Where did the Moon come from? Basic theories of origin. Origin of the Moon

You can come up with anything you want about Distant Space. This is difficult to see and few people know about it. But the Moon hangs over our heads every night, and many have probably wondered how it got there.

According to one of the most popular models of the formation of the Moon, the natural satellite of our planet could have appeared as a result of the collision of a certain cosmic body with the Earth more than 4.5 billion years ago. This body was Theia, a protoplanetary object, with the "embryo" of the Earth. The collision resulted in the ejection of Theia and proto-Earth matter into space, and from this matter the Moon was formed, which explains its amazing geological and chemical similarity to our planet.

However, there is no unanimity within this version. Scientists identify three variants of it.

1. Foreign body
According to one theory, the Moon is nothing more than a fragment of a space object that collided with the Earth more than 4 billion years ago. And scientists even call this object the small planet Theia (according to some assumptions, the size of Mars). As a result of a powerful impact, the cosmic body turned into a huge cloud of debris, which, once in Earth orbit, eventually formed into a satellite. This hypothesis, put forward by two groups of American scientists, successfully explained the iron deficiency on the Moon, in contrast to our planet, and some of the dynamic characteristics of the Earth-Moon system. But there is a weak point in it. Chemical analysis showed the identity of the composition of lunar and terrestrial rocks.

2. Fragment of the Earth
According to this version, during a collision with another celestial body, the proto-earth released a substance from which the Moon was formed. This is exactly how, according to Harvard University staff, the similarity in the chemical composition of the Earth and its satellite can be explained.

3. Two in one
This hypothesis complements the previous one, but it states that as a result of a catastrophic collision, part of the mass of earthly matter and the impactor formed a single substance, ejected in molten form into near-Earth orbit. This material created the satellite. In this interpretation, the collision occurred before the formation of the Earth's core, which explains the low iron content in the lunar soil.

As part of a new study, scientists tried to understand in more detail what the further fate of our satellite was after this event.

During the Katarchaean period (geological eon), the Moon looked completely different from what it looks like today. It was more like a hot lump of lava, possessing an exotic super-dense atmosphere of silicon and metal vapors. It was also located 10 times closer to the surface of the Earth than it is today.

In the study, the team concluded that one of the features of the Moon may indicate that the Earth was deprived of oceans of liquid water during the first 400-500 million years of its existence. And such conclusions, in turn, impose serious restrictions on the time of the origin of life on Earth.

As is now generally accepted, in the next few million years after its formation, the Moon moved quite quickly away from the Earth as a result of tidal forces, until it eventually entered the orbit in which it is located today. Subsequently, when the Moon began to always look at the Earth with only one side, this process slowed down sharply, and now it is moving away from our planet at a speed of about 2-4 centimeters per year.

Zhong and his colleagues revealed one unusual detail of this process, drawing attention to the most mysterious feature of the Moon - its unusual “hump” located at the equator. This structure was discovered by the French astronomer Pierre Laplace two centuries ago. Laplace noticed that the Moon was "flattened" about 17-20 times more than it should have been, given the speed of its rotation on its axis.

“The lunar equatorial hump may hold secrets about the early history of Earth’s evolution that we didn’t even know about,” says researcher Shijie Zhong from the University of Colorado Boulder (USA).

Researchers believe that the existence of this structure indicates that in the distant past the Moon rotated much faster than it does today. American planetary scientists tried to understand how quickly the Moon “slowed down” by studying how this “hump” was structured and trying to reproduce its appearance using a computer model of the development of the Solar System.

These observations unexpectedly showed that the generally accepted theories about the rapid deceleration of the Moon in the first years of its existence were erroneous - the rotation speed of the Earth's satellite remained high for at least the first 400 million years of its existence. Otherwise, the Moon would always remain a “liquid” planet or have a completely different shape and size than it does today.

Such a scenario, as Zhong explains, is only possible if the Earth was not at that time covered by an ocean of water comparable in size to the planet’s current hydrosphere. This means that there was no liquid water on the young Earth. It was either absent from it in principle, or was brought after the formation of the “hump” of the Moon, or was on it in solid form, that is, in the form of ice.

So, as we have already found out, one of the most popular theories about the origin of the Moon is the Giant Collision theory. This theory well explains the size of the Moon and its orbital position, but new research published in the journal Nature suggests the opposite: according to scientists, the interaction of the Earth with the cosmic body was like “hitting a watermelon with a sledgehammer.” Having conducted a detailed study of samples of lunar rocks obtained by expeditions of the Apollo series ships back in the 70s of the last century, experts from the University of Washington refuted the theory of forty years ago.

"If old theory was correct, then more than half of the lunar rocks would consist of material from the planetoid that hit the Earth. But instead we see that the isotopic composition of the fragments of the Moon is very specific. The heavy isotopes of potassium found in the samples could only have formed under the influence of incredibly high temperatures. Only a very powerful collision, in which the planetoid and most of the Earth would evaporate on contact, could cause such an effect. In addition, before cooling and becoming a solid, the vapor formed during the collision would have to occupy an area 500 times larger. more area Earth's surface," explains Kun Wang, an assistant professor at the University of Washington and one of the study's authors.

According to scientists, this discovery will change not only the idea of ​​how the Moon was formed, but also the processes that took place throughout the solar system. However, the data is still insufficient, and in order to formulate a new theory, scientists still have a lot of work to do. analytical work with samples.

But there are other versions.

Centrifugal separation hypothesis

The hypothesis of the separation of the Moon from the Earth under the influence of centrifugal forces was first put forward by George Darwin (son of Charles Darwin) in 1878. According to supporters of this theory, the planet’s rotation speed was fast enough for a fragment of matter to separate from the proto-earth, which subsequently formed the Moon. However, already in the 30s of the 20th century, scientists began to be skeptical about such an idea. They argued that the total rotational moment is insufficient to cause “rotational instability” even in a liquid Earth.

