Was the Moon formed when an object impacted Earth early in its history, in a scenario known as the giant impact theory

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ASTRONOMY AND SPACE EXPLORATION

Was the Moon formed when an object impacted Earth early in its history, in a scenario known as the giant impact theory?

Viewpoint: Yes, the giant impact theory provides a compelling and plausible explanation for the formation of the Moon.

Viewpoint: No, while the giant impact theory is widely regarded as a better explanation for the Moon's formation than previous ones, it still has many problems that must be resolved.

Humanity is perennially involved in a quest for origins, whether our own, that of our universe, or of our own planet. In the case of Earth and its only natural satellite, the Moon, this involves peering back some four and one-half billion years into the past. While the age of the Sun, the solar system, and of Earth is fairly well determined, the origin of the Moon is still uncertain. This debate is, however, an example of a scientific problem where several competing concepts have been largely defeated by observation and where one theory—the giant impact theory—has gradually gained increasing acceptance.

Both of the following essays discuss three competing explanations for the origin of the Moon, which fell from grace in the 1970s, largely as a result of direct exploration of the Moon by the various Apollo missions. Unlike most celestial objects that can only be observed telescopically, humans have actually visited the Moon. And as is often the case, the observations did a marvelous job of foiling the explanations of its origins that had previously been advanced. In response to this, the giant impact theory (GIT) was advanced in the mid-1970s. This theory proposes that an object collided with Earth early in its history, throwing substantial debris into orbit about the planet that eventually coalesced to become the Moon. As the articles point out, this theory is not without its problems, but it is currently the best available explanation for the origin of the Moon. Because this debate is an example of one where a relatively new idea has gained widespread but by no means universal acceptance, it is particularly illustrative of just what is meant by the word theory.

Scientists attempt to explain an observed phenomenon by first formulating a hypothesis, which is a suggestion—educated or otherwise—for the mechanism producing the phenomenon. In the case of the GIT, the phenomenon is "Earth has a large, natural satellite, called the Moon," the question is "Where did the Moon come from?" and the hypothesis might be phrased "The Moon formed after a large object impacted Earth early in its history." At this stage, there is nothing to support or reject the hypothesis, it is simply an idea.

Having formulated a hypothesis, the scientist must then prove (or disprove) it as convincingly as possible. If observations, experiments, and (in recent times) computer modeling support the hypothesis, it may eventually be advanced as a theory. The broader the evidence in support of a hypothesis, the more convincing and widely accepted the theory is like to become.

Hypotheses and theories may be developed in response to observations, or in anticipation of them. A classic example of a theory developed in response to observation is Isaac Newton's theory of universal gravitation, which Newton developed to explain observed motions of the planets. For macroscopic objects moving in a moderate gravitational field, Newton's theory of gravitation works beautifully. It not only explains motion, but it is predictive—it can be used to project the motion of an object moving under the influence of gravity into the future.

Despite this, Newton's theory—or any theory, for that matter—is not fact. It is an intellectual construct that explains a specified set of observations extremely well; so well, in fact, that it is usually described as a law, a generalization that has stood the test of time, is continuously confirmed by new evidence, and which is more certain than either a hypothesis or a theory. Modern scientists universally accept it as the best available explanation of "classical" motion in a gravitational field. There are circumstances, however, where Newtonian gravity does not quite explain the observations. The motion of Mercury is a famous example; it was only explained fully by Albert Einstein's theory of general relativity. In areas of strong gravitational fields, such as near black holes or even sufficiently close to the Sun, Newtonian theory fails to deal correctly with observed motion. Theories, therefore—even the best ones—have regimes under which they are applicable and regimes under which they fail. Any theory is subject to challenge and reevaluation by scientists at any time. It is the nature of scientific inquiry to test and refine theories and to discard those found lacking.

By their nature, newer theories are always subject to greater skepticism than well-established ones. It would be very difficult to challenge Newton's law of gravitation in the regimes for which it is valid, though the American writer Immanuel Velikovsky did just that (among other things) with his controversial book Worlds in Collision (1950). However, Velikovsky's hypotheses were roundly decried and never considered valid. Less thoroughly tested hypotheses and theories are always open to critical review. The giant impact theory is a recent example, and it has met with the usual amount of healthy skepticism from the scientific community. As the essays below discuss, the theory explains certain observations about the Moon, but fails to explain others. Since not all the issues have been resolved, the GIT has not been generally accepted, despite its many compelling features. As with any developing explanations, the GIT can be presented with varying degrees of skepticism, and the viewpoints below reflect this.

