Lunar origin theories

For centuries, humans have had a scientific interest in the moon and how it relates to the Earth and the rest of the solar system. Scientists have sought to understand the moon, as well as the origins of the Earth itself and the planet's place in the universe. The scientific study of the moon's origins is a relatively new field, and research has shed light on the moon's composition, orbit, and surface. Some lunar origin theories have been discounted, while others continue to be explored.

Basic Principles

The study of the moon's origins is an important yet challenging pursuit within the field of astronomy. Scientists have been studying the moon for decades, but even after this long period of time, very little about the moon's composition, history, or origins has been definitively revealed. Americans and Russians have sent missions to and around the moon (including the famous Apollo missions to the moon's surface) and have obtained hundreds of pounds of moon samples. However, analysis continues without conclusive evidence to support a number of lunar origin theories.

The fission theory advocates argue that the moon, during the early formation of the Earth, was composed of a large amount of stray material from Earth and that it spun itself into the spherically shaped satellite seen today. According to the capture theory, the moon was created elsewhere in the solar system and then caught in Earth's gravitational pull as it moved through the system to its present location. In the condensation theory, adherents argue that both Earth and the moon were created from the nebula that formed the rest of the solar system. According to the giant impactor theory, a small planet struck Earth while it was still forming, and the superheated fragments of both the planet and the object spun together to form the moon. Although this theory has been widely accepted, efforts to locate concrete proof for it are ongoing.

Background and History

The study of the moon's origins and its relationship with the Earth dates back to the early seventeenth century. In 1610, Galileo Galilei observed that the moon, like Earth, was covered with mountains, plains, and valleys. This was a considerable departure from previous notions that the moon's environment was completely dissimilar to that of Earth's. For his beliefs, Galileo was faced the Spanish Inquisition of the Roman Catholic Church. (His theories were inspired by Copernicus, whose ideas caused great controversy and were therefore considered taboo.) The Inquisition convinced Galileo to recant his ideas or risk torture. Several decades later, mathematician and philosopher René Descartes, inspired by Galileo's revolutionary viewpoints, introduced his own theory of the moon's origins, suggesting that it had been captured in Earth's gravity.

In 1878, George Darwin (son of naturalist Charles Darwin) suggested that the moon was a part of the Earth, arguing that during its earliest stages of development, Earth had spun so fast that pieces of it had been “pulled” off the planet and ultimately became the moon. This “fission” idea was furthered by English geologist and physicist Osmond Fisher, who claimed that the material that formed the moon left a massive scar on Earth's surface that became the Pacific Ocean.

The fission theory would prevail until the early twentieth century when American astronomer Thomas See speculated that the moon was created somewhere else in the solar system (hence its considerable differences in appearance from that of Earth). See theorized that the moon was captured in Earth's gravitational field and has remained there since. Shortly thereafter, French astronomer and mathematician Édouard Roche speculated that the moon was formed at the same time and from the same materials as the Earth, and that, over time, they simply evolved in a different way but still in tandem. This condensation theory would be added to the number of theories already on hand, but like the others, it lacked complete evidence.

In the late 1950s, the Soviet spacecraft Luna performed the first close-range examinations of the moon and took samples after striking the surface. In the late 1960s and early 1970s, NASA's Apollo missions returned with more than 850 pounds of moon rocks and other lunar samples. After analyzing the samples, scientists began to offer a new theory, which came to be known as the giant impactor theory (or the “big whack”). In this framework, a rogue planetoid of unknown origin collided with the newly forming Earth. The impact obliterated the planetoid and sent fragments of both itself and Earth into space, where they ultimately formed Earth's moon. As the samples from the moon's surface continued to reveal clues, the big whack remained the prevailing theory on the moon's origins.

Giant Impactor Theory

The study of the moon's origins is part of an ongoing effort to understand the history of both Earth and the entire solar system. Although it has not been conclusively verified, the giant impactor theory has been embraced by scientists as the most reasonable hypothesis on which to pursue this study.

The Apollo missions of the late 1960s and early 1970s provided the most supporting evidence. Before these flights, it had been assumed that the moon's surface was dense, that its core was large, and that its composition included volatile elements (basic chemical elements and compounds, such as nitrogen, helium, water, and methane, which have low boiling temperatures). When the Apollo teams landed on the moon, they immediately began searching for evidence of these three characteristics. However, they (and scientists) were surprised to learn that the moon's surface was not very dense. When the astronauts surveyed the moon's core, they were surprised again to discover that the core was considerably smaller than theorized. This unexpected set of discoveries seemed to support the capture theory until further examinations of the samples uncovered many elements that are found on Earth.

