The Origin of the Moon
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According to modern data, in many respects the Moon is very different from the Earth, primarily in its chemical composition: there is practically no water (although noticeable ice reserves have been found in the circumpolar regions[1]), a low content of volatile elements and compounds.
Analysis of lunar rocks suggests that the Moon has undergone a complete meltdown, unlike the Earth.
The density of the Moon is comparable to the density of the Earth's mantle, but it has a very small iron nickel core.
However, a great similarity between the Earth and the Moon has also been discovered.
Radioisotope analysis shows that both celestial bodies have approximately the same age: about 4.5 billion years.
The ratio of stable oxygen isotopes on the Moon and on Earth is the same, at the same time, it is very different from this ratio for all known meteorites.
This indicates that the Earth and the Moon were formed in the neighborhood — from a substance that was at the same distance from the Sun in a protoplanetary cloud.
This combination of common properties and significant differences in the structure of the Moon and the Earth gave rise to three groups of mutually exclusive assumptions about the origin of the Moon:
the joint formation of the Earth and the Moon from one protoplanetary cloud the capture of the already formed Moon by the Earth the formation of the Moon as a result of a giant collision.
Content
1 Historical views on the origin of the Moon 2 The emergence of the Solar System 3 General and special in the properties of the Moon, the Earth and the Earth Moon system 4 Hypotheses of the origin of the Moon 5 Consideration of hypotheses 5.1 The hypothesis of centrifugal separation 5.2 The capture hypothesis 5.3 The hypothesis of joint formation (joint accretion) 5.4 The evaporation hypothesis 5.5 The Hypothesis of many moons 5.6 The collision hypothesis
6 Conclusion 7 Notes 8 Literature 9 References
Historical views on the origin of the Moon[edit / edit wiki text]
The moon has always amazed humanity with its appearance and the fact of its existence.
In ancient times, many peoples worshiped the Moon as a deity.
The ancient Greeks may have been the first to study the moon using a scientific approach.
In the third century BC , Aristarchus of Samos, observing the earth's shadow on the Moon during lunar eclipses, estimated the distance to the Moon at sixty Earth radii (a remarkable result: according to modern data, the radius of the lunar orbit varies from 55 to 63 earth radii).
Plutarch assumed that Selenite people could live on the moon.
It was believed that the dark spots on the Moon are the seas, and the bright places are the land.
In 1609, Galileo Galilei discovered mountains and craters on the moon, having seen the shadows cast by them through a telescope.
Based on his observations, Galileo came to the conclusion that the Moon is a rocky body, like the Earth.
Since then, many generations of scientists have pondered the mystery of the formation of the moon, starting with Immanuel Kant and Rene Descartes.
From the beginning of the seventeenth century to the middle of the twentieth, several basic hypotheses were put forward, which had their supporters and their popularity ups.
A new era in the study of the Moon began in the 1960s, with the flights to the Moon of Soviet automatic stations and American "Apollo".
A new science has appeared — selenology.
Samples of lunar rocks were brought to Earth, which provided rich material for reflection and re evaluation of old ideas.
The emergence of the Solar system[edit / edit wiki text]
The origin of the Solar system began with the gravitational compression of a gas dust cloud, in the center of which the most massive body, the Sun, was formed.
The substance of the protoplanetary disk gathered into small planetesimals, which collided with each other and formed planets.
Some of the planetesimals were ejected from the inner regions into the Kuiper Belt and into the Oort cloud.
General and special in the properties of the Moon, the Earth and the Earth Moon system[edit / edit wiki text]
Any considered hypothesis of the formation of the Moon should not only comply with physical laws, but also explain the following circumstances:
The average density of the Moon is 3.3 g / cm3, significantly inferior to the average density of the Earth 5.5 g/cm3.
The reason is that the Moon has a very small iron nickel core — it makes up only 2-3 % of the total mass of the satellite (according to the NASA Lunar Prospector mission).
The metal core of the Earth makes up about 30 % of the planet's mass.
The moon, in comparison with the Earth, has a very low content of volatile elements, such as hydrogen, nitrogen, fluorine, and inert gases.