Capture theory

IN Lately The version put forward in 1909 by the American astronomer Jackson See is gaining popularity, according to which the Earth and the Moon were formed independently of each other in different parts Solar system. At the moment of the closest passage of the Moon relative to the Earth's orbit, capture by gravitational forces occurred celestial body. This appears to have happened during the human period of Earth's history. Legends of many peoples of the world, in particular the Dogon, tell of times when there was no satellite in the sky. This hypothesis is also indirectly confirmed by the relatively shallow layer of cosmic dust on the surface of the Moon.

"Artificial satellite"

The idea of ​​the artificial origin of the Moon is the most controversial, since the existence of an alien or terrestrial civilization capable of doing this has not yet been proven. Nevertheless, it deserves attention, if only because it was expressed by scientists. In 1960, researchers Mikhail Vasin and Alexander Shcherbakov, studying some of the characteristics of our satellite, came to the idea that it could be of artificial origin. Thus, taking into account the size and depth of the lunar craters formed during the bombardment of cosmic bodies, they suggested that the lunar crust could be made of titanium, the thickness of which, according to preliminary calculations by Soviet scientists, was 32 kilometers. "When I first came across the shocking Soviet theory, explaining the true nature of the Moon, I was shocked, writes American researcher Don Wilson. - At first it seemed incredible to me and, naturally, I rejected it. When our Apollo expeditions brought back more and more evidence confirming the Soviet theory, I was forced to accept it.”

Strange indicators

Adherents of the “artificial Moon” theory drew attention to the very high ratio of the satellite’s mass to the mass of the Earth - 1:81, which is not typical for satellites of other planets in the Solar System. Only Charon and Pluto have higher rates, although the latter is no longer considered a planet. Comparisons of satellite sizes are interesting. For example, the largest satellite of Mars, Phobos, does not exceed 20 km in diameter, while the Moon has this figure of 3560 km. By the way, it is precisely this size of the Moon, which for an earthly observer coincides with the diameter of the Sun, that allows us to periodically see solar eclipses. Finally, the almost perfect circular orbit of the Moon is surprising, while other satellites have an elliptical orbit.

Hollow Moon

Another interesting fact is that the gravitational attraction of the Moon is non-uniform. The crew of Apollo VIII, flying around the satellite, noted that the gravity of the Moon has sharp anomalies - in some places it is “mysteriously intensifying.” Drawing attention to the data of the American crew (which was classified), as well as the low density of the satellite in relation to its mass, nuclear engineer William Bryan stated in 1982 that “the Moon is hollow and relatively rigid.” A number of later studies led scientists to suggest that this cavity is artificial. But the researchers also made bolder conclusions, according to which the Moon was formed “in the opposite direction” - that is, from the surface to the core.

Gas and dust cloud

However, in last years scientists are not ready to seriously consider the version of the artificial origin of the Moon. Much closer to modern scientific views, for example, is the “evaporation theory.” According to this hypothesis, significant masses of matter were released from the earth's plasma, which, when cooled, formed condensate - it became building material for the protomoon. But there is another - a similar idea, put forward in the 18th century. First, the Swedish naturalist Emmanuel Swedenborg and then the French astronomer Pierre-Simon Laplace suggested that interstellar nebulae - clouds of gas and dust in outer space - compress and condense into stars and their surrounding planets. The French scientist also suggested that our satellite could have been formed from this substance. Russian academician E.M. Galimov developed a concept that was temporarily out of fashion, in which the Moon is the result of “fragmentation of a dust condensation.” This hypothesis is based on the results of radioisotope analysis of the satellite and the planet, which show that both bodies have the same age - about 4.5 billion years. In other words, both the Moon and the Earth were formed nearby from matter that was at the same distance from the Sun. According to the scientist, the concept of the origin of the Moon from primary matter, and not from the Earth’s mantle, is in better agreement with the facts than the so far accepted “megaimpact model.”

sources

The moon has a very big size relative to the Earth. The diameter of the Moon at the equator (in the middle part) is 3475 km, which is slightly less than a quarter of the diameter of the Earth. Therefore, some astronomers even believe that the Earth-Moon system should be considered as a double planet. But let us return to the question of the origin of the Moon.

Hypothesis one

In the early stages of Earth's existence, it had a ring system similar to that of Saturn. Perhaps the Moon was formed from them?

Hypothesis two (centrifugal separation)

When the Earth was still very young and consisted of molten rocks, it rotated so quickly that it stretched out, became shaped like a pear, and then the top of this “pear” broke off and turned into the Moon. This hypothesis is jokingly called the “daughter” hypothesis.

Hypothesis three (collisions)

When the Earth was young, it was hit by some celestial body whose size was half the size of the Earth itself. As a result of this collision, a huge amount of material was thrown into outer space, and subsequently the Moon was formed from it.

Hypothesis four (capture)

The Earth and Moon formed independently, in different parts of the solar system. When the Moon passed close to the Earth's orbit, it was captured by the Earth's gravitational field and became its satellite. This hypothesis is jokingly called the “marital” hypothesis.

Hypothesis five (joint education)

The Earth and the Moon formed simultaneously, in close proximity to each other (jokingly - the “sister” hypothesis).

Hypothesis six (many moons)

Several small moons were captured by the Earth's gravity, then they collided with each other, collapsed, and from their debris the present Moon was formed.

Hypothesis seven (evaporation)

From the molten proto-earth, significant masses of matter were evaporated into space, which then cooled, condensed in orbit and formed the proto-moon.

Each of these hypotheses has its pros and cons. Currently, the collision hypothesis is considered the main and more acceptable one. Let's take a closer look at it.

Collision hypothesis

This hypothesis was proposed by William Hartman and Donald Davis in 1975. According to their assumption, a protoplanet (called Theia) approximately the size of Mars collided with the proto-Earth at an early stage of its formation, when the Earth had approximately 90% of its current mass. The blow did not land in the center, but at an angle, almost tangentially. As a result, most of the substance of the impacted object and part of the substance of the earth's mantle were thrown into low-Earth orbit. From these debris, the proto-Moon assembled and began to orbit with a radius of about 60,000 km. As a result of the impact, the Earth received a sharp increase in rotation speed (one revolution in 5 hours) and a noticeable tilt of the rotation axis.

