The Moon

The Moon is the closest astronomical object to the Earth. With the Earth it forms what is almost a double planet for no other body in our solar system has a satellite which is as large in comparison to the size of the planet. The Moon is probably the most satisfying object to look at through any telescope. The craters and mountains can be seen with even a small telescope. The best place to look is near the terminator, where the Sun is either rising or setting. Here the shadows cast by mountains and crater walls are longest and can give very dramatic views. After as short a time as an hour changes in the shadows can be seen as the sunlight reaches or leaves peaks near the terminator.

Because it takes 27.322 days both to rotate on its axis and to orbit Earth, the Moon always shows us the same face. We see the Moon because of reflected sunlight. How much of it we see depends on its position in relation to Earth and the Sun. The 27.322-day number is what scientists call a sidereal month, and it is how long it takes the Moon to orbit the Earth in relation to a fixed star. Another measurement, called a synodic month, is measured between in relation to the Sun and equals 29.53 days. Full moons and new moons are measured by the synodic month. Earth's gravity keeps the Moon in orbit, while the Moon's gravity creates tides on our oceans.

This synchronous rotation is caused by an unsymmetrical distribution of mass in the Moon, which has allowed Earth's gravity to keep one lunar hemisphere permanently turned toward Earth. We can thus only see the 'near side'; the 'far side' is fixed permanently away from us (see animation below and here). Optical librations have been observed telescopically since the mid-17th century. Very small but real librations (maximum about 0°.04) are caused by the effect of the Sun's gravity and the eccentricity of Earth's orbit, perturbing the Moon's orbit and allowing cyclical preponderances of torque in both east-west and north-south directions.

The Lunar Prospector in 1998 provided evidence of ice near the Moon's poles, perhaps as much as 6 billion tonnes of it. However, further evidence suggests otherwise. As the Moon has no water of any kind, either free or chemically combined in the rocks it is therefore unlikely that life could ever have originated on the Moon as water is a substance that is necessary for life. Furthermore, lunar rocks contain only tiny amounts of the carbon and carbon compounds out of which life is built, and most of this carbon is not native to the Moon but is brought to the lunar surface in meteorites and as atoms out of the Sun.

Without an atmosphere, heat is not held near the planet so vary wildly. Daytimes on the sunny side of the Moon reach 134 degrees Celcius. On the dark side it gets as cold as -153 degrees Celcius. (See 'Far Side') Lunar temperatures vary throughout the month-long lunar day, but the lunar year doesn't experience much seasonal variation. Less than a metre beneath the surface there is a relatively constant temperature as compared to its mean surface temperature. The lunar regolith is such a good insulator that it causes it to stay constantly around the -23C mark.

Before the first Moon rocks were collected, we could analyse only two types of bodies in our solar system: our own planet Earth and the meteorites that occasionally fall to Earth from outer space. Now we have learned that the Moon is chemically different from both of these, but it is most like the Earth.

The Moon is made of rocks. The Moon rocks are so much like Earth rocks in their appearance that we can use the same terms to describe both. The rocks are all igneous, which means that they formed by the cooling of molten lava. (No sedimentary rocks, like limestone or shale, which are deposited in water, have ever been found on the Moon)

The total weight of lunar samples brought back to earth by the Apollo astronauts was 381.69 kilograms. It was delivered in 2196 original samples at a cost for the Apollo programme of 24 billion dollars or 62 878 dollars per kilogram. By 1975 this had been split into 35 600 samples. 5 kilograms were consigned to sample displays in museums around the world for public viewing and even touching! A total of 5.1 % or 19.3 kgs were allocated for scientific study, the rest is archived untouched for posterity. About 1.64 kgs were scientifically destroyed.

Maria: The dark, vast regions that form the features of 'The Man in the Moon' are low, level areas covered with layers of basalt lava, a rock similar to the lavas that erupt from terrestrial volcanoes in Hawaii, Iceland, and elsewhere. These cratered areas cover about 16% of the lunar surface and is concentrated on the nearside of the Moon, mostly within impact basins. This concentration may be explained by the fact that the Moon's centre of mass is offset from its geometric centre by about 2 kms in the direction of Earth, probably because the crust is thicker on the far side. It is possible, therefore, that basalt magmas rising from the interior reached the surface easily on the nearside, but encountered difficulty on the far side. Mare rocks are basalt and most date from 3.8 to 3.1 billion years. Some fragments in highland breccias date to 4.3 billion years and high resolution photographs suggest some mare flows actually embay young craters and may thus be as young as 1 billion years. The maria average only a few hundred metres in thickness but are so massive they frequently deformed the crust underneath them which created fault-like depressions and raised ridges.

