Since Bevan M. French wrote the publication "What's New on the Moon?", there has been very little activity on the surface of the Moon. For your information, the Planetary Materials Laboratory at NASA/Johnson Center has up­ dated page 3 of this copy of the publication by adding Russia's Luna-24 landing site to the map of the Moon (Figure 2).

In 1976 Luna-24, a Soviet automated spacecraft, collected and returned 170 grams of lunar . Thus, the total sample return by the three Luna spacecrafts weighed 300 grams.

In order to answer some of the questions posed in the "What Else Can the Moon Tell Us?" section (beginning on page 20), a remote-sensing lunar orbiter called the Lunar Geoscience Observer (LGO) is in the planning stage. The following is an excerpt from the Executive Summary of the 1985 report entitled "Contribu­ tions of a Lunar Geoscience Observer Mission to Fundamental Questions in Lunar Science."

"Because of the well-studied lunar samples, it is clear that our current understanding of the Moon is at a more advanced state than that for any other body in the , save . Continued study of the Moon at this is compelling because we know the right questions to ask and LGO is extremely well matched to providing some of the important answers ....

" ... As the world expands its involvement with science, the Moon will undoubtedly continue to be a centerpiece. Because of our initial investment and the long-term significance of the Moon, both scientifically and politically, it is important that we maintain a vigorous program of lunar science and explora­ tion."

The LGO's scientific instrument package would give us information about the , of the crust and , magmatic history, impact processes, thermal history, paleomagnetism, and the of the Moon. These remote­ sensing instruments will provide information about and the best site for a permanent (staffed) lunar base.

Nl\5/\ What's National Aeronautics and New Space Administration on the Moon?

by Dr. Bevan M. French, Program Chief NASA Extraterrestrial Materials Research Program

Open for Inspection. 10 BEVAN M. FRENCH has studied took this photo soon after lunar samples and terrestrial leaving their . craters for more than 10 years. The line at left runs at about 5 degrees geologist manages NASA's programs west longitu de . of Tr anquility is for research on lunar samples, large dark area near center of photo. . and other extraterrestrial material while continuing work on ancient meteorite craters. In 1973, Dr. French helped discover a Brazilian 25 miles in diameter and 150 million years old.

In 1969 over half a billion people to probe the interior of the Moon. knowledge returned by the Apollo witnessed the "impossible" coming They collected hundreds of pounds astronauts. true as the first men walked on the of lunar and soil, thus beginning The has left us with surface of the Moon. For the next the first attempt to deci pher the origin a large and priceless legacy of lunar three years, people of many nationali­ and geological history of another materials and data. We now have ties watched as one of the great world from actual samples of its Moon ro cks collected from eight dif­ ex plorations of human history was crust. (Figure 1). ferent places on the Moon (Figure 2). displayed on their television screens . The init ial excitement of new suc­ The six Apollo landings returned a col­ Between 1969 and 1972, sup­ cess and discovery has passed. The lection weighing 382 kilograms (843 ported by thousands of scientists and TV sets no longer show astronauts pounds) and consisting of more than engineers back on Earth, 12 astro­ moving across the sunlit lunar ­ 2,000 separate sam ples. Two auto­ nauts ex plored the surface of the scape. But here on Earth, scientists mated Soviet spacecraft named Moon. Protected against the airless­ are only now beginning to under­ ness and the killing he at of the lunar stand the immense treasure of new environment, they stayed on the Moon for days and some of them travelled for miles across its surface in Lunar Rovers. They made scientific observations and set up instruments

Figure 1. The New Explorer. Framed by the arqund the Moon's Littrow Valley, geologist­ Harrison H. Schmitt sweeps with a special rake to collect lunar samples. As it passes through the , the rake sieves and collects rock chips 1 to 2 em (1l2 to one inch) in size. The chips were returned to Earth for analysis.

Figure 2. The Sampling of the Moon. Arrows indicate the locations from which lunar samples have been returned to the Earth for scientific study. The "A" symbols mark the landing sites of the through Apollo 17 missions. (The mission did not land. Enroute to the Moon, an tank exploded; the crew returned safely to Earth.) The "L" symbols indicate the sites near the eastern edge of the Moon from which Russia's automated landers, Luna-16 and Luna-20, returned small samples of lunar soil. WHAT HAS THE APOLLO PROGRAM TOLD US ABOUT THE MOON?

