\n The following material was downloaded from the NASA SpaceLink
\nBBS at the National Aeronautics and Space Administration, George C.
\nMarshall Space Flight Center, Marshall Space Flight Center, Alabama
\n35812 on 11\/16\/88.<\/p>\n
——————————————————————–
\n W H A T ‘ s N E W O N T H E M O O N
\n by Dr. Bevan M. French<\/p>\n
In 1969 over a billion people witnessed the “impossible” coming
\ntrue as the first men walked on the surface of the Moon. For the next
\nthree years, people of many nationalities watched as one of the great
\nexplorations of human history was displayed on their television
\nscreens.<\/p>\n
Between 1969 and 1972, supported by thousands of scientists and
\nengineers back on Earth, 12 astronauts explored the surface of the
\nMoon. Protected against the airlessness and the killing heat of the
\nlunar environment, they stayed on the Moon for days and some of them
\ntravelled for miles across its surface in Lunar Rovers. They made
\nscientific observations and set up instruments to probe the interior
\nof the Moon. They collected hundreds of pounds of lunar rock and
\nsoil, thus beginning the first attempt to decipher the origin and
\ngeological history of another world from actual samples of its crust.<\/p>\n
The initial excitement of new success and discovery has passed.
\nThe TV sets no longer show astronauts moving across the sunlit lunar
\nlandscape. But here on Earth, scientists are only now beginning to
\nunderstand the immense treasure of new knowledge returned by the
\nApollo astronauts.<\/p>\n
The Apollo Program has left us with a large and priceless legacy
\nof lunar materials and data. We now have Moon rocks collected from
\neight different places on the Moon. The six Apollo landings returned
\na collection weighing 382 kilograms (843 pounds) and consisting of
\nmore than 2,000 separate samples. Two automated Soviet spacecraft
\nnamed Luna-16 and Luna-20 returned small but important samples
\ntotalling about 130 grams (five ounces).<\/p>\n
Instruments placed on the Moon by the Apollo astronauts as long
\nago as 1969 are still detecting moonquakes and meteorite impacts,
\nmeasuring the Moon’s motions, and recording the heat flowing out from
\ninside the Moon. The Apollo Program also carried out a major effort
\nof photographing and analyzing the surface of the Moon. Cameras on
\nthe Apollo spacecraft obtained so many accurate photographs that we
\nnow have better maps of parts of the Moon than we do for some areas
\non Earth. Special detectors near the cameras measured the weak X-rays
\nand radioactivity given off by the lunar surface. From these
\nmeasurements, we have been able to determine the chemical composition
\nof about one-quarter of the Moon’s surface, an area the size of the
\nUnited States and Mexico combined. By comparing the flight data with
\nanalyses of returned Moon rocks, we can draw conclusions about the
\nchemical composition and nature of the entire Moon.<\/p>\n
Thus, in less than a decade, science and the Apollo Program have
\nchanged our Moon from an unknown and unreachable object into a
\nfamiliar world.<\/p>\n
WHAT HAS THE APOLLO PROGRAM TOLD US ABOUT THE MOON?<\/p>\n
What have we gained from all this exploration? Before the
\nlanding of Apollo 11 on July 20, 1969, the nature and origin of the
\nMoon were still mysteries. Now, as a result of the the Apollo
\nProgram, we can answer questions that remained unsolved during
\ncenturies of speculation and scientific study:<\/p>\n
(1) Is There Life On The Moon?<\/p>\n
Despite careful searching, neither living organisms nor fossil
\nlife have been found in any lunar samples. The lunar rocks were so
\nbarren of life that the quarantine period for returned astronauts was
\ndropped after the third Apollo landing.<\/p>\n
The Moon has no water of any kind, either free or chemically
\ncombined in the rocks. Water is a substance that is necessary for
\nlife, and it is therefore unlikely that life could ever have
\noriginated on the Moon. Furthermore, lunar rocks contain only tiny
\namounts of the carbon and carbon compounds out of which life is
\nbuilt, and most of this carbon is not native to the Moon but is
\nbrought to the lunar surface in meteorites and as atoms out of the
\nSun.<\/p>\n
(2) What Is The Moon Made Of?<\/p>\n
Before the first Moon rocks were collected, we could analyze
\nonly two types of bodies in our solar system: our own planet Earth
\nand the meteorites that occasionally fall to Earth from outer space.
