The crisis of the classical theory of the origin of the moon

A few decades later, after scientists seem to have agreed on a single theory, a new evidence of a possible way for the appearance of the moon appears that contradicts it.

The artist’s impression of a synestia , a hypothetical object consisting of a vaporized stone that may have spawned the moon

On December 13, 1972, the astronaut Apollo 17 Harrison Schmitt approached a boulder in the Moonsea Sea of ​​Tranquility. “This boulder has a trail coming straight from the hill,” he told his commander, Eugene Cernan, pointing to the marks left by the rolling stone. Cernanus bent down to collect samples, and suggested to Schmitt to imagine what would have happened if they had stood in the way of this stone before it rolled down the slope.

“I would not like to think about it,” said Schmitt.

Astronauts split off pieces of the moon from a boulder. Then, with the help of the Schmitt grabel, he scrubbed the dusty surface, lifting the stone, later called Troctolite 76536 from the regolith and making it into history.

This stone and his brother-boulder tell the story of how the whole moon appeared. In this creation story, described in countless textbooks and explanations of the museum exhibits over the past four decades, the Moon was born in a disastrous clash of the germinal Earth and the rocky world the size of Mars. The second world was named Thea, in honor of the Greek goddess who gave birth to Selena, i.e. The moon Thea hit the Earth with such speed and force that both worlds melted. The remains of Tei’s debris were cooled and hardened, becoming our silver partner.

Modern measurements of the Troctolite 76536 and other stones from the Moon and Mars gave rise to doubts in this story. Over the past five years, a flurry of research has revealed a problem: the canonical hypothesis of shock formation is based on assumptions that do not correspond to the facts. If Thea struck the Earth, and then formed the Moon, the Moon should consist of the same material as Thea. But the Moon does not look like Thea - or like Mars. Up to atoms, it looks almost exactly the same as the Earth.

Astronaut probes stones on the moon

Having met this discrepancy, the researchers of the Moon began to look for new ideas in order to understand how it appeared. The most obvious solution may be the simplest, although it creates other problems with understanding the early stages of the solar system: it is possible that Thea formed the moon, but at the same time Thea consisted of material that was almost identical to the earth. The second possibility is that the collision process is all well mixed, evenly distributed lumps and liquids, like dough for pancakes. This could have happened in an extremely high-energy collision, or in a series of collisions that gave rise to several moons, which later joined together. The third explanation calls into question what we know about the planets. It is possible that the Earth and the Moon have undergone strange metamorphoses and made unusual movements in orbit, which drastically changed their rotation and their future.

Four ways to create the moon

With the emergence of problems in the basic theory of the emergence of the moon, scientists have thrown new theories of its appearance.

Impact formation model

This classical theory, which appeared in the 1970s, claims that a cobblestone the size of Mars, Thea, was banging on young Earth. After the impact, a disk of debris appeared, later united in the moon. But new research has revealed a conflict: computer simulations suggest that the moon should consist of material similar to the Tea material, and a geochemical study of the moon suggests that it consists of material similar to that of the earth.


It is possible that Thea struck the Earth hard enough to allow both objects to evaporate and form a new cosmic structure, a synestium. A rotating cloud of hot debris could thoroughly mix the materials of Tei and the Earth and create the Earth / Moon system with similar geochemistry.

Small moons

It is possible that instead of one serious blow, many comparatively small ones occurred, each of which created the moon. In this model, each collision with an object the size of the moon created a disk of debris, which was gradually collected in the small moon. Then the next blow added debris, and as a result they combined into the big moon.

Clash of twins

Perhaps the simplest alternative would be the one in which Thea consists of the same material as the Earth. But this assumption goes against most of our ideas about the formation of planetary systems.

* * *

To understand what could have happened on Earth’s most memorable day, one needs to understand the youth of the solar system. Four and a half billion years ago, the sun was surrounded by a hot cloud of debris that looked like a donut. The stars generated in the depths of the stars moved around the newly formed Sun, cooling down, and later the epochs, combining - in a process that is not yet clear to us - into lumps, then into planetesimal , then into larger planets. These stony bodies actively and hard collided, evaporating each other. It was in this tough hell of billiards that the Earth and the Moon appeared.

