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."

Re:Not a contradiction

[Quaoar]

This doesn't work. The fractional element abundances depend not only on the location in the protoplantery disk, but on the timescales of accretion, which depend on the mass of the object accreting. Thus, even if you formed Theia in L4/L5, the isotopic ratios should be different, as the two objects will have different masses.

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 [nature.com]

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:An alternate hypothesis.

[idbeholda]

The joke goes back a little bit further than 1991.

http://en.wikipedia.org/wiki/Spaceship_Moon_Theory [wikipedia.org]

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:Genesis 1:16

[Anonymous Coward]

Nothing in the verse that he mentioned actually implies that the moon is its own source of light. It is simply referred to as a luminous body. Which is is, when it's being illuminated by the sun.

Re:Genesis 1:16

[Empiric]

Wake me when the verse widens the scope it's addressing to "lesser, lesser lights".

For the record, I'm an Old Earth Creationist (OEC), not a Young Earth Creationist (YEC).

Some arguments against "creationism" are just too lame to not address, though.

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.