4 Luna-16 and Luna-20 returned small What have we gained from all this bon compounds out of which life is but important samples totalling about exploration? Before the landing of built, and most of this carbon is not 130 grams (five ounces). Apollo 11 on July 20, 1969, the native to the Moon but is brought to Instruments placed on the Moon by and were the lunar surface in meteorites and the Apollo astronauts as long ago as still mysteries. Now, as a resu lt of the as atoms blasted out of the Sun. 1969 are still detecting moonquakes Apollo Program, we can answer and meteorite impacts, measuring questions that remained unsolved (2) What is the Moon made of? the Moon's motions, and recording during centuries of speculation and Before the first Moon rocks were the heat flowing out from the ins ide of scientific study: collected, we could analyze only two the Moon. The Apollo Program also types of bodies in our solar system: carried out a major effort of photo­ (1) Is there life on the Moon? our own Earth and the meteo­ graphing and analyzing the surface Despite careful searching, neither liv­ rites that occasionally fall to Earth of the Moon. Cameras on the Apollo ing organisms nor fossil life have from outer space. Now we have spacecraft obtained so many accu­ been found in any lunar samples. learned that the Moon is chemically rate photographs that we now have The lunar rocks were so barren of life different from both of these, but it is better maps of parts of the Moon that the quarantine period for re­ most like the Earth. than we do for some areas on Earth. turned astronauts was dropped after The Moon is made of rocks. The Special detectors near the cameras the third Apollo landing. Moon rocks are so much like Earth measured the weak X-rays and radio­ The Moon has no water of any rocks in their appearance that we activity given off by the lunar surface. kind, either free or chemically com­ can use the same terms to describe

From these measurements, we have bined in the rocks . Wa ter is a sub­ both. The rocks are all igneo us , been able to determine the chemical stance that is necessary for life, and which means that they formed by the composition of about one-quarter of it is therefore unlikely that life could cooling of molten . (No sedimen­ the Moon's surface, an area the size ever have originated on the Moon. tary rocks, like limestone or shale, of the and Mexico Furthermore, lunar rocks contain only which are deposited in water, have combined. By comparing the flight tiny amounts of the carbon and car­ ever been found on the Moon.). data with analyses of returned Moon The dark regions (called "maria") rocks , we can draw conclusions that form the features of "The Man in . about the chemical composition and nature of the entire Moon. Thus, in less than a decade, sci­ ence and the Apollo Program have changed our Moon from an unknown and unreachable object into a familiar world. the Moon" are low, level areas cov­ ered with layers of lava (Figure 3), a rock similar to the that erupt from terrestrial volcanoes in Hawaii, Iceland, and elsewhere. The light-colored parts of the Moon (called "highlands") are higher, more rugged regions that are older than the maria. These areas are made up of several different kinds of rocks that cooled slowly deep within the Moon (Figure 4). Again using terrestrial terms, we call these rocks , , and . Despite these similarities, Moon

Figure 3. Frozen Lava from the Lunar . This chunk of lunar lava returned by the astronauts still preserves bubbles produced as the lava poured out onto the surface of the Moon more than 3 billion years ago. The lava solidified so quickly that the bubbles, formed by escaping gas, were trapped in the solid rock. Because the Moon rocks contain no water, some other gas must have formed the bubbles. Possible gases are carbon dioxide, sulfur dioxide, or hydro­ chloric acid. The specimen is about 14 centimeters (6 inches) across.

Figure 4. The ancient crust of the Moon. A small rock fragment, collected by the Apollo 17 astronauts, holds the key to the earliest history of the Moon. The rock, called a . is composed mainly of two minerals, yellow-brown and gray . The age of this specimen is measured at 4.6 billion years. It may have been some of the first material to solidify when the Moon ws formed. rocks and Earth rocks are basically rust. A 31/2-billion-year-old this reason. the returned Apollo sam­ different, and it is easy to tell them looks fresher than water-bearing lava ples are carefully stored in an atmos­ apart by analyzing their chemistry or just erupted from a terrestrial vol­ phere of dry nitrogen, and no more of by examining them under a micro­ cano. the lunar material than necessary is scope (Figure 5). The most obvious Another important difference is that exposed to the laboratory atmos­ difference is that Moon rocks have no the Moon rocks formed where there phere while the samples are being water at all, while almost all terrestrial was almost no free oxygen. As a analyzed. rocks contain at least a percent or result, some of the in lunar rocks The Moon rocks are made of the two of water . The Moon rocks are was not oxidized when the lunar same chemical elements that make therefore very well-preserved, be­ lavas formed and still occurs as small up Earth rocks. although the propor­ cause they never were able to react crystals of metallic iron (Figure 6.) tions are different . Moon rocks con­ with water to form clay minerals or Because Moon rocks have never tain more of the common elements been exposed to water or oxygen, , aluminum, and than any contact with the Earth's atmos­ phere could "rust" them badly. For

Figure 5. The Fabric of the Moon. Under an optical microscope, the crystal structures of an lunar basalt are revealed by shining polarized light through a polished slice of rock only a few hundredths of a millimeter thick. The rock contains crystals of three minerals: yellow-brown , clear white feldspar, and black . The intergrown character of the crystals is typical of molten lava that has solidified quickly. The largest crystals are about a millimeter long.