Why is this particular hypothesis about the origin of the Moon considered the main one? She explains everything well known facts O chemical composition and the structure of the Moon, as well as the physical parameters of the Moon-Earth system. Initially, great doubts were raised about the possibility of such a successful collision (oblique impact, low relative speed) of such a large body with the Earth. But then it was suggested that Theia formed in Earth's orbit. This scenario well explains the low impact speed, the impact angle, and the current, almost exactly circular orbit of the Earth.

But this hypothesis also has its vulnerabilities, as, indeed, every hypothesis (after all, HYPOTHESIS translated from ancient Greek means “assumption”).

So, the vulnerability of this hypothesis is as follows: the Moon has a very small iron-nickel core - it makes up only 2-3% of the total mass of the satellite. And the metallic core of the Earth makes up about 30% of the planet’s mass. To explain the iron deficiency on the Moon, we have to accept the assumption that by the time of the collision (4.5 billion years ago) both on Earth and on Theia, a heavy iron core had already been released and a light silicate mantle had formed. But no unambiguous geological evidence for this assumption has been found.

We have already become accustomed to our only natural satellite, which tirelessly circles our planet every 28 days. The moon dominates our night sky, and since ancient times it has touched people's most poetic chords. Although new understandings of many lunar mysteries have been proposed over the past few decades, a large number of Unresolved questions still surround our only natural satellite.

Compared to other planets of ours solar system both the orbital path and size of our Moon are quite significant anomalies. Other planets, of course, also have satellites. But planets with weak gravitational influences, such as Mercury, Venus and Pluto, do not have them. The Moon is one quarter the size of the Earth. Compare this to the huge Jupiter or Saturn, which have several relatively small moons (Jupiter's moon is 1/80 its size), and our Moon seems to be a fairly rare cosmic phenomenon.

Another interesting detail: the distance from the Moon to the Earth is quite small, and in apparent size the Moon is equal to our Sun. This curious coincidence is most obvious during full solar eclipses, when the Moon completely covers our nearest star.

Finally, the Moon's almost perfect circular orbit differs from the orbits of other satellites, which tend to be elliptical.

The gravitational center of the Moon is almost 1,800 m closer to the Earth than its geometric center. With such significant discrepancies, scientists still cannot explain how the Moon manages to maintain its almost perfectly circular orbit.

The gravitational attraction on the Moon is not uniform. The crew aboard Apollo VIII, while flying near the lunar ocean, noticed that the Moon's gravity had sharp anomalies. In some places, gravity seems to mysteriously increase.

The problem of the origin of the Moon is discussed in scientific literature for more than a hundred years. Her solution has great importance to understand the early history of the Earth, the mechanisms of formation of the Solar system, the origin of life.

First a logical explanation for the origin of the Moon was put forward in the 19th century. George Darwin, the son of Charles Darwin, the author of the theory of natural selection, was a famous and authoritative astronomer who carefully studied the Moon and in 1878 came up with the so-called separation theory. Apparently, George Darwin was the first astronomer to establish that the Moon was moving away from the Earth. Based on the speed of divergence of the two celestial bodies, J. Darwin suggested that the Earth and the Moon once formed a single whole. In the distant past, this molten viscous sphere rotated very quickly around its axis, making one full revolution in about five and a half hours.

Darwin suggested that the tidal influence of the Sun subsequently caused the so-called separation: a piece of molten Earth the size of the Moon separated from the main mass and eventually took its position in orbit. This theory looked quite reasonable and became dominant at the beginning of the 20th century. It only came under serious attack in the 1920s, when British astronomer Harold Jeffreys showed that the viscosity of the Earth in a semi-molten state would prevent vibrations strong enough to cause the two celestial bodies to separate.

Second theory, which once convinced a number of specialists, was called the accretion theory. It said that a disk of dense particles, reminiscent of the rings of Saturn, gradually accumulated around the already formed Earth. It was assumed that particles from this disk eventually came together to form the Moon.

There are several reasons why this explanation may not be satisfactory. One of the main ones is the angular momentum of the Earth-Moon system, which would never have become what it is if the Moon had formed from an accretion disk. There are also difficulties associated with the formation of oceans of molten magma on the “newborn” Moon.

Third theory about the origin of the Moon appeared around the time when the first lunar probes were launched; it is called the holistic capture theory. It was assumed that the Moon arose far from the Earth and became a wandering celestial body, which was simply captured by Earth's gravity and entered orbit around the Earth.

Now this theory has also fallen out of fashion for several reasons. The ratio of oxygen isotopes in rocks on Earth and the Moon strongly suggests that they formed at the same distance from the Sun, which could not have been the case if the Moon had formed elsewhere. There are also insurmountable difficulties in trying to construct a model in which a celestial body the size of the Moon could enter a stationary orbit around the Earth. Such a huge object could not carefully “float” to Earth at low speed, like a supertanker mooring to a pier; it almost inevitably had to crash into the Earth at high speed or fly next to it and rush on.

By the mid-1970s, all previous theories of the formation of the Moon had encountered difficulties for one reason or another. This created the almost unthinkable situation where renowned experts could publicly admit that they simply did not know how or why the Moon ended up where it did.

From this uncertainty was born new theory, which is now generally accepted, despite some serious issues. It is known as the "big impact" theory.

The idea originated in the Soviet Union in the 60s. from the Russian scientist B.C. Savronov, who considered the possibility of the emergence of planets from millions of asteroids of different sizes, called planetsimals.

In an independent study, Hartmann and his colleague D.R. Davis suggested that the Moon was formed as a result of the collision of two planetary bodies, one of which was the Earth, and the other was a wandering planet, not inferior in size to Mars. Hartmann and Davis believed that the two planets collided in a specific way, resulting in ejections of material from the mantle of both celestial bodies. This material was thrown into orbit, where it gradually combined and became denser to form the Moon.

