The Legacy of Apollo

By HOMER E. NEWELL

Because we live on it, the Earth is the center of things for us. Around Earth all other celestial bodies circle endlessly, or so it seemed to our forebears. For countless generations men who thought about such matters regarded the Earth as the center of the universe. So satisfying was this view, so entrenched in doctrine and dogma did it become, that when Copernicus and Kepler challenged the idea, they stirred up a hornet’s nest. The concept of the Sun as the central stillness in the solar system around which Earth and all other planets revolve was considered too unsettling to be tolerated, and edict and persecution sought to suppress these dangerous new ideas. But in vain, for the Copernican revolution in human thought continues to this very day. In countless ways it colors the picture men draw of themselves and of man’s place in the universe.

Apollo’s greatest impact was to impress dramatically upon men’s minds, more clearly than ever before, the significance of the Copernican view. The spectacle of a spacecraft leaving Earth with the incredible speed of almost 6 miles per second- thirteen times faster than a rifle bullet- traveling through space like a miniature planet, bearing men for the first time to another world, focused the attention of hundreds of millions of people. We saw Earth as only one of nine planets in the solar system, insignificant, except to us, among the unreachable stars in the vast expanse of the heavens. In cosmic perspective Earth is but a tiny object in a remote corner of space, companion to a modest star, one of a hundred billion stars making up one of billions of galaxies scattered over unimaginable distances to beyond the farthest reaches to which we have been able to peer with the most powerful telescopes.

But while helping to convey Earth’s insignificance in the cosmic scale, Apollo dramatically displayed Earth’s uniqueness and overwhelming significance on the human scale. Standing in imagination on the rocky rubble of a lunar plain, looking through an astronaut’s eye and camera out over the vast arid wasteland of our inhospitable satellite, we saw above the horizon the beautiful, blue, fragile Earth. It awakened a heightened appreciation of and sense of responsibility toward our home in space. In the entire solar system, 6 billion miles across, only Earth so far as we now know nourishes the vast abundance of life that we so casually accept. Only by understanding thoroughly our planet and our place on it can we hope to learn how to use its resources wisely, to preserve for future generations our island in space in its pristine vigor and beauty. And that is where the true significance of Apollo lunar science comes in.

To understand fully our own planet, it is essential that we study many planets, Photography and Spacecraft making comparisons among them. An inevitable myopia interferes when we try to learn about planets from the study of only one. The Moon is a planet in its own right, by reason of its substantial size and mass, and what we learn of lunar science also advances Earth science.

As our nearest neighbor in space, the Moon has long been an object of wonder and study. Through the telescope the astronomer has seen myriads of craters on its surface clear evidence of lava flows, and even some suggestion of current volcanic activity. The Moon is decidedly out of round. The sharpness of telescopic images, and the suddenness with which stars disappear behind the Moon and later reappear show clearly that the Moon has virtually no atmosphere. No mountain systems like the Rockies or the Himalayas could be seen. Without atmospheric erosion and mountain-building activity, we supposed that the Moon would preserve on its face the record of solar system history to the very earliest days. But viewing the Moon from 239,000 miles away left much room for speculation and disagreement. So when rockets became available, plans were quickly laid for investigating the Moon close at hand, eventually by man himself.

IMPLANTING SENSORS ON THE MOON

Serious space probe studies of the Moon began with the Soviet Zond and Luna, and with the U.S. Ranger, Surveyor, and Lunar Orbiter. Armed with the information obtained by unmanned probes, the United States carried out the Apollo manned missions, implanting nuclear-powered geophysical laboratories at several landing sites, including seismometers, magnetometers, plasma and pressure gauges, heat flow instruments, and laser corner reflectors. These laboratories continue to operate long after the astronauts have returned to Earth. On two missions, the lunar surface laboratories were supplemented by satellites left in orbit to support geodetic studies. Detailed studies of lunar surface composition by X-ray fluorescence and radioactivity measurements were made from the Apollo spacecraft itself. Most importantly. the astronauts brought back from their six landing missions hundreds of pounds of lunar rocks and soil, which, together with the small amount returned by Soviet unmanned sample missions, have since been the subject of analysis and study by scientists around the world.

