An isolated system is one so completely sealed off from its environment that neither matter nor energy passes through its boundaries. This is an imaginary construct, however, an idea rather than a reality, because it is impossible to create a situation in which no energy is exchanged between the system and the environment. Under the right conditions it is perhaps conceivable that matter could be sealed out so completely that not even an atom could pass through a barrier, but some transfer of energy is inevitable. The reason is that electromagnetic energy, such as that emitted by the Sun, requires no material medium in which to travel.
In contrast to an isolated system is a closed system, of which Earth is an approximation. Despite its name, a closed system permits the exchange of energy with the environment but does not allow matter to pass back and forth between the external environment and the system. Thus, Earth absorbs electromagnetic energy, radiated from the Sun, yet very little matter enters or departs Earth's system. Note that Earth is an approximation of a closed system: actually, some matter does pass from space into the atmosphere and vice versa. The planet loses traces of hydrogen in the extremities of its upper atmosphere, while meteorites and other forms of matter from space may reach Earth's surface.
Earth more closely resembles a closed system than it does an open one—that is, a system that allows the full and free exchange of both matter and energy with its environment. The human circulatory system is an example of an open system, as are the various "spheres" of Earth (geosphere, hydrosphere, biosphere, and atmosphere) discussed later. Whereas an isolated system is imaginary in the sense that it does not exist, sometimes a different feat of imagination is required to visualize an open system. It is intricately tied to its environment, and therefore the concept of an open system as a separate entity sometimes requires some imagination.
To gain perspective on the use of systems in science as well as the necessity of mentally separating an open system from its environment, consider how these ideas are used in formulating problems and illustrating scientific principles. For example, to illustrate the principle of potential and kinetic energy in physics, teachers often use the example of a baseball dropping from a great height (say, the top of a building) to the ground.
At the top of the building, the ball's potential energy, or the energy it possesses by virtue of its position, is at a maximum, while its kinetic energy (the energy it possesses by virtue of its motion) is equal to zero. Once it is dropped, its potential energy begins to decrease, and its kinetic energy to increase. Halfway through the ball's descent to the ground, its potential and kinetic energy will be equal. As it continues to fall, the potential energy keeps decreasing while the kinetic energy increases until, in the instant it strikes the ground, kinetic energy is at a maximum and potential energy equals zero.
What has been described here is a system. The ball itself has neither potential nor kinetic energy; rather, energy is in the system, which involves the ball, the height through which it is dropped, and the point at which it comes to a stop. Furthermore, because this system is concerned with potential and kinetic energy only in very simple terms, we have mentally separated it from its environment, treating it as though it were closed or even isolated, though in reality it would more likely be an open system.
In the real world, a baseball dropping off the top of a building and hitting the ground could be affected by such conditions as prevailing winds. These possibilities, however, are not important for the purposes of illustrating potential and kinetic energy, and even if they were, they could be incorporated into the larger energy system.
Since kinetic energy and potential energy are inversely related, the potential energy at the top of the building will always equal the kinetic energy at the point of maximum speed, just before impact. This is true whether the ball is dropped from 10 ft. (3 m) or 1,000 ft. (305 m). It may seem almost magical that the sum of potential and kinetic energy is always the same or that the two values are perfectly inverse. In fact, there is nothing magical here: the system has a certain total energy, and this does not change, though the distribution of that energy can and does vary.
Suppose one had a money jar known to contain $20. If one reaches in and grasps a five-dollar bill, two one-dollar bills, three quarters, a dime, and two nickels ($7.95), there must be $12.05 left in the jar. There is nothing magical in this; rather, what has been illustrated is the physical principle of conservation. In physics and other sciences, "to conserve" something means "to result in no net loss of" that particular component. It is possible that within a given system, the component may change form or position, but as long as the net value of the component remains the same, it has been conserved. Thus, the total energy is conserved in the situation involving the baseball, and the total amount of money is conserved in the money-jar.
In the baseball illustration, the distribution between types of energy varies, but the total amount is always the same. Likewise in the money-jar illustration, the total amount of money remains fixed even though the distribution according to various denominations may vary. The same is true of Earth, though here it is the total amount of matter. This includes valuable resources, among them materials that can be mined to produce energy—for instance, fossil fuels such as coal or petroleum—as well as waste products. Because Earth is a closed system, there are no additional resources, nor is there any dumping ground other than the one beneath our feet. Thus, the situation calls for prudence both in the use of the planet's material wealth and in the processing of materials that will leave a byproduct of waste.
The fact that a closed system is by definition finite leads to the principle that the relationships between its constituent parts are likewise finite, and therefore changes in one part of the system are liable to produce effects in another part. Conditions in the baseball or money-jar illustrations are so simple that it is easy to predict the effect of a change. For instance, if we substitute a basketball for a baseball, this will change the total energy, because the latter is a function of the ball's mass. If the denominations making up the $20 in the money jar are replaced with a collection of two-dollar bills and dimes, this will make it impossible to reach in and pull out an odd-numbered value in dollars or cents.
What about the changes that result when one aspect of Earth's system is altered? In some cases, it is easy to guess; in others, the interactions are so complex that prediction requires sophisticated mathematical models. It is perhaps no accident that chaos theory was developed by a meteorologist, the American Edward Lorenz (1917-). Chaos theory, the study of complex systems that appear to follow no orderly laws, involves the analysis of phenomena that appear connected by something than an ordinary cause and effect relationship. The classic example of this is the "butterfly effect, " the idea that a butterfly beating its wings in China can change the weather in New York City. This, of course, is a farfetched scenario, but sometimes changes in one sector of Earth's system can yield amazing consequences in an entirely different part.
