Introduction to How the Moon Was Born

The moon has been an object of wonder and the subject of art and poetry since people first kept written records or decorated their caves with paintings. Many ancient civilizations based their calendars on the cycles of the moon. Lunar tides in seas and harbors helped determine when ships could set off on journeys or float safely into port. Before the development of electric lights and gas lamps, the moon lit the way for travelers at night. People imagined they saw a "man in the moon" and other shapes in the lunar surface, speculated about what the moon was made of, and wondered if it was inhabited. Scientists, too, have been curious about the moon and have scrutinized its cratered landscape with their telescopes. But for a long time, few of them gave much thought to how the moon came to be.

By 1998, the attitudes of scientists had changed. Not only were more researchers interested in the origin of the moon, but nearly all had come to believe in one particular model of lunar origin—the giant-impact theory. According to this theory, the moon was formed when a body about the size of the planet Mars—or a few times larger—struck the young Earth with cataclysmic force. Much support for this theory has come from the study of rock and soil samples from the moon, brought to Earth by U.S. astronauts. In 1998, a satellite called Lunar Prospector, in orbit around the moon, supplied evidence consistent with the theory, as did computer simulations of the giant impact.

Early Theories

Scientists' interest in the origin of the moon in the mid- and late 1900's stood in sharp contrast to attitudes about the moon during the early days of astronomy. The Italian astronomer and physicist Galileo Galilei first gazed at the moon in 1609 with the newly invented telescope, but he offered no theories about the moon's history, nor did most of the other astronomers who came after him.

It was only in 1879 that the British mathematician George H. Darwin, son of the great naturalist Charles Darwin, proposed one of the first theories of lunar origin. According to Darwin's “fission” theory, the material that became the moon was spun off from the very young, fast-spinning Earth, which was still in a molten state. Darwin's theory had little influence on astronomers for almost 100 years, in large part because they continued to be uninterested in the moon's origin.

Scientists finally started to investigate the question in the 1950's. By 1952, a “capture” theory of the moon's origin, proposed by an American chemist, Harold C. Urey, had become the prevailing view. Urey believed that the moon was a primordial (ancient) body that formed at the dawn of the solar system and was later captured by the Earth and pulled into its present orbit.

Urey's views had great influence on the planning of the National Aeronautics and Space Administration's (NASA) Apollo program, the series of manned flights to the moon that took place between 1969 and 1972. Many scientists believed that the moon rock samples that Apollo astronauts were to bring back would reveal the minerals and compounds—incorporated into the primordial, virtually unchanging moon—that were present when the solar system formed. They also expected the samples to help prove or disprove the capture theory. If the moon formed far away from the Earth, they reasoned, it might have a different chemical composition than the Earth.

But long before the lunar samples were gathered, some scientists had doubts about the capture theory. If the moon had formed far away in the solar system, mathematical calculations showed, it would have had to follow a long, looping orbit to reach the Earth, which it would have zipped past at high speed. It would not be possible, the scientists calculated, for a moon captured from such an orbit to end up in its present position, with the Earth and moon spinning at their current rates, once every 24 hours and once every 28 Earth days, respectively.

Some scientists proposed in the early 1960's that the moon formed at roughly the same time and at the same distance from the sun as the Earth, though not alongside it. The infant moon would have been orbiting the sun at nearly the same speed as the Earth and would sometimes move slowly past it. The moon would then have been easy to capture. If this scenario was true, the moon should be made of the same materials as the Earth.

Scientists Meet to Debate Lunar Origins

By 1964, the question of how the moon originated had generated sufficient interest to merit a conference devoted entirely to that subject. At that meeting in New York City, much attention was paid to the fission theory proposed by George Darwin. Darwin's suggestion that the moon was created from molten material spun off by the Earth seemed plausible for a time when our planet rotated much faster than it does now, possibly once every 4 hours instead of once every 24 hours. By the time of the conference, however, calculations had shown that even such a high rotational speed would not have been sufficient to form the moon. Friction in the molten rock would not have permitted a bulge of matter to rise high enough to be flung free of Earth's gravity.

