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Ancient Origins of the Moon: Was it Once Part of Earth?

Ancient Origins of the Moon: Was it Once Part of Earth?

The origin of the Moon has been a subject of research for many years and theories about its creation abound. Hypotheses vary from non-scientific proposals, such as that the Moon is a spaceship, to the currently favored idea that it was formed more than 4.5 billion years ago from a dense cloud of debris created when another planet struck the Earth in its early stages of development. Scientists even have a name for this hypothesized astronomical body, which they believe contained about 10 percent of the mass of the Earth—they call it Theia, after the Greek goddess mother of the Moon goddess Selene.

The initial evidence in favor of this theory was obtained during the Apollo Moon missions, nearly five decades ago. Astronauts brought back about half a tonne of Moon rocks from these missions, and analysis has revealed a striking similarity between the chemical compounds found on the Moon and those found on Earth. Other discoveries also support this theory, including evidence that shows a match between water samples collected from crystalline structures in lunar rocks and water collected on Earth.

The Earth-Theia collision hypothesis, also known as the Giant Impact Theory, has been around for awhile. It reflects the current scientific consensus on this question. But it has not gone unchallenged, by either planetary scientists or by the latest available data.

Shooting a Giant Hole in the Giant Impact Theory

At a 2013 scientific meeting sponsored by the Royal Society of London, astronomical researchers openly challenged the conventional collision theory. Relying on complex simulations, they proposed new scenarios suggesting that things might not be so simple. In an article entitled “ Impact Theory Gets Whacked ,” which appeared in an October 2013 edition of the journal Science, physicist and science reporter Daniel Clery discussed the arguments made by dissenting planetary scientists at this meeting, who asserted that a giant impact event couldn’t have created the Moon as it is seen today.

According to the computer models used by these scientists, a new moon emerging from such an event should be comprised largely of matter harvested from the collider (Theia). Our Moon should be easily identifiable as a remnant of another pulverized astronomical body, yet the similarity in chemical composition found when comparing Moon rocks to Earth rocks suggested greater mixing of materials than the Giant Impact collision theory would allow. And if that is the case, then either there was no collision, or the story of that collision is more complicated than originally believed.

Alternatives to the Giant Impact Theory

To explain the apparent homogeneity between the Earth and the Moon, planetary scientists proposed two modified versions of the collision theory, known as the Fast-Spinning Earth and the Half-Impact Earth . Both were discussed in separate articles appearing in the November 23, 2012 edition of Science.

Based on their simulations, Harvard University astrophysicists Matija Cuk and Sarah Stewart proposed that an impact with a planet just 1/200th the size of Earth could have been led to the creation of the Moon. This theory assumes that our home planet was in a proto stage and therefore spinning at a much faster rate than at present. As a result of this collision, enough material would have been expelled from the fast-spinning Earth’s mantle to account for the mass of the Moon, and for the homogeneity that exists between these two bodies.

Working from opposing assumptions, astrophysicist Robin Canup from the Southwest Research Institute in Boulder, Colorado proposed a different kind of collision. Under the Half-Impact scenario, Earth would have been hit 4.5 billion years ago by a slow-moving planetary object of similar size. The level of destruction of both planets would have been profound and about equal, allowing them to mix together to create a mass of debris with entirely different characteristics than the two planets possessed before they collided.

In the past, theories like these were considered untenable. Existing ideas about orbital mechanics seemed to rule them out, since they suggested that both bodies should be spinning and moving through space much faster than they actually are today.

However, new ideas about how the Sun interacts with the Earth and the Moon gravitationally has changed the picture.

Applying a principle called evection resonance , which would allow the Sun to put a brake on the movements of bodies it holds in its gravitational grasp, some planetary scientists hypothesize that the Earth and Moon could have lost considerable speed (angular momentum) over the past 4.5 billion years. If this is true, there might be no conflict between the current value of the Earth-Moon system’s angular momentum and the predictions of the Fast-Spinning Earth and Half-Impact Earth theories.

Giant Impact Theory Advocates Strike Back

Our current understanding of the solar system and its underlying dynamics could be completely wrong and might need to be reconsidered, if the claims made at the 2013 Royal Society meeting are in fact correct.

But collision/creation theorists have refused to go down without a fight. Another discovery, announced less than a year after the Royal Society gathering, provided new evidence in support of the Earth-Theia hypothesis.

In a June 2104 edition of Science, Dr. Daniel Herwartz from the University of Goettingen in Germany discussed results he and a team of researchers obtained from studying basalt samples collected during the Apollo Moon landings. Inside these samples, they found oxygen isotopes that had different chemical profiles than isotopes collected from the Earth’s mantle.

“We have now found small differences between the Moon and the Earth,” declared Dr. Herwartz . “This confirms the giant impact hypothesis.”

The existence of such isotopes was predicted by the giant impact theory, and that is why these findings by the German team are significant. At the time of the 2013 Royal Society meeting, these discoveries had yet to be revealed publicly, giving attendees no opportunity to assess their impact.

Assessing the Evidence

So where does all this leave us today? Has the discovery of divergent oxygen isotopes in Moon and Earth rocks conclusively proven the reality of the Giant Impact (Earth-Theia) theory? Or, have new discoveries about the potential dynamics of evection resonance given other theories greater explanatory power?

And what about other potential explanations, including those that might be classified as exotic? Could the Moon be an artificial object, engineered and constructed by an advanced civilization and put in orbit around the Earth to make the planet more hospitable to life, or for some other unknown reason?

In his 1975 underground cult favorite “Our Mysterious Spaceship Moon,” author Don Wilson argued that anomalies related to the Moon’s size, shape, location and physical characteristics were consistent with the theory that it was a gigantic spaceship, created by an advanced alien civilization with capabilities well beyond our own. This theme was further developed by British authors Christopher Knight and Alan Butler, who in their 2006 book “Who Built the Moon?” raised the possibility that such a feat might have been achieved by time-traveling humans from a far-distant future. They based this assertion on their discovery of multiple numerical and geometrical synchronicities between the Earth, Sun and Moon, which to them implied intelligent design of the latter.

It is easy to dismiss ideas like this as excessively speculative at best and downright pseudo-scientific at worst. But uncertainties that shroud the truth about the Moon’s origin and encourage such speculation persist.

While it remains the consensus choice among most astrophysicists who’ve pondered the question, the evidence in favor of the Earth-Theia hypothesis is hardly overwhelming. Small differences found in oxygen isotope ratios work in its favor, but this is not enough to rule out other collision scenarios. In fact, more recent evidence, obtained from a 2016 study carried out by researchers from Harvard University and Washington University in the United States, would seem to contradict the conclusions of the German researchers who believed they’d confirmed the Giant Impact Theory.

Using the latest techniques of chemical analysis, the scientists who sponsored this study could find no discernible differences in Earth and Moon rocks. On the contrary, their research revealed that these materials were even more homogeneous than previously believed. Furthermore, their chemical analysis uncovered evidence suggesting the collision that created the Moon was much more energetic and catastrophic than predicted by the Giant Impact model.

Similarly, many astrophysicists and planetary scientists remain skeptical of the evection resonance hypothesis. Knowledge about the specifics of this effect is still limited, with many doubting that it is significant enough to preserve the viability of the Fast-Spinning Earth and Half-Impact Earth collision theories.

Just because certain hypotheses are more popular than others does not mean one or the other is correct. If scientists spend an inordinate amount of time seeking evidence to confirm their personal pet theories, they may not give a fair hearing to other viable alternatives. This can be a particular problem for scientific conundrums that cannot be tested through experiment or resolved by direct observation, which is obviously the case for the processes that created the Moon more than four billion years ago.

A quote from Irwin I. Shapiro, the former director of the Harvard-Smithsonian Center for Astrophysics, perhaps best sums up the ongoing confusion over the Moon’s origin.

“Looking at all the anomalies and unanswered questions about the Moon,” Shapiro said, “the best explanation for the Moon is observational error. It doesn’t exist.”

This statement was undoubtedly made with tongue firmly implanted in cheek. Nevertheless, it accurately conveys the elusiveness of the Moon’s true history.


Origin of the Moon

The origin of the Moon is usually explained by a Mars-sized body striking the Earth, making a debris ring that eventually collected into a single natural satellite, the Moon, but there are a number of variations on this giant-impact hypothesis, as well as alternative explanations, and research continues into how the Moon came to be. [1] [2] Other proposed scenarios include captured body, fission, formed together (condensation theory, Synestia), planetesimal collisions (formed from asteroid-like bodies), and collision theories. [3]

The standard giant-impact hypothesis suggests that a Mars-sized body, called Theia, impacted the proto-Earth, creating a large debris ring around Earth, which then accreted to form the Moon. This collision also resulted in the 23.5° tilted axis of the Earth, thus causing the seasons. [1] The Moon's oxygen isotopic ratios seem to be essentially identical to Earth's. [4] Oxygen isotopic ratios, which may be measured very precisely, yield a unique and distinct signature for each Solar System body. [5] If Theia had been a separate protoplanet, it probably would have had a different oxygen isotopic signature from proto-Earth, as would the ejected mixed material. [6] Also, the Moon's titanium isotope ratio ( 50 Ti/ 47 Ti) appears so close to the Earth's (within 4 parts per million) that little if any of the colliding body's mass could likely have been part of the Moon. [7]


Ancient Origins of the Moon: Was it Once Part of Earth? - History

Where did the Moon come from?

Answer:

Any theory which explains the existence of the Moon must naturally explain the following facts:

  • The Moon's low density (3.3 g/cc) shows that it does not have a substantial iron core like the Earth does.
  • Moon rocks contain few volatile substances (e.g. water), which implies extra baking of the lunar surface relative to that of Earth.
  • The relative abundance of oxygen isotopes on Earth and on the Moon are identical, which suggests that the Earth and Moon formed at the same distance from the Sun.

Various theories had been proposed for the formation of the Moon. Below these theories are listed along with the reasons they have since been discounted.

  • The Fission Theory: This theory proposes that the Moon was once part of the Earth and somehow separated from the Earth early in the history of the solar system. The present Pacific Ocean basin is the most popular site for the part of the Earth from which the Moon came. This theory was thought possible since the Moon's composition resembles that of the Earth's mantle and a rapidly spinning Earth could have cast off the Moon from its outer layers. However, the present-day Earth-Moon system should contain "fossil evidence" of this rapid spin and it does not. Also, this hypothesis does not have a natural explanation for the extra baking the lunar material has received.
  • The Capture Theory: This theory proposes that the Moon was formed somewhere else in the solar system, and was later captured by the gravitational field of the Earth. The Moon's different chemical composition could be explained if it formed elsewhere in the solar system, however, capture into the Moon's present orbit is very improbable. Something would have to slow it down by just the right amount at just the right time, and scientists are reluctant to believe in such "fine tuning". Also, this hypothesis does not have a natural explanation for the extra baking the lunar material has received.
  • The Condensation Theory: This theory proposes that the Moon and the Earth condensed individually from the nebula that formed the solar system, with the Moon formed in orbit around the Earth. However, if the Moon formed in the vicinity of the Earth it should have nearly the same composition. Specifically, it should possess a significant iron core, and it does not. Also, this hypothesis does not have a natural explanation for the extra baking the lunar material has received.

There is one theory which remains to be discussed, and it is widely accepted today.