New information obtained through detailed study of samples from the Moon has almost confirmed the collision theory: 4.57 billion years ago, the protoplanet Earth (Gaia) collided with the protoplanet Theia. The blow did not land in the center, but at an angle (almost tangentially). As a result, most of the substance of the impacted object and part of the substance of the earth's mantle were thrown into low-Earth orbit.

From these debris, the proto-Moon assembled and began to orbit with a radius of about 60,000 km. As a result of the impact, the Earth received a sharp increase in rotation speed (one revolution in 5 hours) and a noticeable tilt of the rotation axis.

In two new studies published in the latest issue of the journal Nature, scientists provide evidence that the chemical similarities between the Earth and the Moon are due to extensive mixing of material formed when the Earth collides with another planet.

Thus, supporters of the main theory of the origin of the earth’s satellite received new confirmation of their correctness, and quite significant ones at that. But, German scientists argue that other theories cannot simply be written off, since new data, although they seriously confirm the main theory, are still not one hundred percent. Therefore, there is still an opportunity to choose for yourself the closest theory of all existing ones, or even come up with a new one!

The most main mystery The moon lies in its origin. We still don't know where the Moon came from. But there are plenty of hypotheses about the origin of the Moon. Let's look at them.

But first

About the Moon

The Earth has only one satellite - the Moon. It moves around the Earth in an orbit at an average distance from it of 376,284 km.

The Earth's gravitational force gradually slows down the rotation of the Moon around its axis, so that now the Moon goes around its entire path around the Earth in exactly the same time as it takes one rotation around its axis. This synchronous rotation means that when we look at the Moon from Earth, we always see only one side of it. Reverse side The moons were only seen by astronauts and spaceships.

As the Moon moves around the Earth, the Sun illuminates different parts of its surface.

Look at the picture. You see on it what the Moon looks like from the same point on the Earth, being at different points its orbit: lunar crescent, half of the lunar disk (first quarter), waxing Moon, full moon, waning Moon, half of the lunar disk (last quarter), lunar crescent.

The Moon is very large relative to the Earth. The diameter of the Moon at the equator (in the middle part) is 3475 km, which is slightly less than a quarter of the diameter of the Earth. Therefore, some astronomers even believe that the Earth-Moon system should be considered as a double planet.

But let us return to the question of the origin of the Moon.

Hypotheses about the origin of the Moon

Hypothesis one

In the early stages of Earth's existence, it had a ring system similar to that of Saturn. Perhaps the Moon was formed from them?

Hypothesis two (centrifugal separation)

When the Earth was still very young and consisted of molten rocks, it rotated so quickly that it stretched out, became shaped like a pear, and then the top of this “pear” broke off and turned into the Moon. This hypothesis is jokingly called the “daughter” hypothesis.

Hypothesis three (collisions)

When the Earth was young, it was hit by some celestial body whose size was half the size of the Earth itself. As a result of this collision, a huge amount of material was thrown into outer space, and subsequently the Moon was formed from it.

Hypothesis four (capture)

The Earth and Moon formed independently, in different parts of the solar system. When the Moon passed close to the Earth's orbit, it was captured by the Earth's gravitational field and became its satellite. This hypothesis is jokingly called the “marital” hypothesis.

Hypothesis five (joint education)

The Earth and the Moon formed simultaneously, in close proximity to each other (jokingly - the “sister” hypothesis).

Hypothesis six (many moons)

Several small moons were captured by the Earth's gravity, then they collided with each other, collapsed, and from their debris the present Moon was formed.

Hypothesis seven (evaporation)

From the molten proto-earth, significant masses of matter were evaporated into space, which then cooled, condensed in orbit and formed the proto-moon.

Each of these hypotheses has its pros and cons. Currently, the collision hypothesis is considered the main and more acceptable one. Let's take a closer look at it.

This hypothesis was proposed by William Hartman and Donald Davis in 1975. According to their assumption, the protoplanet (they called it Theia) about the size of Mars collided with the proto-Earth early in its formation, when the Earth had approximately 90% of its current mass. The blow did not land in the center, but at an angle, almost tangentially. As a result, most of the substance of the impacted object and part of the substance of the earth's mantle were thrown into low-Earth orbit. From these debris, the proto-Moon assembled and began to orbit with a radius of about 60,000 km. As a result of the impact, the Earth received a sharp increase in rotation speed (one revolution in 5 hours) and a noticeable tilt of the rotation axis.

Why is this particular hypothesis about the origin of the Moon considered the main one? It explains well all the known facts about the chemical composition and structure of the Moon, as well as the physical parameters of the Moon-Earth system. Initially, great doubts were raised about the possibility of such a successful collision (oblique impact, low relative speed) of such a large body with the Earth. But then it was suggested that Theia formed in Earth's orbit. This scenario well explains the low impact speed, the impact angle, and the current, almost exactly circular orbit of the Earth.

But this hypothesis also has its vulnerabilities, as, indeed, every hypothesis (after all, HYPOTHESIS translated from ancient Greek means “assumption”).

So, the vulnerability of this hypothesis is as follows: the Moon has a very small iron-nickel core - it makes up only 2-3% of the total mass of the satellite. And the metallic core of the Earth makes up about 30% of the planet’s mass. To explain the iron deficiency on the Moon, we have to accept the assumption that by the time of the collision (4.5 billion years ago) both on Earth and on Theia, a heavy iron core had already been released and a light silicate mantle had formed. But no unambiguous geological evidence for this assumption has been found.

And second: if the Moon had somehow ended up in the Earth’s orbit at such a distant time and after that had not undergone significant shocks, then, according to calculations, a multi-meter layer of dust settling from space would have accumulated on its surface, which was not confirmed during space landings. devices on the lunar surface.

So…

Until the 60s of the 20th century, the main hypotheses of the origin of the Moon were three: centrifugal separation, capture and joint formation. One of the main goals of the American lunar expeditions of 1960-1970 was to find evidence of one of these hypotheses. The first data obtained revealed serious contradictions with all three hypotheses. But during the Apollo flights there was no hypothesis of a giant collision yet. . It is she who is now dominant .