In light of that finding, proponents of the fission theory at the conference suggested another possibility. The material that formed the moon, they said, might have been spun off when the Earth was rotating not once every 4 hours but once every 2.1 hours. If a molten Earth had ever rotated that fast, they pointed out, its mechanical strength and gravity would simply not have been able to hold the planet together. The Earth would have hurled great amounts of matter into space in a process called rotational instability.

Proponents of the modern fission theory calculated exactly how such a process could take place. The birth of the moon would not have occurred as a catastrophic outburst, such as an explosion, but as a progressive change in Earth's shape. First, the Earth's rapid rotation would have flattened the planet at the poles, deforming it into an oblate shape—wider at the equator than across the poles. Next, the spinning, flattened Earth would have produced a neck of material from a point on its equatorial bulge. The neck would have flown off into space as a large blob trailed by smaller blobs that would soon have fused into one. But a major flaw in the fission theory, other scientists noted, was that in order for fission to occur, the Earth would have been spinning so fast that it would still be turning much more rapidly than it is today.

The Coaccretion Theory of the Moon's Origin

Another theory for the origin of the moon discussed at the 1964 conference was that the moon formed together with the Earth at the same time and in the same place. Just as the Earth was believed to have been created by accretion (the accumulation of solid particles and larger objects) about 4.6 billion years ago when the solar system was taking shape, so, according to this theory, was the moon.

But the “coaccretion” theory presented problems as well. Astronomers had calculated the moon's density by using a formula based on the moon's mass, as determined by its distance from the Earth and its orbital velocity, divided by its volume. The moon's density, calculated to be 3.3 times the density of liquid water, is much less than the density of the Earth, which is 5.5 times the density of water. If the Earth and the moon were formed from the same cloud of matter, at the same distance from the sun, how could their composition be so different?

In addition, other doubts plagued scientists as well. If the moon formed near Earth by the same processes that formed the other planets, why don't the other planets of the inner solar system have large satellites like the moon? Mercury and Venus have no moons, and Mars has only two tiny satellites that may be captured asteroids.

For such reasons, some scientists by the time of the 1964 conference were already doubtful about the validity of the coaccretion theory as well as the earlier theory that the moon formed at about the same distance from the sun as the Earth. Instead, most participants at the meeting favored some version of Urey's theory in which the moon was formed elsewhere and then captured by the Earth, or some version of the fission theory. After all, they noted, because fission would take place in the upper layers of the Earth, which are much less dense than its heavy iron core, fission could account for the moon's low density and apparent lack of a significant iron core. Though fission caused by Earth's rapid rotation had been ruled out, the scientists speculated that a portion of the Earth might still have been flung off to form the moon in some other way.

A Revised Capture Scenario

In 1972, Ernst J. Opik, an Estonian-born astrophysicist who worked in Northern Ireland, proposed a totally different theory. Opik suggested that a primordial object may have streaked past the Earth in the early solar system and come within Roche's limit, a boundary located about 18,500 kilometers (11,500 miles) from the center of the Earth. Inside Roche's limit, the Earth's gravity can pull a weak body apart. (Every planet has its own Roche's limit, which varies according to the planet's mass.) Some debris from the disintegrating body may have gone into orbit around the Earth and then coalesced to form the moon.

Scientists agreed that Opik's “disintegrative capture” theory convincingly explained how the moon could have been made from different material than the Earth—material from a distant part of the solar system. And the theory provided an answer to the question of how a large object—or at least part of it—could have been pulled into orbit around the Earth, a test that the previous capture theory had failed. Nonetheless, by 1985, researchers had made detailed calculations showing that a fast-moving object would not have spent enough time within Roche's limit to be pulled apart and leave debris around the Earth before it flew on. Thus, until better calculations might somehow yield contrary results, the disintegrative capture theory was shelved.

