Document created: 04 May 2004
Air University Review, July-August 1971
Dr. William G. Pollard
One of the most significant general results of the space program of the United States has been the new perspective on our earth achieved for all mankind with the magnificent color pictures of the planet brought back from the Apollo missions. The dazzling beauty of the earth, with its swirling white cloud cover and the sparkling azure of her oceans, is breathtaking. From out in space where the earth can be seen as one among many astronomical bodies, no others have anything to compare with her beauty.
The earth has an incredibly long history, and her present adornment of atmosphere, hydrosphere, biosphere, and noosphere is the achievement of that history. It was made possible by a remarkable and delicate combination of circumstances which, when fully appreciated, suggests a largely unrecognized uniqueness for the earth. This can be best appreciated by considering the history of the earth in parallel with the contrasting histories of the other planets in the solar system and of the sun itself. For this purpose a number of other less well-known missions of the U.S. and Soviet space programs have provided much new and relevant information. With this information combined with knowledge derived from other sources, a fairly reliable general account of the history of the solar system can now be given.
Review is pleased to present an adaptation of that lecture.
The Thomas D. White Lectures continued at Air University on 2 March 1971 when the speaker was Dr. William C. Pollard, Executive Director of the Oak Ridge Associated Universities. The
The sun and planets were formed 4.6 billion years ago in the collapse of a great cloud of gas and dust falling in on itself under its own gravity. The gases were mainly hydrogen and helium, with the less abundant carbon, nitrogen, and oxygen present as methane (CH4), ammonia (NH3), and water (H2O). Floating in this gas was a relatively small amount of dust of silicates and metals, mostly iron. The heavy elements in this cloud, from iron through uranium, had been freshly synthesized one or two hundred million years earlier in a vast explosion of a previous star, which became a supernova.
Most of this cloud condensed into a large central body, which became the sun. A few tenths of a percent of it, however, was thrown out by centrifugal force in the rotating, collapsing mass into a great platter in the plane of the
suns equator, like the rings of Saturn. In the meantime the center of the condensing mass became hotter and hotter as the gas was more and more compressed. The temperature at the center of the sun reached the hydrogen-bomb ignition temperature of several million degrees centigrade, and the hydrogen in the central core ignited explosively. This explosion has been going on in the
The young sun was quite active, producing great solar flares and an intense solar wind. In the first hundred million years, this wind swept much of the hydrogen and helium out of the space near the sun well beyond the present orbit of Mars. The heavier materials in the meantime condensed into growing chunks of stone and iron, with a good deal of ice and some entrapped ammonia and methane in them. These in turn fell together in growing bodies that became the lesser planets: Mercury, Venus, Mars, earth, moon, and the asteroids (which were prevented from consolidating into a planet by the powerful gravity of nearby Jupiter). Farther out the very cold hydrogen and helium were gathered under their gravity and condensed into the major planets Jupiter and Saturn, which are low-density bodies very different from the inner planets. They may have central cores of solid helium surrounded by thick mantles of metallic hydrogen with outer atmospheres of hydrogen, ammonia, and methane.
At first all the inner planets, including the earth, must have been much alike: bare, rocky bodies pockmarked with craters resulting from their growth by the falling together of great chunks of rock. The heat of these collisions released gases trapped in the rock so that they all acquired a growing atmosphere of ammonia, methane, and water. In all of them, radioactive materialsmainly uranium, thorium, and potassium generated in the ancestral supernovasteadily generated heat deep in their interiors. Possibly the earth and Venus collected more iron, uranium, and other metals than the others and so experienced greater internal heating than the moon and Mars. In time this internal heat also melted the rock, and the rising pressure released it in volcanoes with great lava flows and quantities of steam, additional carbon dioxide or methane, and nitrogen.
Only the earth was at just the right distance from the sun for the released steam to condense as rain and collect over the surface in rivers, lakes, and eventually oceans. On Venus and Mercury the temperature was always too high for this, and the steam remained in their atmospheres as water vapor. On Mars it came down as snow and collected as ice. Only occasionally in brief warm periods would there have been liquid water such as one sees here on the Greenland ice cap. For the first several billion years the moon may well have been a separate planet in an orbit around the sun near the earths orbit.