To get today's Moon, with its size, rotation and speed of escape from Earth, according to our best computer models, it is necessary that a body the size of Mars collides with Earth. Anything more or less would lead to the emergence of a system with a far greater angular momentum. A larger body would also throw too much iron into Earth’s orbit, and create a moon richer in iron than ours.

The moment of a pulse is a quantity that describes the motion and mass of a rotating object or a system of rotating objects: a rotating Earth, a rotating Moon, orbiting around a rotating Earth, and so on. The moment of the impulse is always preserved, that is, it can be acquired or lost only with the intervention of a third force.

Early geochemical studies of troctolite 76536 and other stones supported this story. They showed that moonstones should have arisen in the ocean of lunar magma, which can only be created during shock formation. Troctolite would have fallen into the molten sea as an iceberg breaking away from Antarctica. Based on physical limitations, scientists argued that the moon consists of the remains of Tei. But there is a problem.

Let us return to the early solar system. After the collision and evaporation of the stony worlds, their contents were mixed, and eventually calmed down in certain areas of space. Closer to the sun, where it is hotter, light elements are more likely to evaporate, leaving an excess of heavy isotopes (variants of elements containing additional neutrons). Further from the sun, the stones managed to keep more water and light isotopes. Therefore, scientists can study the mixture of isotopes of an object and determine from which part of the solar system it has arrived, just as the accent gives a person’s homeland.

Sarah Stewart, a planetologist from the University of California, Davis and her student Simon Locke from Harvard University

These differences are so obvious that they are used to classify the planets and types of meteorites. Mars, for example, is so different from Earth that its meteorites can be determined by simply measuring the ratios of three different oxygen isotopes.

In 2001, using advanced mass spectrometry technology, Swiss scientists re-measured troctolite 76536 and 30 other lunar samples. They found that their oxygen isotopes were indistinguishable from those of the earth. Since then, geochemists have studied titanium, tungsten, chromium, rubidium, potassium and other rare metals of the Earth and the Moon, and in general everything looks the same.

This is bad news for Tei. If Mars is so obviously different from Earth, then Thea - and consequently, the Moon - should also be different. If they are the same, it means that the Moon should have been formed from molten fragments of the Earth. The stones from Apollo contradict what the physicist says.

“The canonical model is undergoing a serious crisis,” said Sarah Stewart, a planetologist from the University of California at Davis. “She has not been killed yet, but at the moment it is not working.”

* * *

Stewart tried to reconcile the physical limitations — the need for a colliding body of a certain size, moving at a certain speed — with new geochemical evidence. In 2012, she and Matya uk [Matija Ćuk], now working at the SETI institute, proposed a new physical model of the formation of the moon. They claim that the early Earth rotated quickly, like a dervish , making one revolution in two or three hours, when Thea collided with it. The collision should have led to the appearance of a disk around the Earth, similar to the rings of Saturn - but it would have survived for no longer than 24 hours. As a result, he would cool and heal, forming the moon.

Supercomputers are not powerful enough to completely model this process, but they showed that a projectile crashing into a rapidly rotating world can cut a decent piece of Earth, destroy most of Tei, and assemble the Moon and Earth with similar isotopes content from these fragments. It is like throwing a piece of not yet frozen clay onto a rapidly rotating potter's wheel.

But for an explanation involving a rapidly rotating Earth to work, it is necessary that something else slows its rotation to today. In 2012, Stuart and Chuk argued that with certain interactions with orbital resonance, the Earth could transmit the angular momentum to the Sun. Later, Jack Wise of MTI proposed several different scenarios for the loss of angular momentum by the Earth / Moon system.

But none of the explanations were satisfactory enough. Stewart says that the 2012 models could not explain the orbit or chemical composition of the moon. Then, last year, Simon Locke, a graduate student at Harvard University who studied with Stewart, proposed an updated model in which a planetary structure that was not previously considered is proposed.