The Giant Impactor Theory (sometimes called The Ejected Ring Theory): This theory proposes that a planetesimal (or small planet) the size of Mars struck the Earth just after the formation of the solar system, ejecting large volumes of heated material from the outer layers of both objects. A disk of orbiting material was formed, and this matter eventually stuck together to form the Moon in orbit around the Earth. This theory can explain why the Moon is made mostly of rock and how the rock was excessively heated. Furthermore, we see evidence in many places in the solar system that such collisions were common late in the formative stages of the solar system. This theory is discussed further below.

More About The Giant Impactor Theory

In the mid-1970s, scientists proposed the giant impact scenario for the formation of the Moon. The idea was that an off-center impact of a roughly Mars-sized body with a young Earth could provide Earth with its fast initial spin, and eject enough debris into orbit to form the Moon. If the ejected material came primarily from the mantles of the Earth and the impactor, the lack of a sizeable lunar core was easily understood, and the energy of the impact could account for the extra heating of lunar material required by analysis of lunar rock samples obtained by the Apollo astronauts.

For nearly a decade, the giant impact theory was not believed by most scientists. However, in 1984, a conference devoted to lunar origin prompted a critical comparison of the existing theories. The giant impact theory emerged from this conference with nearly consensus support by scientists, enhanced by new models of planet formation that suggested large impacts were actually quite common events in the late stages of terrestrial planet formation.

The basic idea is this: about 4.45 billion years ago, a young planet Earth -- a mere 50 million years old at the time and not the solid object we know today-- experienced the largest impact event of its history. Another planetary body with roughly the mass of Mars had formed nearby with an orbit that placed it on a collision course with Earth. When young Earth and this rogue body collided, the energy involved was 100 million times larger than the much later event believed to have wiped out the dinosaurs. The early giant collision destroyed the rogue body, likely vaporized the upper layers of Earth's mantle, and ejected large amounts of debris into Earth orbit. Our Moon formed from this debris.


Image Credit: Joe Tucciarone


Where did the Moon come from?

No other satellite is as large, relative to the planet it orbits, as the Moon. How did the Earth end up with such a whopping neighbour?

The Moon is a mystery. Everyone on Earth can see it, but we only ever see one side of it. It affects the tides of the ocean, when animals have sex and apparently even how people sleep.

Yet until 1969, no one had ever been to the Moon. Even in 2015, almost a half-century later, only a measly 12 people have been there.

Thanks to the astronauts who visited the Moon, along with the many unmanned probes that have also been, we now know a lot about the Moon's makeup. But for all that knowledge, scientists are still struggling with a seemingly simple question: where the Moon came from.

Did it somehow get spun off from the Earth? Was it roaming through the solar system before being grabbed and forced to forever encircle us? Or did something altogether apocalyptic happen to bring it into being?

Our ancestors couldn't get to the Moon, but that didn't stop them pondering where it came from.

After the Second World War a completely different idea took hold

The Italian astronomer, physicist and philosopher Galileo Galilei made an early contribution when he succeeded in making a powerful telescope that showed the Moon in far greater detail than had been possible before.

In the early 1600s, Galileo showed that the Moon had a landscape similar to that of Earth. It was rugged, with mountains and plains. This was the first hint that the Earth and Moon somehow formed together.

Fast forward to the 1800s, and Charles Darwin's son George had an idea. He suggested that when the Earth was young it rotated very quickly, and as a result part of it flew off into space and formed the Moon. The Pacific Ocean is supposedly the scar from this "fission".

This theory didn't get much traction, and after the Second World War a completely different idea took hold.

The chemist Harold Urey proposed instead that the Moon came from another part of the galaxy, and was pulled in by the Earth's gravity as it passed by.

They were unsure if the Earth could capture the Moon without having its orbit disrupted

The capture theory gets a lot right. The Moon is large compared to the Earth, unusually so for a satellite, but if it formed elsewhere that suddenly makes sense. The theory also accounts for the fact that it always faces us with the same side, as this can happen when objects get captured.

Still, some scientists were unconvinced. They were unsure if the Earth could capture the Moon without having its orbit disrupted. They also thought the two would have probably collided.

There was a possible solution. If the Earth's atmosphere was large enough at the time, it could have acted like a giant airbag, slowing the Moon down before it could escape back into space. But this seemed rather unlikely.

The lunar scientists needed a theory that was consistent with several key observations. In particular, the Moon is relatively large. It is also speeding up, which means it is gradually moving away from Earth.

The Apollo astronauts were tasked with bringing back samples of moon rock

One idea put forward was accretion theory. This posits that the Earth and Moon formed together from a giant spinning disk of matter, which surrounded a black hole.

This theory died a quick death. It couldn't explain the speed with which the Moon orbits the Earth. Also, astronomers had calculated that the Moon was half as dense as Earth, suggesting they probably didn't form from the same accretion disk. Finally, there was no sign of the black hole.

This meant Urey's theory of capture remained dominant throughout the 1960s, when the USA began trying to send a manned mission to the Moon. If Urey was right, the Moon ought to have a different chemical composition to the Earth.

In part to test this, the Apollo astronauts were tasked with bringing back samples of moon rock. The data from those rocks blew all the existing theories to pieces.

The first casualty was George Darwin's fission theory. The lunar rock samples showed that the Moon was far older than the Pacific Ocean from which it was supposed to have come.

Urey's capture theory also received a hammer blow

"The oldest rocks on the Moon were these white anorthosites," says Alex Halliday of the University of Oxford in the UK. Because this mineral is not very dense, it normally floats on molten magma, so it would have been found close to Earth's surface rather than deep inside.

However the outermost layer of Earth's crust is only about 200 million years old. It cannot be the source of the Moon's rocks.

Urey's capture theory also received a hammer blow.

To everyone's surprise, the samples of lunar rock and soil revealed that the Moon is almost chemically identical to the Earth. That would be most unlikely if they formed far apart, as Urey had suggested.

Apollo led to "a period of deep confusion"

The rocks also showed that the Moon formed about 29 million years later than other similar-sized objects in the solar system.

It appears to have had a fiery beginning. The dark areas of its surface suggest it was once covered all over by a deep ocean of liquid magma.

Any theory of the Moon's origin would need to account for all of this. None of the existing theories were up to the job, so Apollo led to "a period of deep confusion", according to a 2014 paper by Jay Melosh of Purdue University in West Lafayette, Indiana. "A great many detailed facts about the Moon&hellip were gleaned from the lunar rocks, but no clear picture of its origin emerged."

In 1975, three years after the final Apollo landing, a new idea was put forward. The giant impact hypothesis, as it became known, was distinctly dramatic.

The impact caused part of Earth's outer layer to spin off and form a giant molten ball

When the solar system was forming 4.5 billion years ago, there were all sorts of rocks whizzing around. So William Hartmann and Donald Davis of the Planetary Science Institute in Tucson, Arizona suggested that one of them hit the Earth.

It must have been a seriously big rock: about the size of the planet Mars, which has a mass one-tenth that of Earth. This hypothetical planet, which has been nicknamed Theia, delivered a massive sideways blow rather like the cue ball in a game of pool.

The impact caused part of Earth's outer layer to spin off and form a giant molten ball. This ball would have burned bright, occupying about a third of Earth's sky, until it cooled and moved further away.

This collision has been simulated in computers, and works rather well. For starters, it can account for why the Moon's iron core is about half the size of Earth's. Theia's core accreted into Earth's, so the Moon didn't get much.

Halliday calls the impact the "least worst explanation"

It also explains why the Moon has so few "volatiles", those elements that easily evaporate into gases. The heat of the collision blasted them off into space.

Finally, the relative sizes of Earth and Theia can account for the speed of the Moon's orbit.

As a result, Halliday calls the impact the "least worst explanation". But it still has one big problem.

It's the same issue that derailed Urey's capture theory: the Earth and Moon are just too similar chemically.

If Theia existed, it has left no trace on the Moon

Many elements exist as subtly different variants called isotopes. Each atom is made up of three types of smaller particle, called protons, electrons and neutrons. Every atom of a given element has to have the same number of protons and electrons, but the number of neutrons varies, giving rise to isotopes.

Isotopes act as a kind of chemical fingerprint. If you have a mystery material, looking at the mix of isotopes it contains can give you a clue about where it came from.

In the case of the lunar rocks, some of the isotopes should have come from Earth and some from Theia, so the isotopic composition should be somewhere between the two. But in fact it's almost exactly the same as Earth's. If Theia existed, it has left no trace on the Moon.

This is a big problem for the giant impact hypothesis.

So far, it hasn't killed the impact hypothesis

The isotopes of tungsten and silicon are especially tricky, because they are produced during the formation of planetary cores.

"Every planet has a different history of core formation, so you'd expect to get a different signal," says Halliday. "These isotopes suggest it was the Earth itself that the Moon's atoms came from."

Melosh calls this finding the "isotopic crisis". But so far, it hasn't killed the impact hypothesis.

The simplest possible explanation is that Theia just so happened to have exactly the same isotopic signature as Earth, perhaps because it formed nearby. However, simulations of the early solar system suggested the probability of this happening is less than 1%.

In line with that, there are no other known bodies in the solar system with the same isotopic composition as Earth and the Moon. Scientists would like to collect meteorite samples from Venus and Mercury to see if they share similar isotopes, but it's a long shot.

The problem is that Theia has to have struck Earth a glancing blow

Alternatively, maybe the impact was so severe that Theia and Earth both melted, and their atoms mixed together. That would explain why the Earth and Moon are now so similar, but it's far from clear if such a catastrophic impact happened.

It's also been suggested that the impactor body was mostly made of ice. There are plenty of such ice-balls in the outer solar system, and one could have clobbered the Earth at high speed.

But even then, only 73% of the Moon could be derived from Earth, which is not enough to explain the isotopes. The problem is that Theia has to have struck Earth a glancing blow, otherwise the Moon would have ended up in a different orbit, and this sideways blow messes up the isotopes.

Maybe Theia didn't strike a glancing blow after all. In 2012 Matija Ćuk and Sarah Stewart of Harvard University in Cambridge, Massachusetts came up with a way to avoid it.

Theia could have been far smaller than previously thought

They suggested that the Earth was already spinning very fast when Theia hit it. If Earth was spinning rapidly, there was already enough momentum to send the Moon into the right orbit. There was no need for a glancing blow: Theia could have hit Earth head-on.

That means Theia could have been far smaller than previously thought, about 2% of Earth's mass. In turn, that means the Moon could be primarily made up of material from Earth.

This idea "has shaken the ground beneath all previous approaches," says Melosh.

In April 2015, yet more evidence emerged to support the giant impact hypothesis.

It makes the odd similarity of the Earth and Moon a little easier to explain

Alessandra Mastrobuono-Battisti of the Israel Institute of Technology in Haifa and her colleagues performed a more detailed simulation of the objects buzzing around in the early solar system.

They found that the objects that impacted on planets were much more similar to those planets than previously expected. Instead of just a 1% chance of Theia and Earth being very similar, the odds were more like 20%.

That's still not brilliant odds, but it makes the odd similarity of the Earth and Moon a little easier to explain.

Nevertheless, the job's not quite done. "We are still missing something," says Stewart.

Perhaps we shouldn't be too surprised that part of its origin story relies on blind luck

Most researchers now think the solution will be some version of the giant impact hypothesis, but it still needs some tweaking to convincingly explain the isotopes.

