[Table 7.6]
Five instrument packages were left on the lunar surface, and the last surviving one operated until October of 1977. The seismometers in these packages detected meteoric impacts and many lunar quakes during their operating life span of 8 years.
The Apollo program also carried out an extensive effort to photograph and analyze the lunar surface. The result is maps of some parts of the Moon that are better than those of some areas of the Earth. X-ray and radioactivity studies from orbit have yielded estimates of the chemical composition of about one-quarter of the lunar surface, an area about the size of the United States and Mexico together.
Because of its small mass, the Moon's history has been vastly different from the Earth's. With a small mass comes a weak gravitational attraction; as a result, the Moon retains almost no atmosphere. It has no surface water, either free or chemically combined in the rocks (as in Earth rocks), although some water may be trapped under its surface. It also has no general magnetic field, but its rocks suggest that a strong one existed in the very distant past. However, the Moon is far from a simple, featureless satellite.
Galileo's subdivision of the lunar surface into maria, the low-lying, roughly circular dark regions in Figure 7.16, and terrae, the rough, cratered highlands, is still significant in terms of lunar history and terrain-shaping processes. The maria are covered with layers of basaltic lava similar to the lava that erupt from terrestrial volcanos in Iceland, Hawaii, and elsewhere. The highlands consist of a lighter-colored rock that is older than the rocks of the maria. Highlands constitute about 83 percent of the lunar surface, whereas the maria cover only 17 percent.
[Figure 7.16]
There are other types of features on the Moon than maria and the cratered terrae. Observers over the years have identified a variety of features, such as a range of sizes of impact craters, rugged mountain ranges, and deep, winding canyons, or rilles. The kinds of surface features on the Moon are outlined in Table 7.7.
[Table 7.7]
The lunar mountain ranges tend to be on or near the periphery of the roughly circular maria. The mountains bordering the maria rise more steeply on the side facing them than on the other side. Many have lofty peaks, occasionally rising over 7000 m above the surrounding plains. Although fractures are observed in the lunar crust, there is no evidence that the lunar mountain ranges are folded mountain belts as are those on Earth. Thus, unlike the Earth's mountain ranges, which by and large are the products of collisions between lithospheric plates as discussed earlier, the lunar mountain ranges are the rims of the huge impact basins that contain the maria. Thus their origin is impact cratering and not thermal-tectonic activity as are those on Earth.
Beyond the eastern edge of Mare Imbrium a narrow valley cuts across the Alps Mountains. This feature has long been known from photographs taken from Earth. From photographs taken by an orbiting spacecraft we now know that the Alpine Valley is a deep trough some 3 to 10 km wide and over 100 km long. Narrow channels, known as rilles, which resemble chasms or gorges, cut many kilometers across the lunar terrain, frequently without interruption. Running lengthwise down the middle of the Alpine Valley is a very conspicuous rille, as can be seen in Figure 7.17. Rilles may be lava channels, part or all of which were roofed when filled with flowing lava. Now these tubes have collapsed and are partly choked with rubble from the days of active lava flows.
[Figure 7.17]
Over eons of time meteoroids have pulverized the lunar surface, leaving a dusty layer some 1 to 40 m deep that covers the lunar terrain. Known as the regolith, it is the lunar "soil" on which the astronauts left their footprints (Figure 7.18). Since this soil contains no water or organic matter, it is totally different from soils formed on the Earth by water, wind, and life. More than just bits of ground up lunar rocks, the regolith has also been exposed to cosmic rays, subatomic matter flowing from the Sun, and a fine dust from interplanetary space. Without an atmosphere to shield it, the layers of the regolith contain both the record of lunar events and that of events in the larger Solar System.
[Figure 7.18]
A tremendous number of impact craters pit the Moon, evidence of cataclysms that altered the crust during its past. More than 30,000 are visible by telescope. The total, down to bushel basket size, may well exceed a billion. The great walled plains, or supercraters with low profiles, such as Clavius or Grimaldi (Figure 7.19), have structures similar to those of maria but on a smaller scale. Their diameters are between 200 and 300 km.
