The Origin Of The Moon

[Eclogite]

On another thread, as an aside, the topic of lunar origin came up. I have recently been doing short write ups, for my own edification, on topics scientific and philosophical. Here is one on the origin of the moon, that might interest some of you. (Hello, Turtle. Hello )

 

The overwhelming consensus view among scientists today is that the moon formed following the impact of a Mars sized planet (Theia) on the proto-Earth during the early stages of the formation of the solar system. A huge volume of material was thrown into space, much of it at such speed it was able to escape the gravitational attraction of the Earth. However, some remained in orbit as a debris disc that rapidly (perhaps in less than one hundred years) condensed and accreted to form the moon.

 

Any hypothesis for lunar formation has to account for several features of the moon:

 

• A comparatively large size relative to its parent. (The moon is ~1% of the mass of the Earth.)

• The large angular momentum of the Earth-Moon system

• Low lunar density, implying a major depletion in iron

• A lunar orbit once much closer to the Earth and inclined at 10° to the ecliptic

 

Data from the Apollo samples added further complications: the lunar composition bore striking similarities to terrestrial mantle material, but also striking differences. For example, the oxygen isotope ratios of mantle and lunar rocks differed from chondrites, the posited source for each, by similar amounts; however the moon was clearly seriously depleted in volatiles.

 

Three hypotheses were considered as plausible:

 

• Capture of a body formed elsewhere in the system

• Fission as a consequence of very high rotational speed of the proto-Earth

• Co-formation alongside the Earth

 

Each of these three hypotheses had serious problems. Consequently, two independent groups developed an alternative impact hypothesis: Hartmann and Davis in 1975 and Cameron and Ward in 1976. This idea languished until all four origin hypotheses were examined in detail in a 1984 conference in Hawaii, from which the impact hypothesis emerged as the clear winner.

 

Subsequent advances in computer power, especially using smooth particle hydrodynamics (SPH), have enabled detailed simulations that have refined the hypothesis and removed most of the contradictions, or remaining questions. In parallel with this, work on lunar geochemistry, especially isotope ratios, and the character and formation of the magma ocean, has complimented the dynamical findings. As a consequence, the impact origin of the moon is now generally accepted and work focuses on resolving any remaining inconsistencies.

 

Selected Bibliography:

Cameron, A.G.W. & Ward, W.R. The origin of the Moon. Lunar Sci.7: 120-122 (1976).

Canup,R.M. & Asphaug,E. Origin of the Moon in a giant impact near the end of the Earth's formation Nature 412: 708-712 (2001)

Canup R.M. Dynamics of Lunar Formation Annu.Rev.Astron.Astrophys.42: 441–75 (2004)

Hartmann, W.K. & Davis, D.R. Satellite-sized planetesimals and lunar origin. Icarus24: 504-515 (1975)

Hartmann,W.K.,Phillips R.J.,Taylor G.J.,eds. Origin of the Moon. Houston: LunarPlanet.Inst.781pp (1986)

Warren The Magma Ocean Concept and Lunar Evolution Ann. Rev. Earth Planet. Sci. 1985. 13: 201-40

[Deepwater6]

I thought once we went to the moon and brought samples back, solving this puzzle would be easy, apparently not.

 

I have read somwhere recently (and can't remember where) that there is a dual moon model out there. This one has the early Earth with two moons for a period of time. At some point the other moon was kicked out to who knows where. Are you familiar with this model Eclogite? Is it a mainstream theory or just a shot at trying to come up with another scenario?

[CraigD]

I thought once we went to the moon and brought samples back, solving this puzzle would be easy, apparently not.

I think it’s reasonable to think that, had the Apollo lunar sample return missions of 1969-1972, or the successful unmanned Soviet Luna lunar sample return missions of 1970-1976, not occurred, there wouldn’t have been enough data to reach as certain a consensus on the Moons origin as exists today in the giant impact hypothesis. Although giant impact models had been around since the late 1940s, without hard, chemical data from the moon samples, there just wasn’t enough to decide between them and various other reasonable theories.

 

A key point to note about lunar sample is that we’ve yet to sample, via return mission or remote analysis, its far side. There was a lot of argument about Apollo 17 landing on the farside, but safety concerns (there would have been long Earth-to-LEM radio blackouts, even relaying via the orbiting CSM, and about half the time, no radio link even with the CSM, so relay satellites were considered necessary, but not feasible) won the argument, and Apollo 17 landed, like all its predecessors, on the near side. Had there been more Apollo and Luna missions, a farside sample return might have happened, but when the US-Soviet “moon race” ended, it ended hard.

 

Computer modeling, which benefits from the great improvements in computers over the past few decades, are key to progress in origin of the Moon models. But they can’t replace sample analysis.

 

The lack of details about the composition of the far side of the moon figures importantly into the next subject you raise, Deepwater.