There are two important and somewhat related points to be made about the hydrogen-containing compounds methane, ammonia, and water in the early history of all these planets. At the top of their atmospheres the intense ultraviolet light from the sun continually knocked hydrogen atoms out of all these molecules. The very fast-moving hydrogen atoms, when moving directly away from the planet, would occasionally have enough velocity to escape from it completely. This was very much the case with the moon, for which the escape velocity is only 1.5 miles per second. For Mercury and Mars it is a little over 3 miles per second, but the much higher temperature of Mercury would produce a much more rapid escape of hydrogen from it. For Venus and the earth the escape velocity is about 7 miles per second, so the hydrogen would escape much more slowly.
Ultimately, when all the hydrogen is gone, the oxygen left behind from the water combines with the carbon left behind from the methane to form carbon dioxide, with some remaining as oxygen gas. It also combines with metals to form oxides. The nitrogen left behind from the ammonia remains as nitrogen gas (N2). Thus on any planet no heavier than the earth the atmosphere after four billion years will consist of nitrogen and carbon dioxide.
By now, four billion years later, even the residual nitrogen and carbon dioxide have escaped from the moon and Mercury. They have no atmosphere left at all. Mars has lost practically all its nitrogen but does have some residual carbon dioxide, much of it in the form of dry ice around the polar caps. Venus and the earth, however, have retained all or nearly all of both. The earth, with its liquid water, was able to dispose of its carbon dioxide because it went into solution in the water and there was converted to solid carbonates, such as limestone, of which the earths crust has great quantities. On Venus, in the absence of liquid water, the carbon dioxide remained in the atmosphere. At present Venus has about the same amount of nitrogen as the earth but has an enormous mass of carbon dioxide, so that the atmospheric pressure at the surface of Venus is over a hundred times that on the earth, nearly a ton per square inch. Under this crushing atmospheric canopy, the temperature at the surface is 800ΊF, hot enough to melt lead. There may be mountain-forming volcanoes and earthquakes on Venus as there are on the earth, but there has never been any water erosion with resulting sedimentary beds and ore bodies. But there is probably terrifically powerful wind erosion with high winds blowing over dreadfully hot and totally arid deserts, producing sandblasts of an intensity unimagined here. For all the romanticism of which Venus has been the object in human literature, she is as near hell as one can imagine!
We now turn to the second important point about the hydrogen-containing molecules in the early atmospheres of the planets. As hydrogen atoms were knocked out of them by ultraviolet light, free radicals of carbon, nitrogen, and oxygen were left behind. These are highly active chemically, and they generate in such an atmosphere a great variety of organic compounds basic to life. This process can easily be reproduced in a hydrogen, ammonia, and methane mixture in the laboratory, where extended ultraviolet irradiation produces a variety of amino acids, simple sugars, and bases, like adenine, essential to the formation of nucleic acids. On the earth, and perhaps for a short time on the moon and Mars as well, these compounds were washed out of the atmosphere by rain from the condensing volcanic steam. As the
During the first one and a half billion years of earths existence, these organic materials in the growing oceans of the young earth had somehow become incorporated into simple cellular organisms. This we know from remnants resembling present rodlike bacteria embedded in a black chert 3.2 billion years old in the Fig Tree series of the eastern Transvaal region of Africa. In another billion years a great variety of single-celled filamentous organisms like modern blue-green algae had developed, as we know from their fossil remains in the 1.9-billion-year-old Gunflint iron formation on the northern shores of Lake Superior. These organisms were capable of photosynthesis and so continually converted carbon dioxide into oxygen in the ocean. For these early organisms the oxygen was highly poisonous, and its presence constituted a major crisis in the history of life. At first their only protection from it was its removal from the oceans by conversion of soluble ferrous iron to precipitated ferric iron. Oxygen gas began to accumulate in the atmosphere about two billion years ago, and by 1.3 billion years ago a new kind of living cell, capable of utilizing the oxidation of sugars as an energy source, had developed. The serious oxygen crisis had been successfully passed, and the stage was set for a new phase of life of tremendous potential. An impressive record of this first major ecological crisis that occurred between 2.5 and 1.5 billion years ago is left in the great iron ore beds of Lake Superior and in the Mesabi Range that used to stand in Minnesota.