On the contrary, there is a certain surplus of relatively refractory elements on the moon, for example, titanium, uranium and thorium.
The rocks of the lunar crust and the rocks of the Earth's crust and mantle are almost identical in the ratio of stable oxygen isotopes 16O, 17O, 18O (this ratio is sometimes called the "oxygen signature").
For comparison, meteorites from different parts of the Solar System (including the so called Martian meteorites) have completely different ratios of oxygen isotopes.
This identity indicates that the Earth and the Moon (or, at least, the surface of the Moon) were formed from the same layer of planetesimals — at the same distance from the Sun.
The moon has a powerful strong crust with a thickness of 60-80 kilometers (several times thicker than the Earth's crust), formed from anorthosite rocks products of melting of the lunar mantle.
Therefore, it is believed that the Moon was once heated to complete melting.
The earth is believed to have never been completely molten.
The moon and the Earth have an unusually high ratio of the mass of the satellite to the mass of the planet, equal to 1:81, in comparison with the other satellites of the planets of the Solar system.
(Above — only Charon and Pluto, but the latter is no longer considered a planet);
The Earth Moon system has an unusually high angular momentum (second, again, only to the Pluto Charon system).
The plane of the Moon's orbit (inclination of 5° to the ecliptic) does not coincide with the equatorial plane of the Earth (inclination of 23.5° to the ecliptic).
Hypotheses of the origin of the Moon[edit / edit wiki text]
Based on this, the following hypotheses were put forward:
The hypothesis of centrifugal separation: a piece of matter separated from the rapidly rotating proto earth under the action of centrifugal forces, from which the Moon was then formed.
This hypothesis is jokingly called "child".
Capture hypothesis: The Earth and the Moon were 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 "marital".
The hypothesis of joint formation: The Earth and the Moon were formed simultaneously, in close proximity to each other (jokingly — the "sister" hypothesis).
Evaporation hypothesis: significant masses of matter were evaporated into space from the molten proto earth, which then cooled, condensed in orbit and formed a proto moon.
The hypothesis of many moons: several small moons were captured by the Earth's gravity, then they collided with each other, collapsed, and the current Moon was formed from their fragments.
Collision hypothesis: the proto Earth collided with another celestial body, and the Moon was formed from the matter ejected during the collision.
Consideration of hypotheses[edit / edit wiki text]
Before the Apollo flights, three hypotheses of the formation of the Moon were considered the main ones in the scientific world: centrifugal separation, capture, and joint accretion.
In English language literature, they are called the "Big Three" (eng.
The Big Three).
The hypothesis of centrifugal separation[edit / edit wiki text]
The hypothesis of the separation of the Moon from the Earth was first put forward by George Darwin, the son of the famous Charles Darwin, in 1878.
He suggested that after the formation of the young Earth, it rotated at a very high speed.
Under the influence of centrifugal forces, the planet became so elongated along the equator that a large piece of matter broke off from it (perhaps this was facilitated by the tidal forces of the Sun).
The Moon was subsequently formed from this substance.
This hypothesis was supported in 1882 by geologist Osmond Fisher (English)Russian: in his opinion, the Pacific Ocean basin was formed exactly at the place where the future Moon broke away from the Earth.
The Darwin Fisher hypothesis gained great popularity and remained generally accepted at the beginning of the XX century.
Considerations for and against The separation of matter from an excessively stretched equatorial protrusion well explains the existing size of the Moon.
This hypothesis is also well matched by the lower density of the Moon, since it corresponds to the density of the earth's mantle.
Modern data also confirm the fact of a faster rotation of the Earth in the distant past (see tidal acceleration of the Moon).
However, the rotation speed required for centrifugal separation is excessively high (one rotation of the Earth in 1-2 hours).
The angular momentum of the Earth's rotation in this case should have been 3-4 times higher than the current angular momentum of the Earth — Moon system (which is already unusually high).
The appearance of such a moment of rotation momentum in the formed Earth cannot be explained, just as its subsequent disappearance cannot be explained.
A lower content of volatile elements in the lunar substance than that of the Earth does not fit into this hypothesis.