The biggest problem is to find a theory under which every aspect of the Earth and Moon look reasonably likely. As long as the theory requires Theia to have a particular mass, or hit the Earth in just the right way, it will always be open to doubt.

That being said, part of the reason for all the interest in the formation of the Moon is that it is unusual. Perhaps we shouldn't be too surprised that part of its origin story relies on blind luck.


The Origin of the Moon

Two PSI senior scientists, Dr. William K. Hartmann and Dr. Donald R. Davis, were the first to suggest the leading modern hypothesis of the moon's origin, in a paper published in 1975 in the journal Icarus.

Painting copyright William K. Hartmann

The idea in a nutshell:

At the time Earth formed 4.5 billion years ago, other smaller planetary bodies were also growing. One of these hit earth late in Earth's growth process, blowing out rocky debris. A fraction of that debris went into orbit around the Earth and aggregated into the moon.

Why this is a good hypothesis:

  • The Earth has a large iron core, but the moon does not. This is because Earth's iron had already drained into the core by the time the giant impact happened. Therefore, the debris blown out of both Earth and the impactor came from their iron-depleted, rocky mantles. The iron core of the impactor melted on impact and merged with the iron core of Earth, according to computer models.
  • Earth has a mean density of 5.5 grams/cubic centimeter, but the moon has a density of only 3.3 g/cc. The reason is the same, that the moon lacks iron.
  • The moon has exactly the same oxygen isotope composition as the Earth, whereas Mars rocks and meteorites from other parts of the solar system have different oxygen isotope compositions. This shows that the moon formed form material formed in Earth's neighborhood.
  • If a theory about lunar origin calls for an evolutionary process, it has a hard time explaining why other planets do not have similar moons. (Only Pluto has a moon that is an appreciable fraction of its own size.) Our giant impact hypothesis had the advantage of invoking a stochastic catastrophic event that might happen only to one or two planets out of nine.

What were some earlier ideas?

  1. One early theory was that the moon is a sister world that formed in orbit around Earth as the Earth formed. This theory failed because it could not explain why the moon lacks iron.
  2. A second early idea was that the moon formed somewhere else in the solar system where there was little iron, and then was captured into orbit around Earth. This failed when lunar rocks showed the same isotope composition as the Earth.
  3. A third early idea was that early Earth spun so fast that it spun off the moon. This idea would produce a moon similar to Earth's mantle, but it failed when analysis of the total angular momentum and energy involved indicated that the present Earth-moon system could not form in this way.

Where did the theory come from?

Hartmann and Davis were familiar with the work done in the Soviet Union in the 1960's, on the aggregation of planets out of countless asteroid-like bodies called planetesimals. Much of this work was pioneered by a Russian astrophysicist named V. S. Safronov.

Picking up on Safronov's general ideas, Hartmann and Davis ran calculations of the rate of growth of the 2nd-largest, 3rd largest, etc., bodies in the general vicinity of Earth, as the Earth itself was growing. Just as the asteroid belt today has a largest asteroid (Ceres) at a 1000 km diameter, and several smaller bodies in the 300-500 km diameter range, the region of Earth's orbit would have had several bodies up to about half the size of the growing Earth. Our idea was that in the case of Earth (but not the other planets) the impact happened late enough, and in such a direction relative to Earth's rotation, that abundant enough middle material was thrown out to make a moon.

How did the theory develop?

After we first presented the theory in 1974 at a conference on satellites, Harvard researcher A. G. W. Cameron rose to say that he and William Ward were also working on the same idea, but coming at it from a different motivation -- the study of angular momentum in the system -- and that they had concluded the impacting body had to be roughly Mars size (a third or half the size of Earth). Our paper was published in 1975 (Hartmann and Davis, Icarus, 24, 504-505) Cameron and Ward published an abstract on this idea at the Lunar Science conference in 1976, two years after the PSI paper.


Five Hours After Impact, based on computer modeling by A. Cameron, W. Benz, J. Melosh, and others. Copyright William K. Hartmann

Some work was done by Thompson and Stevenson in 1983 about the formation of moonlets in the disk of debris that formed around Earth after the impact. However, in general the theory languished until 1984 when an international meeting was organized in Kona, Hawaii, about the origin of the moon. At that meeting, the giant impact hypothesis emerged as the leading hypothesis and has remained in that role ever since. Dr. Michael Drake, director of the University of Arizona's Planetary Science Department, recently described that meeting as perhaps the most successful in the history of planetary science.

A collection of papers from that meeting was published by the Lunar and Planetary Institute (Houston) in the 1986 book, Origin of the Moon, edited by PSI scientist William Hartmann, together with Geoffry Taylor and Roger Phillips. This book remains the prime reference on this subject. In the meantime, researchers such as Willy Benz, Jay Melosh, A. G. W. Cameron, and others have attempted computer models of the giant impact, to determine how much material would go into orbit. Some of these results have been used by Hartmann to make the paintings on this web page, attempting to show how the impact would have looked to a human observer (if humans had been around -- they didn't come along until 4.5 billion years later!)

In the 1990's, Dr. Robin Canup wrote a Ph.D. dissertation on the moon's origin and the giant impact hypothesis, which produced new modeling of the aggregation of the debris into moonlets, and eventually, into the moon itself. Dr. Canup is continuing the modeling of the lunar accretion process.

Current status:

In 1997, Dr. Canup's work received a great deal of publicity by media news sources, some of whom mistakenly thought that the giant impact was a brand new idea. Canup's early work, presented in July 1997, suggested the debris from an impact might not make a moon, but only a swarm of moonlets. Her later work (fall 1997) led to more "success" in aggregating the debris into a single moon.

At PSI we have worked with several leading researchers to propose new work or the accretion mechanics using a variant of the PSI planet building model. But this work has not been funded.

Hartmann, W. K. and D. R. Davis 1975 Icarus, 24, 505.

Hartmann, W. K. 1997. A Brief History of the Moon. The Planetary Report. 17, 4-11.

Hartmann, W. K. and Ron Miller 1991. The History of Earth, (New York: Workman Publishing Co.)


Contents

The usual English proper name for Earth's natural satellite is simply the Moon, with a capital M. [18] [19] The noun moon is derived from Old English mōna, which (like all its Germanic cognates) stems from Proto-Germanic *mēnōn, [20] which in turn comes from Proto-Indo-European *mēnsis "month" [21] (from earlier *mēnōt, genitive *mēneses) which may be related to the verb "measure" (of time). [22]

Occasionally, the name Luna / ˈ l uː n ə / is used in scientific writing [23] and especially in science fiction to distinguish the Earth's moon from others, while in poetry "Luna" has been used to denote personification of the Moon. [24] Cynthia / ˈ s ɪ n θ i ə / is another poetic name, though rare, for the Moon personified as a goddess, [25] while Selene / s ə ˈ l iː n iː / (literally "Moon") is the Greek goddess of the Moon.

The usual English adjective pertaining to the Moon is "lunar", derived from the Latin word for the Moon, lūna. The adjective selenian / s ə l iː n i ə n / , [26] derived from the Greek word for the Moon, σελήνη selēnē, and used to describe the Moon as a world rather than as an object in the sky, is rare, [27] while its cognate selenic was originally a rare synonym [28] but now nearly always refers to the chemical element selenium. [29] The Greek word for the Moon does however provide us with the prefix seleno-, as in selenography, the study of the physical features of the Moon, as well as the element name selenium. [30] [31]

The Greek goddess of the wilderness and the hunt, Artemis, equated with the Roman Diana, one of whose symbols was the Moon and who was often regarded as the goddess of the Moon, was also called Cynthia, from her legendary birthplace on Mount Cynthus. [32] These names – Luna, Cynthia and Selene – are reflected in technical terms for lunar orbits such as apolune, pericynthion and selenocentric.

Isotope dating of lunar samples suggests the Moon formed around 50 million years after the origin of the Solar System. [33] [34] Historically, several formation mechanisms have been proposed, [35] but none satisfactorily explained the features of the Earth–Moon system. A fission of the Moon from Earth's crust through centrifugal force [36] would require too great an initial rotation rate of Earth. [37] Gravitational capture of a pre-formed Moon [38] depends on an unfeasibly extended atmosphere of Earth to dissipate the energy of the passing Moon. [37] A co-formation of Earth and the Moon together in the primordial accretion disk does not explain the depletion of metals in the Moon. [37] None of these hypotheses can account for the high angular momentum of the Earth–Moon system. [39]

The prevailing theory is that the Earth–Moon system formed after a giant impact of a Mars-sized body (named Theia) with the proto-Earth. The impact blasted material into Earth's orbit and then the material accreted and formed the Moon [40] [41] just beyond the Earth's Roche limit of

2.56 R . [42] This theory best explains the evidence.

Giant impacts are thought to have been common in the early Solar System. Computer simulations of giant impacts have produced results that are consistent with the mass of the lunar core and the angular momentum of the Earth–Moon system. These simulations also show that most of the Moon derived from the impactor, rather than the proto-Earth. [43] However, more recent simulations suggest a larger fraction of the Moon derived from the proto-Earth. [44] [45] [46] [47] Other bodies of the inner Solar System such as Mars and Vesta have, according to meteorites from them, very different oxygen and tungsten isotopic compositions compared to Earth. However, Earth and the Moon have nearly identical isotopic compositions. The isotopic equalization of the Earth-Moon system might be explained by the post-impact mixing of the vaporized material that formed the two, [48] although this is debated. [49]

The impact released a lot of energy and then the released material re-accreted into the Earth–Moon system. This would have melted the outer shell of Earth, and thus formed a magma ocean. [50] [51] Similarly, the newly formed Moon would also have been affected and had its own lunar magma ocean its depth is estimated from about 500 km (300 miles) to 1,737 km (1,079 miles). [50]

While the giant-impact theory explains many lines of evidence, some questions are still unresolved, most of which involve the Moon's composition. [52]

In 2001, a team at the Carnegie Institute of Washington reported the most precise measurement of the isotopic signatures of lunar rocks. [53] The rocks from the Apollo program had the same isotopic signature as rocks from Earth, differing from almost all other bodies in the Solar System. This observation was unexpected, because most of the material that formed the Moon was thought to come from Theia and it was announced in 2007 that there was less than a 1% chance that Theia and Earth had identical isotopic signatures. [54] Other Apollo lunar samples had in 2012 the same titanium isotopes composition as Earth, [55] which conflicts with what is expected if the Moon formed far from Earth or is derived from Theia. These discrepancies may be explained by variations of the giant-impact theory.