These revelations about the moon's and the Earth's similarities and dissimilarities suggest a framework that is consistent with the big whack. Meanwhile, another discovery concerning other planets in the solar system provided even more convincing evidence: Planets that have a tilt in their axes, such as Mars, Pluto, and Neptune, experienced such shifts after a major event, such as a collision with an asteroid, moon, or another planet. Earth is among those planets with such a tilt. In light of these new discoveries, scientists have largely accepted the giant impactor theory as explaining the moon's origins.

Photo Analysis

Much in the same way that Galileo and other early lunar scientists developed their theories by simply looking skyward, scientists have learned much about the moon simply by studying photographs taken of it. Since Apollo, spacecraft from NASA, the European Space Agency, the Japan Aerospace Exploration Agency (JAXA), and the Russian Federal Space Agency have traveled to and photographed areas of interest on the lunar surface. Various photographic technologies, such as spectral and thermographic equipment, have enabled scientists to capture different types of minerals and find evidence of water, ice, and certain chemical compounds on the lunar surface.

For example, a French study focused on the moon's Aristarchus crater (or plateau). Using ultraviolet and spectral-imaging photographic systems aboard the Clementine orbiting spacecraft and the Lunar Prospector craft (both of which circled the moon in the 1990s), astronomers were able to present a framework of the speed at which the moon's crust develops, how it develops, and how it transforms over time. This analysis of the plateau's crust provided more clues as to how the moon has developed over time. Another set of photographs helped scientists piece together a profile of the moon's topography, surface composition, ice and water volumes, and magnetic fields, all of which provide clues to the moon's history.

Mineralogical Analysis

The collection of hundreds of pounds of moon rocks during the Luna and Apollo missions has revealed invaluable information that continues to be analyzed. For example, a 2010 study of lunar rocks focused on a particular mineral, apatite. Apatite accepts a number of volatile elements into its crystal structure and is prevalent in lunar rocks. The study also revealed that the concentrations of these elements are similar to those found in Earth-based apatite samples. The study also revealed the presence of sulfur in those crystal structures. On Earth, sulfur plays a major role in volcanic activity. This mineralogical revelation could cast light on how the moon's magma, core, and crust could have been formed. Then again, the presence of such large quantities of volatile elements raises questions about how, when the giant impact took place, these elements were able to survive in the moon's core fragments.

Dating Techniques

An important part of the pursuit of the moon's origins is determining the age of this celestial body. The rocks collected during the Apollo missions have provided a great deal of information to this end. Using radioactive isotopes and other dating techniques, scientists have determined the age of some of the rocks collected to be between 4.4 billion and 4.6 billion years. The 200-million-year difference is a vexing disparity.

Decades ago, the rocks that were brought back from the moon were believed to be formed at the moon's core and should have traveled outward to the moon's crust through a theoretical “molten rock ocean” that was the lunar core before it cooled. The rocks crust to the moon's surface. Over time, dating techniques have improved, revealing that the rocks' age is considerably younger. Although the moon's age has yet to be definitively established, the dating technology used to this end may soon solve this mystery. If such dating practices reveal that the rocks are indeed 4.4 billion years old and not 4.6 billion years old, then the widely accepted theory of the molten rock ocean could be called into question, as the particular rocks that were analyzed could not have floated to the moon's crust after the molten core cooled. Using dating practices may answer some questions, but also may raise others about the moon's origins.

In 2023, after analyzing rock samples collected by the Apollo 17 mission in 1972, scientists published research in the journal Geochemical Perspectives Letters that suggested the surface of the moon was 40 million years older than previously thought. This fell in line with a similar study conducted at the University of Hawaiʻi at Mānoa in 2021 that, using similar specimens gathered during the Apollo 17 mission, suggested the cooling period following the formation of the moon lasted approximately 20 million years instead of 100 million years scientists previously thought.

Searching for Water

An important area that researchers explore when studying lunar origins is the probability of water on the moon. The presence and amount of water on the moon is an important clue about how the moon formed. Studying the moon rocks brought back by Apollo missions, scientists uncovered evidence of water. How much water exists on the moon has been an area of considerable debate. However, infrared photographic evidence (first uncovered in the late 1990s) of the moon's south pole suggested the presence of water. In 2009, the NASA Lunar Reconnaissance Orbiter (LRO) probe confirmed the assertion using radar, thermographic imaging, and other sensory equipment. LRO revealed that water was prevalent—an important discovery because, for the giant impactor theory to be viable, there should not be much water on the moon. A giant impact would have vaporized the water.