It is also not the case that once a scientist suggests a theory, he or she will have failed in some way if it turns out to be incorrect. Quite the contrary, establishing that something does not work is often as valuable as establishing that it does. Suppose, for example, that extensive computer models proved beyond a doubt that the GIT was incorrect—that there was no way a huge impact early in Earth's history could have created the Moon. This would be an extremely valuable result, for scientists would then know they should work on other hypotheses, and not waste time investigating something known to be wrong.

Scientists are also very willing to make the statement "this is the best we have," and this is generally said about the GIT. However, saying that the GIT is the best explanation for the formation of the Moon is not the same thing as saying it is an adequate explanation. An essential part of science is the regular acknowledgement of areas where knowledge is simply incomplete. Equally important is the willingness to be open to new ideas, and to discard ideas considered incorrect—a difficult thing to do, given the significant intellectual investment scientists make in their work. Some scientists are more satisfied with the GIT than others, and the two essays below present the various viewpoints in this light, with one striking a more skeptical tone than the other. In this debate, it is not so much a question of whether the GIT is correct or incorrect, but how completely it accounts for the available data.

—JEFFREY HALL

Viewpoint: Yes, the giant impact theory provides a compelling and plausible explanation for the formation of the Moon.

People have accorded religious and mystical significance to the Moon since before recorded history. Eventually, some ancient civilizations attempted to quantify various aspects of the Moon. For example, the ancient Babylonians, Chinese, and Egyptians all strived, with varying success, to predict the occurrence of lunar eclipses. Later on, several ancient Greek astronomers attempted to gauge the distances between various celestial bodies, including the distance between Earth and the Moon. But real progress in our understanding of the Moon began during the European Renaissance, which ushered in the birth of modern science. By the twentieth century, there were three principal scientific hypotheses regarding the Moon's origin. They were: (1) the coaccretion hypothesis (referred to by other titles, including the "double planet hypothesis" and the "planetesimal hypothesis"); (2) the fission hypothesis; and (3) the capture hypothesis. These three hypotheses will be briefly outlined below, followed by selected reasons stating why each of them eventually fell into disfavor within the scientific community.

The coaccretion hypothesis asserts that both Earth and the Moon (and indeed, the other planets and moons in the solar system) coalesced from a rotating disk of "planetesimals," which were a host of celestial bodies thought to have ranged in size from inches to miles across. The planetesimals themselves had formed from a cloud, or nebula, of particles circling the "proto-star" that eventually became our Sun. The hypothesis holds that the Moon and Earth formed at about the same time, revolving about their common center of mass, and from the same small planetesimals.

The fission hypothesis, originally proposed in the late 1800s by G. H. Darwin, son of Charles Darwin, hypothesized that the Moon was thrown out (i.e., "fissioned") from Earth's mantle. According to the fission hypothesis, this ejection of material that became the Moon was due to an extremely fast spin of the primordial Earth.

The capture hypothesis held that the Moon formed farther out in the solar system, away from Earth, where there was a low proportion of the heavier elements, most notably iron. The Moon was subsequently captured by the gravitational attraction of Earth when the two bodies passed close by each other.

Lunar Landings and Their Ramifications

Starting in the 1960s, the use of spacecraft allowed the first direct measurements to be made at the lunar surface itself. By the early 1970s the Apollo astronauts had returned to Earth several hundred pounds of lunar rock and soil. The analysis of those lunar samples helped resolve many questions regarding the Moon. In addition to the soil and rock samples, the Apollo missions left behind various measuring devices on the Moon's surface to relay data back to Earth.

The three main pre-Apollo hypotheses (outlined above) about the Moon's origin were largely discredited after the discoveries of Apollo became known. Several of the most important findings used to refute those hypotheses are summarized below.

