Difference between revisions of "Origin of the Moon"

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(Full rewrite with new sources, balance, and new insights.)
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Scientists now explain the creation of the moon by the "Giant-Impact Model," explained below.  This theory emerged after the Lunar landings disproved previous theories of the moon's creation, that relied upon the assumption that the moon "came," largely, from the earth. The material, to the surprise of scientists, lacked iron that permeates the Earth's crust.
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Over the years, there have been various theories about the '''origin of the Moon'''. Several of these theories were formed before scientists had a chance to study [[Moon]] samples, and the eventual analysis (as well as calculations regarding things like energy and angular momentum) of such samples spoke against many of these old approaches.
  
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Specifically, each of these prevailing scientific theories were disproven by the lunar landings:<ref name="PSI">Planetary Science Institute: [http://www.psi.edu/projects/moon/moon.html The Origin of the Moon]</ref>
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==The "Big Three"==
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Prior to the lunar landings, there had been three major theories that all had been considered valid explanations for the origin of the Moon.
  
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*The Moon formed in an orbit around Earth at the same time that Earth formed.
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===Theory 1: Fission===
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**This theory was disproven upon learning that the Moon has proportionally far less iron in it than the Earth.
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Developed in 1878, this theory by [[George Howard Darwin]] (son of [[Charles Darwin]]) states that the Moon had been a part of a rapidly spinning Earth. This rapid spin, as well as the Sun's gravitational influence, led to a piece of the Earth breaking free and forming the Moon. Osmond Fisher added that the Pacific ocean basin is actually the scar of this process.<ref name="Nova1">PBS NOVA: "To the Moon - Origins" [http://www.pbs.org/wgbh/nova/tothemoon/origins.html (Page 1)]</ref>
  
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*The Moon formed in a part of the solar system that was low in iron, and was later captured into an orbit arount the Earth.
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===Theory 2: Capture===
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**This was also disproven by examination of lunar rocks, which exhibit the same oxygen isotope composition as the Earth, unlike rocks known to come from other sources in the solar system, e.g. Mars.
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In 1909, [[Thomas Jefferson Jackson See]] proposed that the Moon was actually a planet that had been created elsewhere and that had been caught by Earth's gravity after being slowed down by debris.<ref name="Nova1"/>
  
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*The Earth was originally spinning so fast that it spun off a fragment of lower density (iron-poor) material which became the Moon.
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===Theory 3: Coaccretion===
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**This solves the isotopic similarity and the relative lack of iron, but an analysis of the total angular momentum and energy involved ruled out this possibility.
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Many scientists, including [[Edouard Roche|&Eacute;douard Roche]], supported the theory that the Earth and the Moon formed side by side from the material that formed all planets of this solar system. The name comes from the term "accretion disc", which is a disc formed by diffuse material that orbits a central body.<ref name="Nova1"/>
  
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As will be demonstrated, the Giant-Impact Model does away with the need for large amounts of terrestrial material in the moon.
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==Observations and caclulations==
 +
For a long time, these three theories had been accepted as possible lunar origins. However, later research gave a number of points that spoke against these theories.
 +
After the Apollo program, rock samples taken from the Moon revealed two important things:
 +
#The Moon lacks iron. This is important to note because the Earth, in comparison, has a lot of it (in the core).
 +
#The Moon and the Earth have exactly the same oxygen isotope composition.
 +
Along with calculations in the field of energy and angular momentum, these two points spoke against the "Big Three" theories as follows:
  
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== Giant-Impact Model ==
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===Consequences for the Fission Theory===
 +
Since the material that spun off the Earth would have come from the mantle, the lack of iron would not have been a problem (Earth's iron drained towards the core early on, leaving a depleted mantle).<ref name="PSI">Planetary Science Institute: [http://www.psi.edu/projects/moon/moon.html "The Origin of the Moon"]</ref> This would also explain why the Moon has such a small core.<ref name="Nova1"/> For obvious reasons, the theory also covers the issue of the oxygen isotope composition.
  
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For ten years after the lunar landings, non-creationist scientists lacked any accepted theory about the Moon's origin.  In 1984, an international meeting was convened in Kona, Hawaii, for the purpose of agreeing on a new theory for the origin of the Moon.<ref>''Id.''</ref>  The attendees at the meeting agreed upon the following "giant-impact model," which was first proposed in 1975 but which had not yet been generally accepted.<ref>''Id.''</ref>
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However, the total angular momentum today is too small to back this theory.<ref name="NovaCelestia">Nova Celestia: [http://www.novacelestia.com/space_art_solar_system/earth.html#moon_earth_creation "The Creation of the Earth-Moon System"]</ref>
  