Figure 6. Metal from the Moon. Faceted like gems, tiny crystals of iron metal nestling in a cavity in a lunar rock are illuminated in this picture taken with a scanning electron micro­ scope. The crystals, only a few thousandths of a millimeter in size, were probably deposited more than 3 billion years ago by a hot vapor that contained no free oxygen and no water. These fragile crystals have been preserved un­ changed on the dry, airless Moon for billions of years; on Earth, they would be quickly destroyed by exposure to air and water if they were not carefully stored in a dry nitrogen . 8 do most Earth rocks . Rarer elements dark-colored maria contain less of tain kinds of moonquakes occur at li ke hafnium and zirconium, which those elements and more titanium, about the same time every , have high melting points, are also iron, and . suggesting that they are triggered by more plentiful in lunar rocks. How­ repeated tidal strains as the Moon ever, other elements li ke sodium and (3) What is the inside of the Moon moves in its orbit around the Earth . potassium, which have low melting like? A picture of the inside of the Moon points, are scarce in lunar material. Sensitive instruments placed on the has slowly been put together from Because Moon rocks are ric her in lunar surface by the Apollo astro­ the records of thousands of moon­ high-temperature elements and con­ nauts are still recording the tiny vibra­ quakes, meteorite impacts, and the tain less low-temperature elements, tions caused by meteorite impacts on deliberate impacts of discarded scientists believe that the material the surface of the Moon and by small Apollo rocket stages onto the Moon. lhat formed the Moon was once "moonquakes" deep within it. These The Moon is not uniform inside, but is heated to much higher temperatures vibrations provide the data from divided into a series of layers just as than material that formed the Earth. which scientists determine what the the Earth is, although the layers of The chemical composition of the inside of the Moon is like. the Earth and Moon are different. The Moon also is different in different About 3,000 moonquakes are de­ outermost part of the Moon is a crust places. Soon after the Moon formed, tected each year. All of them are very about 60 kilometers (37 miles) thick, various elements sorted themselves weak by terrestrial standards. The probably composed of calcium- and out to form different kinds of rock. average moonquake releases about aluminum-rich rocks like those found The light-colored highlands are rich as much energy as a firecracker, and in the highlands. Beneath this crust is in calcium and aluminum, while the the whole Moon releases less than one-ten-billionth of the earthquake energy of the Earth. The moonquakes occur about 600 to 800 kilometers (370 to 500 miles) deep inside the Moon, much deeper than almost all the qua kes on our own planet. Cer­ a thick layer of denser rock (the result of the Apollo Program was the on the Moon by the Apollo astronauts mantle) which extends down to more discovery of preserved magnetism in have operated much longer than ex­ than 800 kilometers (500 miles). many of the old lunar rocks. One pected. Some of the instruments, (Figure 7). explanation is that the Moon had an originally designed for a one-year The deep interior of the Moon is ancient magnetic field that somehow lifetime, have been operating since still unknown. The Moon may contain disappeared after the old lunar rocks 1969 and 1 970. This long operation a small iron core at its center, and had formed. has provided inforfTlation that we there is some evidence that the Moon One reason we have been able to could not have obtained from shorter may be hot and even partly molten learn so much about the Moon's records. inside. interior is that the instruments placed The long lifetime of the heat flow The Moon does not now have a magnetic field like the Earth's, and so the most baffling and unexpected

Mare (Basalt Lava}

Figure 7. A Slice Through the Moon. The interior of the Moon can now be divided into layers as scientists interpret data stillbeing returned from instrumentson the lunar surface. The Moon has an outer crust, an inner mantle. and an innermost zone which may stillbe partly molten. Within this innermost zone, about 2200kilometers {1400 miles)in diameter. there may exist asmaller metalliccore (dashed circle). Most moonquakes (black squares) occur deep within the Moon. The lunar surface and crustare not uniform. The Earth-facing side is relatively smooth and shows large accumulations of lava (black). The far side is much more rugged, has a thicker crust and almost no lava. (The diagram -EARTH is not to scale and the ruggednessof the Moon ·s far side has beenexaggerated. Adapted from data provided by A. M. Dainty, N. R. Goins. and M. N. Toksoz.) experiments set up by the Apollo 15 vibrations caused by the impacts of changing data, American and Rus­ and 17 missions has made it possi­ small meteorites onto the lunar sur­ sian scientists have not only provided ble to determine more accurately the face. We now have long-term records a small example of international co­ amount of heat coming out of the of how often meteorites strike the operation in space , but they have Moon. This heat flow is a basic Moon, and we have learned that jointly obtained a better picture of the indicator of tbe temperature and these impacts do not always occur at magnetic properties of the Moon and composition of the inside of the random . Some small meteorites seem the space around it. Moon. The new value, about two­ to travel in groups . Several such thirds of the value calculated from swarms, composed of meteorites (4) What is the Moon's surface like? earlier data, is equal to about one­ weighing a few pounds each, struck Long before the Apollo Program sci­ third the amount of heat now coming the Moon in 1975. The detection of entists co uld see that the Moon's out of the inside of the Earth . As a such events is giving scientists new surface was complex. Earth-based result, we can now produce better ideas about the distribution of meteo­ telescopes could distinguish the level models of what the inside of the rites and cosmic dust in the solar maria and the rugged highlands. We Moon is like. system. could recognize countless circular As they probed the lunar interior, The long lifetime of the Apollo craters, rugged ranges, the Apollo instruments have provided instruments has also made several and deep winding canyons or . information about the space environ­ cooperative projects possible. For ex­ Because of the Apollo explorations, ment near the Moon. For example , ample, our instruments were still we have now learned that all these the sensitive devices used to detect making magnetic measurements at lunar landscapes are covered by a moonquakes have also recorded the several Apollo landing sites when, layer of fine broken-up powder and elsewhere on the moon, the Russians rubble about 1 to 20 meters (3 to 60 landed similar instruments attached feet) deep. This layer is usually called to their two automated lunar roving vehicles (Lunokhods). By making si­ multaneous measurements and ex-