In this story, all the particles of Earth and Tei evaporated and formed an inflated cloud in the form of a thick donut. The cloud rotated so fast that it reached the corotation limit [co-rotation limit]. In this state, on the outer edge of the cloud, the evaporated stones rotated so fast that the cloud acquired a new structure in which a thick disk rotates around the inner region. What is important, the disk was not separated from the central region, how the rings were separated from Saturn, and how the disks were separated in the models of the formation of the moon after the collision.

Sinestia could consist of a bagel-shaped mass of evaporated stone surrounding a rocky planet.

The conditions in this structure are hellish to impossibility; there is no surface, instead there are clouds of molten stone, and droplets of molten stone form in each region of the cloud. The moon grew inside this vapor from the drops until the vapor cooled and left the Earth / Moon system behind it.

Given the unusual characteristics of the structure, Locke and Stewart decided that it deserves a new name. They came up with several versions, after which they chose a synesthe, using the Greek prefix syn-, meaning “together,” and the goddess Hestia , the goddess of the family hearth, health and architecture. Stewart says the word means "united structure."

“These bodies are not what you think at first glance. They don't look the way you expected, ”she says.

In May, Locke and Stewart published a paper on the physics of synesthesia; their work on the origin of the moon from synestia is still assessed by experts. They represented this work at conferences of planetologists in the winter and spring, and say that their colleagues were intrigued, but not convinced by this idea. Perhaps because synesthes are still just an idea; unlike planets with rings, of which there are many in our solar system, and protoplanetary disks, of which there are many throughout the universe, no one has yet seen a synestia.

“But this is definitely an interesting way to explain the features of our Moon and help us overcome the difficulty with our non-working model,” says Locke.

* * *

Among the natural satellites in the solar system, the moon stands out for its solitude. Mercury and Venus do not have natural satellites, in particular because of their proximity to the Sun, whose gravitational influence would make the orbits of such moons unstable. Mars has tiny Phobos and Deimos; some believe that they are captured asteroids, while others say that they formed after collisions of bodies with Mars. The gas giants are simply overflowing with moons, some of which are rocky, some are water, some are mixed.

In contrast to these moons, the satellite of the Earth stands out for its size and mass. The mass of the moon is about 1% of the mass of the Earth, while the total mass of the satellites of the outer planets does not exceed one tenth of the mass of their parents. More importantly, the Moon is responsible for 80% of the momentum of the Earth / Moon system, that is, 80% of the movement of the entire system. In outer planets, this figure does not exceed 1%.

But the moon does not necessarily carry such a load. The face of the moon demonstrates that she has been bombarded all her life; Why should we assume that only one stone was cut from the Earth? It is possible that the moon was created by many collisions, says Raluca Rufu, a planetologist from the Weizmann Research Institute in Israel.

In a paper published this winter, she claims that the Moon did not appear all at once on Earth. This collection, created by a thousand cuts - or at least a dozen, according to her simulations. Projectiles flying from different angles and at different speeds, could hit the Earth and form disks collected in small moons, crumbs that are smaller than the current Moon. The interactions between the small moons led to their associations, which formed our present Moon.

Computer simulation: two small moons merge together

Planetologists well accepted her work last year. Robin Kanap, an expert on the Moon from the South-West Research Institute and one of the first authors of the theory of the formation of the Moon, said that it was worth considering. However, additional checks are required. Rufu is not sure if the small moons in their orbit would be fixed in the same way as the moon, always turned to us by one side; if so, she is not sure how they would be united. “That's exactly what we're trying to figure out now,” said Rufu.

Meanwhile, others turned to another explanation of the similarity of the Earth and the Moon, with a very simple answer. Synesthesia, small moons, new physical models - all this is debatable. It is possible that the Moon is very similar to Earth because Thea was also like her.