\nNow we have learned that the Moon is chemically different from both
\nof these, but it is most like the Earth.<\/p>\n
The Moon is made of rocks. The Moon rocks are so much like Earth
\nrocks in their appearance that we can use the same terms to describe
\nboth. The rocks are all IGNEOUS, which means that they formed by the
\ncooling of molten lava. (No sedimentary rocks, like limestone or
\nshale, which are deposited in water, have ever been found on the
\nMoon.).<\/p>\n
The dark regions (called “maria”) that form the features of “The
\nMan in the Moon” are low, level areas covered with layers of basalt
\nlava, a rock similar to the lavas that erupt from terrestrial
\nvolcanoes in Hawaii, Iceland, and elsewhere. The light-colored parts
\nof the Moon (called “highlands”) are higher, more rugged regions that
\nare older than the maria. These areas are made up of several
\ndifferent kinds of rocks that cooled slowly deep within the Moon.
\nAgain using terrestrial terms, we call these rocks gabbro, norite,
\nand anorthosite.<\/p>\n
Despite these similarities, Moon rocks are basically different
\nand it is easy to tell them apart by analyzing their chemistry or by
\nexamining them under a microscope. The most obvious difference is
\nthat Moon rocks have no water at all, while almost all terrestrial
\nrocks contain at least a percent or two of water. The Moon rocks are
\ntherefore very well-preserved, because they never were able to react
\nwith water to form clay minerals or rust. A 3 1\/2-billion-year-old
\nMoon rock looks fresher than water-bearing lava just erupted from a
\nterrestrial volcano.<\/p>\n
Another important difference is that the Moon rocks formed where
\nthere was almost no free oxygen. As a result, some of the iron in
\nlunar rocks was not oxidized when the lunar lavas formed and still
\noccurs as small crystals of metallic iron.<\/p>\n
Because Moon rocks have never been exposed to water or oxygen,
\nany contact with the Earth’s atmosphere could “rust” them badly. For
\nthis reason, the returned Apollo samples are carefully stored in an
\natmosphere of dry nitrogen, and no more of the lunar material than
\nnecessary is exposed to the laboratory atmosphere while the samples
\nare being analyzed.<\/p>\n
The Moon rocks are made of the same chemical elements that make
\nup Earth rocks, although the proportions are different. Moon rocks
\ncontain more of the common elements calcium, aluminum, and titanium
\nthan do most Earth rocks. Rarer elements like hafnium and zirconium,
\nwhich have high melting points, are also more plentiful in lunar
\nrocks. However, other elements like sodium and potassium, which have
\nlow melting points, are scarce in lunar material. Because the Moon
\nrocks are richer in high-temperature elements, scientists believe
\nthat the material that formed the Moon was once heated to much higher
\ntemperatures than material that formed the Earth.<\/p>\n
The chemical composition of the Moon also is different in
\ndifferent places. Soon after the Moon formed, various elements sorted
\nthemselves out to form different kinds of rock. The light-colored
\nhighlands are rich in calcium and aluminum, while the dark-colored
\nmaria contain less of those elements and more titanium, iron, and
\nmagnesium.<\/p>\n
(3) What Is The Inside Of The Moon Like?<\/p>\n
Sensitive instruments placed on the lunar surface by the Apollo
\nastronauts are still recording the tiny vibrations caused by
\nmeteorite impacts on the surface of the Moon and by small moonquakes
\ndeep within it. These vibrations provide the data from which
\nscientists determine what the inside of the Moon is like.<\/p>\n
About 3,000 moonquakes are detected each year. All of them are
\nvery week by terrestrial standards. The average moonquake releases
\nabout as much energy as a firecracker, and the whole Moon releases
\nless than one-ten-billionth of the earthquake energy of the Earth.