A few words from the history of the problem

Of the planets in the inner solar system, which includes Mercury, Venus, Earth and Mars, only Earth has a massive satellite, the Moon. Mars also has satellites: Phobos and Deimos, but these are small bodies irregular shape. The largest of them, Phobos, is only 20 km in maximum dimension, while the diameter of the Moon is 3560 km.

The Moon and Earth have different densities. This is caused not only by the fact that the Earth is large and, therefore, its interior is under greater pressure. The average density of the Earth, normalized to normal pressure (1 atm) is 4.45 g/cm 3 , the density of the Moon is 3.3 g/cm 3 . The difference is due to the fact that the Earth contains a massive iron-nickel core (with an admixture of light elements), which contains 32% of the Earth's mass. The size of the Moon's core remains unclear. But taking into account the low density of the Moon and the limitation imposed by the value of the moment of inertia (0.3931), the Moon cannot contain a core exceeding 5% of its mass. The most probable, based on the interpretation of geophysical data, is considered to be an interval of 1–3%, that is, the radius of the lunar core is 250–450 km.

By the middle of the last century, several hypotheses of the origin of the Moon had been formed: the separation of the Moon from the Earth; accidental capture of the Moon into low-Earth orbit; coaccretion of the Moon and Earth from a swarm of solid bodies. Until recently, this problem was solved by specialists in the field of celestial mechanics, astronomy and planetary physics. Geologists and geochemists did not take part in it, since nothing was known about the composition of the Moon before the start of its study by spacecraft.

Already in the 30s. last century, it was shown that the hypothesis of the separation of the Moon from the Earth, put forward, by the way, by J. Darwin, the son of Charles Darwin, is untenable. The total rotational moment of the Earth and the Moon is insufficient for the occurrence of rotational instability (loss of matter under the influence of centrifugal force) even in the liquid Earth.

In the 60s. Experts in the field of celestial mechanics came to the conclusion that the capture of the Moon into low-Earth orbit is an extremely unlikely event. There remained the coaccretion hypothesis, which was developed by domestic researchers, students of O.Yu. Shmidt V.S. Safronov and E.L. Ruskol. Her weak side– inability to explain the different densities of the Moon and Earth. Clever but implausible scenarios were invented for how the Moon could lose excess iron. When details of the chemical structure and composition of the Moon became known, this hypothesis was finally rejected. Just in the mid-1970s. appeared new script formation of the Moon. American scientists A. Cameron and V. Ward and at the same time V. Hartman and D. Davis in 1975 proposed a hypothesis of the formation of the Moon as a result of a catastrophic collision with the Earth of a large cosmic body the size of Mars (mega-impact hypothesis). As a result, a huge mass of earthly matter and partly the material of the impactor (a celestial body that collided with the Earth) melted and was thrown into low-Earth orbit. This material quickly accumulated into a compact body that became the Moon. Despite its apparent exoticism, this hypothesis became generally accepted because it offered a simple solution to a number of problems. As computer modeling has shown, from a dynamic point of view, the collision scenario is quite feasible. Moreover, he provides an explanation for the increased angular momentum of the Earth-Moon system and the tilt of the Earth's axis. The lower iron content in the Moon is also easily explained, since it is assumed that a catastrophic collision occurred after the formation of the Earth's core. Iron turned out to be mainly concentrated in the Earth's core, and the Moon was formed from the rocky material of the Earth's mantle.

Rice. 1 – The collision of the Earth with a celestial body approximately the size of Mars, which resulted in the ejection of molten matter that formed the Moon (mega-impact hypothesis).
Drawing by V.E. Kulikovsky.

By the mid-1970s, when samples of lunar soil were delivered to Earth, the geochemical properties of the Moon were quite well studied, and in a number of parameters it actually showed good similarity with the composition of the Earth’s mantle. Therefore, such prominent geochemists as A. Ringwood (Australia) and H. Wenke (Germany) supported the mega-impact hypothesis. In general, the problem of the origin of the Moon from the category of astronomical ones rather moved into the category of geological and geochemical ones, since it was geochemical arguments that became decisive in the system of evidence for one or another version of the formation of the Moon. These versions differed only in details: the relative sizes of the Earth and the impactor, what was the age of the Earth when the collision occurred. The strike concept itself was considered unshakable. Meanwhile, some details of the geochemical analysis cast doubt on the hypothesis as a whole.

The problem of "volatile" and isotope fractionation

The issue of iron deficiency on the Moon played a decisive role in the discussion of the origin of the Moon. Another fundamental problem - the extreme depletion of the Earth's natural satellite in volatile elements - remained in the shadows.

The Moon contains many times less K, Na and other volatile elements compared to carbonaceous chondrites. The composition of carbonaceous chondrites is considered to be closest to the original cosmic matter from which the bodies of the Solar System were formed. We usually perceive as “volatile” compounds of carbon, nitrogen, sulfur and water, which easily evaporate when heated to a temperature of 100–200 o C. At temperatures of 300–500 o C, especially under low pressure conditions, for example, in contact with vacuum of space, volatility is characteristic of elements that we usually observe in the composition of solids. The Earth also contains few volatile elements, but the Moon is noticeably depleted in them even compared to the Earth.

It would seem that there is nothing surprising in this. Indeed, in accordance with the impact hypothesis, it is assumed that the Moon was formed as a result of the ejection of molten matter into near-Earth orbit. It is clear that in this case part of the substance could evaporate. Everything would be well explained if not for one detail. The fact is that during evaporation a phenomenon called isotope fractionation occurs. For example, carbon consists of two isotopes 12 C and 13 C, oxygen has three isotopes - 16 O, 17 O and 18 O, the element Mg contains stable isotopes 24 Mg and 26 Mg, etc. During evaporation, the light isotope outstrips the heavy one, so the residual substance must be enriched in the heavy isotope of the element that was lost. The American scientist R. Clayton and his colleagues showed experimentally that with the observed loss of potassium on the Moon, the ratio 41 K/39 K should have changed by 60‰. With the evaporation of 40% of the melt, the isotope ratio of magnesium (26 Mg/ 24 Mg) would change by 11–13‰, and silicon (30 Si/ 28 Si) – by 8–10‰. These are very large shifts, considering that the modern accuracy of measuring the isotopic composition of these elements is no worse than 0.5‰. Meanwhile, no shift in the isotopic composition, that is, any traces of isotopic fractionation of volatiles, was found in the lunar substance.