For three and a half billion years of its history, the earth consisted of oceans and bare sterile land areas subject to rapid erosion by flowing water. Ultraviolet radiation from the sun was intense over the whole land and sea surface of the earth. When the living organisms in the oceans came within several feet of the surface, they were soon destroyed by the ultraviolet radiation. Nothing living existed anywhere on the land. No multi-cellular organisms or creatures moved through the deep waters throughout that immense period of time. No suspicion of what would later be achieved through the further elaboration of DNA codes, developed by then in the single-celled organisms in the oceans, could have been gained from an examination of the earth at that time.
Almost two billion years ago, free oxygen released in photosynthesis began to escape from the oceans and join the nitrogen and other oxygen resulting from the ultraviolet release of hydrogen from water high up in the earths atmosphere. When some ten percent of the present amount had accumulated around a billion years ago, a layer of ozone was formed high in the atmosphere, which absorbs all of the far ultraviolet in the solar radiation. From then on, living systems could rise to the surface of the oceans and later find habitats on the land. At the same time, in response to the dissolved oxygen in the oceans in equilibrium with that in the atmosphere, DNA codes were elaborated for entirely new biological systems for which oxygen was no longer a poison but a benefit. These were the mitochondria, which, by burning sugar with oxygen, could produce the same essential organic compounds as that produced by the ancient and long-standing chloroplasts from carbon dioxide and water using the energy of visible light from the sun. A new and potent energy source was now available for incorporation into living organisms. The stage was set for an astounding new epoch in the history of the earth.
By 650 million years ago, soft-bodied multi-cellular organisms, jellyfish and flatworms, and novel animal forms had developed in the oceans. Their impressions are found in abundance in ancient sandstones in the Ediacara Hills in South Australia. By 600 million years ago some of these animals developed the capacity to make calcium carbonate and could cover themselves with hard protective shells. From then on, the history of life on the earth is recorded in a continuous fossil record. This was the beginning of the geological period known as the Cambrian, which began when the earth was just four billion years old.
The development of living organisms had taken place with almost infinite slowness during those first four billion years. During the Cambrian the rate of development took a quantum jump to a new order of magnitude. Sea creatures developed in great variety and profusion. Plant life began spreading over the land, followed by insects. After 500 million years of such development, the earth of 180 million years ago had acquired a fully developed biosphere, with many of todays features. An Apollo picture of the earth taken then would have looked very much the same as those taken now. Closer up, however, the scene then would have been quite different. The dominant creatures were the great reptiles, mighty dinosaurs, immense and fearful flying reptiles, and numerous reptilian sea monsters. Great conifer forests and other vegetation clothed the land, but as yet no deciduous hardwood or flowering plants and shrubs. Also at that time there was no Atlantic Ocean, and Europe and Africa formed a continuous land mass with North and South America.
Just 70 million years ago the earth entered a new period of her history called the Tertiary. Most reptiles were extinct, and the age belonged to the mammals, which developed in increasing profusion and variety. Plant life had become much as we know it now. The land was graced with a blanket of verdure, great hardwood forests, and high windswept steppes. There were birds and insects in the air, fish in lakes and rivers, and in the sea. Through forest and prairie myriads of animals ranged. By the end of the Tertiary, just two million years ago, these were much the same as we know them today: antelopes, zebras, and horses; a variety of proboscidians in herds; deer, tigers, wolves, foxes, and badgers. Through forest and prairie myriads of animals of the inner energy and dynamism of the phenomenon of life, there was by now in forest and steppe the beauty of flowering plants, shrubs, and trees embracing the whole range of color in every degree of delicacy and brilliance. Only one element was missing from this calm and lovely scene: Man had not yet appeared, and nowhere over the whole earth was there so much as a wisp of smoke rising from a camp fire.
