Origin of the moon

from Wikipedia, the free encyclopedia
The earth moon

The formation of the Earth's moon has been discussed for centuries. Since the mid-1980s, the view has prevailed that the moon was formed after an eccentric collision of the proto-earth with a roughly Mars-sized body called Theia . According to this collision theory , a large part of the material chipped off from both bodies has entered an orbit around the earth and has concentrated there to the moon.

One also speaks of the "origin of the earth-moon system ", because in the entire solar system there is no other satellite (with the exception of Pluto and Charon ) that has a similar size to the orbiting planet or dwarf planet . According to its mass, it also has a particularly large orbital angular momentum . The development of Pluto and Charon took place in the very cool region of the Kuiper Belt , but a similar major collision is increasingly suspected as the cause of their system.

Formation of the solar system

The formation of the solar system began 4.568 billion years ago with the gravitational collapse of the solar nebula , from which the sun emerged as a massive center. Small planetesimals formed from the material (gas and dust) remaining in a protoplanetary disk , and from these, via the intermediate stage of protoplanets, the planets were formed. At the end of the planet formation, most of the remaining planetesimals either fell on the planets or were thrown into the outer solar system ( Kuiper belt and Oort cloud ) or even out of the solar system.

See also: Origin of the Earth

Theories of the origin of the moon

As far as is known, the first considerations about the origin of the moon, which can be regarded as the forerunner of the capture theory , come from René Descartes . They were not published until 1664, some time after Descartes' death.

Since the 19th century several theories about the origin of the earth-moon system have been developed. These are essentially:

  • The separation theory: A "drop" constricted from a hot, (viscous) liquid and rapidly rotating proto-earth and formed the later moon.
  • The capture theory: Earth and moon formed independently in different regions of the solar system; in a close encounter, the earth caught the moon by gravity.
  • The sister planet theory: Earth and moon formed simultaneously and close together.
  • The Öpik theory: The forerunner of the moon arose from matter that evaporated from a hot proto-earth.
  • The many moons theory: Several moons were captured at the same time and collided after a while. Today's moon was formed from the fragments.
  • The collision theory: The proto-earth collided relatively gently with a large body and the moon formed from the matter thrown away.
  • The Synestia Theory: The Proto-Earth was almost completely evaporated by a violent collision; the moon condensed in the outer area of ​​the cloud called Synestia.

A good model must not only be physically possible, but must also be compatible with the properties of the moon and the entire earth-moon system and, if possible, even explain them:

  • The density of the moon is 3.3 g / cm³, significantly lower than that of the earth with 5.5 g / cm³.
  • Compared to the earth, the moon has a slight deficit of volatile elements and substances composed of them, e.g. B. Magnesium , as well as iron.
  • The isotopic composition of the elements in the earth's mantle and on the lunar surface is almost identical, in comparison with the distribution of the conditions in the rest of the solar system.
  • The angular momentum of the earth-moon system is unusually high.

Secession theory

The split theory was developed by George Howard Darwin , son of Charles Darwin , in 1878. According to this, the earth rotated so strongly in its early phase that part of it became detached due to instabilities and the moon formed. In 1882, the geologist Osmond Fisher (1817–1914) took the view that the Pacific Ocean was the scar of this separation that is still visible today. In 1925, the geologist Otto Ampferer also drew the detachment of the moon from the earth as the cause of the uneven distribution of the lithosphere into consideration.

Such a detachment from the extreme equatorial bulge explains the size of the moon quite well. Its lower mean density is also compatible with this, because it corresponds to the density of the earth's mantle. In view of the tidal friction , the earth must have rotated faster in the past, but there is no meaningful explanation for the high rotational speeds (day length of around 2.5 h) that would have been necessary for the current total angular momentum of the earth-moon system. The idea that the Pacific is the scar of this separation has also been refuted by plate tectonics . The orbital plane of the moon is also much too inclined towards the equatorial plane of the earth.

Capture theory

The trapping theory was proposed by Thomas Jefferson Jackson See in 1909 . It says that the moon formed as an independent planetesimal at another location in the solar system and was captured during a close encounter with the earth.

The trapping theory can explain the high angular momentum of the system and the difference in density between the earth and the moon in a very elegant way. However, it requires a very special trapping path, which means a great coincidence. In addition, the moon would have to have survived a brief entry into the Roche border , which cannot yet be explained. Nor does this theory make any statement as to why the moon has a deficit compared to the earth in terms of both volatile elements and iron. Given the similarity of the isotopic composition, the theory fails completely.

Sister planet theory

Immanuel Kant already hypothesized in his Cosmogony of 1755, General Natural History and Theory of Heaven , the first scientific attempt to explain the origin of the celestial bodies, that the earth and moon were formed directly into a double planet from a joint compression of the primordial nebula he postulated . The main mass of the local compression concentrated to the earth and the remaining dust cover to the moon. The sister planet theory was developed quantitatively in 1944 by Carl Friedrich von Weizsäcker , essential preliminary work on stability was carried out by Édouard Albert Roche .