\nThe moonquakes occur about 600 to 800 kilometers (370-500 miles) deep
\ninside the Moon, much deeper than almost all the quakes on our own
\nplanet. Certain kinds of moonquakes occur at about the same time
\nevery month, suggesting that they are triggered by repeated tidal
\nstrains as the Moon moves in its orbits around the Earth.<\/p>\n
A picture of the inside of the Moon has slowly been put together
\nfrom the records of thousands of moonquakes, meteorite impacts, and
\nthe deliberate impacts of discarded Apollo rocket stages onto the
\nMoon. The Moon is not uniform inside, but is divided into a series of
\nlayers just as the Earth is, although the layers of the Earth and
\nMoon are different. The outermost part of the Moon is a crust about
\n60 kilometers (37 miles) thick, probably composed of calcium-and
\naluminium-rich rocks like those found in the highlands. Beneath the
\ncrust is a thick layer of denser rock (the mantle) which extends down
\nto more than 800 kilometers (500 miles).<\/p>\n
The deep interior of the Moon is still unknown. The Moon may
\ncontain a small iron core at its center, and there is some evidence
\nthat the Moon may be hot and even partly molten inside.<\/p>\n
The Moon does not now have a magnetic field like the Earth’s,
\nand so the most baffling and unexpected result of the Apollo Program
\nwas the discovery of preserved magnetism in the many of the old lunar
\nrocks. One explanations is that the Moon had an ancient magnetic
\nfield that somehow disappeared after the old lunar rocks had formed.<\/p>\n
One reason we have been able to learn so much about the Moon’s
\ninterior is that the instruments placed on the Moon by the Apollo
\nastronauts have operated much longer than expected. Some of the
\ninstruments originally designed for a one-year lifetime, have been
\noperating since 1969 and 1970. This long operation has provided
\ninformation that we could not have obtained from shorter records.<\/p>\n
The long lifetime of the heat flow experiments set up by the
\nApollo 15 and 17 missions has made it possible to determine more
\naccurately the amount of heat coming out of the Moon . This heat flow
\nis a basic indicator of the temperature and composition of the inside
\nof the Moon. The new value, about two-thirds of the value calculated
\nfrom earlier data, is equal to about one-third the amount of heat now
\ncoming out of the inside of the Earth. As a result, we can now
\nproduce better models of what the inside of the Moon is like.<\/p>\n
As they probed the lunar interior, the Apollo instruments have
\nprovided information about the space environment near the Moon. For
\nexample, the sensitive devices used to detect moonquakes have also
\nrecorded the vibrations caused by the impacts of small meteorites
\nonto the lunar surface. We now have long-term records of how often
\nmeteorites strike the Moon, and we have learned that these impacts do
\nnot always occur at random. Some small meteorites seem to travel in
\ngroups. Several such swarms, composed of meteorites weighing a few
\npounds each, struck the Moon in 1975. The detection of such events is
\ngiving scientists new ideas about the distribution of meteorites and
\ncosmic dust in the solar system.<\/p>\n
The long lifetime of the Apollo instruments has also made
\nseveral cooperative projects possible. For example, our instruments
\nwere still making magnetic measurements at several Apollo landing
\nsites when, elsewhere on the Moon, the Russians landed similar
\ninstruments attached to their two automated lunar roving vehicles
\n(Lunokhods). By making simultaneous measurements and exchanging data,
\nAmerican and Russian scientists have not only provided a small
\nexample of international cooperation in space, but they have jointly
\nobtained a better picture of the magnetic properties of the Moon and
\nthe space around it.<\/p>\n
(4) What Is The Moon’s Surface Like?<\/p>\n
Long before the Apollo Program scientists could see that the
\nMoon’s surface was complex. Earth-based telescopes could distinguish
\nthe level maria and the rugged highlands. We could recognize
\ncountless circular craters, rugged mountain ranges, and deep winding
\ncanyons or rilles.<\/p>\n
Because of the Apollo explorations, we have now learned that all
\nthese lunar landscapes are covered by a layer of fine broken-up
\npowder and rubble about 1 to 20 meters (3 to 60 feet) deep. This
\nlayer is usually called the “lunar soil,” although it contains no
\nwater or organic material, and it is totally different from soils
\nformed on Earth by the action of wind, water, and life.<\/p>\n
The lunar soil is something entirely new to scientists, for it
\ncould only have been formed on the surface of an airless body like
\nthe Moon. The soil has been built up over billions of years by the
\ncontinuous bombardment of the unprotected Moon by large and small
\nmeteorites, most of which would have burned up if they had entered
\nthe Earth’s atmosphere.<\/p>\n
These meteorites form craters when they hit the Moon. Tiny
\nparticles of cosmic dust produce microscopic craters perhaps 1\/1000
\nof a millimeter (1\/25,000 inch) across, while the rare impact of a
\nlarge body may blasts out a crater many kilometers, or miles, in