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This theory proposes an impact with a large planetesimal (asteroid-like body) released a great quantity of debris into orbit, a portion of which collected under the influence of gravity. The "giant impact theory" explains the following:
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===Consequences for the Capture Theory===
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**The lack of iron, provided the impact of the planetesimal happened relatively late in the earth's formation and after the Earth's iron had sunk towards its core.  In that manner the Moon could pick up mantle from the Earth that has less iron.  Most of the planetesimal itself would have sunk into the earth's core.
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Even ignoring the very small chance of a planet easing into an Earth orbit like our Moon did, this theory fails to explain the oxygen isotope similarity and also leaves open the issue of the Moon's small core.<ref name="Nova1"/>
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**The similar oxygen isotope composition between the Moon and Earth could be explained by such a collision, provided the Moon were then formed from debris dislodged from the Earth by the collision.
 
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**A high-velocity collision might be sufficient to release enough energy to place a substantial mass of the Earth's crust into orbit.
 
  
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Creationists criticize this theory as an inadequate explanation of the moon's geological content, since it seems to differ so greatly from Earth. However, new analyses of the type of impact likely to create a moon-sized object have deduced that the moon is instead 85% composed of the impacting object, and ''only 15%'' composed of terrestrial materials.  This finding, complete with continued scholarship on the subject, suggests that the giant impact model is a complete theory. Like all scientific theories, though, it continues to evolve as new evidence is discovered.<ref>"Linking The Earth To The Moon," Space Daily, 1 August 2006</ref>
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===Consequences for the Coaccretion Theory===
 +
This theory does not require extreme chances like the Capture theory and explains nicely why Earth and Moon share their oxygen isotope composition. However, there are open questions regarding the angular momentum, and the lack of iron spoke against it, too (However, this issue had been addressed recently, see [[#Current Status|Current Status]].).<ref name="Nova1"/>
  
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== Criticism of the Giant-Impact Model ==
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==Giant Impact Theory==
 +
===Development===
 +
In 1974, William Hartmann and Donald Davis suggested the theory that a planetoid (early on estimed to be the size of Mars) impacted with a relatively young Earth and blasted enough mantle material into on orbit, where it would eventually form the Moon. This theory also became known as the "Big Whack" theory. Around the same time, Alastair Cameron and William Ward had come to a similar conclusion. However, their research had been motivated by studies of the angular momentum.<ref name="PSI"/><ref name="Nova2">PBS NOVA: "To the Moon - Origins" [http://www.pbs.org/wgbh/nova/tothemoon/origins2.html (Page 2)]</ref>
  
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Scientists did find that none of three proposed tests are "supportive of the Giant Impact model, but neither do they disprove it."<ref>http://www.lpi.usra.edu/meetings/origin98/pdf/4045.pdf</ref>
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Initially, the the theory was rejected with the claim that a catastrophic event of such a dimension and with such results seemed extremely unlikely.<ref name="Nova2"/> Still, it explained the lack of iron, the oxygen isotope issue and the angular momentum.
  
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Another article noted that the Moon is lacking in siderophilic elements (Au, Co, Fe, Ir, Mn, Mo, Ni, Os, Pd, Pt, Re, Rh, Ru) found on Earth, and this indicates that the Moon was not formed by materials broken off of Earth.<ref>http://www.ias.ac.in/jessci/dec2005/ilc-3.pdf</ref>
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Ten years later, the Giant Impact theory emerged as the leading theory after a conference about the Moon origin in Kona, Hawaii. In the 1990s, Robin Canup picked up the research and created models to determine possible scenarios that might explain the formation of the Moon.<ref name="PSI"/><ref name="Nova2"/>
  
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A third study found that "the bulk composition of the Moon differs significantly from that of the terrestrial mantle" and that "[t]he high
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Canup quickly came to the conclusion that much of the material blasted off by a "Big Whack" would either fall back to Earth or fly off into space, thus requiring a very large impactor (one with a factor of two or three in mass, compared to the initial assumptions). However, she concluded later on that improved computer simulations suggested that the size of the impactor may have been smaller after all, thus leaving all calculations regarding mass and momentum intact.<ref name="PSI"/><ref name="SciAm">Scientific American: [http://www.sciam.com/article.cfm?chanID=sa006&colID=5&articleID=000A90B0-C919-1C6E-84A9809EC588EF21 "Earth-Shattering Theory"]</ref>
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bulk FeO content of the Moon rules out the derivation of the proto-lunar material from any but a small fraction of the terrestrial mantle."<ref>http://www.lpi.usra.edu/meetings/lpsc97/pdf/1070.PDF</ref>
 