If the earth and moon developed closely together, it is absolutely incomprehensible why the density or the proportion of volatile elements and iron in the earth and moon differ so greatly. There is no plausible explanation for the high proportion of the orbital angular momentum of the moon compared to the angular momentum of the earth itself. The five degree inclination of the lunar orbit plane against the orbit plane of the earth is not understandable with it.

Öpik theory

In 1955, Ernst Öpik proposed a theory that can be classified between the split-off and the sister planet theory. The proto-earth, which is surrounded by a ring system of trapped rock debris, heated up in the course of its development due to the permanent impacts to around 2000 ° C and evaporated large amounts of matter again. While the solar wind has blown away the lighter elements, the heavier ones condensed and together with parts of the ring system formed the proto-moon. This heating did not take place until a late phase in the formation of the earth, so that the proportion of iron in the mantle layers of the proto-earth was already significantly reduced due to an already formed earth core.

This theory is very compatible with the observed geochemical properties of the moon, but the momentum problems of the sister planet theory remain unchanged.

Many moons theory

Known in the English-speaking world as many-moons theory , the theory experienced brief popularity after it was proposed by Thomas Gold in 1962 and formalized in the following years by Gordon JF MacDonald . The basic idea is that it is easier for the earth to capture several small celestial bodies than one large one. If six to ten small moons are captured by the earth and orbit it, the orbits of these moons migrate outwards due to the effect of the tides. Over the course of a billion years, the small moons then collide and the fragments form the Earth's moon.

This theory was refuted by the rock samples from the Apollo missions (isotopic composition). It is also not plausible why the union of many moons into a single, unusually large one is said to have taken place only on Earth, while Mars continues to have two separate smaller moons, and the inner planets otherwise have no moons at all. The long period of time that would have to be set for a unification process based on the tidal force suggests that the inner planets are still orbited by an abundance of small moons that have not (yet) been united.

Collision theory

Animated illustration of the formation of the moon through a collision between the earth and Theia as L 4 - Trojans in their relative movement. During her more elliptical orbit of the sun, Theia moved sometimes further away from the sun and sometimes closer to the sun and therefore sometimes slower and sometimes faster than the earth, which led to a constant approach. The speed of the approach at the moment of the collision was accordingly about 14,000 km / h (view of the northern hemispheres).

The collision theory was developed by William K. Hartmann and Donald R. Davis in 1975. According to this theory, in the early phase of planetary development, a roughly Mars-sized protoplanet , sometimes called Theia after the mother of the Greek moon goddess Selene, collided with the proto-earth ( Gaia, after the Greek goddess Gaia ), which at that time was already about 90% of its current position Had mass. The collision did not take place head-on, but grazing, so that large amounts of matter, consisting of parts of the mantle of the impact body and the earth's mantle, were thrown into the earth's orbit while the iron cores united. In orbit, the proto-moon formed almost immediately (i.e. in less than 100 years), which quickly collected all remaining debris and must have condensed to the moon with approximately today's mass after almost 10,000 years. It circled the earth, which was rotating rapidly at that time - also due to the collision - at a distance of only around 60,000 km (see Roche border and double planet ), which must have led to extreme tidal forces . The strong tidal friction led to an initially very rapid deceleration of the earth's rotation with the transfer of the angular momentum to the moon, the orbit radius of which increased rapidly as a result. This interaction with the slowing down of the earth's rotation and the increase in the orbital radius of the moon has been weakened and continues to this day. The synchronized self-rotation of the moon, which means that we can only see one of its sides, is partly due to this, but also has other causes.

The development of this theory is explained in more detail below, as this scenario best describes the facts at hand today.

History of the collision theory

The first suggestion to see the origin of the moon in a cosmic catastrophe was found in a 1946 publication by Reginald Aldworth Daly in the Proceedings of the American Philosophical Society; it went unnoticed , partly because of Immanuel Velikovsky's theories that were spread shortly thereafter .

In the 1960s, the Russian astrophysicist Viktor Safronov developed the theory that the planets were formed by the agglomeration of large numbers of smaller planetesimals . Hartmann and Davis took up this hypothesis and were able to improve Safronov's purely analytical work through computer simulations. They examined the size distribution of the resulting "agglomerations" and obtained a size distribution comparable to that in today's asteroid belt : In addition to a large body (comparable to Ceres with a diameter of around 1000 km), several bodies with about 1/10 of its mass (comparable to Pallas , Vesta and Hygeia with a diameter of 400 to 600 km). The basic idea of ​​the collision theory is that one of these bodies collided almost grazing with the proto-earth only in the final phase of planet formation, whereby part of the total mass was thrown into orbit and formed the moon. Hartmann and Davis published this theory in 1975. Independently of this, Alastair GW Cameron and William Ward came to the same conclusion in 1976 through considerations on angular momentum.