How old is the Moon? This was the first major question. Although a few thought the Moon would prove to be considerably younger than Earth. most felt it would turn out to be of the same vintage as our own planet. From radioactive dating of the lunar rocks and soil it is now established that the Moon formed about 4.6 billion years ago. The Moon is, indeed, very old, as old as Earth, and is telling us much about the earliest years of our planet and of the solar system. But to our great surprise the lunar rocks show that the Moon’s surface is no longer in its original condition, having undergone considerable change in the first 1.5 billion years. A few of the highland rocks in the Apollo samples are older than 4 billion years - at least one is 4.6 billion years old, the probable time of formation and initial melting of the Moon - but most of them have been shocked and remelted by meteoroid impacts 4 billion years ago. Most of the mare lava flows are from 3 to slightly less than 4 billion years old. indicating that substantial melting and flow occurred on the Moon long after its formation. Gravitational energy released during the aggregation of the Moon, heat generated by radioactive elements in the lunar material, and thermal effects of the huge impacts that sculptured the lunar surface all undoubtedly contributed to the early melting of the outer layers.

What is the physical condition of the lunar surface? The question generated countless heated debates before the lunar landings. It was clear from the visible meteorite cratering that the surface must be thoroughly chopped up. How fine would the material be? Would it be loosely or densely packed? Could it hold a spacecraft landed upon it? How thick would the layer of rubble be? Some spoke of deep deposits of finely powdered dust into which a spacecraft would sink out of sight. Now we know that the Moon’s surface is everywhere covered with many feet of fragmented material, or regolith. A sizable fraction of the soil is very fine dust, but the material is sufficiently cohesive and well packed that it easily supported the Apollo spacecraft, the astronauts, and their lunar rover.

Of what is the Moon made? This was a question about which one could only speculate from afar. It could be essentially meteoritic material. It might be like the Earth’s crust. Or it might be something entirely different. Any proposition put forth, however, had to satisfy the constraint that the Moon’s density is only 3.36 , considerably lighter than Earth’s average density of 5.5. One of the major questions was: Is there any water there? Water would be the most important constituent that one might hope to find on the Moon. It would be essential for the support of any microbial life there. It would be an invaluable resource for lunar bases that might be established in the remote future. No one expected to find water exposed on the surface - it would quickly evaporate in the airless, oven-hot environment of the lunar day - but many supposed that there might be substantial subsurface water in the form of ice as a permafrost. Even if free water were not found, it was fully expected that there would be water of crystallization in the lunar minerals.

Learning at firsthand about the stuff of which the Moon is made from the actual lunar material carried back by the Apollo astronauts has been one of the most exciting scientific undertakings of our day. More than 700 scientists, including several hundred from twenty other countries, have spent uncounted hours analyzing samples, conducting laboratory tests, and theorizing over the significance of what was being revealed for the first time. Annually at the Johnson Space Center they have assembled to share their findings and to try to explain them. No water was found. Lunar material is very dry. with practically no water of crystallization. The hydrous minerals so common on Earth arc exceedingly rare in the lunar rocks and soil. Moreover, lunar material is depleted in most volatile elements. suggesting a very hot processing at some time in their history. The rocks in the lunar maria arc similar to, though significantly different from, lavas on Earth. Highland samples show a considerable separation of lunar material into different minerals, showing a differentiation like that responsible for the wide variety of rocks and minerals on Earth. Moon material is neither exactly like the meteorites nor exactly like the Earth’s crust, but all three could have had a common source with a different history.