Science fiction conveys the impression that planets throughout the universe are very similar to the earth; that life has developed on all of them and has finally produced some manlike creature which, although possibly bizarre in appearance, nevertheless thinks, is self-conscious, and can communicate. Beyond science fiction, popular accounts of science in newspapers and magazines instill the same kind of convictions in readers. A good example is Walter Sullivans book We Are Not Alone.1 Even some very distinguished and otherwise highly reliable scientists speak this way. Within the scientific community itself the ideas of the commonness of the earth and the prevalence of life are widely held. As a result there is as yet in the public at large little appreciation of the extraordinary wonder of the earth or of what a rare and precious gem our planet is.
But now, against the background of what we know so far of the history of our solar system, let us examine critically the conditions which must be met for anything comparable to the earth to be achieved elsewhere in the universe. First and most essential is liquid water. This, of course, requires a rather narrow temperature range which must persist for at least half the planets orbit around its star. If the earth were just 10 percent closer to the sun than it is, it would receive 23 percent more solar radiation than it does now. There might then be some liquid water in arctic regions and occasional hot pools elsewhere over the surface which would boil and dry up every summer. There would be some limestone, but the atmosphere would still be like that of Venus, with a great amount of carbon dioxide. If the earth had been 10 percent farther out from the sun, it would receive only 83 percent of the solar energy it does now. In that case most of the water would be in vast ice caps, with some melting on the surface of the ice in summer and perhaps some lakes and rivers in the tropics. Again if the earth were in a highly elliptical orbit around the sun, instead of the near-circular one it has, the oceans would boil vigorously for two or three months of the year and then freeze solid for six to eight months.
If the earth had been much smaller and less massive than it is but otherwise in the same orbit, hydrogen would have escaped much more rapidly from its gravitational hold. In that event it would not have been able to hold sizable quantities of liquid water for more than two or three billion years. By now all the water would have escaped, along with some of the nitrogen and carbon dioxide. Life could have developed up to the stage of the Gunflint algae perhaps but then would have been snuffed out as the last water left. On the other hand, had the earth been much larger and more massive than it is, it would have retained until now the reducing atmosphere of ammonia and methane plus possibly free hydrogen, which seems to have been necessary during its first few billion years for the development of life as we know it. In that case there would be now no free oxygen in the atmosphere and so no ozone layer. There would still be no life on the land. How far life would have developed in the deep oceans, we have no way of knowing. But the earth would obviously be now a very different object with a radically different history.
Those who like to think of the earth as quite common and unexceptional should contemplate quite deeply the significance of what we know now of the moon, Mars, and Venus, to say nothing of Jupiter and Saturn. Even in our own solar system, planets come in a great variety of chemical composition and physical states. Except for the earth, their several histories have led after four and a half billion years to the achievement of nothing like the complexity of organization of matter that we have come to know and take for granted.
But it is not only the planet which is important. The central star is also most important. All main sequence stars like the sun are burning hydrogen into helium in natural hydrogen bombs in their cores. When the hydrogen in the core is used up, they go into a gravitational collapse, which leads to the burning of helium in the core and a tremendous expansion of the outer envelope. The star becomes a red giant. If it previously had a system of planets around it, they are all evaporated and become a part of its outer envelope. This brings all planetary histories, of whatever character, to an abrupt termination. The more massive the star, the more rapid is the burning and the sooner is the red-giant stage reached. A star only fifty percent heavier than the sun would reach the red-giant stage in 2.3 billion years. If it had a planet the size of the earth in the right place, life could have developed to the stage of the Gunflint algae before being destroyed. In order to allow a development of at least 4.6 billion years, the star must be no more massive than 1.25 times the suns mass. The sun itself has several billion years more to go before reaching the red-giant stage.
About halt the stars are double stars or members of systems of three or more stars. In such multistar systems there are no stable near-circular planetary orbits. The other half of the stars that are single probably all have systems of several planets. Moreover, stars less massive than the sun greatly outnumber those that are larger. For example, in our region of space there are three times as many stars with half the mass of the sun there are stars of the same mass as the sun. These smaller stars are much cooler than the sun, and their radiation is much weaker in ultra-violet. Without ultraviolet, both the loss of hydrogen from the original atmosphere would be much slowed and the production of organic materials from that atmosphere would be altered. A planet the size of the earth at the right distance from the star might well develop life, but its evolution over four billion years would probably show a very different history. The mechanism of planetary formation around such a star could well favor hydrogen-helium planets like Jupiter and Saturn close to the star. Such bodies do not offer environment favorable to any very elaborate evolution of complex organisms.