In addition, the modern theory of lithospheric plate tectonics believes that the Pacific basin in its current form has existed for only about 70 million years, and could not have been formed by the separation of matter from the Earth.
Capture hypothesis[edit / edit wiki text]
The capture hypothesis was first put forward in 1909 by the American astronomer Thomas Jefferson Jackson See.
According to this hypothesis, the Moon was formed as an independent planet somewhere in the Solar system, and then, as a result of some perturbations, it moved into an elliptical orbit intersecting with the orbit of the Earth.
During the next approach to the Earth, the Moon was captured by the Earth's gravity and became its satellite.
Considerations for and against For:
Legends of a number of peoples of the Earth, in particular, the Dogons, speak about the times when there was no Moon in the sky yet, and about the appearance of a new luminary (Moon) in the sky.
In preparation for sending the automatic descent station to the Moon, the Design Bureau of S. P. Korolev had to solve the question of its origin.
If we assume that the Moon has been orbiting the Earth for billions of years and, since it does not have a dense atmosphere, a multi meter layer of dust falling from space should have accumulated on its surface, in which a station designed for landing on solid ground would have sunk during landing.
Disputes broke out in the Design Bureau and two approaches appeared to create the descent part of the station.
The first one assumed a layer of lunar dust on the surface and the development of means of landing and movement on such a layer (for example, dust boats).
The second is that the Moon was captured by the Earth relatively recently and therefore the surface of the Moon, respectively, is solid, which can be counted on when landing.
Since there was no confirmed scientific data, neither approach could prevail, and it was impossible to take into account both for technical reasons.
It is known that under these conditions S. P. Korolev made a strong willed decision to consider the surface of the moon solid and calculate the station accordingly:
it is necessary to count on a sufficiently hard soil of the pumice type[1]
Against:
The capture of the Moon by the Earth's gravity could well explain the high angular momentum of the Earth Moon system.
But the simulation results show that the probability of the Earth capturing a passing body with the mass of the Moon is negligible.
It is much more likely that a passing planet would have collided with the Earth or vice versa, would have been thrown by the Earth's gravity far beyond the limits of the Earth's orbit.
The option with a possible capture requires the passage of the Moon at a distance less than the Roche limit, that is, the Moon would probably be torn apart by the action of tidal forces.
If the capture did occur, the Moon would most likely revolve around the Earth in the opposite (retrograde) direction (as is observed in the captured moons of Jupiter), and in a highly elongated elliptical orbit.
The low density of the Moon and its lack of an iron core can be explained if we assume that the Moon was formed outside the zone of the terrestrial planets (Mercury, Venus, Earth, Mars).
But then it is impossible to explain the shortage of volatile elements that are abundant in the zone of giant planets.
It is difficult to find a suitable region in the Solar system with less content of both.
The identity of the ratio of oxygen isotopes on the Moon and on Earth does not fit into this hypothesis at all.
Oleg Sorokhtin and Sergey Ushakov proposed their own version of the capture hypothesis — with the destruction of the captured planet by the tidal forces of the Earth — in 1989.
According to their theory, a planet from a neighboring orbit, called a Proto Moon, was captured by the Earth and moved into a near Earth orbit.
Since the new satellite was orbiting faster than the rotation of the planet, intense tidal forces attracted it to the Earth (at the same time "spinning" the Earth).
Finally, the newfound satellite approached the distance of the Roche limit and began to collapse.
The matter from the Proto moon spiraled towards the Earth.
Then the satellite was almost torn apart, its iron core fell to Earth, and a significant part of the crust remained in orbit.
From these fragments, the Moon began to form, taking on a spherical shape and moving away from the Earth.
The last part of the hypothesis looks weak: why did the Moon begin to move away from the Earth, if before that the Proto Moon was rotating faster than the Earth's rotation period and the tidal forces of the Earth slowed it down, bringing it closer to the Earth?
It is also unclear why it was the iron core that fell to the Earth, and not the substance of the crust.
And finally, the very possibility of such a successful and "smooth" capture of a neighboring planet still looks extremely unlikely.