HOT DEBATES ABOUT VOLCANOES

What have been the relative roles of impact cratering and volcanism? For the crater-ridden Moon the obvious first question is how were the craters formed: by meteorites hitting the surface at high velocity, or as the remains of once active volcanoes? Debates on this subject generated about as much heat as the volcanoes themselves. Many argued for, or gave the appearance of arguing for, one extreme or the other. More sober debaters recognized that both cratering and volcanism played a role and sought to discern their relative importance. It is clear that the Moon has changed considerably since it was first formed, but most of that change occurred more than three billion years ago. Apparently the first 1.5 billion years were a period of violent evolution, involving the Mare Imbrium event and other cataclysmic impacts, followed by the vast flooding of the mare plains with a series of lava flows. In contrast, the last three billion years have been relatively quiet, with occasional impacts like those that generated the craters Copernicus and Tycho, but no great lava events.

The implication of this for Earth is that during its first billion years Earth also must have been subjected to severe bombardment, generating huge craters like those still seen on the Moon, and this may also have been true for other planets as well. Certainly the Mariner 9 pictures of Mars and Mariner 10 pictures of Mercury show that both planets experienced substantial cratering, while terrestrial radars indicate the same for Venus. The evidence mounts that violent meteorite bombardment was widespread in the solar system in times past. On Earth, however, erosion, crumpling of the crust, and subsequent volcanism have erased most of the evidence of this early catastrophic period.

Is the Moon still active today? Infrequent observations of sudden localized glows or hazes on the Moon have caused a stir when they occurred, and gave rise to speculation as to whether these were due to current volcanic activity, or were merely trapped gases shaken loose by moonquakes or meteorite impacts. There was speculation about whether our satellite was a dead planet or still a live one. Five Apollo seismometers were set up at different landing sites, and four of them are still working. At times of lunar perigee, these detect moonquakes of very deep foci centered at 500 to 620 miles below the lunar crust. The energy released over a year by these moonquakes, however, is a billion times less than that released by earthquakes over a similar period. No evidence of current volcanic activity on the Moon has been found from either the unmanned or manned space missions In most places the soil just below the surface appears to have been relatively undisturbed for millions of years, which is consistent with the rarity of large-scale meteorite impacts that Earth experiences today. At the very surface a slow erosion takes place by micrometeorite impacts and solar-wind particles, wearing away on the average a few molecular layers a year on exposed surfaces. Material does move around. Some material falls and slides down slopes, some may be moved around by electrostatic forces, and much of what movement occurs is due to splashes from meteorite impacts. But all in all, the Moon appears to be extremely quiet now, in comparison with its earliest history or with Earth today.

Is the interior of the Moon hot or cold? Discussion of this subject once made the sparks fly. Assuming the Moon contains radioactive elements, as does Earth, then the heat generated by their decay should warm up the interior. But would this source provide enough heat to melt the Moon? Whatever the cause, the lava flows apparent on the surface of the Moon show that at least part of the Moon’s interior actually was molten at one time. But on the other side of the picture, the distinctly out-of-round shape implies a rigidity that a very hot and plastic Moon would not have. The Apollo measurements have added fuel to this flaming controversy. Anomalous concentrations of mass called mascons have been discovered in the great circular mare basins, detected by the way in which they distort the Moon’s gravitational field. In a hot, plastic Moon, these mascons would have sunk until the gravity field was restored to equilibrium. The fact that they persist today indicates a rigid, cool Moon. Yet melting and lava flooding in the upper layers of the Moon are widespread. Electrical conductivities inside the Moon, deduced from the Moon’s reaction to the electrical charges and magnetic fields in the solar wind, indicate that the outer layers, down to 500 to 620 miles in depth, are now well below the melting point. Below those levels, however, is a region where seismic shear waves are markedly attenuated, indicating partial melting of the lunar material there. Moreover while magnetic measurements do not reveal any dipole magnetic field at present like that of the Earth, remnant magnetization in rock samples, and substantial local magnetic fields. suggest that the Moon may well have had a dipole field in the past. Since it is believed that such a planetary magnetic dipole field is generated by circulation in a liquid core, this implies that at one time the core of the Moon was molten. This would further imply that the Moon was at one time quite hot throughout, which gets us right back to the difficulty of explaining how the mascons and the aspherical shape can continue to exist. There is a real puzzle here that needs sorting out.