* * *

The moon is not the only object in the solar system that resembles the earth. In the stones of troctolite 76536, the same ratio of oxygen isotopes as in the rocks of the Earth, and in the group of asteroids called enstatite chondrites . The combination of oxygen isotopes in these asteroids is very similar to the earth, according to Miriam Telas, a cosmochemist who studies meteorites at the Scientific Institute. Carnegie in Washington. “One of the arguments is that they formed in hotter areas of the disk close to the Sun,” she says. They probably formed near the place where the Earth was formed.

Some of these cobblestones formed the Earth, others could form Thea. Enstatite chondrites are detrital material that was never assembled to create a mantle, core, and a complete planet.

In January, Nicholas Daufas, a geophysicist at the University of Chicago, argued that most of the stones that had merged into the Earth were enstatite-type meteorites. He wrote that everything that was formed in the same region should consist of them. The creation of the planets came from those pre-mixed materials that we find now on the Moon and the Earth; they look the same because they are the same. “The gigantic body that crashed and gave birth to the Moon probably possessed an isotopic composition close to that of the earth,” wrote Doufas.

David Stevenson, a Californian Institute of Technology planetary scientist who studied the origin of the moon since first introduced to Tei’s hypothesis in 1974, says he considers this work the most important contribution to the debate over the past year, and claims that it addresses the problem which geochemists have been trying to handle for decades.

Nicholas Daufas holds a piece of enstatite chondrite, such an asteroid, which may consist of material similar to the one that formed the Earth.

“His story is believable. This is a tricky story of how to treat the various elements found on Earth, ”said Stevenson. “And then you can go on to the story of a specific sequence of events that formed the Earth, in which Enstatite chondrites play a large role.”

But not everyone believes in it yet. Questions remain about the number of isotopes of elements such as tungsten - says Stewart. Tungsten-182 is the daughter of hafnium-182, so the ratio of the amount of tungsten to hafnium works like a clock, and determines the age of a particular stone. If there is more tungsten-182 in one stone than in the other, it is safe to say that a tungsten-dominated stone was formed earlier. But the most accurate measurements available show that the ratio of tungsten to hafnium is the same for Earth and the Moon. “It would be a very good coincidence if the two bodies had the same compositions,” concludes Daufas.

* * *

Understanding the Moon - our constant partner, silver sister, the goals of dreamers and researchers from time immemorial - is interesting in itself. But the history of its origin and the history of such stones as troctolite 76536 can become one chapter in a much larger book.

“I see it as a window to a more general question: what happened during the formation of earth-like planets? Said Stephenson. “No one answered him yet.”

Understanding synestia may help in finding the answer; Locke and Stewart believe that synestas in the early solar system could form very quickly when protoplanets collided with each other and melted. Many stony bodies could at the beginning be fat steam halos, so understanding the evolution of synestia could help scientists understand how the moon and other earth-like worlds evolved.

Of course, collecting more samples, especially from the mantles of both bodies, will help the cause, since geochemists will then have more data to work with. They will be able to tell if the oxygen content stored in the depths of the Earth is maintained with depth, or if three common oxygen isotopes dominate each other in different areas.

“When we say that Earth and the Moon are very similar in terms of the content of three oxygen isotopes, we make the assumption that we actually know what the Earth is and what the Moon is,” says Stevenson.

New details in the theory of the origin of the solar system, often based on complex computer simulations, help to understand where the planets were born and where they moved. Scientists are increasingly saying that Mars will not tell this story to us, because it could have been formed not in the same part of the solar system where the Earth, enstatites and Thea appeared. Stephenson says that Mars no longer needs to be used as a barometer of rocky planets.

Moon experts agree that the best answers can be found on Venus, a planet more similar to Earth than others. In her youth, she could have a moon, lost afterwards; she could be very similar to Earth, or unlike. “If we could get a pebble from Venus, it would be very easy for us to answer the question of the origin of the moon. But, unfortunately, nobody has this on the list of priorities, ”says Locke.

The absence of samples from Venus and laboratories capable of testing the unimaginable pressures and temperatures in the center of the collisions leaves the Moon specialists coming up with new models - and revising the original history of the Moon.


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