Figure 8. The Smallest of Craters. The bombardment of the lunar surface by cosmic particles of all sizes is graphically illustrated by Figures 8 and 9. Figure 8 (left) shows a cratered spherule of lunar glass less than a millimeter (1125 inch) in diameter. The spherule, collected from the lunar soil, was once struck and shattered by an even smaller particle of cosmic dust that blasted a crater in the glass surface. 12 the "lunar soil," although it contains ters, or miles, in diameter (Figures 8 huge, whirling, disk-shaped cloud of no water or organic material, and it is and 9). Each of these impacts shat­ gas and dust. Gradually the cloud totally different from formed on ters the solid rock, scatters material collapsed inward. The central part Earth by the action of wind, water, around the crater, and stirs and be came massive and hot, forming and life. mixes the soil. As a result, the lunar the Sun. Around the Sun, the dust The lunar soil is something entirely soil is a well-mixed sample of a large formed small objects that rapidly col­ new to scientists, for it could only area of the Moon, and single sam­ lected together to form the large have been formed on the surface of ples of lunar soil have yielded rock and satellites that we see an airless body li ke the Moon . The fragments whose source was today. soil has been built up over billions of hundreds of kilometers from the col­ By carefully measuring the radioac­ years by the continuous bombard­ lection site. tive elements found in rocks, scien­ ment of the unprotected Moon by However, the lunar soil is more tists can determine how old the rocks large and small meteorites, most of than ground-up and reworked lunar are. Measurements on meteorites in­ which would have burned up if they rock. It is the boundary layer be­ dicate that the formation of the solar had entered the Earth's atmosphere. tween the Moon and outer space, system occurred 4.6 billion years These meteorites form craters and it absorbs the matter and energy ago. There is chemical evidence in when they hit the Moon. Tiny parti­ that strike the Moon from the Sun and both lunar and terrestrial rocks that cles of cosmic dust produce micro­ the rest of the universe. Tiny bits of scopic craters perhaps 1/ 1000 of a cosmic dust and high-energy atomic millimeter (1/25,000 inch) across, particles that would be stopped high while the rare impact of a large body in the Earth's protective atmosphere may blast out a crater many kilome­ rain continually onto the surface of the Moon.

(5) How old is the Moon? Scientists now think that the solar system first came into being as a

Figure 9. The Largest of Craters. This is the mammoth ringed bull's-eye of as first seen by the U.S. in early 1968. The structure, about 750 kilometers in diameter, records the catastrophic effect of an asteroid some 25 kilometers (15 miles) in diameter that impacted about 4 billion years ago.