 

I have read somwhere recently (and can't remember where) that there is a dual moon model out there. This one has the early Earth with two moons for a period of time.

The conventional giant impact models (Including Hartman, Davis, Cameron and Ward’s early ones from the late 1970s) include multiple extra moons, but they’re short lived – no more than 100 years or so after the giant impact.

 

Only recently, I think, a model involving a second moon of about 4% lunar mass colliding with the Moon about 10,000,000 years after the giant impact came out. I read about it a couple of years after Jutzi and Asphaug’s Forming the lunar farside highlands by accretion of a companion moon by was published in August 2011 Nature – a legitimate free copy of the paper can be found here.

 

The driver for a large, long-lived second moon model is mostly the surprising large-scale differences between the geography of the Moon’s near and far sides. The familiar near side has a lot (about 30%) of smooth, lava flood features, while the far side doesn’t (only about 1%). The far side is also substantially higher (about 3,000 m or 0.3%, further from the center of the Moon) than the near.

 

Computer models, and the simple observation that many of the farside craters are big, and thus very old, have pretty much ruled out that the difference is due to the Earth having shielded the nearside from impacts. There are some theories suggesting gravitational effects of Earth are the cause, but they're not very good. Jutzi and Asphaug’s theory is looking good, and from what I can tell, gaining acceptance, aided by the high-resolution maps lunar orbiters have made over the past decade.

 

We could really use a far side sample return mission, though. Computer models and orbital surveys can only take theories so far.

 

At some point the other moon was kicked out to who knows where.

I’m not aware of any models where large, extra moons escape.

[Guest MacPhee]

What if the Moon was originally formed in the asteroid belt, between Mars and Jupiter. Then it spiralled inwards, towards the Sun. Pulled by the Sun's gravity.

 

As it came inwards, the Moon encountered the orbit of Mars - a small planet, with low mass and weak gravity. The Moon slipped easily by it, and continued inwards. Then the Moon encountered the Earth.

 

Earth is a different proposition from Mars. Earth has nearly twice the diameter, and almost ten times the martian mass. With a correspondingly stronger gravitational field. This enabled Earth to capture the Moon, and swing it into a permanent orbit around the Earth.

 

So couldn't our Moon be a captured former asteroid - like Phobos and Deimos?

[CraigD]

What if the Moon was originally formed in the asteroid belt, between Mars and Jupiter

...

So couldn't our Moon be a captured former asteroid - like Phobos and Deimos?

That’s a reasonable and respected hypothesis. Usually called “the capture hypothesis”, when I was an astronomy student in school, in the 1970s and '80s, it was one of the main alternatives taught, with its share of literature, and like all origin of the moon hypotheses, its share of difficulties and successes.

 

As your mention of them alludes to, MacPhee, capture is the favorite hypothesis for the origin of Mars’s 2 tiny moons, Phobos and Deimos. It’s also the leading hypothesis for the origin of Neptune’s largest moon, Triton.

 

The main problem with a capture hypothesis for the Earth's moon, Luna, arises from a few collection of factors:

• An orbit transfer maneuver, which capturing a moon is, requires well-defined non-gravitational change in velocity (the maneuver’s “delta V”)
  
• Since powerful rocket engines and such don’t naturally occur, powerful collisions with other bodies at precisely the right moment are diminishingly rare, and other non-gravitational forces are too small, the only reliable mechanism to accomplish this delta V is friction with the primary body’s atmosphere, AKA aerobraking.
  
• Luna is too large, and the Earth’s present-day atmosphere to small.
  

Capture is plausible for Neptune-Triton, because Neptune’s atmosphere is big enough. It’s plausible for Mars-Phobos, because its mass is small enough. For Earth-Luna, though it’s not, because Earth’s atmosphere isn’t large enough, and Luna’s mass is too large.

 

I expect we could, with a lot of work, set parameters for a simulation in which a Luna-size asteroid, or a former giant planet’s moon, gets captured by Earth, having multiple just-right aerobraking and other-body gravitational “slingshot” interactions, but then the probability of it happening would by tiny. Aerobraking is practical for transferring a spacecraft into orbit around a planet with an atmosphere, but only because it can be very precisely planned and guided.

 

There’s also the problem of the composition of Luna, which from the direct and indirect measurements that have been made of it, fits better the compositions of Earth and the Mars-size “Theia” of the giant impact hypothesis than with the composition of an asteroid. The fit might be better with a giant planet moon.

 

So, short answer: Luna could be a captured body, but from all the simulations and data we have now, that it’s the result of a giant impact is much more likely.

[Deepwater6]

http://www.bbc.com/future/story/20130411-solving-a-moon-mystery

 

This article on BBC gives a version of a planet 5x larger than Mars in a collision.