Measurements showed that the core of the Moon is proportionally much poorer in iron than is Earth's core. Because the coaccretion hypothesis asserts that the Moon and Earth both formed from the same materials, the chemical and elemental composition of the two should be very close. However, this is not the case, as the lack of iron in the Moon demonstrates, and there are other major differences in the chemical composition of the two bodies. Altogether, these findings refute the coaccretion hypothesis.
The fission hypothesis ran into trouble because analysis of the total angular momentum of the present Earth-Moon system is inconsistent with the angular momentum that the original primordial Earth would have had to possess to have "thrown out" the Moon.
Isotope ratios that exist for elements on both Earth and the Moon turn out to be the same for the two bodies. This finding dispels the capture hypothesis, because if the Moon had formed far from Earth, the isotope ratios for elements found on each should have been different. A good illustration of isotopic dissimilarities is that of Mars and Earth. It is now known that oxygen-isotope ratios of Martian soil are different from the oxygen-isotope ratios of Earth soil.

A New Explanation Emerges

In a 1975 paper in the journal Icarus, William K. Hartmann and Donald R. Davis first hypothesized that a sizeable object impacted Earth early in its history. The name of Hartmann and Davis's hypothesis has come to be generally known as the giant impact theory, or GIT. In their paper, the two scientists stated that the primitive Earth experienced an immense collision with another planet-sized body that threw out vast quantities of mantle material from Earth, in turn forming a cloud of debris that encircled Earth. This debris cloud later coalesced to form the Moon.

The most recent version of the giant impact theory proposes that the Moon was formed when an object obliquely (i.e., at an angle) impacted Earth very early in its history—around four and one-half billion years ago. This time frame corresponds to about 50 million years after the birth of the solar system, at a time when Earth is believed to have coalesced to about 90% of its current mass. Thus, Earth was in the latter stages of its development. The collision between Earth and the impacting body (in some theoretical models, this impacting body is proposed to be about the size of the planet Mars) caused an enormously energetic collision. Most of Earth is assumed to have melted, and a small portion of its mass was thrown out into space. This thrown-out debris from the collision formed a ring around Earth, some of which fell back onto Earth; the remainder eventually clumped together to form the Moon. In the GIT model, the Moon was initially close to Earth. Over eons of time the Moon and Earth have slowly moved away from one another, and the rotation rate of Earth has slowed. These processes are still occurring within the Earth-Moon system.

The GIT is able to overcome the shortcomings of the three pre-Apollo hypotheses. Some of its more important features are:

It explains the close distance (approximately 240,000 mi, or 386,000 km) between Earth and the Moon.
It explains why the Moon has such a low density when compared to that of Earth (3.3 g per cu cm for the Moon, 5.5 g per cu cm for Earth) Computer models indicate that debris thrown into orbit from the collision came mainly from the rocky mantles of Earth and the impacting body. For most large objects in our solar system, lighter materials occur away from the core (i.e., in the mantle and crust), while the core itself contains denser materials, such as iron. Earth's iron had already stratified into the core before the impact, leaving only the iron-depleted, rocky mantle to be thrown away from Earth. This ejected iron-poor mantle material, which then formed the Moon.
It explains why the Moon has a low volatile content. When materials were thrown into orbit about Earth, the gaseous volatile materials (such as water, carbon dioxide, and sodium) would have evaporated into space, while less volatile materials would have remained to form the Moon.
It explains the formation of the early magma ocean. The material tossed into orbit around Earth would have been very hot due to the impact with Earth. As this material accumulated to form the Moon it would have become even hotter. This very hot material would produce a thick layer of liquid—the magma ocean that today consists of the lunar anorthosite (the ancient lunar surface rock) crust.
Finally, the giant impact theory explains the high angular momentum of Earth-Moon system. As previously stated, recent versions of the GIT model depict the collision between Earth and the impacting body as being at an oblique angle (i.e., away from vertical). This type of impact would have converted part of the linear momentum of the impacting body to the orbital momentum of the debris ring around Earth, and converted the rest of its momentum to Earth, making it rotate faster.