14 the Earth and Moon also formed at Some Moon rocks preserve traces of have yielded an apparent age of 4.6 that time. However, the oldest known even older lunar events. Studies of billion years, the time at which scien­ rocks on Earth are only 3.8 billion these rocks indicate that widespread tists think that the Moon and the solar years old, and scientists think that the melting and chemical se paration system formed. Early in 1976, scien­ older rocks have been destroyed by were going on within the Moon about tists identified another Apollo 17 crys­ the Earth's continuing , 4.4 billion years ago, or not long after talline rock with the same ancient mountain-building, and . the Moon had formed. age. These pieces may be some of The Moon rocks fill in some of this One of the techniques used to the first material that solidified from gap in time between the Earth's old­ establish this early part of lunar his­ the once-molten Moon. est preserved rocks and the forma­ tory is a new age-dating method tion of the solar system . The lavas (involving the elements neodymium (6) What is the history of the Moon? from the dark maria are the Moon's and samarium) that was not even The first few hundred million years of youngest rocks, but they are as old possible when the first Apollo sam­ the Moon's lifetime were so violent as the oldest rocks found on Earth, ples were returned in 1969. The that few traces of this time remain. with ages of 3.1 to 3.8 billion years. combination of new instruments and Almost immediately after the Moon Rocks from the lunar highlands are careful protection of the lunar sam­ formed, its outer part was completely even older. Most highland sam ples ples from contamination thus make it melted to a depth of several hundred have ages of 4.0 to 4.3 billion years. possible to understand better the kilometers . While this molten layer early history of the Moon. gradually cooled and so lidified into Even more exciting is the discovery different kinds of rocks, the Moon that a few lunar rocks seem to record the actual formation of the Moon. Some tiny green rock fragments col­ lected by the Apollo 17 astronauts was bombarded by huge asteroids great floods of lava rose from inside Moon has preserved features formed and smaller bodies. Some of these the Moon and poured out over its sur­ almost 4 billion years ago, and if men asteroids were the size of small face, fi lling in the large impact basins had landed on the Moon a billion states, li ke Rhode Island or Delaware, to form the dark parts of the Moon that years ago, it would have looked very and their collisions with the Moon we see today. much as it does now. The surface of created huge basins hundreds of As far as we now know , the Moon the Moon now changes so slowly that kilometers across. has been quiet since the last lavas the footprints left by the Apollo astro­ This catastrophic bombardment erupted more than 3 billion years nauts will remain clear and sharp for died away about 4 billion years ago, ago. Since then, the Moon's surface millions of years. leaving the lunar highlands covered has been altered only by rare large This preserved ancient history of with huge overlapping craters and a meteorite impacts and by atomic par­ the Moon is in sharp contrast to the deep layer of shattered and broken ticles from the Sun and the stars. The changing Earth. The Earth still be­ rock (Figure 10). As the bombard­ ment subsided, heat produced by the decay of radioactive elements began to melt the inside of the Moon at depths of about 200 kilometers (125 miles) below its surface. Then, for the next half billion years, from about 3.8 to 3.1 billion years ago,

Figure 10. The Rubble of Ages. This fragile white. feldspar-rich lunar rock. collected by the astronauts, is a sample of the material that covers the light-colored highland regions of the Moon. This unusual specimen, called a , is composed of numerous pieces of many kinds of rock with different sizes, shapes. colors, and compositions. The mixture of broken and crushed rocks in this specimen records the continuous bombardment of the Moon by large cosmic bodies more than 4 billion years ago, an event that left the Moon's surface covered with a thick layer of shattered rubble. The specimen is about 10 centimeters (4 inches) across. WHAT HAS THE MOON TOLD US ABOUT THE EARTH?

16 haves like a young planet. Its internal and that it had formed somewhere It might seem that the active, inhab­ heat is active, and volcanic eruptions else and been captured by the Earth. ited Earth has nothing in common and mountain-building have gone on Scientists still cannot decide be­ with the quiet, lifeless Moon. Never­ continuously as far back as we can tween these three theories. However, theless, the scientific discoveries of decipher the rocks. According to new we have learned that the Moon the Apollo Program have provided a geological theories, even the present formed as a part of our solar system new and unexpected look into the ocean basins are less than about 200 and that it has existed as an individ­ early history of our own planet . Scien­ million years old, having formed by ual body for 4.6 billion years. Separa­ tists think that all the planets formed the slow separation of huge moving tion from the Earth is now considered in the same way, by the rapid accu­ plates that ma ke up the Earth's crust. less li kely because there are many mulation of small bodies into larger basic differences in chemical compo­ ones about 4.6 billion years ago. The (7) Where did the Moon come from? sition between the two bodies, such Moon's rocks contain the traces of Before we explored the Moon, there as the absence of water on the this process of planetary creation . were three main suggestions to ex­ Moon. But the other two theories are The same catastrophic impacts and plain its existence: that it had formed still evenly matched in their strengths widespread melting that we recog­ near the Earth as a separate body; and weaknesses. We will need more nize on the Moon must also have that it had separated from the Earth; data and perhaps some new theories dominated the Earth during its early before the origin of the Moon is settled.

Figure 11. An Ancient Impact Scar on the Earth. Viewed from an altitude of 500 miles, the circular Manicouagan structure in Quebec, Canada, stands out clearly in this picture taken from NASA's Land­ sat-1 satellite. Geological studies have shown that this structure, formed about 200 million years ago b} the impact of a large asteroid, was once a crater about 80 kilometers (50 miles) in diameter, about thE size of many large . Although deeply eroded since it formed, the central region (sur­ rounded by the black-appearing waters of a reser­ voir) still preserves rocks that were shattered and melted by the catastrophic force of the impact.