  
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Nearly two decades after the Giant-Impact Model was embraced at the Hawaii convention, ''Scientific American'' observed that:<ref>http://www.sciam.com/article.cfm?chanID=sa006&colID=5&articleID=000A90B0-C919-1C6E-84A9809EC588EF21</ref>
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===Criticism===
 +
The theory is far from complete, though. Despite addressing the issues raised in the light of the first lunar landings, the Giant Impact theory leaves quite a few issues currently not fully addressed:
 +
*One study suggested three tests of the theory, based on things that should have occurred after such an impact. The result of these tests did not prove the Giant Impact theory, but it also did not disprove it. Still, the paper raised a few points that have to be factored into the theory.<ref>J. H. Jones: [http://www.lpi.usra.edu/meetings/origin98/pdf/4045.pdf "Tests of the Giant Impact Hypothesis"]</ref>
 +
*A study analysing the bulk composition has come to the conclusion that "the high bulk FeO content of the Moon rules out the derivation of the proto-lunar material from any but a small fraction of the terrestrial mantle".<ref>S. R. Taylor: [http://www.lpi.usra.edu/meetings/lpsc97/pdf/1070.PDF "The Bulk Composition of the Moon"]</ref>
 +
*Addressing the above concern, the Giant Impact theory raised the point that the impactor also contributed a large share of the base material that formed the Moon. But as another paper pointed out, the Moon is lacking siderophilic elements that should be present in such a case.<ref>Galimov and Krivtsov: [http://www.ias.ac.in/jessci/dec2005/ilc-3.pdf "Origin of the Earth–Moon system"]</ref>
  
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:Unfortunately, researchers have had trouble getting the giant-impact model to work without the contrivances that scuttled earlier theories.
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==Current Status==
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As of now, the Giant Impact theory still stands. Research on it is not yet complete, and the theory is currently neither proven nor disproven. To date, it is also the most plausible theory considered by scientists.
  
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:Four facts and three parameters is a recipe for contradiction. To explain the moon's low iron content, you need to avoid a grazing collision (corresponding to a large impact angle), lest too much of the impactor's iron spill into orbit. Then, to explain the angular momentum, you need to compensate for the smallish angle with a hefty impactor. Then, to explain the moon's mass, you need to adjust the proto-Earth's mass. In the end, you might find that the total mass is incorrect.
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Independently of the Giant Impact theory, Peter Noerdlinger from Saint Mary's University in Halifax, Canada proposed an expansion of the Coaccretion theory. He suggested that the Moon formed next to the Earth, complete with an iron core. However, this core was then ripped out by the Earth's gravity shortly after the formation, leaving an iron-less Moon behind to orbit the Earth.<ref>New Scientist: [http://space.newscientist.com/article/mg19325875.700-did-the-new-moon-lose-its-iron-heart.html "Did the new moon lose its iron heart?"]</ref>
  
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''Scientific American'' concluded:<ref>''Id.''</ref>
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Further research will show if either of these theories is correct, or if scientists are going to develop a new theory based on new observations.
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:Considering all the twists and turns in lunar science, nobody claims that the models are complete just yet.
 
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Of course, all of these criticisms are answered by the fact that the moon is composed of mostly extraterrestrial rock from the object that impacted the Earth, creating the moon.<ref>See ''Space Daily'', supra.</ref>  It should also be recognized that a theory's imcompleteness is never a full challenge to it.  By virtue of the nature of scientific theories, they continue to evolve and "learn" from their mistakes.
 
  
 
== Reference ==
 
== Reference ==
 
<references/>
 
<references/>

Revision as of 16:27, March 20, 2007

Over the years, there have been various theories about the origin of the Moon. Several of these theories were formed before scientists had a chance to study Moon samples, and the eventual analysis (as well as calculations regarding things like energy and angular momentum) of such samples spoke against many of these old approaches.

The "Big Three"

Prior to the lunar landings, there had been three major theories that all had been considered valid explanations for the origin of the Moon.

Theory 1: Fission

Developed in 1878, this theory by George Howard Darwin (son of Charles Darwin) states that the Moon had been a part of a rapidly spinning Earth. This rapid spin, as well as the Sun's gravitational influence, led to a piece of the Earth breaking free and forming the Moon. Osmond Fisher added that the Pacific ocean basin is actually the scar of this process.[1]

Theory 2: Capture

In 1909, Thomas Jefferson Jackson See proposed that the Moon was actually a planet that had been created elsewhere and that had been caught by Earth's gravity after being slowed down by debris.[1]

Theory 3: Coaccretion

Many scientists, including Édouard Roche, supported the theory that the Earth and the Moon formed side by side from the material that formed all planets of this solar system. The name comes from the term "accretion disc", which is a disc formed by diffuse material that orbits a central body.[1]

Observations and caclulations

For a long time, these three theories had been accepted as possible lunar origins. However, later research gave a number of points that spoke against these theories. After the Apollo program, rock samples taken from the Moon revealed two important things:

  1. The Moon lacks iron. This is important to note because the Earth, in comparison, has a lot of it (in the core).
  2. The Moon and the Earth have exactly the same oxygen isotope composition.