The Terrae: Bright upland regions consisting of large and small circular craters, many overlapping. The craters and basins in these highland areas are formed by meteorite impact and are thus older than the maria, having accumulated more craters. The dominant rock type in this region contain high contents of plagioclase feldspar (a mineral rich in calcium and aluminum) and are a mixture of crustal fragments brecciated by meteorite impacts. Most terrae breccias are composed of still older breccia fragments. Other terrae samples are fine-grained crystalline rocks formed by shock melting due to the high pressures of an impact event. Nearly all of the highland breccias and impact melts formed about 4.0 to 3.8 billion years ago. The intense bombardment began 4.6 billion years ago, which is the estimated time of the Moon's origin.

Mountains or 'Highlands': The light-coloured parts of the Moon 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. Again using terrestrial terms, we call these rocks gabbro, norite, and anorthosite. there are chains of these mountain areas; one such is the 'Appenine' range.

Despite these similarities, Moon rocks are basically different and it is easy to tell them apart by analysing their chemistry or by examining them under a microscope. The most obvious difference is that Moon rocks have no water at all, while almost all terrestrial rocks contain at least a percent or two of water. The Moon rocks are therefore very well-preserved, because they never were able to react with water to form clay minerals or rust. A 3,5 billion year old Moon rock looks fresher than water-bearing lava just erupted from a terrestrial volcano.

Because Moon rocks have never been exposed to water or oxygen, any contact with the Earth's atmosphere could 'rust' them badly. For this reason, the returned Apollo samples are carefully stored in an atmosphere of dry nitrogen, and no more of the lunar material than necessary is exposed to the laboratory atmosphere while the samples are being analysed.

The Moon rocks are made of the same chemical elements that make up Earth rocks, although the proportions are different. Moon rocks contain more of the common elements calcium, aluminum, and titanium than do most Earth rocks. Rarer elements like hafnium and zirconium, which have high melting points, are also more plentiful in lunar rocks. However, other elements like sodium and potassium, which have low melting points, are scarce in lunar material. Because the Moon rocks are richer in high-elements, scientists believe that the material that formed the Moon was once heated to much higher s than material that formed the Earth.

The chemical composition of the Moon also is different in different places. Soon after the Moon formed, various elements sorted themselves out to form different kinds of rock. The light-coloured highlands are rich in calcium and aluminum, while the dark-coloured maria contain less of those elements and more titanium, iron and magnesium.

Scientists can estimate the volume of the Moon by dividing its mass by its volume. Using this formula, they have calculated a figure of 3.34 grams per cubic centimetre. For comparison, the density of water is 1 gram per cubic centimeter, and the density of a typical Earth rock is about 3 grams per cubic centimeter.

Sensitive instruments placed on the lunar surface by the Apollo astronauts are still recording the tiny vibrations caused by meteorite impacts on the surface of the Moon and by small moonquakes deep within it. These vibrations provide the data from which scientists determine what the inside of the Moon is like.

About 3,000 moonquakes are detected each year. All of them are very weak by terrestrial standards. The average moonquake releases about as much energy as a firecracker, and the whole Moon releases less than one-ten-billionth of the earthquake energy of the Earth. The moonquakes occur about 600 to 800 kilometres deep inside the Moon; much deeper than almost all the quakes on our own planet. Certain kinds of moonquakes occur at about the same time every month, suggesting that they are triggered by repeated tidal strains as the Moon moves in its orbits around the Earth.

A picture of the inside of the Moon has slowly been put together from the records of thousands of moonquakes, meteorite impacts, and the deliberate impacts of discarded Apollo rocket stages onto the Moon. Four nuclear powered seismic stations were installed during the Apollo project to collect seismic data about the interior of the Moon. There is only residual tectonic activity due to cooling and tidal forcing. It is thought that the Moon is not uniform inside but is divided into a series of layers just like the Earth, although the layers of the Earth and Moon are different. The outermost part of the Moon is a crust about 60 kilometres thick (at the centre of the near side) probably composed of calcium- and aluminium-rich rocks like those found in the highlands. Beneath the crust is a thick layer of denser rock (the mantle) which extends down to more than 800 kilometres. If the crust is uniform over the Moon, it would constitute about 10% of the Moon's volume as compared to the less than 1% on Earth. The seismic determinations of a crust and mantle on the Moon indicate a layered planet with differentiation by igneous processes. There is no evidence for an iron-rich core unless it were a small one. Seismic information has influenced theories about the formation and evolution of the Moon.