18 years, and about 4 billion years ago record of later impacts remains, how­ One of the most exciting results of the Earth may have looked much the ever, in nearly 100 ancient impact the Apollo Program is that, by going same as the Moon does now. structures that have been recognized to the Moon, we have also been able The two worlds then took different on the Earth in recent years. Some of to collect samples of the Sun. paths. The Moon be came quiet while these structures are the deeply The surface of the Moon is contin­ the Earth continued to generate eroded remnants of craters as large ually exposed to the solar wind, a mountains, volcanoes, oceans, an at­ as those of the Moon (Figure 11) and stream of atoms boiled into space mosphere, and life. The Moon pre­ they give us a way to study on Earth from the Sun's atmosphere. Since the served its ancient rocks, while the the process that once dominated Moon formed, the lunar soil has trap­ Earth's older rocks were continually both the Eart h and Moon. ped billions of tons of these atoms destroyed and recreated as younger Lunar science is also making other eje cted from the Sun. The soil also ones. contributions to the study of the contains traces of co smic rays pro­ The Earth's oldest preserved ro cks, Earth. The new techniques devel­ duced outside our own solar system. 3.3 to 3.8 billion years old, occur as oped to analyze lunar samples are These high-energy atoms, pro bably small remnants in Greenland , Minne­ now being applied to terrestrial rocks. produced inside distant stars, leave sota, and . These rocks are not Chemical analyses can now be made permanent tracks when they stri ke li ke the lunar lava fl ows of the same on samples weighing only 0.001 particles in the lunar soil (Figure 12). age The Earth's most ancient rocks gram (3/ 1 00,000 ounce) and the By analyzing the soil samples re­ are granites and sediments, and they ages of terrestrial rocks can now be turned from the Moon, we have been tell us that the Earth already had measured far more accurately than able to determine the chemical com­ mountain-building, running water, before Apollo. These new techniques position of the matter ejected by the oceans, and life at a time when the are already helping us to better un­ last lava flows were pouring out derstand the origin of terrestrial vol­ across the Moon. canic rocks, to identify new occur­ In the same way, all traces of any rences of the Earth's oldest rocks, intense early bombardment of the and to probe further into the origin of Earth have been destroyed . The terrestrial life more than 3 billion years ago. Sun and thus learn more about how the Sun operates. A major surprise was the discovery that the material in the solar wind is not the same as that in the Sun itself. The ratio of hydro­ gen to helium atoms in the solar wind that reaches the Moon is about 20 to 1. But the ratio of these atoms in the Sun, as measured with Earth-based instruments, is only 1 0 to 1. Some unexplained process in the Sun's outer atmosphere apparently oper­ ates to eject the lighter hydrogen atoms in preference to the heavier helium atoms. Even more important is the fact that the lunar soil still preserves ma­ terial ejected by the Sun in the past.

Figure 12. The Tracks of the Stars. Short tracks made over millions of years by fast­ moving charged atomic particles that were shot out of the Sun and other stars to eventually strike the surface of the Moon crisscross a tiny crystal collected from the lunar soil. The picture shows an area about a tenth of a millimeter across. The tracks, made visible by etching the crystal, are only a few hundredths of a millimeter long. Such grains, which may have trapped atomic particles for millions of years. preserve among other things. a unique record of the past activity of our Sun.

(Photograph courtesy of Dr. R. L.Fieischer, from Science, v. 167, p. 569, 30 January 1970. (c) The American Association for the Advancement of Science. Used by permission.) WHAT ELSE CAN THE MOON TELL US?

20 We now have a unique to and 17 missions, three long cores of Although the Apollo Program officially study the past behavior of the Sun. lunar soil were obtained by drilling ended in 1972, the active study of Our very existence depends on the hollow tu bes into the soil layer. These the Moon goes on. More than 125 Sun's activity, and by understanding core tubes penetrated as much as teams of scientists ar e studying the the Sun's past history, we can hope three meters (1 0 feet) deep. The returned lunar samples and an alyzing to predict better its future behavior. layers of soil in these cores contain a the information that co ntinues to These studies of the lunar soil ar e well-preserved history of the Moon come from the instruments on the only beginning, but what we have and the Sun that may extend as far Moon. Less than 10 per cent of the learned about the Sun so far is back as one and a half bi llion years. lunar sample material has yet been reassuring. Such chemical features No single terrestrial sample contains studied in detail, and more results will as the ratio of hydrogen to helium such a long record, and no one emerge as new ro cks and soil sam­ and the amount of iron in solar knows how much can be learned ples ar e examined. material show no change for at least when all the cores ar e carefully The scientific results of the Apollo the past few hundred thousand opened and studied. Certainly we will Program have spread far beyond the years. The lunar samples ar e telling learn more about the an cient history Moon itself. By studying the Moon, us that the Sun, in the recent past, of the Sun and Moon. We may even we have learned how to go about the has behaved very much as it does find traces of the movement of the business of exploring other planets. today, making us optimistic that the Sun and the solar system through The Apollo Program proved that we Sun will rem ain the same for the different regions of our Milky Way could apply to another world the foreseeable future. Galaxy. methods that we have used to learn As far as the ancient history of the about the Earth. Now the knowledge Sun is concerned, the most exciting gained from the Moon is being used lunar samples have not yet been fully with the photographs returned by examined. During the Apollo 15, 16, Mariners 9 and 10 to understand the histories of and and to interpret the data returned by the Viking mission to Mars. The Moon has thus become an WHAT MYSTERIES REMAIN ABOUT THE MOON?

