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What's New on the Moon?

by Dr. Bevan M. French

In 1969 over a billion people witnessed the "impossible" coming true as the first men walked on the surface of the Moon. For the next three years, people of many nationalities watched as one of the great explorations of human history was displayed on their television screens.

Between 1969 and 1972, supported by thousands of scientists and engineers back on Earth, 12 astronauts explored the surface of the Moon. Protected against the airlessness and the killing heat of the lunar 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 to probe the interior of the Moon. They collected hundreds of pounds of lunar rock and soil, thus beginning the first attempt to decipher the origin and geological history of another world from actual samples of its crust.
Image from NASA Spacelink The initial excitement of new success and discovery has passed. The TV sets no longer show astronauts moving across the sunlit lunar landscape. But here on Earth, scientists are only now beginning to understand the immense treasure of new knowledge returned by the Apollo astronauts.

The Apollo Program has left us with a large and priceless legacy of lunar materials and data. We now have Moon rocks collected from eight different places on the Moon. The six Apollo landings returned a collection weighing 382 kilograms (843 pounds) and consisting of more than 2,000 separate samples. Two automated Soviet spacecraft named Luna-16 and Luna-20 returned small but important samples totalling about 130 grams (five ounces).

Instruments placed on the Moon by the Apollo astronauts as long ago as 1969 are still detecting moonquakes and meteorite impacts, measuring the Moon's motions, and recording the heat flowing out from inside the Moon. The Apollo Program also carried out a major effort of photographing and analyzing the surface of the Moon. Cameras on the Apollo spacecraft obtained so many accurate photographs that we now have better maps of parts of the Moon than we do for some areas on Earth. Special detectors near the cameras measured the weak X-rays and radioactivity given off by the lunar surface. From these measurements, we have been able to determine the chemical composition of about one-quarter of the Moon's surface, an area the size of the United States and Mexico combined. By comparing the flight data with analyses of returned Moon rocks, we can draw conclusions about the chemical composition and nature of the entire Moon.

Thus, in less than a decade, science and the Apollo Program have changed our Moon from an unknown and unreachable object into a familiar world.

What Has the Apollo Program Told Us About the Moon?

What have we gained from all this exploration? Before the landing of Apollo 11 on July 20, 1969, the nature and origin of the Moon were still mysteries. Now, as a result of the the Apollo Program, we can answer questions that remained unsolved during centuries of speculation and scientific study:

(1) Is There Life On The Moon?

Despite careful searching, neither living organisms nor fossil life have been found in any lunar samples. The lunar rocks were so barren of life that the quarantine period for returned astronauts was dropped after the third Apollo landing.

(2) What Is The Moon Made Of?

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 dark regions (called "maria") 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. 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. Again using terrestrial terms, we call these rocks gabbro, norite, and anorthosite.

Despite these similarities, Moon rocks are basically different and it is easy to tell them apart by analyzing 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 1/2-billion-year-old Moon rock looks fresher than water-bearing lava just erupted from a terrestrial volcano.

(3) What Is The Inside Of The Moon Like?

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 kilometers (370-500 miles) 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. The Moon is not uniform inside, but is divided into a series of layers just as the Earth is, although the layers of the Earth and Moon are different. The outermost part of the Moon is a crust about 60 kilometers (37 miles) thick, 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 kilometers (500 miles).

The deep interior of the Moon is still unknown. The Moon may contain a small iron core at its center, and there is some evidence that the Moon may be hot and even partly molten inside.

(4) What Is The Moon's Surface Like?

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 recognize countless circular craters, rugged mountain ranges, and deep winding canyons or rilles.

Because of the Apollo explorations, we have now learned that all these lunar landscapes are covered by a layer of fine broken-up powder and rubble about 1 to 20 meters (3 to 60 feet) deep. This layer is usually called the "lunar soil," although it contains no water or organic material, and it 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 millimeter (1/25,000 inch) across, while the rare impact of a large body may blasts out a crater many kilometers, or miles, 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 kilometers 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.

(5) How Old Is The Moon?

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.

(6) What Is The History Of The 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 kilometers. 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 states, like Rhode Island or Delaware, and their collisions with the Moon created huge basins hundreds of kilometers 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 kilometers (125 miles) 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 that the footprints left by the Apollo astronauts will remain clear and sharp for millions of years.

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.

(7) Where Did The Moon Come From?

Before we explored the Moon, there were three main suggestions to explain its existence: that it had formed near the Earth as a separate body; that it had separated from the Earth; and that is had formed somewhere else and been captured by the Earth.

Scientists still cannot decide among these three theories.

What Has the Moon Told Us About the Earth?

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 recognize 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, 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 recognized 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 (3/100,000 ounce) 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.

What Else Can the Moon Tell Us?

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 percent 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 Program 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 Program 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.

Note for Scientists and Educators

The Lunar Science Institute in Houston, Texas can provide further information about lunar science and about data resources that are available for scientific and educational purposes. In particular, the Institute maintains lists of available books, articles, photographs, maps, and other materials dealing with the Moon and the Apollo missions. For further information, contact:

LUNAR SCIENCE INSTITUTE
Data Center, Code L
3303 NASA Road #1
Houston, TX 77058
Phone (713) 488-5200


taken from WHAT'S NEW ON THE MOON?, B.M. French, NASA 1.19:131, c1978

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