The Moon is a very slightly scalene ellipsoid due to tidal stretching, with its long axis displaced 30° from facing the Earth, due to gravitational anomalies from impact basins. Its shape is more elongated than current tidal forces can account for. This 'fossil bulge' indicates that the Moon solidified when it orbited at half its current distance to the Earth, and that it is now too cold for its shape to adjust to its orbit. [56]

Internal structure

Lunar surface chemical composition [57]
Compound Formula Composition
Maria Highlands
silica SiO2 45.4% 45.5%
alumina Al2O3 14.9% 24.0%
lime CaO 11.8% 15.9%
iron(II) oxide FeO 14.1% 5.9%
magnesia MgO 9.2% 7.5%
titanium dioxide TiO2 3.9% 0.6%
sodium oxide Na2O 0.6% 0.6%
99.9% 100.0%

The Moon is a differentiated body that was initially in hydrostatic equilibrium but has since departed from this condition. [58] It has a geochemically distinct crust, mantle, and core. The Moon has a solid iron-rich inner core with a radius possibly as small as 240 kilometres (150 mi) and a fluid outer core primarily made of liquid iron with a radius of roughly 300 kilometres (190 mi). Around the core is a partially molten boundary layer with a radius of about 500 kilometres (310 mi). [59] [60] This structure is thought to have developed through the fractional crystallization of a global magma ocean shortly after the Moon's formation 4.5 billion years ago. [61]

Crystallization of this magma ocean would have created a mafic mantle from the precipitation and sinking of the minerals olivine, clinopyroxene, and orthopyroxene after about three-quarters of the magma ocean had crystallised, lower-density plagioclase minerals could form and float into a crust atop. [62] The final liquids to crystallise would have been initially sandwiched between the crust and mantle, with a high abundance of incompatible and heat-producing elements. [1] Consistent with this perspective, geochemical mapping made from orbit suggests a crust of mostly anorthosite. [14] The Moon rock samples of the flood lavas that erupted onto the surface from partial melting in the mantle confirm the mafic mantle composition, which is more iron-rich than that of Earth. [1] The crust is on average about 50 kilometres (31 mi) thick. [1]

The Moon is the second-densest satellite in the Solar System, after Io. [63] However, the inner core of the Moon is small, with a radius of about 350 kilometres (220 mi) or less, [1] around 20% of the radius of the Moon. Its composition is not well understood, but is probably metallic iron alloyed with a small amount of sulfur and nickel analyses of the Moon's time-variable rotation suggest that it is at least partly molten. [64] The pressure at the lunar core is estimated to be 5 GPa . [65]

Magnetic field

The Moon has an external magnetic field of generally less than 0.2 nanoteslas, [66] or less than one hundred thousandth that of Earth. The Moon does not currently have a global dipolar magnetic field and only has crustal magnetization likely acquired early in its history when a dynamo was still operating. [67] [68] However, early in its history, 4 billion years ago, its magnetic field strength was likely close to that of Earth today. [66] This early dynamo field apparently expired by about one billion years ago, after the lunar core had completely crystallized. [66] Theoretically, some of the remnant magnetization may originate from transient magnetic fields generated during large impacts through the expansion of plasma clouds. These clouds are generated during large impacts in an ambient magnetic field. This is supported by the location of the largest crustal magnetizations situated near the antipodes of the giant impact basins. [69]

Surface geology

The topography of the Moon has been measured with laser altimetry and stereo image analysis. [70] Its most extensive topographic feature is the giant far-side South Pole–Aitken basin, some 2,240 km (1,390 mi) in diameter, the largest crater on the Moon and the second-largest confirmed impact crater in the Solar System. [71] [72] At 13 km (8.1 mi) deep, its floor is the lowest point on the surface of the Moon. [71] [73] The highest elevations of the Moon's surface are located directly to the northeast, which might have been thickened by the oblique formation impact of the South Pole–Aitken basin. [74] Other large impact basins such as Imbrium, Serenitatis, Crisium, Smythii, and Orientale possess regionally low elevations and elevated rims. [71] The far side of the lunar surface is on average about 1.9 km (1.2 mi) higher than that of the near side. [1]

The discovery of fault scarp cliffs suggest that the Moon has shrunk by about 90 metres (300 ft) within the past billion years. [75] Similar shrinkage features exist on Mercury. Mare Frigoris, a basin near the north pole long assumed to be geologically dead, has cracked and shifted. Since the Moon doesn't have tectonic plates, its tectonic activity is slow and cracks develop as it loses heat. [76]

Volcanic features

The dark and relatively featureless lunar plains, clearly seen with the naked eye, are called maria (Latin for "seas" singular mare), as they were once believed to be filled with water [77] they are now known to be vast solidified pools of ancient basaltic lava. Although similar to terrestrial basalts, lunar basalts have more iron and no minerals altered by water. [78] The majority of these lava deposits erupted or flowed into the depressions associated with impact basins. Several geologic provinces containing shield volcanoes and volcanic domes are found within the near side "maria". [79]

Almost all maria are on the near side of the Moon, and cover 31% of the surface of the near side [80] compared with 2% of the far side. [81] This is likely due to a concentration of heat-producing elements under the crust on the near side, which would have caused the underlying mantle to heat up, partially melt, rise to the surface and erupt. [62] [82] [83] Most of the Moon's mare basalts erupted during the Imbrian period, 3.0–3.5 billion years ago, although some radiometrically dated samples are as old as 4.2 billion years. [84] As of 2003, crater counting studies of the youngest eruptions appeared to suggest they formed no earlier than 1.2 billion years ago. [85]

In 2006, a study of Ina, a tiny depression in Lacus Felicitatis, found jagged, relatively dust-free features that, because of the lack of erosion by infalling debris, appeared to be only 2 million years old. [86] Moonquakes and releases of gas also indicate some continued lunar activity. [86] Evidence of recent lunar volcanism has been identified at 70 irregular mare patches, some less than 50 million years old. This raises the possibility of a much warmer lunar mantle than previously believed, at least on the near side where the deep crust is substantially warmer because of the greater concentration of radioactive elements. [87] [88] [89] [90] Evidence has been found for 2–10 million years old basaltic volcanism within the crater Lowell, [91] [92] inside the Orientale basin. Some combination of an initially hotter mantle and local enrichment of heat-producing elements in the mantle could be responsible for prolonged activities on the far side in the Orientale basin. [93] [94]

The lighter-colored regions of the Moon are called terrae, or more commonly highlands, because they are higher than most maria. They have been radiometrically dated to having formed 4.4 billion years ago, and may represent plagioclase cumulates of the lunar magma ocean. [84] [85] In contrast to Earth, no major lunar mountains are believed to have formed as a result of tectonic events. [95]

The concentration of maria on the near side likely reflects the substantially thicker crust of the highlands of the Far Side, which may have formed in a slow-velocity impact of a second moon of Earth a few tens of millions of years after the Moon's formation. [96] [97] Alternatively, it may be a consequence of asymmetrical tidal heating when the Moon was much closer to the Earth. [98]

Impact craters

A major geologic process that has affected the Moon's surface is impact cratering, [99] with craters formed when asteroids and comets collide with the lunar surface. There are estimated to be roughly 300,000 craters wider than 1 km (0.6 mi) on the Moon's near side. [100] The lunar geologic timescale is based on the most prominent impact events, including Nectaris, Imbrium, and Orientale structures characterized by multiple rings of uplifted material, between hundreds and thousands of kilometers in diameter and associated with a broad apron of ejecta deposits that form a regional stratigraphic horizon. [101] The lack of an atmosphere, weather, and recent geological processes mean that many of these craters are well-preserved. Although only a few multi-ring basins have been definitively dated, they are useful for assigning relative ages. Because impact craters accumulate at a nearly constant rate, counting the number of craters per unit area can be used to estimate the age of the surface. [101] The radiometric ages of impact-melted rocks collected during the Apollo missions cluster between 3.8 and 4.1 billion years old: this has been used to propose a Late Heavy Bombardment period of increased impacts. [102]

Blanketed on top of the Moon's crust is a highly comminuted (broken into ever smaller particles) and impact gardened surface layer called regolith, formed by impact processes. The finer regolith, the lunar soil of silicon dioxide glass, has a texture resembling snow and a scent resembling spent gunpowder. [103] The regolith of older surfaces is generally thicker than for younger surfaces: it varies in thickness from 10–20 km (6.2–12.4 mi) in the highlands and 3–5 km (1.9–3.1 mi) in the maria. [104] Beneath the finely comminuted regolith layer is the megaregolith, a layer of highly fractured bedrock many kilometers thick. [105]

High-resolution images from the Lunar Reconnaissance Orbiter in the 2010s show a contemporary crater-production rate significantly higher than was previously estimated. A secondary cratering process caused by distal ejecta is thought to churn the top two centimeters of regolith on a timescale of 81,000 years. [106] [107] This rate is 100 times faster than the rate computed from models based solely on direct micrometeorite impacts. [108]

Gravitational field

The gravitational field of the Moon has been measured through tracking the Doppler shift of radio signals emitted by orbiting spacecraft. The main lunar gravity features are mascons, large positive gravitational anomalies associated with some of the giant impact basins, partly caused by the dense mare basaltic lava flows that fill those basins. [109] [110] The anomalies greatly influence the orbit of spacecraft about the Moon. There are some puzzles: lava flows by themselves cannot explain all of the gravitational signature, and some mascons exist that are not linked to mare volcanism. [111]

Lunar swirls

Lunar swirls are enigmatic features found across the Moon's surface. They are characterized by a high albedo, appear optically immature (i.e. the optical characteristics of a relatively young regolith), and have often a sinuous shape. Their shape is often accentuated by low albedo regions that wind between the bright swirls. They are located in places with enhanced surface magnetic fields and many are located at the antipodal point of major impacts. Well known swirls include the Reiner Gamma feature and Mare Ingenii. They are hypothesized to be areas that have been partially shielded from the solar wind, resulting in slower space weathering. [112]

Presence of water

Liquid water cannot persist on the lunar surface. When exposed to solar radiation, water quickly decomposes through a process known as photodissociation and is lost to space. However, since the 1960s, scientists have hypothesized that water ice may be deposited by impacting comets or possibly produced by the reaction of oxygen-rich lunar rocks, and hydrogen from solar wind, leaving traces of water which could possibly persist in cold, permanently shadowed craters at either pole on the Moon. [113] [114] Computer simulations suggest that up to 14,000 km 2 (5,400 sq mi) of the surface may be in permanent shadow. [115] The presence of usable quantities of water on the Moon is an important factor in rendering lunar habitation as a cost-effective plan the alternative of transporting water from Earth would be prohibitively expensive. [116]

In years since, signatures of water have been found to exist on the lunar surface. [117] In 1994, the bistatic radar experiment located on the Clementine spacecraft, indicated the existence of small, frozen pockets of water close to the surface. However, later radar observations by Arecibo, suggest these findings may rather be rocks ejected from young impact craters. [118] In 1998, the neutron spectrometer on the Lunar Prospector spacecraft showed that high concentrations of hydrogen are present in the first meter of depth in the regolith near the polar regions. [119] Volcanic lava beads, brought back to Earth aboard Apollo 15, showed small amounts of water in their interior. [120]

The 2008 Chandrayaan-1 spacecraft has since confirmed the existence of surface water ice, using the on-board Moon Mineralogy Mapper. The spectrometer observed absorption lines common to hydroxyl, in reflected sunlight, providing evidence of large quantities of water ice, on the lunar surface. The spacecraft showed that concentrations may possibly be as high as 1,000 ppm. [121] Using the mapper's reflectance spectra, indirect lighting of areas in shadow confirmed water ice within 20° latitude of both poles in 2018. [122] In 2009, LCROSS sent a 2,300 kg (5,100 lb) impactor into a permanently shadowed polar crater, and detected at least 100 kg (220 lb) of water in a plume of ejected material. [123] [124] Another examination of the LCROSS data showed the amount of detected water to be closer to 155 ± 12 kg (342 ± 26 lb). [125]

In May 2011, 615–1410 ppm water in melt inclusions in lunar sample 74220 was reported, [126] the famous high-titanium "orange glass soil" of volcanic origin collected during the Apollo 17 mission in 1972. The inclusions were formed during explosive eruptions on the Moon approximately 3.7 billion years ago. This concentration is comparable with that of magma in Earth's upper mantle. Although of considerable selenological interest, this announcement affords little comfort to would-be lunar colonists – the sample originated many kilometers below the surface, and the inclusions are so difficult to access that it took 39 years to find them with a state-of-the-art ion microprobe instrument.