[Moontanman]

http://www.bbc.com/future/story/20130411-solving-a-moon-mystery

 

This article on BBC gives a version of a planet 5x larger than Mars in a collision.

 

 

Um... your link didn't say anything about the origin of the moon...

[Deepwater6]

Page two fourth paragraph down, under "living and breathing"

[CraigD]

http://www.bbc.com/future/story/20130411-solving-a-moon-mystery

 

This article on BBC gives a version of a planet 5x larger than Mars in a collision.

The model the BBC article mentions proposes that the Earth and Moon were formed not from the collision of a roughly 1 Earth-mass early Earth and a 1 Mars-mass (about 0.1 Earth-mass) "Theia", as in the Giant Impact hypothesis, but from 2 similar, about 5 Mars-mass early planets.

 

That's a pretty big change!

 

This 10/30/2012 NASA news article has a bit more detail. The paper on it, by SRIR's Robin Canup, rather than NASA's Richard Elphic, in in Science Online. Hopefully, some of us will have a chance to read it soon.

[Moontanman]

Page two fourth paragraph down, under "living and breathing"

 

 

I missed it sorry..

[Lancewen]

On another thread, as an aside, the topic of lunar origin came up. I have recently been doing short write ups, for my own edification, on topics scientific and philosophical. Here is one on the origin of the moon, that might interest some of you. (Hello, Turtle. Hello )

 

The overwhelming consensus view among scientists today is that the moon formed following the impact of a Mars sized planet (Theia) on the proto-Earth during the early stages of the formation of the solar system. A huge volume of material was thrown into space, much of it at such speed it was able to escape the gravitational attraction of the Earth. However, some remained in orbit as a debris disc that rapidly (perhaps in less than one hundred years) condensed and accreted to form the moon.

 

Any hypothesis for lunar formation has to account for several features of the moon:

 

• A comparatively large size relative to its parent. (The moon is ~1% of the mass of the Earth.)

• The large angular momentum of the Earth-Moon system

• Low lunar density, implying a major depletion in iron

• A lunar orbit once much closer to the Earth and inclined at 10° to the ecliptic

 

Data from the Apollo samples added further complications: the lunar composition bore striking similarities to terrestrial mantle material, but also striking differences. For example, the oxygen isotope ratios of mantle and lunar rocks differed from chondrites, the posited source for each, by similar amounts; however the moon was clearly seriously depleted in volatiles.

 

Three hypotheses were considered as plausible:

 

• Capture of a body formed elsewhere in the system

• Fission as a consequence of very high rotational speed of the proto-Earth

• Co-formation alongside the Earth

 

Each of these three hypotheses had serious problems. Consequently, two independent groups developed an alternative impact hypothesis: Hartmann and Davis in 1975 and Cameron and Ward in 1976. This idea languished until all four origin hypotheses were examined in detail in a 1984 conference in Hawaii, from which the impact hypothesis emerged as the clear winner.

 

Subsequent advances in computer power, especially using smooth particle hydrodynamics (SPH), have enabled detailed simulations that have refined the hypothesis and removed most of the contradictions, or remaining questions. In parallel with this, work on lunar geochemistry, especially isotope ratios, and the character and formation of the magma ocean, has complimented the dynamical findings. As a consequence, the impact origin of the moon is now generally accepted and work focuses on resolving any remaining inconsistencies.

 

I have a couple of questions that haven't been made very clear yet. First, what happened to Theia after the impact? On a program I watched they claimed that Theia's core merged with Earth's core. Then they also said the computer models showed the collision had to be a glancing blow to produce a debris field that would coalesce into a moon. It doesn't seem like those two events could both happen. In a glancing blow Theia would be off somewhere else and not merged with the Earth. Second, Nothing has been said about the composition of Theia. Wouldn't a great deal of Theia's mass also be included in the moons formation?

[Moontanman]

I have a couple of questions that haven't been made very clear yet. First, what happened to Theia after the impact? On a program I watched they claimed that Theia's core merged with Earth's core. Then they also said the computer models showed the collision had to be a glancing blow to produce a debris field that would coalesce into a moon. It doesn't seem like those two events could both happen. In a glancing blow Theia would be off somewhere else and not merged with the Earth. Second, Nothing has been said about the composition of Theia. Wouldn't a great deal of Theia's mass also be included in the moons formation?

 

 

Most of Theia's mass was captured by the Earth including it's core, this gave the Earth a significantly bigger core than say Venus but it is possible that a considerable amount of Theia was lost to space in the impact, as a side note it has been discovered that Moon has quite a bit more volatiles than was once thought contained in microscopic sphericals, as much as lava in underwater volcanoes on the Earth. This is not inconsistent with the impact theory.

 

http://www.dailymail.co.uk/sciencetech/article-2322388/Water-moon-come-Earth-Scientists-analysis-moon-rocks-lunar-moisture-identical-ours.html