Meanwhile, a 2010 chemical analysis of the moon's relatively high chlorine content posited that there may not be as much water under the moon's surface as previously thought. During intense heat conditions (such as volcanic eruptions), chlorine and water cancel each other out: Higher levels of chlorine in the moon's mantle indicate that there is not much hydrogen in the moon's interior, and the possibility exists that the water may have come from elsewhere. Clearly, the presence of water casts doubt on the giant impactor theory. As such, this area of lunar origin studies continues to be a source of more mystery than evidence.

Relevant Groups and Organizations

National governments play perhaps the most integral role in the exploration of the moon's origins. NASA is not the only government agency with activity in this arena. For example, the European Space Agency (ESA) is an intergovernmental organization dedicated to the exploration of space. In 2003, ESA launched the SMART-1 probe (the first of the Small Missions for Advanced Research in Technology program), which was the first European spacecraft to orbit the moon. The craft cataloged ice and used innovative X-ray cameras to capture the reflections of iron, calcium, and other elements in the sunlight. In 2008, the Indian Space Research Organization (ISRO) launched Chadrayaan-1, a probe that detected water molecules when it touched down on the moon's surface. In 2007, China launched a probe, Chang'e 1, which mapped landing sites for a 2012 program to return humans to the moon's surface. As the world prepares for this return, governments will play the most significant role in bringing such plans to fruition.

Although NASA and other government space agencies maintain research laboratories to examine lunar and data collected from moon missions, universities also play important roles in the analysis of this data. Many major public and private universities around the world have gained access to these small souvenirs from the Apollo missions. Massachusetts Institute of Technology, Cambridge University, University of Tokyo, and other major universities have all gained access to these rocks and to the data gathered by orbiting probes; the universities conduct experiments and uncover a wide range of information about the moon's origins.

Most of the equipment used in the pursuit of information about the moon's origins is manufactured by private technology and engineering companies. From the onboard equipment used in the spacecraft (and the crafts themselves) to the analytical technologies used in the laboratory, such machinery is produced by government contractors, built to the specification (and maintained on site) by these private (or semiprivate) organizations.

Implications and Future Prospects

Since 1972, scientists have continued to analyze the samples that the Apollo and Luna missions brought home. When water molecules were discovered in those samples, the public's interest was again piqued. When it was learned that a great deal more ice and water exists on the moon's surface (and that more may lie beneath it), that interest turned into a growing desire to return to the moon.

In the fall of 2011, NASA launched the Gravity Recovery and Interior Laboratory (GRAIL) mission. The mission's pair of probes mapped the moon's gravity and topography with more complexity than ever before. Additionally, the probes carried a system called MoonKam (Moon Knowledge Acquired by Middle school students). With MoonKam, young people were able to take images and video themselves on the moon's surface, enabling nonscientists to study the moon using the same technologies that scientists have used for decades. Chinese missions to the lunar surface (to explore how to use the moon one day as a launching point to deep space), began in 2007 and continued through 2018.

Begun in 2017, the Artemis program presented a recommitment by the global community to lunar study. NASA, JAXA, ESA, and the Canadian Space Agency (CSA) worked together to support plans for increased human and robotic exploration of the moon. In 2022, Artemis 1 launched a spacecraft into lunar orbit, with the next phase of Artemis planned to put humans in a spacecraft in lunar orbit. Lunar exploration was once again at the forefront of scientific study

Principal Terms

Apollo: the National Aeronautics and Space Administration (NASA) lunar expedition program active from 1963 to 1972, which included the 1969 Apollo 11 mission that placed humans on the moon for the first time

capture theory: a lunar origin theory suggesting that the moon was created elsewhere in the solar system and was drawn into Earth's orbit

Chandrayaan-1: an Indian lunar probe that landed on the moon in 2008

Chang'e 1: a Chinese lunar orbiter launched in 2007 to map the moon's surface in preparation for a piloted mission to the moon in 2012

condensation theory: a lunar origin theory suggesting that the Earth and its moon were created from the same nebula materials that formed the rest of the solar system

fission theory: a lunar origin theory suggesting that the moon and the spinning Earth were separated from each other as the solar system formed

giant impactor theory: a lunar origin theory suggesting that the moon was formed from fragments after another planetoid collided with the still-forming Earth

Luna: a Soviet lunar orbit expedition that culminated in the first impact on the moon's surface in 1959

Lunar Reconnaissance Orbiter: a 2009 craft that found evidence of large amounts of ice at the moon's south pole

SMART-1: a satellite from the European Space Agency's Small Missions for Advanced Research in Technology program, which launched in 2003 and was deliberately crashed into the moon in 2006, putting a European spacecraft on the moon for the first time

volatile elements: basic chemical elements and compounds, such as nitrogen, helium, water, and methane, which have low boiling temperatures

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