ROLE OF THE ATMOSPHERE: GREENHOUSE EFFECT
If the earth had no atmosphere, life would not exist here. The insulating blanket of air surrounding us maintains a temperature range favorable for life because the sun's radiant energy, which is primarily in the visible wavelengths, is absorbed by and warms the ground, which in turn reradiates energy in the infrared region of the electromagnetic spectrum. (The reradiated energy is in the infrared because the warming of the surface by sunlight maintains it at a temperature of not quite 300 K.) The reradiated infrared photons' passage outward into space, however, is restricted by carbon dioxide and water-vapor molecules in the atmosphere; they absorb the energy and reradiate much of it back to the surface of the earth. This is called the greenhouse effect, after the similarity of the action to that of the glass in a greenhouse, which prevents heat radiation produced inside the greenhouse from escaping.
Without its atmosphere the earth's average temperature would be about 20° to 30° lower than its present value of 15°C (288 K). Since water would be frozen at that temperature, it could not effect the development and maintenance of life as it does in a liquid form. Worldwide circulation of the atmosphere also transports thermal energy and helps to moderate the extremes in temperature that would otherwise exist.
Moreover, 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 that we call cosmic rays. Finally the atmosphere creates the soft blue appearance of the sky: atmospheric gases scatter the photons of sunlight in the blue region much more efficiently than they do photons of longer wavelengths.
In all, the earth's atmosphere plays a very vital role beyond the obvious one of providing the oxygen that we breath. One of mankind's most important challenges is in understanding all aspects of our atmosphere and preserving it; for our continued existence depends upon that knowledge.
PHYSICAL PROPERTIES OF THE ATMOSPHERE
The total mass of the atmosphere is about onemillionth of the total mass of the earth. It has several layers, each with distinctive thermal, physical, chemical, and electrical properties. Approximately half the atmosphere is contained in the first 5.6 kilometers, and 99 percent of it lies below 35 kilometers.
Our weather takes place in the bottom layer, called the troposphere. Eleven kilometers up, the temperature drops to -55°C. Above this region lies a 40kilometer-thick layer, the stratosphere, where the temperature slowly rises, reaching a maximum of about O°C at 50 kilometers, and 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 -85°C at its upper limit, 90 kilometers.
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 th roughout the thermosphere and into the exosphere, the atmospheric fringe several hundred kilometers above sea level.
CHEMICAL COMPOSITION OF THE ATMOSPHERE
Up to about 90 kilometers gravitational settling causes no significant separation of the 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 kilometers 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 kilometers, 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 that introduce a particular molecule into the atmosphere and those that remove the molecule from the atmosphere. Probably the most meaningful 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 extend by plant life on land. A little oxygen comes from the direct dissociation of atmospheric water molecules by ultraviolet photons from the sun. Oxygen is chemically quite an active molecule, combining readily with a number of different atoms. Thus the formation of oxides in rocks removes 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.
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 with 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 volcanoes and the escape of gases from the crust. The gaseous emission from presentday volcanoes includes water vapor, carbon dioxide, nitrogen, inert gases, and small amounts of methane, ammonia, and sulfur compounds. It is estimated that on a lifeless earth with no significant amounts of liquid water the dominant atmospheric constituent would be 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's being some hundred times denser that ours is. 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 oxygenproducing photosynthesis by green plants.
IONOSPHERE
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 meters, or frequencies less than about 30 megahertz, are turned back by the ionospheric layers. Shorter wavelengths, or higher frequencies, such as radar signals, pass through the ionosphere into space with little or no bending.