There are so many stars now on the main sequence in our galaxy that the probability is great that somewhere another earth-like planet has held liquid water for billions of years and has enjoyed a history that could have clothed it with verdure and produced another gem of rare beauty like the earth in the vast reaches of space. But the conditions required for such an outcome, as we have considered them, suggest that such objects are quite rareprobably none at the stage a development presently reached by the earth within several hundred light years of us. If that is the case, then the earth for all practical purposes is unique. There is no other creative achievement of such a high organization of matter within any conceivable reach of us.
The preceding account of the earths history goes through the Tertiary to the beginning of the present geological epoch, the Pleistocene, which began just two million years ago. This sequence was followed to point up the major character of the turning point in this long history which the appearance of man on the planet represents. Only three or four other turning points of comparable magnitude can be identified. One was certainly the point over three billion years ago at which nucleic acid and protein first became organized in living cells. The second was the achievement of photosynthesis around 2.5 billion years ago, with its attendant oxygen crisis. The third was the transformation 600 million years ago from the pre-Cambrian to the Cambrian, when multicellular organisms appeared and the earth began to acquire a biosphere. A fourth could have been the earths capture of the moon sometime between these two turning points, followed by the moons subsequent close approach to the earth and causing immense scouring tidal waves. The fifth is the very recent appearance of man, as a result of which the earth has acquired what Teilhard de Chardin aptly calls the noosphere i.e., a blanket of mind and spirit covering the earth. The planetary impact and crisis proportions of this transformation of the biosphere into the noosphere are just beginning to be felt in this century.
Early in the Pleistocene one stem of the evolving and diversifying branch of primates took a fateful step. As at so many other points in the history of life, a door opened briefly and this primate stepped through it. Had he passed by instead to go through some other door, the opportunity for man could well have been gone forever. The result was the first of the species Homo. He left the trees and began to learn to walk on two feet. He originated in south central Africa and is called Homo habilis. After a long period of continuous development and diversification, a quite new version called Homo erectus appeared on the scene 300,000 years ago, after which all forms of Homo habilis became extinct. In time he showed the very human trait of wanderlust, and he migrated to the Middle East, Europe, England, China, and Southeast Asia. In Europe he is Heidelberg man, in China Sinanthropus, and in Java Pithecanthropus. By 100,000 years ago a still more human version, Homo neanderthalensis, emerged, and soon Homo erectus became extinct. Neanderthal man was the first creature in the whole history of life on earth to bury his dead. From this fact alone we know we are dealing with a self-conscious being who anticipates in anxiety or hope. Yet if we could see a Neanderthal man today, with his massive jaw and absence of forehead, we would not consider him human at all.
Some 40,000 years ago one of the diversifying lines of development in Neanderthal man made another leap, and our species, Homo sapiens, first appeared on the scene. After his arrival Homo neanderthalensis became extinct. Homo sapiens not only buried his dead but was an artist as well, and we still marvel at the remarkable, dynamic paintings of mammoth and reindeer that he left in the caves of southern France. But for the next 30,000 years he remained a hunter and gatherer of food, like other animals, and did not appreciably alter the pre-existing balance of nature into which he was born. Then some 10,000 years ago a drastic change began to materialize in his way of life. Settled villages were formed, based on the first agriculture and domestication of animals. In another 5000 years, another major step was taken in the emergence of the first civilizations in Egypt and Mesopotamia, leading to cities, nations, and empires, with a division of labor from slave to king, and attendant professions. Just 200 years ago, the industrial revolution ushered in a new era of mechanical power and invention, leading in the last 50 years to an efflorescence in science and technology, consumption and pollution, and, above all, population explosion. By now there is widespread recognition of the fact that the noosphere is interacting in a major and decisive way with the biosphere and that this interaction is bound to reach a crisis level before the end of the century.