The hypothesis of joint formation (joint accretion)[edit / edit wiki text]
For the first time, such a hypothesis was presented by Immanuel Kant in his work on cosmogony, in 1755.
He suggested that all the celestial bodies appeared as a result of the compression of a dust cloud, and the Moon and the Earth formed together, from a single dust clot: first the Earth, then, from the remaining matter, the Moon.
A great supporter of the joint accretion hypothesis was the famous astronomer Eduard Roche.
In the Soviet Union, the coaccretion hypothesis was actively developed by the school of Otto Schmidt (Viktor Safronov, Yevgenia Ruskol, etc.).
Until the 1970s, the joint accretion hypothesis was considered the most developed.
The hypothesis suggests that the Earth and the Moon simply "grew" in the same orbit as a double planet, from the original protoplanetary swarm of solid particles.
Proto Earth was the first to form.
When it gained sufficient mass, the particles from the protoplanetary swarm were captured by its attraction and began to rotate around the embryo of the planet in independent elliptical orbits.
These particles formed their own near planetary swarm.
The swarm particles collided with each other, some lost speed and fell to the proto Earth.
The orbits of others were averaged among themselves — the swarm acquired an orbit close to circular.
Then the embryos of the future satellite, the Moon, began to form from this swarm.
Considerations for and against If the Earth and the Moon were formed in close proximity, then the identity of the oxygen isotope ratio is easily explained.
But then the difference in the density of the two bodies, as well as the lack of iron and volatile elements on the Moon, become completely incomprehensible.
According to William Hartman, " it is difficult to imagine that two celestial bodies grow side by side from the same orbital layer of matter, but at the same time one of them takes all the iron, and the second remains practically without it."
Proponents of the hypothesis explain this by the fact that pieces of swarm matter were crushed during collisions, then heavy iron particles fell to the Ground, and silicate dust remained in orbit.
This explanation can hardly be considered satisfactory: for this, almost all the particles of the swarm had to first collapse to the state of dust.
Similarly, this hypothesis explains the lack of volatile substances — they evaporated during collisions and crushing of swarm particles.
But to do this, the particles would have to collide at high relative speeds, and they are all supposed to be turning in the same direction.
Moreover, a similar process should have occurred during the formation of the Earth and other planets of the Earth group, but the results of this are not observed.
This hypothesis also could not give a clear explanation for either the large angular momentum of the Earth Moon system, or the inclination of the lunar orbit in 5° to the plane of the Earth's orbit.
The evaporation hypothesis[edit / edit wiki text]
In 1955, Ernst Julius Epic put forward a hypothesis that partially combines the hypotheses of centrifugal separation and joint formation.
According to his version, the proto Earth, surrounded by a ring of stone particles bombarding it, warmed up to a high temperature from constant blows — about 2000 °C. Significant masses of matter were evaporated back into near Earth space.
The solar wind blew away the volatile elements, and the heavier components condensed and combined with the material of the rotating rings, which then merged into one large piece of matter — the Moon.
If the Earth was heated at a late stage of its formation, then by this time heavy iron rocks had already sunk into the core, and the iron content in the surface layers of the Earth was significantly less than the initial one.
Considerations for and against The evaporation hypothesis explains the data on the chemical composition of the Moon very well, but it cannot solve either the problem of high angular momentum or the problem of the inclination of the lunar orbit.
Geological data also do not confirm such a strong warming of the Earth at the stage of formation: the composition of the rocks of the Earth's crust indicates that the Earth has never been completely melted.
The hypothesis of many moons[edit / edit wiki text]
The hypothesis of the formation of one large moon from several satellites was presented in the 1960s by Thomas Gold and Gordon MacDonald.
Their main idea was that it would be much easier for the Earth to capture several small celestial bodies flying by separately than one large one.
If the Earth "caught" from six to ten small moons, then their orbits could later be changed by tidal forces.
For about a billion years, the moons could have collided with each other, and the Moon would have formed from their fragments.
Considerations for and against The very possibility of the Earth capturing a large number of satellites with their subsequent destruction looks improbable.
Mars has two small moons (Phobos and Deimos), which still coexist in near Martian orbits.