The Moon does not now have a magnetic field like the Earth's, and so the most baffling and unexpected result of the Apollo Program was the discovery of preserved magnetism in the many of the old lunar rocks. One explanation is that the Moon had an ancient magnetic field that somehow disappeared after the old lunar rocks had formed. As there is no native magnetic field on the Moon so there are no magnetic poles on the surface (Source)

One reason we have been able to learn so much about the Moon's interior is that the instruments placed on the Moon by the Apollo astronauts have operated much longer than expected. Some of the instruments originally designed for a one-year lifetime, have been operating since 1969 and 1970. This long operation has provided information that we could not have obtained from shorter records.

The long lifetime of the heat flow experiments set up by the Apollo 15 and 17 missions has made it possible to determine more accurately the amount of heat coming out of the Moon . This heat flow is a basic indicator of the and composition of the inside of the Moon. The new value, about two-thirds of the value calculated from earlier data, is equal to about one-third the amount of heat now coming out of the inside of the Earth. As a result, we can now produce better models of what the inside of the Moon is like.

As they probed the lunar interior, the Apollo instruments have provided information about the space environment near the Moon. For example, the sensitive devices used to detect moonquakes have also recorded the vibrations caused by the impacts of small meteorites onto the lunar surface. We now have long-term records of how often meteorites strike the Moon, and we have learned that these impacts do not always occur at random. Some small meteorites seem to travel in groups. Several such swarms, composed of meteorites weighing a few kilograms each, struck the Moon in 1975. The detection of such events is giving scientists new ideas about the distribution of meteorites and cosmic dust in the solar system.

The extended lifetime of the Apollo instruments has also made several co-operative projects possible. For example, our instruments were still making magnetic measurements at several Apollo landing sites when, elsewhere on the Moon, the Russians landed similar instruments attached to their two automated lunar roving vehicles (Lunokhods). By making simultaneous measurements and exchanging data, American and Russian scientists have not only provided a small example of international co-operation in space, but they have jointly obtained a better picture of the magnetic properties of the Moon and the space around it.

No. radioactive dating of crustal material from Apollo lunar rocks absolutely dates the formation of the moon from about 60 million years after the formation of the earth some 4.4 billion years ago. No other 'clock' matters when you can perform this type of analysis.

Long before the Apollo Program scientists could see that the Moon's surface was complex. Earth-based telescopes could distinguish the level maria and the rugged highlands. We could recognise countless circular craters, rugged mountain ranges and deep winding canyons or rilles. Scientists estimate that the Moon has over 30 thousand billion craters.

Like the four inner planets, the Moon is rocky. Its pockmarked with craters formed by asteroid impacts millions of years ago. Because there is no weather, the craters have not eroded. The Apollo and Luna missions returned 382 kilograms of rock and soil from which three major surface materials have been studied: the regolith, the maria, and the terrae. Micrometeorite bombardment has thoroughly pulverised the surface rocks into a fine-grained debris called the regolith. The regolith, or lunar soil, is unconsolidated mineral grains, rock fragments, and combinations of these which have been welded by impact-generated glass. It is found over the entire Moon, with the exception of steep crater and valley walls. It is 2 to 8 metres thick on the maria and may exceed 15 metre on the terrae, depending on how long the bedrock underneath it has been exposed to meteoritic bombardment. It contains no water or organic material however, and is totally different from soils formed on Earth by the action of wind, water and life.

The lunar soil is something entirely new to scientists, for it could only have been formed on the surface of an airless body like the Moon. The soil has been built up over billions of years by the continuous bombardment of the unprotected Moon by large and small meteorites, most of which would have burned up if they had entered the Earth's atmosphere.

These meteorites form craters when they hit the Moon. Tiny particles of cosmic dust produce microscopic craters perhaps 1/1000 of a millimetre across, while the rare impact of a large body may blast out a crater many kilometres in diameter. Each of these impacts shatters the solid rock, scatters material around the crater, and stirs and mixes the soil. As a result, the lunar soil is a well-mixed sample of a large area of the Moon, and single samples of lunar soil have yielded rock fragments whose source was hundreds of kilometres from the collection site.