Along with calculations in the field of energy and angular momentum, these two points spoke against the "Big Three" theories as follows:

Consequences for the Fission Theory

Since the material that spun off the Earth would have come from the mantle, the lack of iron would not have been a problem (Earth's iron drained towards the core early on, leaving a depleted mantle).[2] This would also explain why the Moon has such a small core.[1] For obvious reasons, the theory also covers the issue of the oxygen isotope composition.

However, the total angular momentum today is too small to back this theory.[3]

Consequences for the Capture Theory

Even ignoring the very small chance of a planet easing into an Earth orbit like our Moon did, this theory fails to explain the oxygen isotope similarity and also leaves open the issue of the Moon's small core.[1]

Consequences for the Coaccretion Theory

This theory does not require extreme chances like the Capture theory and explains nicely why Earth and Moon share their oxygen isotope composition. However, there are open questions regarding the angular momentum, and the lack of iron spoke against it, too (However, this issue had been addressed recently, see Current Status.).[1]

Giant Impact Theory

Development

In 1974, William Hartmann and Donald Davis suggested the theory that a planetoid (early on estimed to be the size of Mars) impacted with a relatively young Earth and blasted enough mantle material into on orbit, where it would eventually form the Moon. This theory also became known as the "Big Whack" theory. Around the same time, Alastair Cameron and William Ward had come to a similar conclusion. However, their research had been motivated by studies of the angular momentum.[2][4]

Initially, the the theory was rejected with the claim that a catastrophic event of such a dimension and with such results seemed extremely unlikely.[4] Still, it explained the lack of iron, the oxygen isotope issue and the angular momentum.

Ten years later, the Giant Impact theory emerged as the leading theory after a conference about the Moon origin in Kona, Hawaii. In the 1990s, Robin Canup picked up the research and created models to determine possible scenarios that might explain the formation of the Moon.[2][4]

Canup quickly came to the conclusion that much of the material blasted off by a "Big Whack" would either fall back to Earth or fly off into space, thus requiring a very large impactor (one with a factor of two or three in mass, compared to the initial assumptions). However, she concluded later on that improved computer simulations suggested that the size of the impactor may have been smaller after all, thus leaving all calculations regarding mass and momentum intact.[2][5]

Criticism

The theory is far from complete, though. Despite addressing the issues raised in the light of the first lunar landings, the Giant Impact theory leaves quite a few issues currently not fully addressed:

  • One study suggested three tests of the theory, based on things that should have occurred after such an impact. The result of these tests did not prove the Giant Impact theory, but it also did not disprove it. Still, the paper raised a few points that have to be factored into the theory.[6]
  • A study analysing the bulk composition has come to the conclusion that "the high bulk FeO content of the Moon rules out the derivation of the proto-lunar material from any but a small fraction of the terrestrial mantle".[7]
  • Addressing the above concern, the Giant Impact theory raised the point that the impactor also contributed a large share of the base material that formed the Moon. But as another paper pointed out, the Moon is lacking siderophilic elements that should be present in such a case.[8]

Current Status

As of now, the Giant Impact theory still stands. Research on it is not yet complete, and the theory is currently neither proven nor disproven. To date, it is also the most plausible theory considered by scientists.

Independently of the Giant Impact theory, Peter Noerdlinger from Saint Mary's University in Halifax, Canada proposed an expansion of the Coaccretion theory. He suggested that the Moon formed next to the Earth, complete with an iron core. However, this core was then ripped out by the Earth's gravity shortly after the formation, leaving an iron-less Moon behind to orbit the Earth.[9]

Further research will show if either of these theories is correct, or if scientists are going to develop a new theory based on new observations.

Reference

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 PBS NOVA: "To the Moon - Origins" (Page 1)
  2. ↑ 2.0 2.1 2.2 2.3 Planetary Science Institute: "The Origin of the Moon"
  3. ↑ Nova Celestia: "The Creation of the Earth-Moon System"
  4. ↑ 4.0 4.1 4.2 PBS NOVA: "To the Moon - Origins" (Page 2)
  5. ↑ Scientific American: "Earth-Shattering Theory"
  6. ↑ J. H. Jones: "Tests of the Giant Impact Hypothesis"
  7. ↑ S. R. Taylor: "The Bulk Composition of the Moon"
  8. ↑ Galimov and Krivtsov: "Origin of the Earth–Moon system"
  9. ↑ New Scientist: "Did the new moon lose its iron heart?"
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