\ndiameter. Each of these impacts shatters the solid rock, scatters
\nmaterial around the crater, and stirs and mixes the soil. As a
\nresult, the lunar soil is a well-mixed sample of a large area of the
\nMoon, and single samples of lunar soil have yielded rock fragments
\nwhose source was hundreds of kilometers from the collection site.<\/p>\n
However, the lunar soil is more than ground-up and reworked
\nlunar rock. It is the boundary layer between the Moon and outer
\nspace, and it absorbs the matter and energy that strikes the Moon fro
\nthe Sun and the rest of the universe. Tiny bits of cosmic dust and
\nhigh-energy atomic particles that would be stopped high in the
\nEarth’s protective atmosphere rain continually onto the surface of
\nthe Moon.<\/p>\n
(5) How Old Is The Moon?<\/p>\n
Scientists now think that the solar system first came into being
\nas a huge, whirling, disk-shaped cloud of gas and dust. Gradually the
\ncloud collapsed inward. The central part became masssive and hot,
\nforming the Sun. Around the Sun, the dust formed small objects that
\nrapidly collected together to form the large planets and satellites
\nthat we see today.<\/p>\n
By carefully measuring the radioactive elements found in rocks,
\nscientists can determine how old the rocks are. Measurements on
\nmeteorites indicate that the formation of the solar system occurred
\n4.6 billion years ago. There is chemical evidence in both lunar and
\nterrestrial rocks that the Earth and Moon also formed at that time.
\nHowever, the oldest known rocks on Earth are only 3.8 billion years
\nold, and scientists think that the older rocks have been destroyed by
\nthe Earth’s continuing volcanism, mountain-building, and erosion.<\/p>\n
The Moon rocks fill in some of this gap in time between the
\nEarth’s oldest preserved rocks and the formation of the solar system.
\nThe lavas from the dark maria are the Moon’s youngest rocks, but they
\nare as old as the oldest rocks found on Earth, with ages of 3.1 to
\n3.8 billion years. Rocks from the lunar highlands are even older.
\nMost highland samples have ages of 4.0 to 4.3 billion years. Some
\nMoon rocks preserve traces of even older lunar events. Studies of
\nthese rocks indicate that widespread melting and chemical separation
\nwere going on within the Moon about 4.4 billion years ago, or not
\nlong after the Moon had formed.<\/p>\n
One of the techniques used to establish this early part of lunar
\nhistory is a new age-dating method (involving the elements neodymium
\nand samarium) that was not even possible when the first Apollo
\nsamples were returned in 1969. The combination of new instruments and
\ncareful protection of the lunar samples from contamination thus make
\nit possible to understand better the early history of the Moon.<\/p>\n
Even more exciting is the discovery that a few lunar rocks seem
\nto record the actual formation of the Moon. Some tiny green rock
\nfragments collected by the Apollo 17 astronauts have yielded an
\napparent age of 4.6 billion years, the time at which scientists think
\nthat the Moon and the solar system formed. Early in 1976, scientists
\nidentified another Apollo 17 crystalline rock with the same ancient
\nage. These pieces may be some of the first material that solidified
\nfrom the once-molten Moon.<\/p>\n
(6) What Is The History Of The Moon?<\/p>\n
The first few hundred million years of the Moon’s lifetime were
\nso violent that few traces of this time remain. Almost immediately
\nafter the Moon formed, its outer part was completely melted to a
\ndepth of several hundred kilometers. While this molten layer
\ngradually cooled and solidfied into different kinds of rocks, the
\nMoon was bombarded by huge asteroids and smaller bodies. Some of
\nthese asteroids were the size of small states, like Rhode Island or
\nDelaware, and their collisions with the Moon created huge basins
\nhundreds of kilometers across.<\/p>\n
The catastrophic bombardment died away about 4 billion years
\nago, leaving the lunar highlands covered with huge overlapping
\ncraters and a deep layer of shattered and broken rock. As the
\nbombardment subsided, heat produced by the decay of radioactive
\nelements began to melt the inside of the Moon at depths of about 200
\nkilometers (125 miles) below its surface. Then, for the next half
\nbillion years, from about 3.8 to 3.1 billion years ago, great floods
\nof lava rose from the inside the Moon and poured out over its
\nsurface, filling in the large impact basins to form the dark parts of
\nthe Moon that we see today.<\/p>\n
As far as we know, the Moon has been quiet since the last lavas
\nerupted more than 3 billion years ago. Since then, the Moon’s surface
\nhas been altered only by rare large meteorite impacts and by atomic
\nparticles from the Sun and the stars. The Moon has preserved featured
\nformed almost 4 billion year ago, and if men had landed on the Moon a
\nbillion years ago, it would have looked very much as it does now. The
\nsurface of the Moon now changes so slowly that the footprints left by
\nthe Apollo astronauts will remain clear and sharp for millions of
\nyears.<\/p>\n
This preserved ancient history of the Moon is in sharp contrast
\nto the changing Earth. The Earth still behaves like a young planet.