However, the lunar soil is more than ground-up and reworked lunar rock. It is the boundary layer between the Moon and outer space, and it absorbs the matter and energy that strikes the Moon fro the Sun and the rest of the universe. Tiny bits of cosmic dust and high-energy atomic particles that would be stopped high in the Earth's protective atmosphere rain continually onto the surface of the Moon. The surface is glassier due to the superheated nature of the asteroid ejecta and the subsequent quick cooling.

The fine, powdery material that makes up the lunar surface is most commonly called 'regolith' in scientific and engineering literature. The Apollo missions were surprised by the difficulty of extracting subsurface samples. While the top was powdery and soft, attempts to drill into the surface and extract subsurface material resulted in seizing of drill tubes which could not be removed and had to be abandoned. It is now thought that underneath the very top layers, lunar soil is actually more dense than equivalent Earth soils at the same depth (Source)

Scientists now think that the solar system first came into being as a huge, whirling, disk-shaped cloud of gas and dust. Gradually the cloud collapsed inward. The central part became masssive and hot, forming the Sun. Around the Sun, the dust formed small objects that rapidly collected together to form the large planets and satellites that we see today.

By carefully measuring the radioactive elements found in rocks, scientists can determine how old the rocks are. Measurements on meteorites indicate that the formation of the solar system occurred 4.6 billion years ago. There is chemical evidence in both lunar and terrestrial rocks that the Earth and Moon also formed at that time. However, the oldest known rocks on Earth are only 3.8 billion years old, and scientists think that the older rocks have been destroyed by the Earth's continuing volcanism, mountain-building, and erosion.

The Moon rocks fill in some of this gap in time between the Earth's oldest preserved rocks and the formation of the solar system. The lavas from the dark maria are the Moon's youngest rocks, but they are as old as the oldest rocks found on Earth, with ages of 3.1 to 3.8 billion years. Rocks from the lunar highlands are even older. Most highland samples have ages of 4.0 to 4.3 billion years. Some Moon rocks preserve traces of even older lunar events. Studies of these rocks indicate that widespread melting and chemical separation were going on within the Moon about 4.4 billion years ago, or not long after the Moon had formed.

One of the techniques used to establish this early part of lunar history is a new age-dating method (involving the elements neodymium and samarium) that was not even possible when the first Apollo samples were returned in 1969. The combination of new instruments and careful protection of the lunar samples from contamination thus make it possible to understand better the early history of the Moon.

Even more exciting is the discovery that a few lunar rocks seem to record the actual formation of the Moon. Some tiny green rock fragments collected by the Apollo 17 astronauts have yielded an apparent age of 4.6 billion years, the time at which scientists think that the Moon and the solar system formed. Early in 1976, scientists identified another Apollo 17 crystalline rock with the same ancient age. These pieces may be some of the first material that solidified from the once-molten Moon.

The first few hundred million years of the Moon's lifetime were so violent that few traces of this time remain. Almost immediately after the Moon formed, its outer part was completely melted to a depth of several hundred kilometres. While this molten layer gradually cooled and solidfied into different kinds of rocks, the Moon was bombarded by huge asteroids and smaller bodies. Some of these asteroids were the size of small US states, like Rhode Island or Delaware, and their collisions with the Moon created huge basins hundreds of kilometres across.

The catastrophic bombardment died away about 4 billion years ago, leaving the lunar highlands covered with huge overlapping craters and a deep layer of shattered and broken rock. As the bombardment subsided, heat produced by the decay of radioactive elements began to melt the inside of the Moon at depths of about 200 kilometres below its surface. Then, for the next half billion years, from about 3.8 to 3.1 billion years ago, great floods of lava rose from the inside the Moon and poured out over its surface, filling in the large impact basins to form the dark parts of the Moon that we see today.

As far as we know, the Moon has been quiet since the last lavas erupted more than 3 billion years ago. Since then, the Moon's surface has been altered only by rare large meteorite impacts and by atomic particles from the Sun and the stars. The Moon has preserved featured formed almost 4 billion year ago, and if men had landed on the Moon a billion years ago, it would have looked very much as it does now. The surface of the Moon now changes so slowly as a result of there being no atmosphere that the footprints left by the Apollo astronauts will remain clear and sharp for centuries.