Earth-Moon Isotope Ratios

Similarities between oxygen isotope ratios on Earth and Moon deserve to be explored in greater depth because of their importance to the giant impact theory. Indeed, a major reason why the this explanation has gained so much support in recent years is the evidence gathered concerning oxygen isotopes of materials from both Earth and the Moon. In the October 12, 2001 issue of the science magazine Science ETH Zurich, researchers showed that the Moon and Earth possess identical ratios of oxygen isotopes. Using laser fluorination, a technique developed only in the 1990s, some 31 samples of various types of lunar rocks returned from the Apollo missions were analyzed. The group performing the research was able to measure the isotope ratios of O16, O17, and O18 (denoting different isotopes of oxygen). This research was important because it was done with a precision 10 times greater than that attainable with previous techniques. According to Uwe Wiechert, the primary author of the article, scientists already knew that Earth and the Moon were similar to each other with respect to oxygen isotope ratios. However, because the instruments and techniques previously used were not very accurate, no one could determine whether or not the two bodies shared basically identical materials. This question now seems to be settled—Earth and Moon did indeed form from the same material.

Evidence gleaned from NASA's Lunar Prospector spacecraft presents additional empirical evidence that the Moon was formed by a massive Earth-and-protoplanet collision. At a presentation of the Lunar and Planetary Sciences Conference in Houston in March 1999, scientists with the Lunar Prospector project said gravity and magnetometer measurements indicated the existence of a lunar iron core of between 140 to 280 mi (225 to 450 km) in radius. From this information, the mass of the lunar core is estimated to be between just 2 and 4% of the total mass of the Moon. By comparison, around 30% of Earth's mass is contained in its iron-nickel core. The relatively small size of the lunar core is seen as evidence that the Moon was formed when a planet-sized body (currently thought to be about the size of Mars) struck Earth late in the formation of the solar system. The massive collision stripped off the upper, lighter layers of Earth, which later formed the Moon. "This impact occurred after Earth's iron core had formed, ejecting rocky, iron-poor material from the outer shell into orbit," said Prospector principal investigator Alan Binder. This material—which formed the Moon—contained little iron that could sink down and form the lunar core.

Advances in Modeling the Giant Impact

Any hypothesis about the Moon's formation can only be tested indirectly. One cannot go back in time to witness the development of the solar system, or perform an experiment by crashing one planet into another (at least not yet!). So the planetary scientist must turn to theoretical models to "test" various hypotheses and theories. The advent of the modern digital computer has proven to be indispensable for this task. As computing power has increased, so has the sophistication of the mathematical models being tested. And recent computer "simulations" (i.e., computer-modeling runs) have added more credence to the giant impact theory.

In recent computer-modeling research into lunar formation, Robin Canup, senior research scientist of Space Studies at Southwest Research Institute in Boulder, Colorado, added support to the giant impact theory in her 2001 article in the journal Nature. Canup's research involved a collision simulation that showed how the Moon could have formed from a large impact with a "proto-Earth" (i.e., a body that would eventually form into the planet Earth). Although the GIT had earlier encountered difficulties when reconciling some characteristics of Earth, and with the development of an integrated model of the Earth-Moon system, Canup and other researchers have computer-modeled the planetary dynamics involved in the interaction of the impacting body and the "proto-Earth" to show that the giant impact theory can convincingly account for the origin of the Moon, as well as the Earth-Moon system.

In other research, a relatively new model used by researchers at the Southwest Research Institute in Boulder, and the University of California at Santa Cruz, has resulted in high-resolution computer simulations showing that an oblique impact by an object with just 10% of the mass of Earth could have ejected enough iron-free material into space to eventually coalesce into the Moon. This would also have left Earth with its present mass and rotation rate, the researchers report.

Conclusion

The giant impact theory is the leading explanation of the Moon's formation. Planetary scientists have largely discredited others, especially coaccretion, capture, and fission, which seemed more or less plausible until direct evidence from the Moon showed otherwise. Since its proposal in the mid-1970s, the ascension of the GIT to preeminence can be attributed to two main factors: its correlation with the known characteristics of the Moon, and improvements in mathematical modeling techniques, due in large part to the enormous increase in computing power over the decades since its introduction. The exact sequence of events that led to the formation of the Moon may never be fully known. However, mounting physical evidence, coupled with sophisticated computer simulations, has eliminated many previously popular hypotheses, while reinforcing the general validity of the giant impact theory.

—PHILIP KOTH

Viewpoint: No, while the giant impact theory is widely regarded as a better explanation for the Moon's formation than previous ones, it still has many problems that must be resolved.