\nIts internal heat is active, and volcanic eruptions and
\nmountain-building have gone on continuously as far back as we can
\ndecipher the rocks. According to new geological theories, even the
\npresent ocean basins are less than about 200 million years old,
\nhaving formed by the slow separation of huge moving plates that make
\nup the Earth’s crust.<\/p>\n
(7) Where Did The Moon Come From?<\/p>\n
Before we explored the Moon, there were three main suggestions
\nto explain its existence: that it had formed near the Earth as a
\nseparate body; that it had separated from the Earth; and that is had
\nformed somewhere else and been captured by the Earth.<\/p>\n
Scientists still cannot decide among these three theories.
\nHowever, we have learned that the Moon formed as a part of our solar
\nsystem and that it has existed as an individual body for 4.6 billion
\nyears. Separation from the Earth is now considered less likely
\nbecause there are many basic differences in chemical composition
\nbetween the two bodies, such as the absence of water on the Moon. But
\nthe other two theories are still evenly matched in their strengths
\nand weaknesses. We will need more data and perhaps some new theories
\nbefore the origin of the Moon is settled.<\/p>\n
WHAT HAS THE MOON TOLD US ABOUT THE EARTH?<\/p>\n
It might seem that the active, inhabited Earth has nothing in
\ncommon with the quiet, lifeless Moon. Nevertheless, the scientific
\ndiscoveries of the Apollo Program have provided a new and unexpected
\nlook into the early history of our own planet. Scientists think that
\nall the planets formed in the same way, by the rapid accumulation of
\nsmall bodies into large ones about 4.6 billion years ago. The Moon’s
\nrocks contain the traces of this process of planetary creation. The
\nsame catastrophic impacts and widespread melting that we recognize on
\nthe Moon must also have dominated the Earth during its early years,
\nand about 4 billion years ago the Earth may have looked much the same
\nas the Moon does now.<\/p>\n
The two worlds then took different paths. The Moon became quiet
\nwhile the Earth continued to generate mountains, volcanoes, oceans,
\nan atmosphere, and life. The Moon preserved its ancient rocks, while
\nthe Earth’s older rocks were continually destroyed and recreated as
\nyounger ones.<\/p>\n
The Earth’s oldest preserved rocks, 3.3 to 3.8 billion years
\nold, occur as small remnants in Greenland, Minnesota, and Africa.
\nThese rocks are not like the lunar lava flows of the same age. The
\nEarth’s most ancient rocks are granites and sediments, and they tell
\nus that the Earth already had mountain-building, running water,
\noceans, and life at a time when the last lava flows were pouring out
\nacross the Moon.<\/p>\n
In the same way, all traces of any intense early bombardment of
\nthe Earth have been destroyed. The record of later impacts remains,
\nhowever, in nearly 100 ancient impact structures that have been
\nrecognized on the Earth in recent years. Some of these structures are
\nthe deeply eroded remnants of craters as large as those of the Moon
\nand they give us a way to study on Earth the process that once
\ndominated both the Earth and Moon.<\/p>\n
Lunar science is also making other contributions to the study of
\nthe Earth. The new techniques developed to analyze lunar samples are
\nnow being applied to terrestrial rocks. Chemical analyses can now be
\nmade on samples weighing only 0.001 gram (3\/100,000 ounce) and the
\nages of terrestrial rocks can now be measured far more accurately
\nthan before Apollo. These new techniques are already helping us to
\nbetter understand the origin of terrestrial volcanic rocks, to
\nidentify new occurrences of the Earth’s oldest rocks, and to probe
\nfurther into the origin of terrestrial life more than 3 billion years
\nago.<\/p>\n
WHAT HAS THE MOON TOLD US ABOUT THE SUN?<\/p>\n
One of the most exciting results of the Apollo Program is that,
\nby going to the Moon, we have also been able to collect samples of
\nthe Sun.<\/p>\n
The surface of the Moon is continually exposed to the solar
\nwind, a stream of atoms boiled into space from the Sun’s atmosphere.