[Figure 7.19]
Next in size on the Moon's front side are some three dozen impact craters from 80 to 200 km in diameter. A third of them have conspicuous light-colored streaks, called rays, radiating outward in all directions up to several hundred kilometers long, such as the well-known rayed craters Tycho, Copernicus, Kepler, and Aristarchus. Many of the small secondary craters, as well as the ray systems, were apparently formed by a rain of debris ejected from the primary crater after a large body struck the surface.
Impact craters are reasonably circular, with the interior rim steeper than the outer rim. The larger craters have terraces on their inner walls and frequently have a fairly smooth floor from which a few low peaks rise. Beyond the craters the terrain is hummocked and overlain with the ejected material from the cratering process. In those craters having a central peak, the peak is believed to have been created by the elastic rebound of rock from below the surface after the initial impact. Others have bare floors, presumably because they were flooded with lava; the crater Plato is a good example.
The impact craters are not all of the same age, as one can see in Figure 7.19 with the crater Clavius. The rim of the major crater is eroded and worn, whereas the half dozen or so small ones in the center have sharper and higher rims. Clearly, the impacting bodies that produced the small craters superimposed on the rim of the large crater must have fallen more recently than that one that formed the large crater. As an example of ages, the craters Copernicus and Tycho are about 600 and 200 million years old, respectively.
Volcanically produced craters may have formed during the Moon's early history (Figure 7.20). But if so, they are present in considerable fewer numbers than those of impact origin.
[Figure 7.20]
Topography on the far side of the Moon appears to the eye to be strikingly different from that on the near side (Figure 7.21). This is in the sense that impact craters are everywhere, but there are no large lava-flooded basins (maria) comparable with Mare Imbrium on the near side, although some small ones do exist. The far side thus lacks the near side's extensive lava flooding. As a consequence, the back side has no extensive mountain ranges.
[Figure 7.21]
The Moon's center of mass is displaced from its geometric center about 2 km earthward. One consequence is that the lunar crust facing the Earth is about half as thick as that of the far side. Perhaps this variation explains why the near side had more volcanic activity in the distant past, which produced the large deposits of dark maria material. It may also help explain why the large impact basins on the far side are only partially filled.
7.7. Evolution of the Lunar Surface
The Moon appears to have formed from the same chemical elements, although in somewhat different proportions, as those that formed the Earth. It has less iron, more of the substances that are hard to melt--such as calcium, aluminum, and titanium--and less of the easily vaporized substances--such as sodium and potassium--than does the Earth. The relative abundances of oxygen's three isotopes, however, is extremely close to that of terrestrial rocks. In general, the composition of the Moon mimics that suspected for the Earth's mantle, although it is not a perfect match.
The most common surface rocks are anorthosites (silicates mainly of aluminum and calcium) from the highlands, iron-rich basalt from the maria, and thorium-rich and uranium-rich rocks. No traces of water and no organic compounds, the indicators of living processes, were discovered in any lunar samples. In fact, the Apollo lunar rocks contain only tiny amounts of carbon and carbon-based compounds from which life originates. With no water or oxygen present, the minerals in lunar rocks could not react with water to form clays or rust, nor did iron react with oxygen to form oxides. The lunar highlands, which cover about four-fifths of the lunar surface, are the oldest preserved terrain.
From the findings on the chemical composition of lunar samples coupled with radioactive dates for these rocks, what can be deduced about the history of the lunar surface?
Radioactive dating of lunar rocks points to the formation of the Moon from materials that were much like in composition those forming the Earth. Nearly all the lunar samples are enriched in a kind of mineralogical slag that could only have formed if the Moon underwent a molten phase as did the Earth. This evidence suggests that the Moon probably underwent a global melting to a depth of at least several hundred kilometers, followed by chemical differentiation (a separation of the chemical elements by gravitational settling) shortly after formation. A few of the lunar rock samples are about 4.6 billion years old, the same age as the Earth (even though Earth rocks no longer exist that are older than 3.5 to 3.8 billion years).