A dramatic situation arose. On the one hand, the impact hypothesis was proclaimed unshakable, especially in the American scientific literature, on the other hand, it was not combined with isotopic data.

R. Clayton (1995) noted: "These isotopic data are inconsistent with almost all proposed mechanisms for depletion of volatile elements by evaporation of condensed matter." H. Jones and H. Palme (2000) concluded that "evaporation cannot be considered as a mechanism leading to volatile depletion due to irreducible isotopic fractionation."

Moon formation model

Ten years ago, I put forward a hypothesis, the meaning of which was that the Moon was formed not as a result of a catastrophic impact, but as a binary system simultaneously with the Earth as a result of the fragmentation of a cloud of dust particles. This is how double stars are formed. Iron, which the Moon is depleted of, was lost along with other volatiles as a result of evaporation.

Rice. 2 – Formation of the Earth and the Moon from a common dust disk in accordance with the author’s hypothesis about the origin of the Earth and the Moon as a binary system.

But can such fragmentation actually occur at the values ​​of mass, angular momentum, and other things that the Earth-Moon system has? This remained unknown. Several researchers joined a group to study this problem. It included well-known experts in the field of space ballistics: academician T.M. Eneev, back in the 70s. who investigated the possibility of accumulation of planetary bodies by combining dust concentrations; famous mathematician academician V.P. Myasnikov (unfortunately, has already passed away); a major specialist in the field of gas dynamics and supercomputers, Corresponding Member of the Russian Academy of Sciences A.V. Zabrodin; Doctor of Physical and Mathematical Sciences M.S. Easy access; Doctor of Chemical Sciences Yu.I. Sidorov. Later we were joined by Doctor of Physical and Mathematical Sciences, specialist in the field of computer modeling A.M. Krivtsov from St. Petersburg, who made a significant contribution to solving the problem. Our efforts were aimed at solving the dynamic problem of the formation of the Moon and Earth.

However, the idea of ​​the Moon losing iron through evaporation would seem to be in as much conflict with the lack of traces of isotopic fractionation on the Moon as the impact hypothesis. In fact, there was a remarkable difference here. The fact is that isotope fractionation occurs when isotopes irreversibly leave the surface of the melt. Then, due to the greater mobility of the light isotope, a kinetic isotope effect occurs (the above values ​​of isotope shifts are due precisely to this effect). But another situation is possible when evaporation occurs in a closed system. In this case, the evaporated molecule can return to the melt again. Then some equilibrium is established between the melt and steam. It is clear that more volatile components accumulate in the vapor phase. But due to the fact that there is both direct and reverse transition of molecules between steam and melt, the isotope effect turns out to be very small. This is a thermodynamic isotope effect. At elevated temperatures it can be negligible. The idea of ​​a closed system is not applicable to a melt ejected into low-Earth orbit and evaporating into outer space. But it fully corresponds to the process occurring in a cloud of particles. The evaporating particles are surrounded by their vapor, and the cloud as a whole is in a closed system.

Rice. 3 – Kinetic and thermodynamic isotope effects: a) the kinetic isotope effect during melt evaporation leads to the enrichment of the steam with light isotopes of volatile elements, and the melt with heavy isotopes; b) thermodynamic isotope effect that occurs when there is equilibrium between liquid and vapor. It may be negligible at elevated temperatures; c) a closed system of particles surrounded by their own vapor. Evaporated particles can return to the melt again.

Let us now assume that the cloud is compressed as a result of gravity. It collapses. Then the part of the substance that has turned into vapor is squeezed out of the cloud, and the remaining particles turn out to be depleted of volatiles. In this case, almost no fractionation of isotopes is observed!

Several versions of the solution to the dynamic problem were considered. The most successful model of particle dynamics (a variant of the molecular dynamics model) proposed by A.M. Krivtsov.

Let's imagine that there is a cloud of particles, each of which moves in accordance with the equation of Newton's second law, as is known, including mass, acceleration and the force causing the movement. The force of interaction between each particle and all other particles f includes several components: gravitational interaction, elastic force acting upon collision of particles (manifests itself at very small distances), and the inelastic part of the interaction, as a result of which the collision energy is converted into heat.

It was necessary to accept certain initial conditions. The solution was carried out for a cloud of particles that has the mass of the Earth–Moon system and has angular momentum characterizing the system of these bodies. In fact, these parameters for the initial cloud could differ slightly, both up and down. Based on the convenience of computer calculations, a two-dimensional model was considered - a disk with an unevenly distributed surface density. In order to describe the behavior of a real three-dimensional object in the parameters of a two-dimensional model, similarity criteria were introduced using dimensionless coefficients. Another condition: it was necessary to attribute to the particle, in addition to the angular velocity, a certain chaotic velocity. Mathematical calculations and some other technical details can be omitted here.

A computer calculation of a model based on the above principles and conditions well describes the collapse of a cloud of particles. In this case, a central body of elevated temperature was formed. However, the main thing was missing. There was no fragmentation of the particle cloud, that is, one body arose, and not the Earth-Moon binary system. Generally speaking, there was nothing unexpected in this. As already mentioned, attempts to simulate the formation of the Moon by breaking away from the rapidly rotating Earth have previously been unsuccessful. The angular momentum of the Earth-Moon system was insufficient to split the overall body into two fragments. The same thing happened with the cloud of particles.

However, the situation changed radically when the phenomenon of evaporation was taken into account.

The process of evaporation from the particle surface causes a repulsion effect. The force of this repulsion is inversely proportional to the square of the distance from the evaporating particle:

where λ is a proportionality coefficient that takes into account the magnitude of the flow evaporating from the surface of the particle; m is the mass of the particle.