important key to solving several fun­ strength varies, or why it reverses Despite the great scientific return damental questions about the other itself every few hundred thousand from the Apollo Program, there are planets: years or so . still many unanswered questions One way to learn about the Eart h's about the Moon: (1) What is the early history of magnetic field is to study the mag­ other planets? netic field of other planets. In this (1) What is the chemical composi­ The first half-billion years of the respect, the Moon is surprising. It has tion of the whole Moon? Moon's lifetime were dominated by no magnetic field today, but its rocks We have sampled only eight places intense and widespread melting, by suggest that it had a strong magnetic on the Moon, with six Apollo and two catastrophic meteorite impacts, and field in the past. If the Moon did have Luna landings. The chemical an aly­ by great eruptions of lava. Now an ancient magnetic field that some­ ses made from or bit cover only about close-up pictures of the planets Mer­ how "switched off" about 3 billion a quarter of the Moon's surface. We cury and Mars show heavily-cratered years ago, then continued study of still know little about the far side of regions and definite volcanic struc­ the Moon may help us learn how the Moon and nothing whatever tures, indi cating that these planets magnetic fields are produced in other about the Moon's polar regions. also have been affected by the same planets, including our own. processes that shaped the Moon (2) Why is the Moon uneven? when it was young. Such episodes of (3) How did life originate? Orbiting Apollo spacecraft used a early bombardment and volcanic Even the lifeless lunar soil contains laser device to me asure accurately eruption seem to be part of the life simple molecules formed by react ion the heights of peaks and valleys over story of planets. Our own Earth must between the soil particles and atoms much of the lunar surface. From have had a similar history, even of carbon, oxygen, and nitrogen that these careful measurements, scien­ though the traces of these primordial come from the Sun. In a more favora­ tists have learned that the Moon is events have been removed by later ble environment, these simple mole­ not a perfect sphere . It is slightly egg­ changes. cules might react further, forming the shaped, with the small end of the more complex molecules ("building egg pointing toward the Earth and (2) How do planets develop mag­ blocks") needed for the development the larger end facing away from it. netic fields? of life. The sterile Moon thus sug­ There are other major differences We have known for centuries that the gests that the basic ingredients for Earth has a strong magnetic field. life are common in the universe, and However, we still do not know exactly further study of the lunar soil will tell how the Earth's field formed, why its us about the chemical reactions that occur in space before life develops . 22 between the two sides of the Moon . forces that have kept it trapped in make more measurements of the The front (Earth-facing side), which is that position for billions of years. magnetism on the lunar surface, we the small end of the egg, is covered Perhaps lava erupted only on the may find a definite answer to the with large dark areas which were front side because the crust was baffling question. produced by great eruptions of ba­ th inner there . These differences could salt lava between ;3 and 4 billion tell us much about the early years of (5) How old are the youngest lunar years ago. However, the , if we could understand rocks? the Moon is almost ent irely com­ them. The youngest rocks collected from posed of light-colored, rugged, and the Moon were formed 3.1 billion heavily cratered terrain identical to (3) Is the Moon now molten inside? years ago. We cannot determine how the highland regions on the front We know that there were great vol­ the Moon heated up and then cooled side, and there are only a few canic eruptions on the Moon billions again until we know whether these patches of dark lava-like material. of years ago, but we do not know eruptions were the last or whether Furthermore, the Moon's upper layer how long they continued. To under­ volcanic activity continued on the (the crust), is also uneven. On the stand the Moon's history completely, Moon for a much longer time. front side, where the maria are, the we need to find out if the inside of lunar crust is about 60 kilometers (37 the Moon is still hot and partly mol­ (6) Is the Moon now really "dead"? miles) thick. On the back side, it is ten. More information about the heat Unexplained occurrences of reddish over 100 kilometers (62 miles) thick flow coming out of the Moon may glows, clouds, and mists have been (Figure 7) . help provide an answer. reported on the Moon's surface for We still do not know enough to over 300 years. These "lunar tran­ explain these different observations. (4) Does the Moon have an iron sient events," as they are called, are Perhaps the Moon points its small core like the Earth? still not explained. It is important to end toward the Earth because of tidal This question is critical to solving the determine what they are, because puzzle of ancient lunar magnetism. At they may indicate regions where the moment, we have so little data gases and other materials are still that we can neither rule out the coming to the surface from inside the possible existence of a small iron Moon . core nor prove that one is present. If we can determine more accurately the nature of the Moon's interior and WHAT DO WE DO NOW?