Analysis of the findings of the Moon Mineralogy Mapper (M3) revealed in August 2018 for the first time "definitive evidence" for water-ice on the lunar surface. [127] [128] The data revealed the distinct reflective signatures of water-ice, as opposed to dust and other reflective substances. [129] The ice deposits were found on the North and South poles, although it is more abundant in the South, where water is trapped in permanently shadowed craters and crevices, allowing it to persist as ice on the surface since they are shielded from the sun. [127] [129]

In October 2020, astronomers reported detecting molecular water on the sunlit surface of the Moon by several independent spacecraft, including the Stratospheric Observatory for Infrared Astronomy (SOFIA). [130] [131] [132] [133]

Surface conditions

The surface of the Moon is an extreme environment with temperatures that range from 140 °C down to −171 °C , an atmospheric pressure of 10 −10 Pa, and high levels of ionizing radiation from the Sun and cosmic rays. The exposed surfaces of spacecraft are considered unlikely to harbor bacterial spoors after just one lunar orbit. [134] The surface gravity of the Moon is approximately 1.625 m/s 2 , about 16.6% that on Earth's surface or 0.166 ɡ . [4]

Atmosphere

The Moon has an atmosphere so tenuous as to be nearly vacuum, with a total mass of less than 10 tonnes (9.8 long tons 11 short tons). [137] The surface pressure of this small mass is around 3 × 10 −15 atm (0.3 nPa) it varies with the lunar day. Its sources include outgassing and sputtering, a product of the bombardment of lunar soil by solar wind ions. [14] [138] Elements that have been detected include sodium and potassium, produced by sputtering (also found in the atmospheres of Mercury and Io) helium-4 and neon [139] from the solar wind and argon-40, radon-222, and polonium-210, outgassed after their creation by radioactive decay within the crust and mantle. [140] [141] The absence of such neutral species (atoms or molecules) as oxygen, nitrogen, carbon, hydrogen and magnesium, which are present in the regolith, is not understood. [140] Water vapor has been detected by Chandrayaan-1 and found to vary with latitude, with a maximum at

60–70 degrees it is possibly generated from the sublimation of water ice in the regolith. [142] These gases either return into the regolith because of the Moon's gravity or are lost to space, either through solar radiation pressure or, if they are ionized, by being swept away by the solar wind's magnetic field. [140]

Studies of Moon magma samples retrieved by the Apollo missions demonstrate that the Moon had once possessed a relatively thick atmosphere for a period of 70 million years between 3 and 4 billion years ago. This atmosphere, sourced from gases ejected from lunar volcanic eruptions, was twice the thickness of that of present-day Mars. The ancient lunar atmosphere was eventually stripped away by solar winds and dissipated into space. [143]

A permanent Moon dust cloud exists around the Moon, generated by small particles from comets. Estimates are 5 tons of comet particles strike the Moon's surface every 24 hours, resulting in the ejection of dust particles. The dust stays above the Moon approximately 10 minutes, taking 5 minutes to rise, and 5 minutes to fall. On average, 120 kilograms of dust are present above the Moon, rising up to 100 kilometers above the surface. Dust counts made by LADEE's Lunar Dust EXperiment (LDEX) found particle counts peaked during the Geminid, Quadrantid, Northern Taurid, and Omicron Centaurid meteor showers, when the Earth, and Moon pass through comet debris. The lunar dust cloud is asymmetric, being more dense near the boundary between the Moon's dayside and nightside. [144] [145]

Lunar distance

Scale model of the Earth–Moon system: Sizes and distances are to scale.

Orbit

Because of tidal locking, the rotation of the Moon around its own axis is synchronous to its orbital period around the Earth. The Moon makes a complete orbit around Earth with respect to the fixed stars about once every 27.3 days, [g] its sidereal period. However, because Earth is moving in its orbit around the Sun at the same time, it takes slightly longer for the Moon to show the same phase to Earth, which is about 29.5 days [h] its synodic period. [80] [146]

Unlike most satellites of other planets, the Moon orbits closer to the ecliptic plane than to the planet's equatorial plane. The Moon's orbit is subtly perturbed by the Sun and Earth in many small, complex and interacting ways. For example, the plane of the Moon's orbit gradually rotates once every 18.61 years, [147] which affects other aspects of lunar motion. These follow-on effects are mathematically described by Cassini's laws. [148]

The Moon's axial tilt with respect to the ecliptic is only 1.5427°, [8] [149] much less than the 23.44° of Earth. Because of this, the Moon's solar illumination varies much less with season, and topographical details play a crucial role in seasonal effects. [150] From images taken by Clementine in 1994, it appears that four mountainous regions on the rim of the crater Peary at the Moon's north pole may remain illuminated for the entire lunar day, creating peaks of eternal light. No such regions exist at the south pole. Similarly, there are places that remain in permanent shadow at the bottoms of many polar craters, [115] and these "craters of eternal darkness" are extremely cold: Lunar Reconnaissance Orbiter measured the lowest summer temperatures in craters at the southern pole at 35 K (−238 °C −397 °F) [151] and just 26 K (−247 °C −413 °F) close to the winter solstice in the north polar crater Hermite. This is the coldest temperature in the Solar System ever measured by a spacecraft, colder even than the surface of Pluto. [150] Average temperatures of the Moon's surface are reported, but temperatures of different areas will vary greatly depending upon whether they are in sunlight or shadow. [152]

Relative size

The Moon is an exceptionally large natural satellite relative to Earth: Its diameter is more than a quarter and its mass is 1/81 of Earth's. [80] It is the largest moon in the Solar System relative to the size of its planet, [i] though Charon is larger relative to the dwarf planet Pluto, at 1/9 Pluto's mass. [j] [153] The Earth and the Moon's barycentre, their common center of mass, is located 1,700 km (1,100 mi) (about a quarter of Earth's radius) beneath the Earth's surface.

The Earth revolves around the Earth-Moon barycentre once a sidereal month, with 1/81 the speed of the Moon, or about 12.5 metres (41 ft) per second. This motion is superimposed on the much larger revolution of the Earth around the Sun at a speed of about 30 kilometres (19 mi) per second.

The surface area of the Moon is slightly less than the areas of North and South America combined.

Appearance from Earth

The synchronous rotation of the Moon as it orbits the Earth results in it always keeping nearly the same face turned towards the planet. However, because of the effect of libration, about 59% of the Moon's surface can actually be seen from Earth. The side of the Moon that faces Earth is called the near side, and the opposite the far side. The far side is often inaccurately called the "dark side", but it is in fact illuminated as often as the near side: once every 29.5 Earth days. During new moon, the near side is dark. [154]

The Moon originally rotated at a faster rate, but early in its history its rotation slowed and became tidally locked in this orientation as a result of frictional effects associated with tidal deformations caused by Earth. [155] With time, the energy of rotation of the Moon on its axis was dissipated as heat, until there was no rotation of the Moon relative to Earth. In 2016, planetary scientists using data collected on the 1998-99 NASA Lunar Prospector mission, found two hydrogen-rich areas (most likely former water ice) on opposite sides of the Moon. It is speculated that these patches were the poles of the Moon billions of years ago before it was tidally locked to Earth. [156]

The Moon has an exceptionally low albedo, giving it a reflectance that is slightly brighter than that of worn asphalt. Despite this, it is the brightest object in the sky after the Sun. [80] [k] This is due partly to the brightness enhancement of the opposition surge the Moon at quarter phase is only one-tenth as bright, rather than half as bright, as at full moon. [157] Additionally, color constancy in the visual system recalibrates the relations between the colors of an object and its surroundings, and because the surrounding sky is comparatively dark, the sunlit Moon is perceived as a bright object. The edges of the full moon seem as bright as the center, without limb darkening, because of the reflective properties of lunar soil, which retroreflects light more towards the Sun than in other directions. The Moon does appear larger when close to the horizon, but this is a purely psychological effect, known as the Moon illusion, first described in the 7th century BC. [158] The full Moon's angular diameter is about 0.52° (on average) in the sky, roughly the same apparent size as the Sun (see § Eclipses).

The Moon's highest altitude at culmination varies by its phase and time of year. The full moon is highest in the sky during winter (for each hemisphere). The orientation of the Moon's crescent also depends on the latitude of the viewing location an observer in the tropics can see a smile-shaped crescent Moon. [159] The Moon is visible for two weeks every 27.3 days at the North and South Poles. Zooplankton in the Arctic use moonlight when the Sun is below the horizon for months on end. [160]

The distance between the Moon and Earth varies from around 356,400 km (221,500 mi) to 406,700 km (252,700 mi) at perigee (closest) and apogee (farthest), respectively. On 14 November 2016, it was closer to Earth when at full phase than it has been since 1948, 14% closer than its farthest position in apogee. [161] Reported as a "supermoon", this closest point coincided within an hour of a full moon, and it was 30% more luminous than when at its greatest distance because its angular diameter is 14% greater and 1.14 2 ≈ 1.30 approx 1.30> . [162] [163] [164] At lower levels, the human perception of reduced brightness as a percentage is provided by the following formula: [165] [166]

When the actual reduction is 1.00 / 1.30, or about 0.770, the perceived reduction is about 0.877, or 1.00 / 1.14. This gives a maximum perceived increase of 14% between apogee and perigee moons of the same phase. [167]

There has been historical controversy over whether features on the Moon's surface change over time. Today, many of these claims are thought to be illusory, resulting from observation under different lighting conditions, poor astronomical seeing, or inadequate drawings. However, outgassing does occasionally occur and could be responsible for a minor percentage of the reported lunar transient phenomena. Recently, it has been suggested that a roughly 3 km (1.9 mi) diameter region of the lunar surface was modified by a gas release event about a million years ago. [168] [169]

The Moon's appearance, like the Sun's, can be affected by Earth's atmosphere. Common optical effects are the 22° halo ring, formed when the Moon's light is refracted through the ice crystals of high cirrostratus clouds, and smaller coronal rings when the Moon is seen through thin clouds. [170]

The illuminated area of the visible sphere (degree of illumination) is given by ( 1 − cos ⁡ e ) / 2 = sin 2 ⁡ ( e / 2 ) (e/2)> , where e is the elongation (i.e., the angle between Moon, the observer on Earth, and the Sun).