Before the Moon was visited by U.S. astronauts of the Apollo missions, three general explanations about the Moon's origin were considered. The fission hypothesis claimed that the Moon was spun off from Earth's mantle and crust at a time when Earth was still forming and rotating rapidly on its axis. Since both objects were presumed to originate as one body, this idea gained support because the Moon's density is similar to the density of rocks just below Earth's crust. One difficulty with this explanation is that the angular momentum of Earth—in order for a large portion to break off—must have been much greater than the angular momentum of the present Earth-Moon system. Calculations show that there is no reasonable way for Earth to have spun at this required rate. The fission hypothesis does explain why the Moon's core is so small (or perhaps even absent), and why Earth and the Moon are so close together. It does not explain why the Moon has few volatile materials, such as water, carbon dioxide, and sodium, and why the magma ocean formed. The binary accretion hypothesis maintained that the Moon coalesced ("accreted") as an independent protoplanet in orbit around Earth. This hypothesis proposed that Earth, the Moon, and all other bodies of the solar system, condensed independently out of a primordial solar nebula. The binary accretion hypothesis was able to explain why Earth and the Moon formed so close together, but had difficulty explaining the discrepancy in chemical content between both bodies, the origin of the magma lunar ocean, the Moon's lack of iron (and a substantial core); and the lack of appreciable amounts of volatile materials. The capture explanation contended that the Moon formed independently from Earth, elsewhere in the solar system, and was eventually "captured" by Earth. This hypothesis has difficulty explaining how the capture took place from an orbital dynamics point of view, and also fails to explain why lunar rocks share the same isotope composition as Earth. Like the accretion hypothesis, the capture hypothesis does not explain why the Moon has so little iron in its core, why it has so few volatile materials, and why the lunar magma ocean formed.

The Giant Impact Theory

Given that each of the three explanations had its own particular strengths and weaknesses, it was hoped that the research and exploration of the Moon by the Apollo astronauts, and the instruments used in lunar orbit and on the Moon, would indicate which was correct. This, however, did not happen. After studying Moon rocks and close-up pictures of the Moon, scientists, for the most part, disclaimed the three hypotheses (which were now considered inadequate) and proposed what is now regarded as a more probable explanation of the Moon's formation, the giant impact theory, or GIT.

In 1975 Planetary Science Institute (PSI) senior scientists William K. Hartmann and Donald R. Davis proposed that early in Earth's history, over 4 billion years ago, a large object struck Earth. Their work was based on research performed in the Soviet Union during the 1960s concerning the formation of planets from countless asteroid-like bodies called planetesimals. The Russian astrophysicist V. S. Safronov pioneered much of this work.

Working independently from the PSI scientists, Alfred Cameron and William Ward, both of Harvard University's Smithsonian Center for Astrophysics, concluded—by studying the angular momentum in the Earth-Moon system—that an impact from a body at least as large as Mars could have supplied the rough material for the Moon and also given system its observed angular momentum. Much of Earth's crust and mantle, along with most of the impact planetesimal, disintegrated and was blasted into orbit thousands of miles high. Loose material in orbit can coalesce if it is outside the "Roche limit," the distance interior to which tidal forces from the central body prevent a body from forming. The material outside this limit formed the Moon; the material inside the limit fell back to Earth. Early estimates for the size of the impact planetesimal were comparable to the size of Mars, but computer simulation models by U.S. scientists in 1997 showed that the body would have had to be at least two-and-a-half to three times the size of Mars.

Early Reactions to the Giant Impact Theory

The GIT was viewed skeptically for about a decade after its introduction, because most planetary researchers generally dislike catastrophic solutions to geophysical problems. Hartmann, in fact, said that such solutions were "too tidy." On the other hand, proponents suggested that such a catastrophic event is actually very random, since by the time the planets were near the end of their formation, there were not many large objects left in the solar system. Earth, it is assumed, just happened to be the planet struck by this large planetesimal. Experts predicted that if the formation of the solar system could be "rerun," Venus or Mars might have ended up with a large moon instead. The chemical composition of Earth and the Moon are clearly predicted to be similar in this model, since a portion of Earth went into forming the Moon and a portion of the impact planetesimal remained in Earth. The Moon would be deficient in iron and similar metals if the impact occurred after those elements had largely sunk to the center of Earth. The Moon should also be quite dry because the material from which the Moon formed was heated to a high temperature in the impact, evaporating all the water.