New Evidence From the Moon

Even while scientists were evaluating Opik's theory, the first physical evidence from the moon had become available. From 1969 to 1972, Apollo astronauts landed on the moon six times, collecting more than 2,000 rock and soil samples. Researchers who analyzed the samples found that the specimens shared a unique characteristic: The relative amounts of oxygen isotopes (atoms of the same element that differ in atomic weight) in the moon rocks proved to be almost identical to those found in the Earth's mantle, a thick layer of hot rock between the Earth's outer core and its crust. The relative amounts of these isotopes are believed to have varied in different parts of the solar system, so the oxygen-isotope ratios strongly implied that the moon and Earth's mantle had a common origin.

The moon rocks were also extremely dry, whereas a great many rock types and minerals on Earth contain water. Compared with Earth rocks, there were very few other substances in moon rocks that, like water, are volatile (vaporize easily).

Finally, the dating of radioactive isotopes in the lunar samples told the scientists how long it had been since minerals in the samples had solidified from a molten state. (A radioactive isotope is one that decays spontaneously to become another substance. For example, an isotope of hafnium decays to form an isotope of tungsten.) By comparing the relative number of such isotopes, scientists can tell the age of the sample. The dating process showed that all or most of the moon had been molten at one time, solidifying by 4.4 billion years ago. This finding, in particular, convinced most scientists that the moon could not be the primordial body that Urey thought had been captured by the Earth.

Thus, studies of the moon rocks clearly showed that the moon could not be an intact, or nearly intact, remnant from the earliest days of the solar system. The evidence suggested that the moon was born from the Earth's mantle by a process that released enormous heat. The heat would have driven off the volatiles and vaporized the rock.

The Giant-impact Theory

Influenced by the information from the moon rocks, in the mid-1970's two teams of planetary scientists separately proposed what is now called the “giant-impact” theory of the origin of the moon. The groups were led by A. G. W. “Al" Cameron and William R. Ward at Harvard University in Cambridge, Massachusetts, and William K. Hartmann and Donald R. Davis at the Planetary Science Institute in Tucson, Arizona. The researchers suggested that a huge Mars-sized object struck the Earth, blasting away enough of the planet's crust and mantle to create the moon from its debris. According to the theory, the object struck the Earth a glancing blow, rather than hitting it head-on. The blow set the Earth spinning faster than before. This may explain why the angular momentum of the Earth-moon system (a measure of the rate at which both bodies are rotating and orbiting) is unusually high, compared with other planets and moons in the solar system.

At first, many scientists were reluctant to accept the giant-impact theory, because the theory was based on a single event—a catastrophic collision—that most researchers believed had occurred very rarely in the solar system. However, the more that lunar and planetary scientists thought about the theory, the better they liked it. Surveys of lunar geography made with both Earth-based telescopes and telescopes and cameras on spacecraft showed that strikes on the moon by asteroids and meteoroids (metal or rocky objects smaller than asteroids) after it had formed and solidified had produced many huge craters, called impact basins. Why couldn't the impact of an even more massive object on the Earth have made the moon itself?

Collision With A Protoplanet?

At the same time, new findings about the formation of the solar system suggested that large numbers of massive objects, called protoplanets or planetesimals, had been orbiting the sun at the time the Earth formed. Gradually, scientists came to believe, these objects collided with one another and were broken up, or massed together to make the planets, or were flung out of the solar system.

At the second major conference on the origin of the moon, held in Kona, Hawaii, in 1984, the giant-impact theory was the center of attention. It became the prevailing view of most astronomers.

As more researchers tested the giant-impact theory, they had to contend with the fact that many of the early calculations of the theory were relatively crude. Although the calculations showed that an enormous collision could have blown enough matter out of the Earth (and from the vaporized outer layers of the impacting object itself) to make the moon, they did not trace the events in detail. So while the theory was very promising, astronomers did not know how realistic it was.