Venus, whose mass is close to Earth's, has no satellites at all, just like Mercury.
This hypothesis also does not explain the identity of the isotopic oxygen composition of the Moon and the Earth.
Collision hypothesis[edit / edit wiki text]
The collision of Theia with the Earth, as a result of which the Moon is supposed to have appeared
Main article: The Giant collision hypothesis
The collision hypothesis was proposed by William Hartman and Donald R. Davis in 1975.
According to their assumption, a protoplanet (it was called Theia) about the size of Mars collided with the proto Earth at an early stage of its formation, when our planet had about 90% of the current mass.
The blow fell not in the center, but at an angle (almost tangentially).
As a result, most of the matter of the impacted object and part of the matter of the Earth's mantle were thrown into near Earth orbit.
From these fragments, the proto moon gathered 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 the speed of rotation (one revolution in 5 hours) and a noticeable tilt of the axis of rotation.
Considerations for and against The collision hypothesis is currently considered the main one, since it well explains all the known facts about the chemical composition and structure of the Moon, as well as the physical parameters of the Earth — Moon system.
Initially, the possibility of such a successful collision (oblique impact, low relative speed)caused great doubts such a large body with the Earth.
But then it was assumed that Theia was formed in the Earth's orbit, at one of the Lagrange points of the Sun — Earth system.
Such a scenario well explains both the low collision velocity, the angle of impact, and the current, almost exactly circular orbit of the Earth.
Vulnerabilities of the collision hypothesis[source not specified 1095 days]:
to explain the iron deficiency on the Moon, we have to assume that by the time of the collision (4.5 billion years ago), both on Earth and on Theia had already undergone gravitational differentiation, that is, a heavy iron core was released and a light silicate mantle was formed.
Unambiguous geological evidence for this assumption has not been found.
if the Moon somehow appeared in the Earth's orbit at such a distant time and did not undergo significant shocks after that, then according to calculations, a multi meter layer of dust settling from space would have accumulated on its surface, which was not confirmed when spacecraft landed on the lunar surface.[source not specified 1095 days]
Conclusion[edit / edit wiki text]
One of the main goals of the American lunar expeditions of 1960-1970 was to find evidence for one of the three leading hypotheses of the "Big Three" (the hypothesis of centrifugal separation, capture and joint accretion).
But the first data obtained revealed serious contradictions with all three hypotheses.
All the facts accumulated to date are believed to support a hypothesis that did not yet exist during the Apollo flights: the hypothesis of a giant collision.
Notes[edit / edit wiki text]
1 1 2 The Star Road of the Middle Kingdom (interview with D. A. Gulutin, who worked for 15 years as a test engineer and design engineer at the State Space Research and Production Center named after M. V. Khrunichev.
Now Dmitry Gulyutin is the head of the Department of Museum Pedagogy of the Memorial Museum of Cosmonautics.
// Nezavisimaya Gazeta, 2012, April 25
Literature[edit / edit wiki text]
W. K. Hartmann & D R. Davis: Satellite Sized Planetesimals and Lunar Origins, Icarus 24 (1975): 504—515.
Hartmann, W. K., et al., eds 1986: Origin of the Moon (Houston: Lunar and Planetary Institute)
Dana Mackenzie, «The Big Splat, or How Our Moon Came to Be», 2003, John Wiley & Sons, ISBN 0-471-15057-6.
Links[edit / edit wiki text]
"The Origin of planets and Satellites", E. V. Ruskol, O. Y. Schmidt Joint Institute of Earth Physics of the Russian Academy of Sciences — on the hypothesis of joint accretion of the Moon and the Earth "V. S. Safronov's model of planet formation and the global evolution of the Earth", O. G. Sorokhtin, Institute of Oceanology of the Russian Academy of Sciences — on the hypothesis of capture and destruction of the Proto Moon The Origin of the Moon.
The Russian concept against the "American" - " ZiV " No. 6/2005 E. M. Galimov, Academician, GEOHI RAS.
NASA: The moon appeared as a result of a collision with a huge celestial body Russia Today, December 12, 2012
Moon
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