For all we have learned about the movements contain important clues Other spacecraft, like the Russian 2 Moon, the exploration of our nearest about what the inside of the Moon is Luna-16 and Luna-20 landers, could neighbor world has only just begun. like. return small samples from locations Much of the returned lunar sample The laser reflectors, which need no never before visited: the far side, the material remains to be studied, and power, will last on the Moon for more poles, or the sites of the puzzling we will continue to analyze the data than a century before being covered transient events. Because of the from the instruments on the Moon as with slow-moving lunar dust. Long Apollo Program, we now know how to long as they operate. before that, continuous measure­ analyze such small samples and how From what we have learned, we ments should make it possible to to interpret correctly the data we can now confidently plan ways to use understand the internal structure of obtain. Each landing and sample the Moon to help us understand the Moon. It may even be possible to return would have a double purpose: better the behavior of our own planet. use the Moon to measure the slow to teach us more about the Moon, One such project involves using sev­ movements of the Earth's continents and to help us design the machines eral reflectors that were placed on and oceans as they converge and that might return samples from the the Moon by Apollo astronauts. By separate. surfaces of Mars, Mercury, or the bouncing a laser beam off these To further explore the Moon itself, of . reflectors and back to Earth, we can we can send machines in place of Finally, we may see man return to measure variations in the Earth-Moon men. An unmanned spacecraft could the Moon, not as a passing visitor but distance (about 400,000 kilometers or circle the Moon from pole to pole, as a long-term resident, building 250,000 miles) with an accuracy of a measuring its chemical composition, bases from which to explore the few centimeters (a few inches, or one radioactivity, gravity, and magnetism. Moon and erecting astronomical in­ part in 10 billion). Continued meas­ This mission would carry on the tasks struments that use the Moon as a urement of the Earth-Moon distance begun by the Apollo Program and platform from which to see deeper as the Moon moves in its orbit would produce physical and chemi­ into the mysterious universe that sur­ around us will make it possible to cal maps of the whole Moon. Such rounds us. recognize tiny variations that exist in an orbiter could also serve as a the Moon's motions. These variations prototype for later spacecraft and occur because the Moon is not quite instruments to be put into orbit a uniform sphere, and these minor around Mars or Mercury to map and study those planets as we have mapped and explored the Moon. SUGGESTIONS NOTE FOR FOR FURTHER SCIENTISTS AND READING EDUCATORS.

24 Lewis, Robert S. (1974) . The Voyages Short, Nicholas M. (1975). Planetary The Lu nar Science Institute in Hous­ of Apollo: The Exploration of the Geology. Englewood Cliffs, N.J., ton, Texas can provide furt her infor­ Moon. New York, Quadrangle (The Prentice-Hall , Inc., 361 p., price mation about lunar science and New Yo rk Ti mes Book Co.), 308 p., $1 7.95 . A college-level textbook that about data resou rces that are av aila­ pri ce $12.50. A history of the entire summarizes the application of terres­ ble for scientific and educational pur­ Apollo program and the scientifi c dis­ tri al geology to lunar studies, the scien­ poses. In particular, the Institute coveries made about the Moon, from tific results from the Apollo program, maintains lists of av ailable boo ks, the first steps of Apollo 11 to the and the new knowledge obtained articles, photographs, maps, and splashdown of Apollo 17. about other planets by automated other materials dealing with the Moon Collins. Michael (1 974). Carrying the spacecraft. and the Apollo missions. For further Fire: An Astronaut's Journeys. New Ki ng, El bert A. (1976). Space information, contact : York, Ballantine Books, 488 p., paper­ Geology. New Yo rk, John Wiley and Lunar Science Institute back, price $1.95. A biographical Sons, Inc., 349 p., pnce $16.95. A Data Center, Code L memoir by an Apollo 11 astronaut, text for the advanced geology student, 3303 NASA Ro ad #1 descri bi ng his progress from test pi lot this book presents in detail our current , TX 7705 8 through the Gemini and Apollo knowledge about the geology of the Phone (713) 488-5200 missions. An entertaining presentation solar system without discussing the Inquiries about participation in NASA­ of the personalities and technicalities history and technical details of space supported programs of rese arch on i,nvolved ingoing to the Moon. exploration. The chapters on mete­ lunar samples and other as pects of orites, impact craters, th e moon, Mars, Taylor, S. R. (1975). Lunar Science: A lunar science should be addr es sed to: and the asteroids are excellent Post-Apollo View. New York, Perga­ Deputy Di rector summaries of the exciti ng scientific mon Press, 372 p., paper back, price Lunar and Planetary Programs Offi ce discoveries that have been made in $9.50. An extensive textbook that de­ Code SL the last few years . scribes in detail the scientific results NASA Headquarters from the Apollo program and the state (SP 350) Apollo Expeditions to the Washington, DC 205 46 of present knowledge about the na­ Moon. Was hington, D.C., Phone (202) 755-3730. ture, origin, and history of the Moon. Su perintendent of Documents, U.S. For scientists and advanced science Government Printing Office, 32 6 p., students. price $8.90. Stock Number 033--000- -00 630-6. A history of Apollo as told by the Apollo astronauts and key NASA personnel. Four-color illustratio ns .

Earthrise. Photograph by Apollo 12 crew captured a Earth rising above the lunar horizon.

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