The highland areas are apparently about 4.0 to 4.3 billion years old. After formation of the lunar crust, it was continually modified as a result of impact cratering by material from elsewhere in the Solar System. The cratering record preserved in early crustal units represents a distinct phase of early intense impact cratering, which occurred very early in the history of the Solar System and began to decline about 3.8 billion years ago (Figure 7.22). Although volcanic processes may have operated during this early period, the surface history of the Moon is primarily that due to impact cratering. As mentioned earlier, this phase in the Earth's history has been almost completely erased. Impact cratering continues on the Moon today, but at a drastically reduced rate from what it must have been billions of years ago.
[Figure 7.22]
The next stage in lunar history was dominated by the formation of dark mare plains, which cover about 17 percent of the lunar surface. These structures are relatively thin ponds of basaltic lava which taken together are less than one percent of the crust's volume. Rocks from maria suggest that the major outpouring of lava occurred between 3.9 and 3.2 billion years ago. Although some mare deposits may be as young as 2 billion years, there appears to have been no extensive lava outpouring on the lunar surface for the last 3 billion years. Thus the shaping of the present lunar terrain is almost the opposite of that of Earth's--the Moon dominated by impact cratering and the Earth by thermal-tectonic activity.
7.8. Internal Structure of the Moon
There is no general lunar magnetic field as large as approximately one ten-thousandth that of the Earth. This seems to indicate that the Moon does not possess a molten iron core comparable with that of the Earth, which is thought to be necessary to produce a magnetic field. But evidence suggests that the Moon may have had a stronger magnetic field early in its history. Random magnetic fields up to about 0.6 percent of the Earth's field intensity were detected at different sites by Apollo astronauts, but we do not know the reason for such magnetic anomalies.
From seismographs left on its surface we know that seismic events on the Moon follow patterns different from those here on Earth. Vibrations from moonquakes or rare meteorite impacts are transmitted very slowly through lunar material. They build gradually and then take hours to subside. Some seismic disturbances have been traced to geologic movements in the rilles; others, to occasional impacts of meteoroid swarms. Moonquakes frequently coincide with tidal stresses triggered by the varying distance between the Moon and the Earth. They occur at depths of 600 to 900 km, much deeper than earthquakes. Almost a 100 sources for these deep moonquakes have been discovered so far. But compared with the Earth's seismic activity, the Moon's is fairly subdued; the whole Moon releases less than one ten-billionth of the Earth's earthquake energy.
About 35 shallow quakes, presumably tectonic events, have been detected. Thus in the last 3 billion years, any thermal and geologic activity has been relatively rare. As we have mentioned, most volcanic activity appears to have ceased about 3 billion years ago, but some minor activity may still be going on.
Seismic data tell us that the crust is about 60 km thick, twice the thickness of the Earth's crust. Heat flow from the deep interior through the lunar crust is no more than about a third of that for the Earth. Thus thermally driven processes cannot be nearly as important as for the Earth.
The Moon's mantle, nearly 800 to 1000 km deep under the lunar crust, is uniformly structured. Most of it may be pyroxene and olivine, minerals containing silicon, oxygen, calcium, magnesium, and iron. The seismic data reinforce the view that the Moon's core is unlike the Earth's metallic core. Probably the lunar core consists of partly molten silicates, with a small metallic center, as shown in Figure 7.23. At present, scientists can neither rule out the existence of a small iron core nor prove that one does exist.
[Figure 7.23]
Only a few decades ago the Moon was a light in the sky that, even though near, was still part of the remote cosmos. Now it is a place that has been visited by human beings and studied to such an extent that it is no longer a remote cosmic body. What is its future role in human affairs? There are proposals to establish permanent bases on the Moon for astronomical observatories, mining operations, ore refining, or manufacturing; and it is not impossible that the Moon may become the departure station for manned exploration of the Solar System. From any point of view, however, the Moon is no longer a strange and distant world.