The systems approach is relatively new to the earth sciences, themselves a group of disciplines whose diversity reflects the breadth of possible approaches to studying Earth (see Studying Earth). At one time, earth scientists tended to investigate specific aspects of Earth without recognizing the ways in which these aspects connect with one another; today, by contrast, the paradigm of the earth sciences favors an approach that incorporates the larger background.
Given the complexities of Earth itself, as well as the earth sciences, it is helpful to apply a schema (that is, an organizational system) for dividing larger concepts and entities into smaller ones. For this reason, earth scientists tend to view Earth in terms of four interconnected "spheres. " One of these terms, atmosphere, is a familiar one, while the other three (geosphere, hydrosphere, and biosphere) may sound at first like mere scientific jargon.
In fact, each sphere represents a sector of existence on the planet that is at once clearly defined and virtually inseparable from the others. Each is an open system within the closed system of Earth, and overlap is inevitable. For example, the seeds of a plant (biosphere) are placed in the ground (geosphere), from which they receive nutrients for growth. In order to sustain life, they receive water (hydrosphere) and carbon dioxide (atmosphere). Nor are they merely receiving: they also give back oxygen to the atmosphere, and by providing nutrition to an animal, they contribute to the biosphere.
Each of the spheres, or Earth systems, is treated in various essays within this book. These essays examine these subsystems of the larger Earth system in much greater depth; what follows, by contrast, is the most cursory of introductions. It should be noted also that while these four subsystems constitute the entirety of Earth as humans know and experience it, they are only a small part of the planet's entire mass. The majority of that mass lies below the geosphere, in the region of the mantle and core.
As a passing curiosity, it is interesting to note that modern scientists have identified four subsystems and given them the name spheres. As discussed in the essay Earth, Science, and Nonscience, the ancient Greeks were inclined to divide natural phenomena into fours, a practice that reached its fullest expression in the model of the universe developed by the Greek philosopher Aristotle (384-322 B.C. ) He even depicted the physical world as a set of spheres and suggested that the heaviest material would sink to the interior of Earth while the lightest would rise to the highest points.
These points of continuity with ancient science are notable because almost everything about Aristotle's system was wrong, and, indeed, the differences between his model of the physical world and the modern one are instructive. There are four spheres in the modern earth sciences because these four happen to be useful ways of discussing the larger Earth system—not, as in the case of the Greeks, because the number four represents spiritual perfection. Furthermore, scientists understand these spheres to be artificial constructs, at least to some extent, rather than a key to some deeper objective reality about existence, as the ancients would have supposed.
Nor are the spheres of the modern earth sciences literally spheres, as Aristotle's concentric orbits of the planets around Earth were. If anything, the use of the term sphere represents a holdover from the Greek way of viewing the material world. Finally, unlike such ancient notions as the concept of the four elements, four qualities, or four humors, the idea of the four spheres is not simply the result of pure conjecture. Instead, the concept of these four interrelated systems came about by application of the scientific method and entered the vocabulary of earth scientists because the ideas involved clearly reflected and illustrated the realities of Earth processes.
The geosphere itself may be defined as the upper part of the planet's continental crust, the portion of the solid earth on which human beings live, which provides them with most of their food and natural resources. Even with the exclusion of the mantle and core, the solid earth portion of Earth's system is still by far the most massive. It is estimated that the continental and oceanic crust to a depth of about 1.24 mi. (2 km) weighs 6 × 10 21 kg—about 13,300 billion billion pounds. The mass of the biosphere, by contrast, is about one millionth that figure. If the mass of all four spheres were combined, the geosphere would account for 81.57%, the hydrosphere 18.35%, the atmosphere 0.08%, and the biosphere a measly 0.00008%. (Of that last figure, incidentally, animal life—of which humans are, of course, a very small part—accounts for less than 2%.)
Not only is the geosphere the largest, it is also by far the oldest of the spheres. Its formation dates back about four billion years, or within about 0.5 billion years of the planet's formation. As Earth cooled after being formed from the gases surrounding the newborn Sun, its components began to separate according to density. The heaviest elements, such as iron and nickel, drifted toward the core, while silicon rose to the surface to form the geosphere.
In that distant time Earth had an atmosphere in the sense that there was a blanket of gases surrounding the planet, but the atmospheric composition was quite different from today's mixture of nitrogen (78%), oxygen (21%), argon (0.93%), and other substances that include water vapor, carbon dioxide, ozone, and noble gases such as neon, which together comprise 0.07%. The atmosphere then consisted largely of carbon dioxide from Earth's interior as well as gases brought to Earth by comets. Elemental hydrogen and helium escaped the planet, and much of the carbon was deposited in what became known as carbonate rocks. What remained was a combination of hydrogen compounds, including methane, ammonia, nitrogen-and sulfur-rich compounds expelled by volcanoes, and (most important of all) H 2 O, or water.
Simultaneous with these developments, the gases of Earth's atmosphere cooled and condensed, taking the form of rains that, over millions of years, collected in deep depressions on the planet's surface. This was the beginning of the oceans, the largest but far from the only component of Earth's hydrosphere, which consists of all the planet's water except for water vapor in the atmosphere. Thus, the hydrosphere includes not only saltwater but also lakes, streams, groundwater, snow, and ice.
Water, of course, is necessary to life, and it was only after its widespread appearance that the first life-forms appeared. This was the beginning of the biosphere, which consists of all living organisms as well as any formerly living material that has not yet decomposed. (Typically, following decomposition an organism becomes part of the geosphere.) Over millions of years, plants formed, and these plants gradually began producing oxygen, helping to create the atmosphere as it is known today—an example of interaction between the open systems that make up the larger Earth system.