This preserved ancient history of the Moon is in sharp contrast to the changing Earth. The Earth still behaves like a young planet. Its internal heat is active, and volcanic eruptions and mountain-building have gone on continuously as far back as we can decipher the rocks. According to new geological theories, even the present ocean basins are less than about 200 million years old, having formed by the slow separation of huge moving plates that make up the Earth's crust.

Scientists still cannot decide among these three theories. However, we have learned that the Moon formed as a part of our solar system and that it has existed as an individual body for 4.6 billion years. Separation from the Earth is now considered less likely because there are many basic differences in chemical composition between the two bodies, such as the absence of water on the Moon. But the other two theories are still evenly matched in their strengths and weaknesses. We will need more data and perhaps some new theories before the origin of the Moon is settled.

It might seem that the active, inhabited Earth has nothing in common with the quiet, lifeless Moon. Nevertheless, the scientific discoveries of the Apollo Program have provided a new and unexpected look into the early history of our own planet. Scientists think that all the planets formed in the same way, by the rapid accumulation of small bodies into large ones about 4.6 billion years ago. The Moon's rocks contain the traces of this process of planetary creation. The same catastrophic impacts and widespread melting that we recognise on the Moon must also have dominated the Earth during its early years, and about 4 billion years ago the Earth may have looked much the same as the Moon does now.

The two worlds then took different paths. The Moon became quiet while the Earth continued to generate mountains, volcanoes, oceans, an atmosphere and life. The Moon preserved its ancient rocks, while the Earth's older rocks were continually destroyed and recreated as younger ones.

The Earth's oldest preserved rocks, 3.3 to 3.8 billion years old, occur as small remnants in Greenland, Minnesota (US) and Africa. These rocks are not like the lunar lava flows of the same age. The Earth's most ancient rocks are granites and sediments, and they tell us that the Earth already had mountain-building, running water, oceans, and life at a time when the last lava flows were pouring out across the Moon.

In the same way, all traces of any intense early bombardment of the Earth have been destroyed. The record of later impacts remains, however, in nearly 100 ancient impact structures that have been recognised on the Earth in recent years. Some of these structures are the deeply eroded remnants of craters as large as those of the Moon and they give us a way to study on Earth the process that once dominated both the Earth and Moon.

Lunar science is also making other contributions to the study of the Earth. The new techniques developed to analyze lunar samples are now being applied to terrestrial rocks. Chemical analyses can now be made on samples weighing only 0.001 gram and the ages of terrestrial rocks can now be measured far more accurately than before Apollo. These new techniques are already helping us to better understand the origin of terrestrial volcanic rocks, to identify new occurrences of the Earth's oldest rocks, and to probe further into the origin of terrestrial life more than 3 billion years ago.

The surface of the Moon is continually exposed to the solar wind, a stream of atoms boiled into space from the Sun's atmosphere. Since the Moon formed, the lunar soil has trapped billions of tonnes of these atoms ejected from the Sun. The soil also contains traces of cosmic rays produced outside our own solar system. These high-energy atoms, probably produced inside distant stars, leave permanent tracks when they strike particles in the lunar soil.

By analyzing the soil samples returned from the Moon, scientists have been able to determine the chemical composition of the matter ejected by the 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 hydrogen 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 10 to 1. Some unexplained process in the Sun's outer atmosphere apparently operates 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 material ejected by the Sun in the past. We now have a unique opportunity to study the past behavior of the Sun. Our very existence depends on the Sun's activity, and by understanding the Sun's past history, we can hope to predict better its future behaviour.

These studies of the lunar soil are only beginning, but what we have learned about the Sun so far is reassuring. Such chemical features as the ratio of hydrogen to helium and the amount of iron in solar material show no change for at least the past few hundred thousand years. The lunar samples are telling us that the Sun, in the recent past, has behaved very much as it does today, making us optimistic that the Sun will remain the same for the foreseeable future.

As far as the ancient history of the Sun is concerned, the most exciting lunar samples have not yet been fully examined. During the Apollo 15, 16, and 17 missions, three long cores of lunar soil were obtained by drilling hollow tubes into the soil layer. These core tubes penetrated as much as three metres deep. The layers of soil in these cores contain a well-preserved history of the Moon and the Sun that may extend as far back as one and a half billion years. No single terrestrial sample contains such a long record, and no one knows how much can be learned when all the cores are carefully opened and studied. Certainly we will learn more about the ancient history of the Sun and Moon. We may even find traces of the movement of the Sun and the solar system through different regions of our Milky Way Galaxy.