The Help of Computer Simulations

During the 1990's, a new generation of planetary scientists, including Robin M. Canup, now at the Southwest Research Institute in Boulder, Colorado, developed the theory further. They made improved calculations with better computers and more advanced programs that more realistically simulated cosmic collisions and their consequences. By 1996, Canup and her associate Larry W. Esposito of the University of Colorado in Boulder had demonstrated that the giant-impact theory required a much more massive impacting object than previously suspected. They found that the object had to be at least twice as massive as Mars, or even larger, for the debris from the collision to coalesce into the single moon that we have today. Otherwise, the calculations indicated, there would be several moons circling the Earth.

Though Canup and Esposito's calculations were more sophisticated than those of the past, they still were not precise enough. The team used a simplifying assumption, called gas dynamic theory, that represented the processes in the formation of the moon after the giant impact on Earth as though the debris were all gas. In reality, even though the material hurled up by the collision was so hot that it took the form of vapor, it soon cooled and condensed. So the actual conditions must have been much more complicated than those represented by the gas dynamic calculations, with clumps of rock cooling, solidifying, and colliding. In fact, the various proponents of the giant-impact theory had shown only that an enormous collision could have produced the necessary conditions to form the moon. But they had not proven that such an event had actually occurred.

In 1997, the Japanese geophysicist Shigeru Ida at the Tokyo Institute of Technology, together with Canup and Canup's University of Colorado colleague Glen R. Stewart, presented a more advanced simulation of the impact. They used a mathematical technique called the N-body method. This model recreated the processes inside the impact debris cloud as a series of interactions between a large number (“N”) of individual clumps or objects that collided with one another and were affected by one another's gravitational forces. Thus, the N-body method represented real processes in space much better than the gas dynamic technique or other methods did. The Ida team found that the debris from a giant impact with the Earth would indeed have formed a large moon and that this process might have taken as little as one year. Refined calculations by the team and others showed that the most likely mass for the body that collided with Earth was three times that of Mars.

Further Support For the Giant-impact Theory

Besides conducting computer simulations, researchers pursued other ways of testing the giant-impact theory. During 1998, Lunar Prospector, NASA's first moon mission since Apollo, was launched to gather a variety of data about the moon. As Prospector orbited just 100 kilometers (60 miles) above the moon's surface, researchers used Earth-based radio telescopes to monitor the moon's gravitational pull on the spacecraft. By mapping the results, the scientists were able to get a far more accurate picture of how the moon's mass is distributed than they had had before. The researchers calculated that the moon has a core, most likely composed of iron, with a radius of 220 kilometers to 450 kilometers (135 miles to 280 miles). This is well within the range expected if the moon had formed as the result of a devastating impact on Earth and thus is consistent with the giant-impact theory.

In December 1998, at an origin-of-the-moon conference in Monterey, California, Canup and planetary scientists Craig Aignor of the University of Colorado at Boulder and Harold Levison of the Southwest Research Institute announced further calculations in support of the giant-impact theory. They used a computer method called “symplectic integration,” which allowed them to simulate much greater lengths of time and thousands more sun-orbiting objects than ever before possible. Canup's group found that impacts on Earth, such as the one that may have formed the moon, were almost certainly common during the first 100 million years after the birth of the solar system. This conclusion was among the strongest indications available to astronomers that the giant-impact theory was not based on an unusual accident of nature, a finding that made the theory even more credible.

Not A Trouble-free Theory

Nevertheless, the theory faced the same stumbling block that early theories did. The impact of a body three times the mass of Mars would have set the Earth spinning so rapidly that, even today, it would be turning faster than it is. Cameron and others proposed variations of the theory to get around this problem. One possibility they explored was that another large object might have struck the Earth from the opposite direction after the moon was formed, slowing the planet's spin.

Despite the one remaining difficulty, astronomers in 1999 considered the giant-impact theory the best explanation for the origin of the moon. They think they can now sketch fairly accurately how the moon was born and how it developed over time into the body that today illuminates our night sky.