In 1983, AC Thompson and David J. Stevenson published a study of the formation of smaller bodies from collision material in orbit, but few have seriously considered collision theory. The breakthrough came in 1984 in Kailua-Kona , Hawaii , at an international conference on the origins of the moon. The discussion of the first investigations of the lunar rocks brought back to Earth by the Apollo missions led most scientists to believe that the collision theory describes the facts much better than any other theories about the formation of the moon. In particular, it was shown that the isotopic composition of the elements of the lunar rock is essentially the same as that of terrestrial rock. For example, the oxygen isotope ratios of earthly rock, Apollo samples and lunar meteorites are on a common fractionation line , which shows that the oxygen - as the most common element in the earth-moon system - must come from a shared, mixed reservoir. In contrast, the oxygen isotope ratios of other meteorites lie on different fractionation lines depending on their origin.

In the 1990s there was a setback for the theory when the first simulation calculations required the impact of a body with three times the mass of Mars in order to bring enough material into orbit. This impact, at a point in time when the Proto-Earth had reached about half its current size, would have transmitted far too much angular momentum; therefore another severe impact would have been necessary towards the end of the earth's accretion phase . In 2001, however, Robin M. Canup and Erik Asphaug were able to use improved models to show that a single impact towards the end of the accretion phase is sufficient to explain both the mass and geochemistry of the moon and the angular momentum of the earth-moon system. According to these simulations, the best results are obtained for an impact body that is slightly larger than Mars and collides with a relative speed of less than four kilometers per second (14,400 km / h) at a collision angle of about 45 °. By comparing the niobium-tantalum ratio of the moon and the earth with the niobium-tantalum ratio of the rest of the solar system, it has now been shown that the moon consists of at least half of earth material. The age of the moon was determined in November 2005 in a rock-analytical study by scientists from ETH Zurich and the Universities of Cologne, Münster and Oxford by radiometric dating using tungsten -182 (which is made from hafnium -182 by β - decay with a half-life of 9 Million years ago) is determined to be 4.527 billion years (± 0.01).

A majority of scientists agree that the collision theory agrees very well with the observations, even if a great deal of detailed work is still necessary. In the simulation calculations in particular, very strong simplifications are still being used and there are still no consistent mathematical models for the formation and structure of the orbital disk after the impact. Despite the uncertainties about the exact course of the impact and the low probability of such a collision with a body of exactly the right size at exactly the right time with exactly the right impact parameters, in contrast to the other proposed hypotheses, there are at least no major contradictions to the Observations. Although the model of a single impact can explain the formation of the moon very well, further early collisions of large bodies from space, both with the moon and with the earth, cannot be ruled out. A final clarification of these old processes is expected in the future from the lunar geology , which, for example, through drilling on the moon and investigations into its internal composition, can provide empirical data that allows conclusions to be drawn about its true history.

A 2013 discovery published in Nature Geoscience showed that lunar rocks, which are believed to represent the original lunar crust, have a surprisingly high water content. This raised new questions about the formation of the moon, as this finding is difficult to reconcile with the well-established collision theory.

Originally two moons

Another theory says that the earth should have had a smaller second moon about 1200 km in diameter in addition to the moon . This is said to have collided with the larger one after several million years, which could explain the different-looking halves of the moon.

Synestia hypothesis

A synestia is the state of a rocky planet after a very high- energy, off-center collision: evaporated rock expands to a multiple of the original radius; the inner part rotates quickly and uniformly, the outer part forms a thick, optically dense disk with slightly suborbital velocities, since the gas pressure is not negligible there either. A joint simulation of the dynamics and phase equilibrium, followed by the geochemistry and isotope fractionation, showed: External cooling leads to radial transport for the mixing of the two starting materials, moonlets are created in the disk, while there are still vapor pressures of several megapascals, which is the moderate depletion of volatile elements explained; also, the range of the collision parameters for a plausible result is not as narrow as under the Theia hypothesis.