Several aspects of this crisis call for consideration against the background of the total history of the earth. Perhaps the first of these which stands out is the extraordinary acceleration which the phenomenon of life has manifested in its history. Some sense of this acceleration is evident in the accompanying table; what stands out is the extraordinary compression in the time scale that has marked each new stage in this history of life. Measured first in billion-year periods, it moves to 100 million, to 10 million, to million-year spans. Thereafter for man it goes from 100,000-year to 10,000-year spans, and then from millennia to centuries, to mere decades. The acceleration of life has become breathtaking if not intolerable. We do indeed live in a time of change more rapid by orders of magnitude than any which the phenomenon of life on this planet has experienced before.
Another feature that stands out is the immense potentiality of matter as organized on the earth to rise to ever more complex modes of organization, culminating in the phosphorescence of thought and spirit which now envelops the earth in her noosphere. What stands out here is the extreme rarity of conditions throughout the universe in which this inherent, almost unlimited potentiality of matter can be realized. One need only think of the moon, Mars, and Mercury to find matter severely limited in possibility. Jupiter and Saturn may consist almost entirely of hydrogen and helium; and although for them such possibilities as solid helium and metallic hydrogen, unknown to us, are actualized, additional possibilities for new developments of matter are almost nonexistent. Matter in stars can occasionally be assembled into the rare and unusual forms of atoms above iron all the way to uranium and californium when the star becomes a supernova. But for the most part it is severely limited in the variety of possibilities open to it, regardless of the length of time available.
It is awesome to contemplate the immense creative investment that has gone into bringing the earth to her present stage of beauty and fulfillment. The slow but ever accelerating elaboration of information coded on DNA over an unimaginably vast reach of time has by now produced, suspended in the alien reaches of space, a magical garden and placed within it that strangest achievement of any of the manifold DNA codes, man. This was possible only because of a most delicate balance of gravity, heat, and light realized on the earth but only very rarely on the other planets. This uniqueness and the wonder of the creative achievement which it has made possible mean that the earth is a rare gem of fantastic beauty and that its desecration or destruction by any being is an act of awful sacrilege against which the heart of all meaning and purpose in the entire universe must cry out in anguish.
If, as I believe, the human species stands on the threshold of the next great step in its evolution, then this view of the earth may well prove to be a decisive element in the possibility of that step. A full appreciation of the beauty of the earth and of the immense creative investment that has gone into producing it, including as an integral component man himselfan appreciation of all this is essential to mans continued occupation of this planet. With such an appreciation, man will know how to love the earth as she deserves, to woo her into ever greater and more wonderful creative achievements, to celebrate the wonder of the achievement already realized, and to have a holy fear of desecrating her. To accomplish this, it is first necessary for man to recover widely and generally his lost sense of transcendent reality. The process of this recovery is now well under way and will be much accelerated during the time of judgment that is now upon us, with its destruction of secular hopes and confidences and its acute raising of the issues of meaning and purpose.
As men regain this essentially theological perspective on nature and their proper place within it, they will be enabled to respond more and more fully to the creative energies now at work in the world and to play their part in bringing into being the new creation that is now in preparation. For those meek enough to be guided in judgment, the prospect of what is coming in the world during the remainder of this century does not lead to despair. Rather their hope is deeply grounded in and sustained by the knowledge that the creative energy which has been able to accomplish such amazing results through four and a half billion years is not exhausted. Rather, in concert with man, that same energy is even now at work preparing the next great step in this long process.
Oak Ridge, Tennessee
1. Walter Sullivan, We Are Not Alone (New York: McGraw-Hill, 1964).
Dr. William G. Pollard is Executive Director, Oak Ridge Associated Universities. After graduating Phi Beta Kappa from the University of Tennessee and earning the Ph.D. from Rice University, he taught physics at the former, 1936-1947. While on leave in 1944-45 he worked on the Manhattan Project. An ordained priest in the Episcopal Church, he has served on the faculty, Graduate School of Theology, University of the South. He holds some dozen honorary doctorates in science, divinity, law, and letters and has written several books. Dr. Pollard was a member of the Board of Visitors to Air University from 1967 to 1970.
The conclusions and opinions expressed in this document are those of the author cultivated in the freedom of expression, academic environment of Air University. They do not reflect the official position of the U.S. Government, Department of Defense, the United States Air Force or the Air University.
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