Eclipses

Eclipses only occur when the Sun, Earth, and Moon are all in a straight line (termed "syzygy"). Solar eclipses occur at new moon, when the Moon is between the Sun and Earth. In contrast, lunar eclipses occur at full moon, when Earth is between the Sun and Moon. The apparent size of the Moon is roughly the same as that of the Sun, with both being viewed at close to one-half a degree wide. The Sun is much larger than the Moon but it is the vastly greater distance that gives it the same apparent size as the much closer and much smaller Moon from the perspective of Earth. The variations in apparent size, due to the non-circular orbits, are nearly the same as well, though occurring in different cycles. This makes possible both total (with the Moon appearing larger than the Sun) and annular (with the Moon appearing smaller than the Sun) solar eclipses. [172] In a total eclipse, the Moon completely covers the disc of the Sun and the solar corona becomes visible to the naked eye. Because the distance between the Moon and Earth is very slowly increasing over time, [173] the angular diameter of the Moon is decreasing. Also, as it evolves toward becoming a red giant, the size of the Sun, and its apparent diameter in the sky, are slowly increasing. [l] The combination of these two changes means that hundreds of millions of years ago, the Moon would always completely cover the Sun on solar eclipses, and no annular eclipses were possible. Likewise, hundreds of millions of years in the future, the Moon will no longer cover the Sun completely, and total solar eclipses will not occur. [174]

Because the Moon's orbit around Earth is inclined by about 5.145° (5° 9') to the orbit of Earth around the Sun, eclipses do not occur at every full and new moon. For an eclipse to occur, the Moon must be near the intersection of the two orbital planes. [175] The periodicity and recurrence of eclipses of the Sun by the Moon, and of the Moon by Earth, is described by the saros, which has a period of approximately 18 years. [176]

Because the Moon continuously blocks the view of a half-degree-wide circular area of the sky, [m] [177] the related phenomenon of occultation occurs when a bright star or planet passes behind the Moon and is occulted: hidden from view. In this way, a solar eclipse is an occultation of the Sun. Because the Moon is comparatively close to Earth, occultations of individual stars are not visible everywhere on the planet, nor at the same time. Because of the precession of the lunar orbit, each year different stars are occulted. [178]

Tidal effects

The gravitational attraction that masses have for one another decreases inversely with the square of the distance of those masses from each other. As a result, the slightly greater attraction that the Moon has for the side of Earth closest to the Moon, as compared to the part of the Earth opposite the Moon, results in tidal forces. Tidal forces affect both the Earth's crust and oceans.

The most obvious effect of tidal forces is to cause two bulges in the Earth's oceans, one on the side facing the Moon and the other on the side opposite. This results in elevated sea levels called ocean tides. [173] As the Earth rotates on its axis, one of the ocean bulges (high tide) is held in place "under" the Moon, while another such tide is opposite. As a result, there are two high tides, and two low tides in about 24 hours. [173] Since the Moon is orbiting the Earth in the same direction of the Earth's rotation, the high tides occur about every 12 hours and 25 minutes the 25 minutes is due to the Moon's time to orbit the Earth. The Sun has the same tidal effect on the Earth, but its forces of attraction are only 40% that of the Moon's the Sun's and Moon's interplay is responsible for spring and neap tides. [173] If the Earth were a water world (one with no continents) it would produce a tide of only one meter, and that tide would be very predictable, but the ocean tides are greatly modified by other effects: the frictional coupling of water to Earth's rotation through the ocean floors, the inertia of water's movement, ocean basins that grow shallower near land, the sloshing of water between different ocean basins. [179] As a result, the timing of the tides at most points on the Earth is a product of observations that are explained, incidentally, by theory.

While gravitation causes acceleration and movement of the Earth's fluid oceans, gravitational coupling between the Moon and Earth's solid body is mostly elastic and plastic. The result is a further tidal effect of the Moon on the Earth that causes a bulge of the solid portion of the Earth nearest the Moon. Delays in the tidal peaks of both ocean and solid-body tides cause torque in opposition to the Earth's rotation. This "drains" angular momentum and rotational kinetic energy from Earth's rotation, slowing the Earth's rotation. [173] [180] That angular momentum, lost from the Earth, is transferred to the Moon in a process (confusingly known as tidal acceleration), which lifts the Moon into a higher orbit and results in its lower orbital speed about the Earth. Thus the distance between Earth and Moon is increasing, and the Earth's rotation is slowing in reaction. [180] Measurements from laser reflectors left during the Apollo missions (lunar ranging experiments) have found that the Moon's distance increases by 38 mm (1.5 in) per year (roughly the rate at which human fingernails grow). [181] [182] [183] Atomic clocks also show that Earth's day lengthens by about 17 microseconds every year, [184] [185] [186] slowly increasing the rate at which UTC is adjusted by leap seconds. This tidal drag would continue until the rotation of Earth and the orbital period of the Moon matched, creating mutual tidal locking between the two and suspending the Moon over one meridian (this is currently the case with Pluto and its moon Charon). However, the Sun will become a red giant engulfing the Earth-Moon system long before this occurrence. [187] [188]

In a like manner, the lunar surface experiences tides of around 10 cm (4 in) amplitude over 27 days, with three components: a fixed one due to Earth, because they are in synchronous rotation, a variable tide due to orbital eccentricity and inclination, and a small varying component from the Sun. [180] The Earth-induced variable component arises from changing distance and libration, a result of the Moon's orbital eccentricity and inclination (if the Moon's orbit were perfectly circular and un-inclined, there would only be solar tides). [180] Libration also changes the angle from which the Moon is seen, allowing a total of about 59% of its surface to be seen from Earth over time. [80] The cumulative effects of stress built up by these tidal forces produces moonquakes. Moonquakes are much less common and weaker than are earthquakes, although moonquakes can last for up to an hour – significantly longer than terrestrial quakes – because of scattering of the seismic vibrations in the dry fragmented upper crust. The existence of moonquakes was an unexpected discovery from seismometers placed on the Moon by Apollo astronauts from 1969 through 1972. [189]

According to recent research, scientists suggest that the Moon's influence on the Earth may contribute to maintaining Earth's magnetic field. [190]

Before spaceflight

One of the earliest-discovered possible depictions of the Moon is a 5000-year-old rock carving Orthostat 47 at Knowth, Ireland. [191] [192]

Understanding of the Moon's cycles was an early development of astronomy: by the 5th century BC , Babylonian astronomers had recorded the 18-year Saros cycle of lunar eclipses, [193] and Indian astronomers had described the Moon's monthly elongation. [194] The Chinese astronomer Shi Shen (fl. 4th century BC) gave instructions for predicting solar and lunar eclipses. [195] ( p411 ) Later, the physical form of the Moon and the cause of moonlight became understood. The ancient Greek philosopher Anaxagoras (d. 428 BC) reasoned that the Sun and Moon were both giant spherical rocks, and that the latter reflected the light of the former. [196] [195] ( p227 ) Although the Chinese of the Han Dynasty believed the Moon to be energy equated to qi, their 'radiating influence' theory also recognized that the light of the Moon was merely a reflection of the Sun, and Jing Fang (78–37 BC) noted the sphericity of the Moon. [195] ( pp413–414 ) In the 2nd century AD, Lucian wrote the novel A True Story, in which the heroes travel to the Moon and meet its inhabitants. In 499 AD, the Indian astronomer Aryabhata mentioned in his Aryabhatiya that reflected sunlight is the cause of the shining of the Moon. [197] The astronomer and physicist Alhazen (965–1039) found that sunlight was not reflected from the Moon like a mirror, but that light was emitted from every part of the Moon's sunlit surface in all directions. [198] Shen Kuo (1031–1095) of the Song dynasty created an allegory equating the waxing and waning of the Moon to a round ball of reflective silver that, when doused with white powder and viewed from the side, would appear to be a crescent. [195] ( pp415–416 )

In Aristotle's (384–322 BC) description of the universe, the Moon marked the boundary between the spheres of the mutable elements (earth, water, air and fire), and the imperishable stars of aether, an influential philosophy that would dominate for centuries. [199] However, in the 2nd century BC , Seleucus of Seleucia correctly theorized that tides were due to the attraction of the Moon, and that their height depends on the Moon's position relative to the Sun. [200] In the same century, Aristarchus computed the size and distance of the Moon from Earth, obtaining a value of about twenty times the radius of Earth for the distance. These figures were greatly improved by Ptolemy (90–168 AD): his values of a mean distance of 59 times Earth's radius and a diameter of 0.292 Earth diameters were close to the correct values of about 60 and 0.273 respectively. [201] Archimedes (287–212 BC) designed a planetarium that could calculate the motions of the Moon and other objects in the Solar System. [202]

During the Middle Ages, before the invention of the telescope, the Moon was increasingly recognised as a sphere, though many believed that it was "perfectly smooth". [203]

In 1609, Galileo Galilei used an early telescope to make drawings of the Moon for his book Sidereus Nuncius, and deduced that it was not smooth but had mountains and craters. Thomas Harriot had made, but not published such drawings a few months earlier. Telescopic mapping of the Moon followed: later in the 17th century, the efforts of Giovanni Battista Riccioli and Francesco Maria Grimaldi led to the system of naming of lunar features in use today. The more exact 1834–1836 Mappa Selenographica of Wilhelm Beer and Johann Heinrich Mädler, and their associated 1837 book Der Mond, the first trigonometrically accurate study of lunar features, included the heights of more than a thousand mountains, and introduced the study of the Moon at accuracies possible in earthly geography. [204] Lunar craters, first noted by Galileo, were thought to be volcanic until the 1870s proposal of Richard Proctor that they were formed by collisions. [80] This view gained support in 1892 from the experimentation of geologist Grove Karl Gilbert, and from comparative studies from 1920 to the 1940s, [205] leading to the development of lunar stratigraphy, which by the 1950s was becoming a new and growing branch of astrogeology. [80]

1959–1970s

Between the first human arrival with the robotic Soviet Luna program in 1958, to the 1970s with the last Missions of the crewed U.S. Apollo landings and last Luna mission in 1976, the Cold War-inspired Space Race between the Soviet Union and the U.S. led to an acceleration of interest in exploration of the Moon. Once launchers had the necessary capabilities, these nations sent uncrewed probes on both flyby and impact/lander missions.

Soviet missions

Spacecraft from the Soviet Union's Luna program were the first to accomplish a number of goals: following three unnamed, failed missions in 1958, [206] the first human-made object to escape Earth's gravity and pass near the Moon was Luna 1 the first human-made object to impact the lunar surface was Luna 2, and the first photographs of the normally occluded far side of the Moon were made by Luna 3, all in 1959.

The first spacecraft to perform a successful lunar soft landing was Luna 9 and the first uncrewed vehicle to orbit the Moon was Luna 10, both in 1966. [80] Rock and soil samples were brought back to Earth by three Luna sample return missions (Luna 16 in 1970, Luna 20 in 1972, and Luna 24 in 1976), which returned 0.3 kg total. [207] Two pioneering robotic rovers landed on the Moon in 1970 and 1973 as a part of Soviet Lunokhod programme.

Luna 24 was the last Soviet mission to the Moon.