The Turning Point

In October 1984 a conference on the origin of the Moon was held in Kailua-Kona, Hawaii. Discussions held during the conference added consensus to the validity of the new giant impact theory, and further dismissed the three traditional explanations of fission, binary accretion, and capture. Although these had added much information to the explanation of the Moon's formation, they were set aside in favor of a better explanation that coincided with better technology now available to planetary scientists. Computer methods had improved significantly over the years, and more advanced computer simulations of the proposed giant impact could now be performed. The understanding of impact processes has also improved due to experiments and studies of large terrestrial craters. The study of planet formation had also provided more knowledge, specifically on how planets formed from objects that were themselves still forming. Such information led to the idea that several large bodies could easily form near each other.

During the 1990s, Robin Canup wrote her Ph.D. dissertation concerning computer modeling of debris collected into "moonlets" (as she called them), and eventually collected into the Moon. Later, as an astrophysicist at the Southwest Research Institute in Boulder, Colorado, Canup admitted that, "At first [the giant impact theory] was seen as ad hoc, probably unlikely, possibly ridiculous." But the evolving giant impact theory was eventually able to resolve many of the problems associated with the earlier three hypotheses. For instance, it explained why Earth and the Moon were so close, why the Moon has little or no iron core; why the moon has a low volatile content; how the early magma ocean formed; and why the Earth-Moon system had a very high angular momentum.

The Skeptics

Although the GIT explains many of the mysteries that the former three theories could not, it is still incomplete, and therefore is not yet a generally accepted theory. Even though an advocate for the GIT, Canup has a list of research questions she would like to see addressed. These include: (1) make the giant impact model work with just a single impact, rather than with multiple impacts; (2) explain how a planetesimal formed elsewhere in the solar system; (3) explain the formation of Charon, Pluto's moon, which scientists think might also have been a result of a giant impact; and (4) chemically match the Moon's characteristics with what should have happened in the proto-lunar debris cloud.

One major problem with the GIT is that it seems to require that Earth be completely melted after the impact, as this would be the only way the huge crater caused by the impact could have been erased. Earth's geochemistry, however, does not indicate such a radical melting. There is an intense effort underway to understand the processes that might have operated within Earth at its formation and during its development. Until this happens, the chemical evidence for or against the GIT has not been proven.

Planetary scientists at a 1998 planetary conference in Monterey, California, raised further doubts about the validity of the GIT in three main areas: the evidence for a giant impact, the extent of melting in early Earth, and how Earth's core was formed. Research carried by Alfred Cameron at Harvard, in collaboration with Canup, stated that there were numerous parameters still to be tested by computer simulations. Cameron admitted that he has not yet researched all the possibilities of the ratio of the mass of the growing Earth to that of the impact planetesimal, or the total range in angular momentum of the Earth-Moon system. Further, Jay Melosh of the University of Arizona pointed out that the physical properties (called the equation of state) that he, Cameron, and others use in impact simulations is far from perfect and might lead to unrealistic results. Cameron agreed, noting that he "considers this game [of impact simulations] very primitive so far."

As computers become more complex, better computer simulations will be created. However, the solar system's chaotic past makes it impossible to repeat history. Planetary scientist David Stevenson of the California Institute of Technology says, "None of the scenarios for the Moon's formation is highly likely." Currently, no simulations of collisions can form the Moon out of debris thrown into orbit.

Planet formation, and the later formation of crusts, mantles, and cores, is so complicated that much of the evidence for or against the GIT has already been destroyed. Furthermore, insufficient data is available to test all the possibilities for this explanations. One of its most important assumptions is that Earth would have been mostly molten when it formed. This scenario would logically lead to a nearly complete separation of the elements present when the core formed. The densest materials would eventually settle at Earth's core, less dense materials would settle further away from the core, and the least dense materials would settle at or near the surface of Earth. However, the composition of Earth's upper mantle today suggests incomplete core formation. In addition, the degree of separation of elements varies with pressure, temperature, and the amount of available oxygen and sulfur. Because experiments to determine such factors are extremely difficult to carry out, the molten nature of early Earth and the early element concentrations are still a mystery, and one important prediction of the GIT cannot be verified scientifically.