The structure of the formula characterizing gas-dynamic repulsion looks similar to the expression for the gravitational force, if instead of λ we substitute γ - the gravitational constant. Strictly speaking, there is no complete similarity of these forces, since the gravitational interaction is long-range, and the repulsive force of evaporation is local. However, as a first approximation, they can be combined:

This yields a certain effective constant γ", less than γ.

It is clear that a decrease in the coefficient γ will lead to the appearance of rotational instability at lower values ​​of angular momentum. The question is what should be the evaporation flux so that the requirements for the initial angular velocity of the cloud decrease so much that the real angular momentum of the Earth-Moon system turns out to be sufficient for fragmentation to occur.

The estimates performed showed that the flow should be very small and fit into quite plausible values ​​of time and mass. Namely, for chondrules (spherical particles that make up chondrite meteorites) with a size of approximately 1 mm, with a temperature of the order of 1000 K and a density of ~ 2 g/cm 3 , the flux should be approximately 10–13 kg/m 2 s. In this case, a decrease in the mass of the evaporating particle by 40% will take a time of the order of (3 - 7) 10 4 years, which is consistent with the possible order of 10 5 years for the time scale of the initial accumulation of planetary bodies. Computer simulations using real parameters clearly showed the emergence of rotational instability, culminating in the formation of two heated bodies, one of which would become the Earth, and the other the Moon.

Rice. 4 – Computer model of the collapse of a cloud of evaporating particles. The successive phases of cloud fragmentation (a–d) and the formation of a binary system (e–f) are shown. The calculations used real parameters characterizing the Earth–Moon system: kinetic moment K = 3.45 10 34 kg m 2 s –1 ; total mass of the Earth and Moon M = 6.05 10 24 kg, radius of a solid body with the total mass of the Earth and Moon Rc = 6.41 10 6 m; gravitational constant "gamma" = 6.67 10 –11 kg –1 m 3 s –2; initial cloud radius R0 = 5.51 Rc; the number of calculated particles is N = 10 4, the value of the evaporation flux is 10 –13 kg m –2 s –1, corresponding to approximately 40% of the evaporation of the mass of particles with a chondrule size of about 1 mm over 10 4 – 10 5 years. An increase in temperature is conventionally shown by a change in color from blue to red.

Thus, the proposed dynamic model explains the possibility of the emergence of the Earth-Moon binary system. In this case, evaporation leads to the loss of volatile elements under conditions of a practically closed system, which ensures the absence of a noticeable isotope effect.

Iron deficiency problem

The explanation of the iron deficiency on the Moon compared to the Earth (and the primary cosmic matter - carbonaceous chondrites) at one time became the most convincing argument in favor of the impact hypothesis. It is true that the impact hypothesis has difficulties here too. Indeed, the Moon contains less iron than the Earth, but more than the Earth's mantle from which it is thought to have formed. Perhaps the Moon additionally inherited the impactor iron. But then it should be enriched not only with iron relative to the earth’s mantle, but also with siderophile elements (W, P, Mo, Co, Cd, Ni, Pt, Re, Os, etc.) accompanying iron. In iron-silicate melts they join the iron phase. Meanwhile, the Moon is depleted in siderophile elements, although it contains more iron than the Earth's mantle. In order to reconcile the impact hypothesis with observations, the latest models increasingly increase the mass of the impactor that collided with the Earth, and conclude that its predominant contribution to the composition of the Moon's material is made. But here a new complication arises for the impact hypothesis. The substance of the Moon, as follows from isotopic data, is strictly related to the substance of the Earth. Indeed, the isotopic compositions of samples from the Moon and Earth lie on the same line in the coordinates δ 18 O and δ 17 O (the ratio of the oxygen isotopes 17 O and 18 O to 16 O). This is how samples belonging to the same cosmic body behave. Samples of other cosmic bodies occupy other lines. As long as the Moon was considered to have formed from mantle material, the coincidence of isotopic characteristics supported this hypothesis. However, if the substance of the Moon is substantially formed from the substance of an unknown celestial body, the coincidence of isotopic characteristics no longer supports the impact hypothesis.

Rice. 5 – Comparative content of iron (Fe) and iron oxide (FeO) in the Earth and the Moon.

Rice. 6 – Diagram of oxygen isotope ratios δ 17 O and δ 18 O (δ 17 O and δ 18 O are values ​​characterizing the shifts in oxygen isotope ratios 17 O/ 16 O and 18 O/ 16 O, relative to the accepted SMOW standard). In this diagram, samples from the Moon and Earth fall along a common fractionation line, indicating the genetic relatedness of their composition.

The extreme depletion of the Moon in volatile elements and the role of evaporation in the dynamics of the formation of the Earth-Moon system allow us to interpret the problems of iron deficiency in a completely different way.

Based on our model, it is necessary to find out how the Moon is depleted in iron, and why the Moon is depleted in iron, but the Earth is not, despite the fact that as a result of fragmentation, two bodies with similar formation conditions arise.

Laboratory experiments have shown that iron is also a relatively volatile element. If you evaporate a melt that has a primary chondritic composition, then after the evaporation of the most volatile components (compounds of carbon, sulfur and a number of others), alkaline elements (K, Na) will begin to evaporate, and then it will be iron’s turn. Further evaporation will lead to volatilization of Si, followed by Mg. Ultimately, the melt will be enriched in the most difficult to volatile elements Al, Ca, Ti. The listed substances are among the rock-forming elements. They are part of the minerals that make up the bulk (99%) of rocks. Other elements form impurities and minor minerals.

Rice. 7 – After the formation of two hot nuclei (red spots), a significant part of the cooler (green and blue) material of the initial cloud of particles remains in the surrounding space (particle sizes are increased).

Note: The Earth's core (its mass is taken into account, which is 32% of the planet's mass) contains, in addition to iron, nickel and other siderophile elements, as well as up to 10% of light elements. It can be oxygen, sulfur, silicon, and, less likely, impurities of other elements. Data for the Moon are taken from S. Taylor (1979). Estimates of the composition of the Moon vary greatly among different authors. It seems to us that S. Taylor’s assessments are the most justified (Galimov, 2004).