How Scientists Think the Impact Occurred

Here's the scenario: A little more than 4.5 billion years ago, a young, hot Earth, constantly bombarded by thousands of asteroid-sized objects, had grown to almost its present size. Most of the iron it contained had sunk toward the center, forming a huge iron core that was much denser than the rest of the planet. Surrounding the molten core was a slowly hardening mantle of lighter rock.

Suddenly, a round, fast-moving, fully formed planet the size of Mars or perhaps larger, with its own iron core and rocky mantle, loomed from space. Traveling at a speed of 40,000 kilometers (25,000 miles) per hour, it struck the Earth a glancing blow. The object's kinetic energy (energy of motion) was instantly converted into heat that vaporized much of the object's mantle along with a good part of the Earth's. The collision produced a great, expanding cloud of fiery vapor composed of gasified rock. Thrown to a height of perhaps 22,500 kilometers (14,000 miles), much of the vapor formed a diffuse cloud that orbited the Earth. At the same time, most of the iron core of the body that struck Earth looped around the planet and struck again, this time penetrating and merging with the Earth.

Over the course of about a year, the debris in the cloud condensed into solid particles and formed a ring around the Earth. The particles slowly clumped together, forming tiny rocks, then bigger and bigger ones. For a time there were thousands of these “moonlets” orbiting Earth. But over a period of less than 100 years, the larger moonlets swept up the smaller ones, until they all merged into one large body. The ring was gone, and in its place there was the infant moon. At that point, the Earth-moon system resembled a double planet. The moon circled Earth rapidly at a small fraction of its present distance, and Earth spun rapidly, thanks to the blow it had suffered in the impact.

The Later Development of the Moon

The newborn moon was at first covered by magma (molten rock). This feature, which geologists call the magma ocean, was at least a few hundred kilometers deep. The magma ocean was created by heat from the many large final impacts of moonlets on the by-then largely formed moon. Liquid iron sank to the moon's center, and electrical currents in the molten core generated a magnetic field. As the magma ocean cooled, about 4 billion years ago, it solidified. Heavier minerals sank, while lighter ones rose to form a crust.

Even as the lunar surface hardened, it was being peppered by meteoroids, asteroids, and comets. The largest of these objects produced huge basins up to 2,500 kilometers (1,600 miles) across. Later, heat released by radioactivity deep inside the moon caused magma to well up from the interior, partially filling and leveling many of the basins.

Bodies large enough to carve out basins stopped striking the moon by about 3.2 billion years ago, but smaller objects continued to hit the surface, forming many craters. At the centers of some craters the rebound of surface material after the impact created a mountain. The effects of impacts also produced mountain chains at the boundaries of many basins and craters. No lunar mountains were formed by the folding and upthrust of surface layers, as occurs to form many mountains on Earth. Nor did any large volcanic mountains develop on the moon.

The countless impacts of meteoroids slowly fragmented the lunar surface. This created a regolith (surface layer) of broken rocks and soil particles as deep as 15 meters (50 feet) in some areas.

The Moon As It Is Today

Over 4.4 billion years, lunar tides caused the Earth to slow to its present spin rate of once every 24 hours and the moon to move gradually away from the Earth to its present distance of about 385,000 kilometers (240,000 miles). Even now, the process continues, though more slowly, as the moon recedes from the Earth at 3.75 centimeters (1.5 inches) per year, or about 3.6 meters (4 yards) per century.

The moon has changed little for several billion years. Newer craters have formed atop older ones and the regolith has gradually deepened, but there are no more magma flows–most scientists think the moon long ago became totally cold and solid—and no more huge impacts.

Scientists believe this is probably how the moon was created and how it developed, but even now they aren't certain that they have learned the true history of the moon. The giant-impact theory best fits the evidence we have, but as scientists continue their research, other theories may yet replace it. However, one thing will probably never change—the sense of wonder that people feel when they gaze up at the moon.