\nSince the Moon formed, the lunar soil has trapped billions of tons of
\nthese atoms ejected from the Sun. The soil also contains traces of
\ncosmic rays produced outside our own solar system. These high-energy
\natoms, probably produced inside distant stars, leave permanent tracks
\nwhen they strike particles in the lunar soil.<\/p>\n
By analyzing the soil samples returned from the Moon, we have
\nbeen able to determine the chemical composition of the matter ejected
\nby the Sun and thus learn more about how the Sun operates. A major
\nsurprise was the discovery that the material in the solar wind is not
\nthe same as that in the Sun itself. The ratio of hydrogen to helium
\natoms in the solar wind that reaches the Moon is about 20 to 1. But
\nthe ratio of these atoms in the Sun, as measured with Earth-based
\ninstruments, is only 10 to 1. Some unexplained process in the Sun’s
\nouter atmosphere apparently operates to eject the lighter hydrogen
\natoms in preference to the heavier helium atoms.<\/p>\n
Even more important is the fact that the lunar soil still
\npreserves material ejected by the Sun in the past. We now have a
\nunique opportunity to study the past behavior of the Sun. Our very
\nexistence depends on the Sun’s activity, and by understanding the
\nSun’s past history, we can hope to predict better its future
\nbehavior.<\/p>\n
These studies of the lunar soil are only beginning, but what we
\nhave learned about the Sun so far is reassuring. Such chemical
\nfeatures as the ratio of hydrogen to helium and the amount of iron in
\nsolar material show no change for at least the past few hundred
\nthousand years. The lunar samples are telling us that the Sun, in the
\nrecent past, has behaved very much as it does today, making us
\noptimistic that the Sun will remain the same for the foreseeable
\nfuture.<\/p>\n
As far as the ancient history of the Sun is concerned, the most
\nexciting lunar samples have not yet been fully examined. During the
\nApollo 15, 16, and 17 missions, three long cores of lunar soil were
\nobtained by drilling hollow tubes into the soil layer. These core
\ntubes penetrated as much as three meters (10 feet) deep. The layers
\nof soil in these cores contain a well-preserved history of the Moon
\nand the Sun that may extend as far back as one and a half billion
\nyears. No single terrestrial sample contains such a long record, and
\nno one knows how much can be learned when all the cores are carefully
\nopened and studied. Certainly we will learn more about the ancient
\nhistory of the Sun and Moon. We may even find traces of the movement
\nof the Sun and the solar system through different regions of our
\nMilky Way Galaxy.<\/p>\n
WHAT ELSE CAN THE MOON TELL US?<\/p>\n
Although the Apollo Program officially ended in 1972, the active
\nstudy of the Moon goes on. More than 125 teams of scientists are
\nstudying the returned lunar samples and analyzing the information
\nthat continues to come from the instruments on the Moon. Less than 10
\npercent of the lunar sample material has yet been studied in detail,
\nand more results will emerge as new rocks and soil samples are
\nexamined.<\/p>\n
The scientific results of the Apollo Program have spread far
\nbeyond the Moon itself. By studying the Moon, we have learned how to
\ngo about the business of exploring other planets. The Apollo Program
\nproved that we could apply to another world the methods that we have
\nused to learn about the Earth. Now the knowledge gained from the Moon
\nis being used with the photographs returned by Mariner 9 and 10 to
\nunderstand the histories of Mercury and Mars and to interpret the
\ndata returned by the Viking mission to Mars.<\/p>\n
The Moon has thus become an important key to solving several
\nfundamental questions about the other planets.<\/p>\n
(1) What Is The Early History Of Other Planets?<\/p>\n
The first half-billion years of the Moon’s lifetime were
\ndominated by intense and widespread melting, by catastrophic
\nmeteorite impacts and by great eruptions of lava. Now close-up
\npictures of the planets Mercury and Mars show heavily-cratered
\nregions and definite volcanic structures, indicating that these
\nplanets also have been affected by the same processes that shaped the
\nMoon when it was young. Such episodes of early bombardment and
\nvolcanic eruptions seem to be part of the life story of planets. Our