The Moon is depleted in Fe and enriched in difficultly volatile elements: Al, Ca, Ti. The higher content of Si and Mg in the Moon is an illusion caused by iron deficiency. If the loss of volatiles is due to the evaporation process, then the content of only the most difficult to volatile elements will remain unchanged in relation to the original composition. Therefore, in order to make comparisons between chondrites (CI), the Earth and the Moon, all concentrations should be attributed to an element whose abundance is assumed to be constant.

Then the depletion of the Moon not only in iron, but also in silicon and magnesium is clearly revealed. Based on experimental data, this should be expected given a significant loss of iron during evaporation.

A. Hashimoto (1983) evaporated a melt that initially had a chondritic composition. An analysis of his experiment reveals that at 40% evaporation, the residual melt acquires a composition almost similar to that of the Moon. Thus, the composition of the Moon, including the observed iron deficiency, can be obtained during the formation of the Earth's satellite from primordial chondritic material. And then there is no need for the catastrophic impact hypothesis.

Asymmetry of growth of embryos of the Earth and the Moon

The second of the questions asked above remains - why the Earth is not depleted in iron, as well as silicon and magnesium, to the same extent as the Moon. The answer required solving another computer problem. First of all, we note that after fragmentation and the formation of two hot bodies in a collapsing cloud, a large amount of matter remains in the cloud of particles surrounding them. The surrounding mass of matter remains cold compared to the relatively high-temperature consolidated nuclei.

Rice. 8 – Computer simulations show that the larger of the resulting nuclei (red) develops much faster and accumulates most of the remaining initial cloud of particles (blue).

Initially, both fragments, both the one that was to become the Moon and the one that was to become the Earth, were depleted in volatiles and iron by almost to the same degree. However, computer modeling showed that if one of the fragments turned out to be (by chance) slightly larger in mass than the other, then further accumulation of matter proceeds extremely asymmetrically. Germ bigger size grows much faster. As the difference in size increases, the difference in the rates of accumulation of matter from the remaining part of the cloud increases like an avalanche. As a result, the smaller embryo only slightly changes its composition, while the larger embryo (the future Earth) accumulates almost all of the primary matter of the cloud and ultimately acquires a composition very close to that of the primary chondritic matter, with the exception of the most volatile components, irrevocably leaving the collapsing cloud. Let us note again that the loss of volatile elements in this case occurs not due to evaporation in space, but due to the squeezing out of the residual vapor by the collapsing cloud.

Thus, the proposed model explains the super-depletion of the Moon in volatiles and the deficiency of iron in it. The main feature of the model is the introduction into consideration of the evaporation factor, and under conditions that exclude or reduce to small values ​​the fractionation of isotopes. This overcomes the fundamental difficulty faced by the mega-impact hypothesis. The evaporation factor made it possible for the first time to obtain a mathematical solution to the development of the Earth-Moon binary system under real physical parameters. It seems to us that the new concept we have proposed of the origin of the Moon from primordial matter, and not from the Earth’s mantle, is in better agreement with the facts than the American mega-impact hypothesis.

Upcoming Challenges

Although many questions have been answered, many still remain and a major new problem is emerging. It is as follows. In our calculations, we proceeded from the fact that the Earth and the Moon, at least their embryos measuring 2–3 thousand km, arose from a cloud of particles. Meanwhile, the existing theory of planetary accumulation describes the formation of planetary bodies as a result of the collision of solid bodies (planetesimals), first meter-long, then kilometer-long, hundred-kilometer, etc. sizes. Consequently, our model requires that during the early stage of development of a protoplanetary disk, large concentrations of dust, rather than an ensemble of solid bodies, arise in it and grow to an almost planetary mass. If this is really the case, then we're talking about not only about the model of the origin of the Earth-Moon system, but also about the need to revise the theory of planetary accumulation as a whole.

Questions remain regarding the following aspects of the hypothesis:

• a more detailed calculation of the temperature profile in a collapsing cloud is required, combined with a thermodynamic analysis of the distribution of elements in the particle-vapor system different levels this profile (until this is done, the model remains rather a qualitative hypothesis);
• it is necessary to obtain a more rigorous expression for gas-dynamic repulsion, taking into account the local nature of the action of this force, in contrast to gravitational interaction.
• The model leaves aside the question of the influence of the Sun, the radius of the disk is chosen arbitrarily, and the deforming influence of the collision of clumps during the formation of the disk is not considered.
• to obtain a more rigorous solution, it would be important to move to a three-dimensional formulation of the problem and increase the number of model particles;
• it is necessary to consider cases of the formation of a binary system from a protodisk of less mass than the total mass of the Earth and the Moon, since it is likely that the accumulation process occurred in two stages - at the early stage - the collapse of the dust concentration with the formation of a binary system, and at the late stage - additional growth due to the collision of solid bodies formed by that time in the Solar System;
• In the dynamic part of our model, the question of the reason for the high value of the initial moment of rotation of the Earth-Moon system and the noticeable inclination of the Earth’s axis to the ecliptic plane remains undeveloped, while the mega-impact hypothesis offers such a solution.

The answers to these questions largely depend on general solution the above-mentioned problem of the evolution of condensations in a protoplanetary gas-dust disk around the Sun.

Finally, it should be borne in mind that our hypothesis assumes some elements of heterogeneous accretion (layer-by-layer formation of a celestial body), although in the opposite sense to that accepted. Proponents of heterogeneous accretion assumed that planets first form an iron core in one way or another, and then a predominantly silicate mantle shell grows. In our model, an iron-depleted embryo initially appears, and only subsequent accumulation brings forth iron-enriched material. It is clear that this significantly modifies the process of core formation and the associated conditions for fractionation of siderophile elements, and other geochemical parameters. Thus, the proposed concept opens up new aspects of research in the dynamics of the formation of the solar system and in geochemistry.