United States missions

During the late 1950s at the height of the Cold War, the United States Army conducted a classified feasibility study that proposed the construction of a staffed military outpost on the Moon called Project Horizon with the potential to conduct a wide range of missions from scientific research to nuclear Earth bombardment. The study included the possibility of conducting a lunar-based nuclear test. [208] [209] The Air Force, which at the time was in competition with the Army for a leading role in the space program, developed its own similar plan called Lunex. [210] [211] [208] However, both these proposals were ultimately passed over as the space program was largely transferred from the military to the civilian agency NASA. [211]

Following President John F. Kennedy's 1961 commitment to a manned Moon landing before the end of the decade, the United States, under NASA leadership, launched a series of uncrewed probes to develop an understanding of the lunar surface in preparation for human missions: the Jet Propulsion Laboratory's Ranger program produced the first close-up pictures the Lunar Orbiter program produced maps of the entire Moon the Surveyor program landed its first spacecraft four months after Luna 9. The crewed Apollo program was developed in parallel after a series of uncrewed and crewed tests of the Apollo spacecraft in Earth orbit, and spurred on by a potential Soviet lunar human landing, in 1968 Apollo 8 made the first human mission to lunar orbit. The subsequent landing of the first humans on the Moon in 1969 is seen by many as the culmination of the Space Race. [212]

Neil Armstrong became the first person to walk on the Moon as the commander of the American mission Apollo 11 by first setting foot on the Moon at 02:56 UTC on 21 July 1969. [213] An estimated 500 million people worldwide watched the transmission by the Apollo TV camera, the largest television audience for a live broadcast at that time. [214] [215] The Apollo missions 11 to 17 (except Apollo 13, which aborted its planned lunar landing) removed 380.05 kilograms (837.87 lb) of lunar rock and soil in 2,196 separate samples. [216] The American Moon landing and return was enabled by considerable technological advances in the early 1960s, in domains such as ablation chemistry, software engineering, and atmospheric re-entry technology, and by highly competent management of the enormous technical undertaking. [217] [218]

Scientific instrument packages were installed on the lunar surface during all the Apollo landings. Long-lived instrument stations, including heat flow probes, seismometers, and magnetometers, were installed at the Apollo 12, 14, 15, 16, and 17 landing sites. Direct transmission of data to Earth concluded in late 1977 because of budgetary considerations, [219] [220] but as the stations' lunar laser ranging corner-cube retroreflector arrays are passive instruments, they are still being used. Ranging to the stations is routinely performed from Earth-based stations with an accuracy of a few centimeters, and data from this experiment are being used to place constraints on the size of the lunar core. [221]

1970s – present

In the 1970s, after the Moon race, the focus of astronautic exploration shifted, as probes like Pioneer 10 and the Voyager program were sent towards the outer solar system. Years of near lunar quietude followed, only broken by a beginning internationalization of space and the Moon through, for example, the negotiation of the Moon treaty.

Since the 1990s, many more countries have become involved in direct exploration of the Moon. In 1990, Japan became the third country to place a spacecraft into lunar orbit with its Hiten spacecraft. The spacecraft released a smaller probe, Hagoromo, in lunar orbit, but the transmitter failed, preventing further scientific use of the mission. [222] In 1994, the U.S. sent the joint Defense Department/NASA spacecraft Clementine to lunar orbit. This mission obtained the first near-global topographic map of the Moon, and the first global multispectral images of the lunar surface. [223] This was followed in 1998 by the Lunar Prospector mission, whose instruments indicated the presence of excess hydrogen at the lunar poles, which is likely to have been caused by the presence of water ice in the upper few meters of the regolith within permanently shadowed craters. [224]

The European spacecraft SMART-1, the second ion-propelled spacecraft, was in lunar orbit from 15 November 2004 until its lunar impact on 3 September 2006, and made the first detailed survey of chemical elements on the lunar surface. [225]

The ambitious Chinese Lunar Exploration Program began with Chang'e 1, which successfully orbited the Moon from 5 November 2007 until its controlled lunar impact on 1 March 2009. [226] It obtained a full image map of the Moon. Chang'e 2, beginning in October 2010, reached the Moon more quickly, mapped the Moon at a higher resolution over an eight-month period, then left lunar orbit for an extended stay at the Earth–Sun L2 Lagrangian point, before finally performing a flyby of asteroid 4179 Toutatis on 13 December 2012, and then heading off into deep space. On 14 December 2013, Chang'e 3 landed a lunar lander onto the Moon's surface, which in turn deployed a lunar rover, named Yutu (Chinese: 玉兔 literally "Jade Rabbit"). This was the first lunar soft landing since Luna 24 in 1976, and the first lunar rover mission since Lunokhod 2 in 1973. Another rover mission (Chang'e 4) was launched in 2019, becoming the first ever spacecraft to land on the Moon's far side. China intends to following this up with a sample return mission (Chang'e 5) in 2020. [227]

Between 4 October 2007 and 10 June 2009, the Japan Aerospace Exploration Agency's Kaguya (Selene) mission, a lunar orbiter fitted with a high-definition video camera, and two small radio-transmitter satellites, obtained lunar geophysics data and took the first high-definition movies from beyond Earth orbit. [228] [229] India's first lunar mission, Chandrayaan-1, orbited from 8 November 2008 until loss of contact on 27 August 2009, creating a high-resolution chemical, mineralogical and photo-geological map of the lunar surface, and confirming the presence of water molecules in lunar soil. [230] The Indian Space Research Organisation planned to launch Chandrayaan-2 in 2013, which would have included a Russian robotic lunar rover. [231] [232] However, the failure of Russia's Fobos-Grunt mission has delayed this project, and was launched on 22 July 2019. The lander Vikram attempted to land on the lunar south pole region on 6 September, but lost the signal in 2.1 km (1.3 mi). What happened after that is unknown.

The U.S. co-launched the Lunar Reconnaissance Orbiter (LRO) and the LCROSS impactor and follow-up observation orbiter on 18 June 2009 LCROSS completed its mission by making a planned and widely observed impact in the crater Cabeus on 9 October 2009, [233] whereas LRO is currently in operation, obtaining precise lunar altimetry and high-resolution imagery. In November 2011, the LRO passed over the large and bright crater Aristarchus. NASA released photos of the crater on 25 December 2011. [234]

Two NASA GRAIL spacecraft began orbiting the Moon around 1 January 2012, [235] on a mission to learn more about the Moon's internal structure. NASA's LADEE probe, designed to study the lunar exosphere, achieved orbit on 6 October 2013.

Future

Upcoming lunar missions include Russia's Luna-Glob: an uncrewed lander with a set of seismometers, and an orbiter based on its failed Martian Fobos-Grunt mission. [236] Privately funded lunar exploration has been promoted by the Google Lunar X Prize, announced 13 September 2007, which offers US$20 million to anyone who can land a robotic rover on the Moon and meet other specified criteria. [237]

NASA began to plan to resume human missions following the call by U.S. President George W. Bush on 14 January 2004 for a human mission to the Moon by 2019 and the construction of a lunar base by 2024. [238] The Constellation program was funded and construction and testing begun on a crewed spacecraft and launch vehicle, [239] and design studies for a lunar base. [240] That program was cancelled in 2010, however, and was eventually replaced with the Donald Trump supported Artemis program, which plans to return humans to the Moon by 2025. [241] India had also expressed its hope to send people to the Moon by 2020. [242]

On 28 February 2018, SpaceX, Vodafone, Nokia and Audi announced a collaboration to install a 4G wireless communication network on the Moon, with the aim of streaming live footage on the surface to Earth. [243]

Recent reports also indicate NASA's intent to send a woman astronaut to the Moon in their planned mid-2020s mission. [244]

Planned commercial missions

In 2007, the X Prize Foundation together with Google launched the Google Lunar X Prize to encourage commercial endeavors to the Moon. A prize of $20 million was to be awarded to the first private venture to get to the Moon with a robotic lander by the end of March 2018, with additional prizes worth $10 million for further milestones. [245] [246] As of August 2016, 16 teams were reportedly participating in the competition. [247] In January 2018 the foundation announced that the prize would go unclaimed as none of the finalist teams would be able to make a launch attempt by the deadline. [248]

In August 2016, the US government granted permission to US-based start-up Moon Express to land on the Moon. [249] This marked the first time that a private enterprise was given the right to do so. The decision is regarded as a precedent helping to define regulatory standards for deep-space commercial activity in the future. Previously, private companies were restricted to operating on or around Earth. [249]

On 29 November 2018 NASA announced that nine commercial companies would compete to win a contract to send small payloads to the Moon in what is known as Commercial Lunar Payload Services. According to NASA administrator Jim Bridenstine, "We are building a domestic American capability to get back and forth to the surface of the moon.". [250]

Human impact

Beside the traces of human activity on the Moon, there have been some intended permanent installations like the Moon Museum art piece, Apollo 11 goodwill messages, six Lunar plaques, the Fallen Astronaut memorial, and other artifacts.

Infrastructure

Longterm missions continuing to be active are some orbiters such as the 2009-launched Lunar Reconnaissance Orbiter surveilling the Moon for future missions, as well as some Landers such as the 2013-launched Chang'e 3 with its Lunar Ultraviolet Telescope still operational. [251]

There are several missions by different agencies and companies planned to establish a longterm human presence on the Moon, with the Lunar Gateway as the currently most advanced project as part of the Artemis program.

Astronomy from the Moon

For many years, the Moon has been recognized as an excellent site for telescopes. [252] It is relatively nearby astronomical seeing is not a concern certain craters near the poles are permanently dark and cold, and thus especially useful for infrared telescopes and radio telescopes on the far side would be shielded from the radio chatter of Earth. [253] The lunar soil, although it poses a problem for any moving parts of telescopes, can be mixed with carbon nanotubes and epoxies and employed in the construction of mirrors up to 50 meters in diameter. [254] A lunar zenith telescope can be made cheaply with an ionic liquid. [255]

In April 1972, the Apollo 16 mission recorded various astronomical photos and spectra in ultraviolet with the Far Ultraviolet Camera/Spectrograph. [256]

Living on the Moon

Humans have stayed for days on the Moon, such as during Apollo 17. [257] One particular challenge for astronauts' daily life during their stay on the surface is the lunar dust sticking to their suits and being carried into their quarters. Subsequently, the dust was tasted and smelled by the astronauts, calling it the "Apollo aroma". [258] This contamination poses a danger since the fine lunar dust can cause health issues. [258]

In 2019 at least one plant seed sprouted in an experiment, carried along with other small life from Earth on the Chang'e 4 lander in its Lunar Micro Ecosystem. [259]

Although Luna landers scattered pennants of the Soviet Union on the Moon, and U.S. flags were symbolically planted at their landing sites by the Apollo astronauts, no nation claims ownership of any part of the Moon's surface. [260] Russia, China, India, and the U.S. are party to the 1967 Outer Space Treaty, [261] which defines the Moon and all outer space as the "province of all mankind". [260] This treaty also restricts the use of the Moon to peaceful purposes, explicitly banning military installations and weapons of mass destruction. [262] The 1979 Moon Agreement was created to restrict the exploitation of the Moon's resources by any single nation, but as of January 2020, it has been signed and ratified by only 18 nations, [263] none of which engages in self-launched human space exploration. Although several individuals have made claims to the Moon in whole or in part, none of these are considered credible. [264] [265] [266]

In 2020, U.S. President Donald Trump signed an executive order called "Encouraging International Support for the Recovery and Use of Space Resources". The order emphasizes that "the United States does not view outer space as a 'global commons ' " and calls the Moon Agreement "a failed attempt at constraining free enterprise." [267] [268]

The Declaration of the Rights of the Moon [269] was created by a group of "lawyers, space archaeologists and concerned citizens" in 2021, drawing on precedents in the Rights of Nature movement and the concept of legal personality for non-human entities in space. [270]

Coordination

In light of future development on the Moon some international and multi-space agency organizations have been created:

Mythology

The contrast between the brighter highlands and the darker maria creates the patterns seen by different cultures as the Man in the Moon, the rabbit and the buffalo, among others. In many prehistoric and ancient cultures, the Moon was personified as a deity or other supernatural phenomenon, and astrological views of the Moon continue to be propagated.

