Findings Cast Doubt On Moon Origins

sciencehabit writes "A new analysis of isotopes found in lunar minerals challenges the prevailing view of how Earth's nearest neighbor formed. Geochemists looked at titanium isotopes in 24 separate samples of lunar rock and soil, and found that the moon's proportion was effectively the same as Earth's and different from elsewhere in the solar system. This contradicts the so-called Giant Impact Hypothesis, which posits that Earth collided with a hypothetical, Mars-sized planet called Theia early in its existence, and the resulting smash-up produced a disc of magma orbiting our planet that later coalesced to form the moon."

Comments

In other words...

[Anonymous Coward]

That's no moon!

Where is it ? (my keys)

[fluffythedestroyer]

We have the technology to find and look very deep or far where isotopes are or where the fartest solar system is. But yet, I can't find my damn keys in my house sometimes.

Re:Where is it ? (my keys)

[stoofa]

That's because you look for your keys with sight. 'Fartest' solar systems are detected with smell.

Re:Where is it ? (my keys)

[ShanghaiBill]

We have the technology to find and look very deep or far where isotopes are or where the fartest solar system is. But yet, I can't find my damn keys in my house sometimes.

If technology isn't solving your problem, you aren't using enough: Put an RFID tag on your key chain. While you are at it, you should tag the TV remote too.

Re:Where is it ? (my keys)

[fleeped]

Obviously using the Smell-O-Scope

Re:What are the implications?

[Anonymous Coward]

Anthropogenic global warming.

Not a contradiction

[Anonymous Coward]

It doesn't contradict it at all. The current version of the impactor theory pre-supposes that Theia was formed at Earth's L4 or L5 point. There, the fractional distillation effect in the solar nebula would give the same Ti isotope ratios as in Earth, since Theia would be orbiting at the same distance. Formation at L4 or L5 also gives a nicely low impact energy, agreeing with what is needed to form the moon.

Re:Not a contradiction

[icebike]

The location of Theia's formation at 4 or L5 would be close enough to earth that the accretion would of the same material. Further if Theia were at L4 it would lead earth in the orbital path, 60 degrees ahead, and would tend to preferentially sweep the protoplanetary disk, before earth's mass rendered any advantage. Any differences in ratios would be small at the time of impact.

Bear in mind that anything at the the Lagrange points must necessarily be insignificantly small relative to the earth. As soon as it stops being so, the likelihood of it staying at the Lagrange point becomes nil. I remain unconvinced that a planet could form at L4 or L5 and become large enough such that any impact would eject a mass as large as the moon. Drift should occur long before it acquired enough mass. (Earths orbit is not circular, rather it is elliptical, and as such the Lagrange points are really unstable Lagrange "areas").

Disregarding my doubts, when a body formed at L4 or L5 does drift, and impact earth, that impact would scatter its content over the surface of the earth such that we would, after all these billions of years, be hard pressed to distinguish it from earth's original composition. Similarly, the moon would be composed of the same material sources, a combination of both Theia and Earth materials.

Any subtle differences in accretion would be completely masked by impact mixing.

However, the same could be said about any body impacting the earth. The likelihood of such a body remaining intact (bottling up any difference in isotopic ratios) is virtually nil, and both earth and moon are going to be covered with the same relative ratios in any method which postulates the moon being formed from ejecta from an earth impact.

At best this finding puts to rest the long discredited "captured moon" theory.

Re:Not a contradiction

[Quaoar]

The problem is that the ratio of Earth's mantle to Theia's mantle matters in the combination, even if mixing is efficient. The Earth's mantle is fully convective, and around 6 times the mass of the impactor's mantle, which means that you have to really fine-tune the conditions to achieve the exact right mixture. A good analogy would be trying to mix milk and water in a glass such that the fluid that splashes out of the glass has the same fraction of milk to water as the fluid remaining in the glass. With the original Oxygen isotope constraint, 95%+ of the lunar mantle needed to originate from the Earth, which is in direct conflict with the giant impact simulations that have been performed (which find 80% coming from the *impactor*), even for iron-rich impactors that preferentially remove Earth's mantle. This new constraint, if I am reading the paper correctly, is even stronger than the Oxygen isotope constraint, being at the part per million level rather than the part per ten thousand level.

Re:Not a contradiction

[icebike]

The Earth's mantle is fully convective, and around 6 times the mass of the impactor's mantle,

Wait, what? Where did you get 6 times? And where did the impactor get a mantle? That number is sheer conjecture, and the existence of a mantle makes so sense until you have an impactor large enough to have a differentiated body. That hasn't been proven.