Does the Moon have any atmosphere at all? Before Apollo it was already clear that the Moon could have very little atmosphere. It was expected that heavier gases like argon and krypton would be found clinging close to the surface, but in the lower gravitational field lighter gases would long since have escaped. In large measure this has been confirmed. Argon generated by radioactivity in the lunar crust, and hydrogen, helium, and neon from the solar wind, account for most of what little atmosphere there is, and that is over a billion times less than the Earth’s atmosphere.

ANCIENT TRACKS OF RADIATION

What are the principal effects of the Sun upon the Moon, and how do they differ from the effects of the Sun on Earth? Protected by Earth’s atmosphere, we are not directly exposed to the lethal part of the Sun’s radiations. But the airless Moon is starkly exposed. By a neat twist, this situation gives us a way of checking up on the Sun’s activity over the past few million years. Cosmic rays from the Sun and galaxy leave tracks in the soil grains and rock surfaces on which they impact; these tracks can be enhanced by etching, and counted. The track intensities indicate the intensity of the radiation to which the materials were exposed. By analyzing the rate at which lunar material is brought out onto the surface and then later buried again, it is possible to estimate the times when different layers in the lunar regolith were exposed, and for how long. Thus sample cores of the lunar regolith taken by the Apollo astronauts enable us to look back in time at the radiation environment experienced by the Moon. No significant changes in galactic cosmic radiation are seen, but there is a suggestion that there might have been variation in solar cosmic rays over the last one to ten million years. This would imply changes in solar activity, which in turn may have had effects on Earth.

Is there life on the Moon? Some of the bitterest exchanges took place over this question If there were life, no matter how primitive, we would want to study it carefully and compare it with Earth life. This would require very difficult and expensive sterilization of all materials and equipment landed on the Moon, so as not to contaminate Moon life with Earth life. Also there was the question of contamination of Earth by Moon life, possibly a serious hazard to us on Earth. So difficult, time-consuming, and expensive quarantine procedures were urged. On the other side of this argument were those who pointed to the hostile conditions on the Moon: virtually no atmosphere, probably no water, temperatures ranging from 150° C during lunar night to more than 120° C at lunar noon, merciless exposure to lethal doses of solar ultraviolet, X-rays, and charged particles. No life, they argued, could possibly exist under these conditions. The biologists countered: water and moderate temperature conditions below ground might sustain primitive life forms. And so the argument went on and on, until finally Apollo flew to the Moon with careful precautions against back contamination of Earth, but with limited effort to protect the Moon.

It turns out that there is no evidence that indigenous life exists now or has ever existed on the Moon. A careful search for carbon was made, since Earth life is carbon based. In the lunar samples one hundred to two hundred parts per million of carbon were found; of this no more than a few tens of parts per million are indigenous to the lunar material, the rest being brought in by the solar wind. None of the carbon appears to derive from life processes. As a consequence, after the first few Apollo flights, even the back-contamination quarantine procedures were dropped.

THE NAGGING QUESTION OF ORIGIN

Where did the Moon come from? This is the big puzzler of them all. There were three major theories. The Moon came from Earth, possibly wrenched from what is now the Pacific Ocean. Or the Moon formed in the vicinity of Earth at the same time that Earth was forming. Or the Moon formed somewhere else in the solar system, and was later captured by Earth. Most students of the subject reject the first possibility, because it proves very difficult to explain all the steps that must have occurred to bring this about. That leaves various versions of the last two as principal contenders. Apollo has done little to favor one over the other. Indeed, some feel that we will never be able to say for sure. But the question is a nagging one, and scientists will continue to argue over it.