In Proto-Indo-European religion, the Moon was personified as the male god *Meh1not. [271] The ancient Sumerians believed that the Moon was the god Nanna, [272] [273] who was the father of Inanna, the goddess of the planet Venus, [272] [273] and Utu, the god of the Sun. [272] [273] Nanna was later known as Sîn, [273] [272] and was particularly associated with magic and sorcery. [272] In Greco-Roman mythology, the Sun and the Moon are represented as male and female, respectively (Helios/Sol and Selene/Luna) [271] this is a development unique to the eastern Mediterranean [271] and traces of an earlier male moon god in the Greek tradition are preserved in the figure of Menelaus. [271]

In Mesopotamian iconography, the crescent was the primary symbol of Nanna-Sîn. [273] In ancient Greek art, the Moon goddess Selene was represented wearing a crescent on her headgear in an arrangement reminiscent of horns. [274] [275] The star and crescent arrangement also goes back to the Bronze Age, representing either the Sun and Moon, or the Moon and planet Venus, in combination. It came to represent the goddess Artemis or Hecate, and via the patronage of Hecate came to be used as a symbol of Byzantium.

An iconographic tradition of representing Sun and Moon with faces developed in the late medieval period.

The splitting of the Moon (Arabic: انشقاق القمر ‎) is a miracle attributed to Muhammad. [276] A song titled 'Moon Anthem' was released on the occasion of landing of India's Chandrayan-II on the Moon. [277]

Calendar

The Moon's regular phases make it a convenient timepiece, and the periods of its waxing and waning form the basis of many of the oldest calendars. Tally sticks, notched bones dating as far back as 20–30,000 years ago, are believed by some to mark the phases of the Moon. [278] [279] [280] The

30-day month is an approximation of the lunar cycle. The English noun month and its cognates in other Germanic languages stem from Proto-Germanic *mǣnṓth-, which is connected to the above-mentioned Proto-Germanic *mǣnōn, indicating the usage of a lunar calendar among the Germanic peoples (Germanic calendar) prior to the adoption of a solar calendar. [281] The PIE root of moon, *méh1nōt, derives from the PIE verbal root *meh1-, "to measure", "indicat[ing] a functional conception of the Moon, i.e. marker of the month" (cf. the English words measure and menstrual), [282] [283] [284] and echoing the Moon's importance to many ancient cultures in measuring time (see Latin mensis and Ancient Greek μείς (meis) or μήν (mēn), meaning "month"). [285] [286] [287] [288] Most historical calendars are lunisolar. The 7th-century Islamic calendar is an example of a purely lunar calendar, where months are traditionally determined by the visual sighting of the hilal, or earliest crescent moon, over the horizon. [289]

Lunar effect

The lunar effect is a purported unproven correlation between specific stages of the roughly 29.5-day lunar cycle and behavior and physiological changes in living beings on Earth, including humans.

The Moon has long been particularly associated with insanity and irrationality the words lunacy and lunatic (popular shortening loony) are derived from the Latin name for the Moon, Luna. Philosophers Aristotle and Pliny the Elder argued that the full moon induced insanity in susceptible individuals, believing that the brain, which is mostly water, must be affected by the Moon and its power over the tides, but the Moon's gravity is too slight to affect any single person. [290] Even today, people who believe in a lunar effect claim that admissions to psychiatric hospitals, traffic accidents, homicides or suicides increase during a full moon, but dozens of studies invalidate these claims. [290] [291] [292] [293] [294]


Going Moon-Mad

People once believed that moonlight had a powerful effect on human behavior. Those who acted strangely were said to be "moonstruck," and lunacy, a term for madness, comes from Luna, the Latin name for the moon goddess. The Japanese believed that the moon was a god with powers to foretell the future. Priests would study the moon's reflection in a mirror, believing that if they gazed directly at the moon, it might drive them mad. Superstitions about the moon's evil influence made some people refuse to sleep in a place where moonbeams could touch them. In the 1200s, the English philosopher Roger Bacon wrote, "Many have died from not protecting themselves from the rays of the moon."

lunar relating to the moon

immortality ability to live forever

A myth from the Indonesian island of Java tells how Nawang Wulan, the moon goddess, came to earth to bathe in a lake. A man stole her cloak of swan's feathers so she could no longer fly back up into the sky, and she stayed on earth and married him. Nawang Wulan used her magic powers to feed the household every day with just a single grain of rice. When her husband discovered her secret, she lost her magic power and had to gather and pound rice every day like all other wives. However, she did find her swan-feather cloak and used it to return to the sky. She stayed there at night but spent the daylight hours on earth with her husband and daughter.


Co-formation theory

Moons can also form at the same time as their parent planet. Under such an explanation, gravity would have caused material in the early solar system to draw together at the same time as gravity bound particles together to form Earth. Such a moon would have a very similar composition to the planet, and would explain the moon's present location. However, although Earth and the moon share much of the same material, the moon is much less dense than our planet, which would likely not be the case if both started with the same heavy elements at their core.

In 2012, researcher Robin Canup, of the Southwest Research Institute in Texas, proposed that Earth and the moon formed at the same time when two massive objects five times the size of Mars crashed into each other.

"After colliding, the two similar-sized bodies then re-collided, forming an early Earth surrounded by a disk of material that combined to form the moon," NASA said. "The re-collision and subsequent merger left the two bodies with the similar chemical compositions seen today.


An Ancient Greek Philosopher Was Exiled for Claiming the Moon Was a Rock, Not a God

Close to the north pole of the moon lies the crater Anaxagoras, named for a Greek philosopher who lived in the fifth century B.C. The eponym is fitting, as Anaxagoras the man was one of the first people in history to suggest the moon was a rocky body, not all too dissimilar from Earth. Streaks of material thrown out during the impact that formed the crater extend 560 miles southward to the rim of another crater, this one named for Plato.

Like Plato, Anaxagoras the scholar did most of his work in Athens, but the similarities between the two men stop there. Influenced strongly by the Pythagoreans, Plato posited a mystical universe based on sacred geometric forms, including perfectly circular orbits. Plato eschewed observation and experimentation, preferring to pursue a pure knowledge he believed was innate in all humans. But Anaxagoras, who died around the time Plato was born, had a knack for astronomy, an area of study that requires careful observational and calculation to unlock the mysteries of the universe.

During his time in Athens, Anaxagoras made several fundamental discoveries about the moon. He reiterated and expended upon an idea that likely emerged among his predecessors but was not widely accepted in antiquity: that the moon and sun were not gods, but rather objects. This seemingly innocuous belief would ultimately result in Anaxagoras’ arrest and exile.

Anaxagoras crater near the north pole of the moon, imaged by the Lunar Orbiter 4 spacecraft in 1967. (NASA)

Piecing together the lives of early philosophers such as Anaxagoras, who is thought to have written just one book, lost to us today, can be a major challenge for historians. Modern scholars have only “fragments” to describe the life of Anaxagoras—brief quotes from his teachings and short summaries of his ideas, cited within the works of scholars from later generations, such as Plato and Aristotle.

Through persistent observation, Anaxagoras came to believe that the moon was a rock, not totally unlike the Earth, and he even described mountains on the lunar surface. The sun, he thought, was a burning rock. In fragment 18, Anaxagoras says, “It is the sun that puts brightness into the moon.” While Anaxagoras was not the first to realize that moonlight is reflected light from the sun, he was able to use this concept to correctly explain additional natural phenomena, such as eclipses and lunar phases.

Hailing from Clazomenae in the Ionian lands east of the Greek mainland, Anaxagoras grew up during the Ionian Enlightenment, an intellectual revolution that began around 600 B.C. As a young man, he saw Athens and Sparta align to drive the Persian Empire out of Ionia. When he relocated to Athens, Anaxagoras and his contemporaries brought philosophy to the budding Athenian democracy. Although many Greek philosophers of the sixth and fifth centuries B.C. believed in one or a few fundamental elements—such as water, air, fire and earth—Anaxagoras thought there must be an infinite number of elements. This idea was his way of resolving an intellectual dispute concerning the nature of existence that had emerged between the naturalistic-minded philosophers of Ionia to the east and the mystical-minded philosophers to the west, in Greek-colonized Italy, such as Pythagoras and his followers.

Daniel Graham, a professor of philosophy at Brigham Young University and one of the few Anaxagoras experts in the world, says that of the Italian-based philosophers, Parmenides in particular influenced Anaxagoras and his ideas about astronomy.

“Anaxagoras turns the problem of lunar light into a problem of geometry,” Graham says. He noted that when the moon is on the opposite side of the Earth than the sun, the full face is illuminated, “[producing] a model of the heavens that predicts not only phases of the moon, but how eclipses are possible.”

The moon’s phases, Anaxagoras realized, were the result of different portions of the celestial object being illuminated by the sun from Earth’s perspective. The philosopher also realized that the occasional darkening of the moon must result from the moon, sun and Earth lining up such that the moon passes into the Earth’s shadow—a lunar eclipse. When the moon passes directly in front of the sun, the skies darken during the day, a phenomenon Anaxagoras also described and we now call a solar eclipse.

The total lunar eclipse of October 8, 2014, as photographed from California. When the shadow of the Earth covers the moon, only light filtered through Earth's atmosphere reaches the lunar surface, casting the moon in a reddish glow. (Alfredo Garcia, Jr. / Flickr under CC BY-SA 2.0)

Anaxagoras also wrestled with the origins and formation of the moon, a mystery that still challenges scientists today. The philosopher proposed that the moon was a big rock which the early Earth had flung into space. This concept anticipated a scenario for the moon’s origin that physicist George Darwin, son of Charles Darwin, would propose 23 centuries later. Known as the fission hypothesis, Darwin’s idea was that the moon began as a chunk of Earth and was hurled into space by the Earth’s rapid rotation, leaving behind the Pacific basin. (Today, many astronomers believe that a Mars-sized body slammed into the early Earth, expelling material that then coalesced into the moon, though other theories exist for the origin of our natural satellite.)

By describing the moon as a rock of terrestrial origin, and the sun as a burning rock, Anaxagoras moved beyond earlier thinkers, even those who realized the moon was a kind of reflector. This forward thinking got Anaxagoras labeled as a chief denier of the idea that the moon and sun were deities.

Such an idea should have been welcome in democratic Athens, but Anaxagoras was a teacher and friend of the influential statesman Pericles, and political factions would soon conspire against him. In power for over 30 years, Pericles would lead Athens into the Peloponnesian wars against Sparta. While the exact causes of these conflicts are a matter of debate, Pericles’ political opponents in the years leading to the wars blamed him for excessive aggression and arrogance. Unable to hurt the Athenian leader directly, Pericles’ enemies went after his friends. Anaxagoras was arrested, tried and sentenced to death, ostensibly for breaking impiety laws while promoting his ideas about the moon and sun.

“In the Athenian democracy, with its ‘democratic’ trials before large juries on criminal charges being brought by private citizens—there was no district attorney—all trials were basically political trials,” Graham says. “They were often disguised as being about religion or morality, but they aimed at embarrassing some public figure by going after him directly if he was vulnerable, or a member of his circle if he was not. If you wanted to attack Pericles, but he was too popular to attack directly, you found the weakest link in his group. As a foreigner and intellectual with unorthodox new ideas, Pericles’ friend and ‘science advisor’ Anaxagoras was an obvious target.”

Still holding some political sway, Pericles was able to free Anaxagoras and prevent his execution. Though his life was spared, the philosopher who questioned the divinity of the moon found himself in exile in Lampsacus at the edge of the Hellespont. But his ideas regarding eclipses and lunar phases would live on to this day, and for his recognition of the true nature of the moon, a lunar crater, visited by orbiting spacecraft some 2,400 years later, bears the name Anaxagoras.

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