"It's good news that the best model [the giant impact theory] gives the most plausible result," says planetary scientist David Stevenson of the California Institute of Technology, "[b]ut this will not be the last word on the subject. The models still have their limitations," and "[t]hey may not be capturing all the dynamics of the impact correctly."

Conclusion

What processes formed the Moon and Earth? As humans were not there at the time, the most that can be done is to outline a possible course of events which does not contradict physical laws and observed facts. This was done with the earlier three explanations, and as new information was discovered, those were succeeded by the giant impact theory. For the present, rigorous mathematical methods cannot deduce the exact history of lunar formation, and thus cannot verify the validity of the GIT or fully determine the various steps of the origin of the Moon. However, it may be possible to show which steps are likely and which steps are unlikely. Although not proven to everyone's satisfaction, the GIT explains much about Earth and the Moon. Combined with our current understanding of accretion, it leads to a dynamic and somewhat terrifying picture of the first several hundred million years for both bodies.

—WILLIAM ARTHUR ATKINS

KEY TERMS

ANGULAR MOMENTUM:

The measure of motion of objects in curved paths, including both rotation and orbital motion. For Earth and the Moon, angular momentum is the spin of each planet plus the orbital motion of the Moon around Earth.

ANORTHOSITE:

The ancient lunar surface rock, made up of igneous or magmatic rock (usually called "plutonic" rock).

DENSITY:

Mass of a given particle or substance per unit volume.

ISOTOPE:

Two or more forms of an element with the same atomic number (same number of protons) but with different numbers of neutrons.

LINEAR MOMENTUM:

Sometimes called simply "momentum"; for a single (nonrelativistic) particle, the product of the particle's mass and its velocity.

MAGMA:

Molten matter beneath Earth's crust that forms igneous rock when cooled.

MAGNETOMETER:

Instrument that measures the magnitude and sometimes the direction of a magnetic field, such as the Moon's magnetic field.

PLANETESIMAL:

One of an enormous number of small bodies supposed to have orbited the Sun during the formation of the planets.

PRIMORDIAL:

Existing at the beginning of time.

PRIMORDIAL SOLAR NEBULA:

Cloud of interstellar gas and/or dust that was disturbed, collapsed under its own gravity, and eventually formed the Sun, the planets, and all other objects within the solar system.

ROCHE LIMIT:

The restrictive distance below which a body orbiting a celestial body would be disrupted by the tidal forces generated by the gravitational attraction of the celestial body. For Earth, the Roche limit is about three Earth radii.

VOLATILE:

Capable of being readily and easily vaporized, or evaporated.

MYTHS ABOUT THE MOON

There have been many beliefs and myths about the Moon, such as its association with mental illness, werewolves, and increases in crime and accidents. People once believed that a full Moon (likened with the goddess Luna) caused people to go insane. The Moon was also thought to have been Diana, in her incarnations as the goddess of the woodland, an evil magical witch, and the goddess of the sky.

In ancient times the heavens were considered to be perfect, but it can be easily seen that the Moon has dark patches. To explain these imperfections, the belief of the "Man in the Moon" became popular. In ancient times, the figure was visualized as a man leaning on a fork upon which he carried a bundle of sticks. The biblical origin of this fable is from Numbers 15:32-36 (in which God commands Moses to gather the community to stone to death a man caught gathering wood on the Sabbath). Others thought the Man in the Moon looked like a rabbit. In one Hindu story, the god Indra posed as a beggar. A hare could not find food so threw itself into a fire. Indra took the body and placed it on the Moon for all to see. Native Americans thought that a brave had become so angry with his mother-in-law that he killed her and threw the body into the sky. It landed on the Moon so that all would see the crime. Danish folklore is thought to have originated the belief that the Moon was a wheel of curing cheese. This fable is thought to have been a precursor to the belief that the Moon is made of green cheese.

In any case, professors Ivan Kelly at the University of Saskatchewan, James Rotton at Florida International College, and Roger Culter at Colorado State University have examined over 100 studies on lunar effects and concluded that there was no significant correlation between the Moon and any of the beliefs cited in popular folklore.

—William Arthur Atkins

Science in Dispute