Moon's core is different from earth's by our best guesses. But the surface accretion in the eons after any impact is going to accumulate the same combination of protoplanetary disk material and ejecta material.

We've barely scratches the surface of earth, let alone the moon. These isotope measurements are akin to determining the structure of a large building by examining a paint chip scraped off of each.

And using hind sight, doesn't ANY outcome appear to be the result of "fine tuning"? Isn't any such argument just another form of intelligent creation dogma?

Re:Not a contradiction

[Quaoar]

The preferred giant impact model has a Mars-sized impactor with a core-to-mantle ratio equal to the Earth's, with approximately 30% of its mass being in an iron core (http://adsabs.harvard.edu/abs/2004Icar..168..433C). Mars is ~1/6th the Earth's mass. In this impact, the material liberated that eventually forms the moon is iron poor, as the iron core of the impactor sinks into the Earth. That has been the interpretation as to why the Moon's iron core is so small (no more than 3% its total mass), so in this sense the giant impact model produces a satisfactory outcome. Some fraction of the lunar surface is accumulated over the 4 billion years since the Moon formed, but this layer is thought to be very thin, and the meteorites + Apollo samples we use to measure the moon's isotopic ratios come from a range of depths that probe significantly deeper than this surface layer. The fine-tuning argument comes from the fact that for an arbitrary combination of impactor + Earth mass, impact angle, velocity, etc, you'd expect a scatter in the isotope ratios consistent with the typical scatter measured between other bodies in the solar system (say that between Mars and the Earth). Fine-tuning is often employed in intelligent design arguments as they rely on the anthropic principle, but as there's no reason to require the Earth and Moon to have identical isotopic compositions to explain the existence of life, there is no particular reason to favor any particular outcome over the myriad of other outcomes for this particular measurement.

Good conclusion bad logic (or writing)

[vlm]

Conclusion sounds good, written logic is horrible.

found that the moon's proportion was effectively the same as Earth's

This contradicts the so-called Giant Impact Hypothesis, which posits that Earth collided with a hypothetical, Mars-sized planet called Theia early in its existence, and the resulting smash-up produced a disc of magma orbiting our planet that later coalesced to form the moon.

Does not explain why that doesn't work. The summary makes it sound very likely that something "smooshed off" the earth and became the moon, because both have the same ratios. Also does a poor job of explaining the more likely alternative explanation, by not discussing it at all. Fail.

I think part of the fail is assuming:

different from elsewhere in the solar system

That means we've sampled everything in the entire solar system both now and infinitely in the past? ha ha I think not.

Headline vs. Article

[Sean_Inconsequential]

From the article: "[...]and the researchers aren't claiming to have refuted the giant impact hypothesis."

Working Article Link

[Quaoar]

http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo1429.html

Re:What are the implications?

[jdgeorge]

So, if it wasn't a big impact, what was it? What's the next best theory?

Well, according to TFA:
"One possibility is that a glancing blow from a passing body left Earth spinning so rapidly that it threw some of itself off into space like a shot put, forming the disk that coalesced into the moon. This would explain why the moon seems to be made entirely of Earth material. But there are problems with this model, too, such as the difficulty of explaining where all the extra angular momentum went after the moon formed, and the researchers aren't claiming to have refuted the giant impact hypothesis."

Re:Occam's Razor

[ShanghaiBill]

Is there evidence to suggest that the simplest explanation (accretion disk formed the earth and the moon at roughly the same time, along with all the other rocky planets) is not the correct one?

Computer simulations have shown that the accretion disk theory is unlikely. The moon is HUGE. Compared to the size of the mother planet, it is by far the biggest in the solar system. It is also really far from the earth, nearly 400,000km. By comparison, the distance from Mars to Phobos is less than 10,000km. Most of the mass in an accretion disk should have fallen to earth, with a small amount forming a few very small moons, orbiting closely.

Re:An alternate hypothesis.

[elgeeko.com]

That's the silliest thing I've ever read. Anyone with half a mind knows it wasn't built by Aliens, it was built by a previously advanced civilization on Earth that now controls our governments from the safety of their Lunar Habitat. Geesh, get an education or at least watch the History Channel!

Re:What are the implications?

[VernonNemitz]

The giant impact scenario can still make sense. All we need to do is assume both the Earth and the other object formed in the same zone (distance from sun). That's the most critical thing, since we can expect any one zone, all around the sun, to be fairly consistent in its isotopic composition. So, each gathered up lots of debris while forming, and their collision constituted one of the last events that made the Earth a planet (per modern definition: a planet has to clear its zone of all large debris).