Although the Apollo Program officially ended in 1972, the active study of the Moon goes on. More than 125 teams of scientists are studying the returned lunar samples and analyzing the information that continues to come from the instruments on the Moon. Less than 10 % of the lunar sample material has yet been studied in detail, and more results will emerge as new rocks and soil samples are examined.

The scientific results of the Apollo Programme have spread far beyond the Moon itself. By studying the Moon, we have learned how to go about the business of exploring other planets. The Apollo Programme proved that we could apply to another world the methods that we have used to learn about the Earth. Now the knowledge gained from the Moon is being used with the photographs returned by Mariner 9 and 10 to understand the histories of Mercury and Mars and to interpret the data returned by the Viking mission to Mars.

The first half-billion years of the Moon's lifetime were dominated by intense and widespread melting, by catastrophic meteorite impacts and by great eruptions of lava. Now close-up pictures of the planets Mercury and Mars show heavily-cratered regions and definite volcanic structures, indicating that these planets also have been affected by the same processes that shaped the Moon when it was young. Such episodes of early bombardment and volcanic eruptions seem to be part of the life story of planets. Our own Earth must have had a similar history, even though the traces of these primordial events have been removed by later changes.

One way to learn about the Earth's magnetic field is to study the magnetic field of other planets. In this respect, the Moon is surprising. It has no magnetic field today, but its rocks suggest that it had a strong magnetic field in the past. If the Moon did have an ancient magnetic field that somehow 'switched off' about 3 billion years ago, then continued study of the Moon may help us learn how magnetic fields are produced in other planets, including our own.

Even the lifeless lunar soil contains simple molecules formed by reaction between the soil particles and atoms of carbon, oxygen, and nitrogen that come from the Sun. In a more favourable environment, these simple molecules might react further, forming the more complex molecules ('building blocks') needed for the development of life. The sterile Moon thus suggests that the basic ingredients for life are common in the universe, and further study of the lunar soil will tell us about the chemical reactions that occur in space before life develops.

Samples have been taken from only eight places on the Moon, with six Apollo and two Luna landings. The chemical analyses made from orbit cover only about a quarter of the Moon's surface. We still know little about the far side of the Moon and nothing whatever about the Moon's polar regions.

Orbiting Apollo spacecraft used a laser device to measure accurately the heights of peaks and valleys over much of the lunar surface. From these careful measurements, scientists have learned that the Moon is not a perfect sphere. It is slightly egg-shaped, with the small end of the egg pointing toward the Earth and the larger end facing away from it.

There are other major differences between the two sides of the Moon. The front (Earth-facing side), which is the small end of the egg, is covered with large dark areas which were produced by great eruptions of basalt lava between 3 and 4 billion years ago. However, the far side of the Moon is almost entirely composed of light-colored, rugged, and heavily cratered terrain identical to the highland regions on the front side, and there are only a few patches of dark lava-like material. Furthermore, the Moon's upper layer (the crust), is also uneven. On the front side, where the maria are, the lunar crust is about 60 kilometres thick. On the back side, it is over 100 kilometres thick.

We still do not know enough to explain these different observations. Perhaps, the Moon points its small end toward the Earth because of tidal forces that have kept it trapped in that position for billions of years. Perhaps lava erupted only on the front side because the crust was thinner there. These differences could tell us much about the early years of the Moon, if we could understand them.

We know that there were great volcanic eruptions on the Moon billions of years ago, but we do not know how long they continued. To understand the Moon's history completely, we need to find out if the inside of the Moon is still hot and partly molten. More information about the heat flow coming out of the Moon may help provide an answer.

This question is critical to solving the puzzle of ancient lunar magnetism, At the moment, we have so little data that we can neither rule out the possible existence of a small iron core nor prove that one is present. If we can determine more accurately the nature of the Moon's interior and make more measurements of the magnetism on the lunar surface, we may find a definite answer to the baffling question.

The youngest rocks collected from the Moon were formed 3.1 billion years ago. We cannot determine how the Moon heated up and then cooled again until we know whether these eruptions were the last or whether volcanic activity continued on the Moon for a much longer time.

Unexplained occurrences of reddish clouds, and mists have been reported on the Moon's surface for over 300 years. These 'lunar transient events', as they are called, are still not explained. It is important to determine what they are, because they may indicate regions where gases and other materials are still coming to the surface from inside the Moon.