The deep significance of the Apollo investigations lies in the fact that these measurements and observations give us a detailed insight into a planetary body other than Earth, thereby helping us to understand better our own planet. Before space probes, lunar and planetary science had for a long time been inactive, due in part to lack of new data to spark serious thought. With the vast quantities of lunar data returned by Apollo and other lunar missions, together with rich new space- probe data from Mars, Venus, Mercury. and Jupiter, and new discoveries on Earth such as the slow spreading of ocean floors and the drifting of continents, a new field of comparative study of planets has virtually exploded into world science. In this comparative investigation of the planets, the Moon is an important link.

Already it is clear that bodies of planetary size will undergo considerable evolution after their formation. Most of this evolution takes place early, and it is probably less in the case of a Moon-sized planet, leaving the planet relatively quiet for most of its history. A planet the size of Mars, though substantially smaller than Earth, remains active for much longer than a Moon-sized body, as the Mariner 9 pictures clearly show. With its huge volcanoes, its giant canyon several times deeper than the Grand Canyon and long enough to span the United States, its variable polar caps, its suggestion of colliding crustal plates, Mars is clearly still an active planet. Venus, the size of the Earth, with its very hot surface and extremely dense and dynamic atmosphere, may well prove to be more active volcanically and tectonically than Earth. Mercury, intermediate between Mars and the Moon in size, is heavily cratered and seems to be much like the lunar highlands.

It will be fascinating when we can complete this perspective by studying the giant outer planets on the one hand, and the very small bodies like the comets and asteroids on the other. As Pioneer 10 and 11 have shown, Jupiter is extremely dynamic. We may expect the same to prove true of the other giant planets when we get a chance to see them close at hand. Moreover, the large planets, consisting, as they do, mainly of the lighter elements like hydrogen and helium, should provide a revealing insight into conditions in the solar nebula at the time the planets of the solar system were condensing out of the nebula. The same should be true of the comets and asteroids. The asteroids in particular are probably too small to have undergone any gravitational or radioactive melting, and therefore should in their interiors be as they were at the time of their formation, unless they, too, turn out to be fragments of what were once larger bodies.

LIFE IN AN OCEAN OF GAS

Our atmosphere is the breath of life to everything we do. It is an ocean of gas in which we live as fish live in an ocean of water. It is an integral part of our planet, and we cannot understand Earth’s history without also understanding its atmosphere. In seeking to do this, the wide variation from the airless Moon to the exotic and turbulent atmosphere of Jupiter provides an invaluable context and perspective. As it happens, the solar system has several planetary atmospheres between these extremes.

Earth’s atmosphere lies intermediate in surface pressure between that of Venus and Mars, Venus’ atmosphere being about 100 times that of Earth, and Mars’ about one one-hundredth. The composition of the atmospheres of Venus and Mars is predominantly carbon dioxide, while Earth’s today is predominantly nitrogen and oxygen. On the other hand, the amount of carbon dioxide tied up in the carbonate of the Earth’s crust is comparable to what we see free in the atmosphere of Venus today. Presumably on Earth carbon dioxide exhaled from volcanoes, in the presence of the abundant water, was soon taken up into the crustal rocks. Life on Earth doubtless accounts for the presence of so much oxygen in the atmosphere, most of which was probably generated by photosynthesis. From the Mariner observations of Mercury, it appears that Mercury is closer to the Moon than to Mars as far as atmosphere is concerned.

Much patient work remains to bring out the full significance of Apollo observations. Nevertheless it is already clear that Apollo has made a great contribution to the development of the new field of comparative planetary studies, a field that over the years will provide us priceless insights into our own planet. In days of deep concern over the Earth’s environment and finite resources, comparative planetology is a new science of incalculable importance, and Apollo led the way.

PHOTOGRAPHY AND SPACECRAFT…

…Brought the Moon Closer

Space has brought dramatic new views of Earth…

Shown us mysterious super-canyons on Mars…

Given exciting views of giant, turbulent Jupiter…

Revealed atmospheric circulations on overheated Venus…

Unveiled the scarred, pocked face of Mercury…

And will soon visit Saturn’s strange rings.