\nown Earth must have had a similar history, even though the traces of
\nthese primordial events have been removed by later changes.<\/p>\n
(2) How Do Planets Develop Magnetic Fields?<\/p>\n
We have known for centuries that the Earth has a strong magnetic
\nfield. However, we still do not know exactly how the Earth’s field
\nformed, why its strength varies, or why it reverses itself every few
\nhundred thousand years or so.<\/p>\n
One way to learn about the Earth’s magnetic field is to study
\nthe magnetic field of other planets. In this respect, the Moon is
\nsurprising. It has no magnetic field today, but its rocks suggest
\nthat it had a strong magnetic field in the past. If the Moon did have
\nan ancient magnetic field that somehow “switched off” about 3 billion
\nyears ago, then continued study of the Moon may help us learn how
\nmagnetic fields are produced in other planets, including our own.<\/p>\n
(3) Even the lifeless lunar soil contains simple molecules formed by
\nreaction between the soil particles and atoms of carbon, oxygen, and
\nnitrogen that come from the Sun. In a more favorable environment,
\nthese simple molecules might react further, forming the more complex
\nmolecules (“building blocks”) needed for the development of life. The
\nsterile Moon thus suggests that the basic ingredients for life are
\ncommon in the universe, and further study of the lunar soil will tell
\nus about the chemical reactions that occur in space before life
\ndevelops.<\/p>\n
WHAT MYSTERIES REMAIN ABOUT THE MOON?<\/p>\n
Despite the great scientific return from the Apollo Program,
\nthere are still many unanswered questions about the Moon:<\/p>\n
(1) What Is The Chemical Composition of the Whole Moon?<\/p>\n
We have sampled only eight places on the Moon, with six Apollo
\nand two Luna landings. The chemical analyses made from orbit cover
\nonly about a quarter of the Moon’s surface. We still know little
\nabout the far side of the Moon and nothing whatever about the Moon’s
\npolar regions.<\/p>\n
(2) Why Is The Moon Uneven?<\/p>\n
Orbiting Apollo spacecraft used a laser device to measure
\naccurately the heights of peaks and valleys over much of the lunar
\nsurface. From these careful measurements, scientists have learned
\nthat the Moon is not a perfect sphere. It is slightly egg-shaped,
\nwith the small end of the egg pointing toward the Earth and the
\nlarger end facing away from it.<\/p>\n
There are other major differences between the two sides of the
\nMoon. The front (Earth-facing side), which is the small end of the
\negg, is covered with large dark areas which were produced by great
\neruptions of basalt lava between 3 and 4 billion years ago. However,
\nthe far side of the Moon is almost entirely composed of
\nlight-colored, rugged, and heavily cratered terrain identical to the
\nhighland regions on the front side, and there are only a few patches
\nof dark lava-like material. Furthermore, the Moon’s upper layer (the
\ncrust), is also uneven. On the front side, where the maria are, the
\nlunar crust is about 60 kilometers (37 miles) thick. On the back
\nside, it is over 100 kilometers (62 miles) thick .<\/p>\n
We still do not know enough to explain these different
\nobservations. Perhaps, the Moon points its small end toward the Earth
\nbecause of tidal forces that have kept it trapped in that position
\nfor billions of years. Perhaps lava erupted only on the front side
\nbecause the crust was thinner there. These differences could tell us
\nmuch about the early years of the Moon, if we could understand them.<\/p>\n
(3) Is The Moon Now Molten Inside?<\/p>\n
We know that there were great volcanic eruptions on the Moon
\nbillions of years ago, but we do not know how long they continued. To
\nunderstand the Moon’s history completely, we need to find out if the
\ninside of the Moon is still hot and partly molten. More information
\nabout the heat flow coming out of the Moon may help provide an
\nanswer.<\/p>\n
(4) Does The Moon Have An Iron Core Like The Earth?<\/p>\n
This question is critical to solving the puzzle of ancient lunar
\nmagnetism, At the moment, we have so little data that we can neither
\nrule out the possible existence of a small iron core nor prove that
\none is present. If we can determine more accurately the nature of the
\nMoon’s interior and make more measurements of the magnetism on the