Summary

The main scientific goal of the Apollo missions - as part of the race to the moon  - was to find clues about its formation on the moon based on its composition. Clear geochemical evidence was sought for one of the big three theories (split-off theory, capture theory, sister planet theory), but the evaluations only raised new contradictions for all three. Instead, further ideas were developed on the basis of the salvaged lunar rocks, which in principle consist of parts of the capture and splitting theory. Rock samples from other landing sites, including from the far side of the moon, would help.

literature

Individual evidence

  1. ↑ Confirmed: Moon was created by collision.
  2. ↑ George Howard Darwin: On the Precession of a Viscous Spheroid. In: Nature. Volume 18, 1878, pp. 580-582, doi : 10.1038 / 018580a0 .
  3. ↑ George Howard Darwin: On the Precession of a Viscous Spheroid, and on the Remote History of the Earth. In: Philosophical Transactions of the Royal Society of London. Volume 170, 1879, pp. 447-538, doi : 10.1098 / rstl.1879.0073 .
  4. ^ Geologist Osmond Fisher: On the Physical Cause of the Ocean Basins. In: Nature. Volume 25, 1882, pp. 243-244, doi : 10.1038 / 025243a0 .
  5. ↑ Otto Ampferer: About continent shifts. In: Natural Sciences. Volume 13, no. 31, 1925, S. 669-675 ( digizeitschriften.de ), pp 672nd
  6. ↑ Thomas Jefferson Jackson See: Origin of the lunar terrestrial system by capture, with further considerations on the theory of satellites and on the physical cause which has determined the directions of the rotations of the planets about their axes. In: Astronomical News. Volume 181, number 23, 365-386, 1909, pp. 365-386, doi : 10.1002 / asna.19091812302 .
  7. ↑ Carl Friedrich von Weizsäcker: About the emergence of the planetary system. In: Journal of Astrophysics. Volume 22, 1944, pp. 319-355.
  8. ^ Édouard Roche: Essai sur la constitution et l'origine du systeme solaire. In: Académie des sciences et lettres de Montpellier. Mémoires de la Section des Sciences. Volume 8, 1783, pp. 235-324 (online).
  9. ^ Ernst Öpik: The Origin of the Moon. In: Irish Astronomical Journal. Volume 3, Number 8, 1955, pp. 245-248, (online).
  10. ^ Gordon JF MacDonald: Origin of the Moon: Dynamical Considerations. In: Annals of the New York Academy of Sciences. Volume 118, 1965, pp. 742-782, doi : 10.1111 / j.1749-6632.1965.tb40150.x .
  11. ^ William K. Hartmann, Donald R. Davis: Satellite-sized planetesimals and lunar origin. In: Icarus. Volume 24, number 4, 1975, pp. 504-515, doi : 10.1016 / 0019-1035 (75) 90070-6 .
  12. ^ Reginald Aldworth Daly: Origin of the Moon and Its Topography. In: Proceedings of the American Philosophical Society. Volume 90, Number 2, 1946, pp. 104-119, JSTOR.
  13. ^ Victor S. Safronov: Sizes of the largest bodies falling onto the planets during their formation. In: Soviet Astronomy. Volume 9, 1966, pp. 987-991, (online).
  14. ^ Alastair GW Cameron, William Ward: The Origin of the Moon. In: Abstracts of the Lunar and Planetary Science Conference. Volume 7, 1976, pp. 120-122, (online).
  15. ^ AC Thompson, David J. Stevenson: Two-Phase Gravitational Instabilities in Thin Disks with Application to the Origin of the Moon. In: Abstracts of the Lunar and Planetary Science Conference. Volume 14, 1983, pp. 787-788.
  16. ↑ Robin Canup, Erik Asphaug: Origin of the Moon in a giant impact near the end of the Earth's formation. In: Nature. Volume 412, 2001, pp. 708-712, doi : 10.1038 / 35089010 , see also
    Robin M. Canup: Simulations of a late lunar-forming impact. Icarus, Vol. 168, 2004, pp. 433–456, online (PDF; 2.1 MB).
  17. ↑ Thorsten Kleine, Herbert Palme, Klaus Mezger, Alex N. Halliday: Hf-W Chronometry of Lunar Metals and the Age and Early Differentiation of the Moon. In: Science. Volume 310, number 5754, 2005, pp. 1671–1674, doi : 10.1126 / science.1118842 .
  18. ^ Water on the moon: It's been there all along. At: ScienceDaily.com. February 18, 2013, accessed October 26, 2017.
  19. ^ Martin Jutzi, Erik Asphaug: Forming the lunar farside highlands by accretion of a companion moon . In: Nature . No. 476, August 2011, pp. 69-72. doi : 10.1038 / nature10289 .
  20. ↑ Jan Oliver Löfken: New evidence: Second moon once orbited the earth. weltderphysik.de, August 3, 2011, accessed April 18, 2015 .
  21. ↑ Simon J. Lock, Sarah T. Stewart: The structure of terrestrial bodies: Impact heating, corotation limits, and synestias. Journal of Geophysical Research: Planets, 2017, doi: 10.1002 / 2016JE005239 , arxiv: 1705.07858 .
  22. ↑ Simon J. Lock et al .: The Origin of the Moon Within a Terrestrial Synestia. Journal of Geophysical Research: Planets, 2018, doi: 10.1002 / 2017JE005333 , arxiv: 1802.10223 .

Web links

Commons : Origin of the Moon  - Album with pictures, videos and audio files
This version was added to the list of articles worth reading on November 27, 2010 .