\nlunar surface, we may find a definite answer to the baffling
\nquestion.<\/p>\n
(5) How Old Are The Youngest Lunar Rocks?<\/p>\n
The youngest rocks collected from the Moon were formed 3.1
\nbillion years ago. We cannot determine how the Moon heated up and
\nthen cooled again until we know whether these eruptions were the last
\nor whether volcanic activity continued on the Moon for a much longer
\ntime.<\/p>\n
(6) Is The Moon Now Really “Dead”?<\/p>\n
Unexplained occurrences of reddish clouds, and mists have been
\nreported on the Moon’s surface for over 300 years. These “lunar
\ntransient events,” as they are called, are still not explained. It is
\nimportant to determine what they are, because they may indicate
\nregions where gases and other materials are still coming to the
\nsurface from inside the Moon.<\/p>\n
WHAT DO WE DO NOW?<\/p>\n
For all we have learned about the Moon, the exploration of our
\nnearest neighbor world has only just begun. Much of the returned
\nlunar sample material remains to be studied, and we will continue to
\nanalyze the data from the instruments on the Moon as long as they
\noperate.<\/p>\n
From what we have learned, we can now confidently plan ways to
\nuse the Moon to help us understand better the behavior of our own
\nplanet. One such project involves using several reflectors that were
\nplaced on the Moon by Apollo astronauts. By bouncing a laser beam off
\nthese reflectors and back to Earth, we can measure variations in the
\nEarth-Moon distance (about 400,000 kilometers or 250,000 miles) with
\nan accuracy of a few centimeters (a few inches, or one part in 10
\nbillion). Continued measurement of the Earth-Moon distance as the
\nMoon moves in its orbit around us will make it possible to recognize
\ntiny variations that exist in the Moon’s motions. These variations
\noccur because the Moon is not quite a uniform sphere, and these minor
\nmovements contain important clues about what the inside of the Moon
\nis like.<\/p>\n
The laser reflectors, which need no power, will last on the Moon
\nfor more than a century before being covered with slow-moving lunar
\ndust. Long before that, continuous measurements should make it
\npossible to understand the internal structure of the Moon. It may
\neven be possible to use the Moon to measure the slow movements of
\nEarth’s continents and oceans as they converge and separate.<\/p>\n
To further explore the Moon itself, we can send machines in
\nplace of men. An unmanned spacecraft could circle the Moon from pole
\nto pole, measuring its chemical composition, radioactivity, gravity,
\nand magnetism. This mission would carry on the tasks begun by the
\nApollo Program and would produce physical and chemical maps of the
\nwhole Moon. Such an orbiter could also serve as a prototype for later
\nspacecraft and instruments to be put into orbit around Mars or
\nMercury to map and study those planets as we have mapped and explored
\nthe Moon.<\/p>\n
Other spacecraft, like the Russian Luna-16 and Luna-20 landers,
\ncould return small samples from locations never before visited: the
\nfar side, the poles, or the sites of the puzzling transient events.
\nBecause of the Apollo Program, we now know how to analyze such small
\nsamples and how to interpret correctly the data we obtain. Each
\nlanding and sample return would have a double purpose: to teach us
\nmore about the Moon, and help us design the machines that might
\nreturn samples from the surfaces of Mars, Mercury, or the moons of
\nJupiter.<\/p>\n
Finally, we may see man return to the Moon, not as a passing
\nvisitor but as a long-term resident, building bases from which to
\nexplore the Moon and erecting astronomical instruments that use the
\nMoon as a platform from which to see deeper into the mysterious
\nuniverse that surrounds us.<\/p>\n
NOTE FOR SCIENTISTS AND EDUCATORS<\/p>\n
The Lunar Science Institute in Houston, Texas can provide
\nfurther information about lunar science and about data resources that
\nare available for scientific and educational purposes. In particular,
\nthe Institute maintains lists of available books, articles,
\nphotographs, maps, and other materials dealing with the Moon and the
\nApollo missions. For further information, contact:<\/p>\n
LUNAR SCIENCE INSTITUTE
\n Data Center, Code L
\n 3303 NASA Road #1
\n Houston, TX 77058
\n Phone (713) 488-5200<\/p>\n
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