A new view of the Moon's formation: Crucial difference in 'fingerprints' confirms explosive, interconnected past

April 8, 2015, University of Maryland
This artist's rendering shows the collision of two planetary bodies. A collision like this is believed to have formed the moon within the first 150 million years after our solar system formed. Credit: NASA/JPL-Caltech

Within the first 150 million years after our solar system formed, a giant body roughly the size of Mars struck and merged with Earth, blasting a huge cloud of rock and debris into space. This cloud would eventually coalesce and form the moon.

For almost 30 years, planetary scientists have been quite happy with this explanation—with one major exception. Although this scenario makes sense when you look at the size of the moon and the physics of its orbit around Earth, things start to break down a little when you compare their isotopic compositions—the geological equivalent of a DNA "fingerprint." Specifically, Earth and the moon are too much alike.

The expectation has long been that the moon should carry the isotopic "fingerprint" of the foreign body, which scientists have named Theia. Because Theia came from elsewhere in the , it probably had a much different isotopic fingerprint than early Earth.

Now, a team of scientists at the University of Maryland has generated a new isotopic fingerprint of the moon that could provide the missing piece of the puzzle. By zeroing in on an isotope of Tungsten present in both the moon and Earth, the UMD team is the first to reconcile the accepted model of the moon's formation with the unexpectedly similar isotopic fingerprints of both bodies. The results suggest that the impact of Theia into early Earth was so violent, the resulting debris cloud mixed thoroughly before settling down and forming the moon. The findings appear in the April 8, 2015 advance online edition of the journal Nature.

"The problem is that Earth and the moon are very similar with respect to their isotopic fingerprints, suggesting that they are both ultimately formed from the same material that gathered early in the solar system's history," said Richard Walker, a professor of geology at UMD and co-author of the study. "This is surprising, because the Mars-sized body that created the moon is expected to have been very different. So the conundrum is that Earth and the moon shouldn't be as similar as they are."

The UMD team examined the tungsten isotopic composition of two moon rocks collected by the Apollo 16 mission, including sample 68815, seen here. When corrected for meteoritic additions to Earth and the moon after formation of the moon, the two bodies were found to have identical Tungsten isotopic compositions. Credit: NASA/JSC

Several different theories have emerged over the years to explain the similar fingerprints of Earth and the moon. Perhaps the impact created a huge cloud of debris that mixed thoroughly with the Earth and then later condensed to form the moon. Or Theia could have, coincidentally, been isotopically similar to young Earth. A third possibility is that the moon formed from Earthen materials, rather than from Theia, although this would have been a very unusual type of impact.

To tease out an explanation, Walker and his team looked to another well-documented phenomenon in the early history of the solar system. Evidence suggests that both Earth and the moon gathered additional material after the main impact, and that Earth collected more of this debris and dust. This new material contained a lot of Tungsten, but relatively little of this was of a lighter isotope known as Tungsten-182. Taking these two observations together, one would expect that Earth would have less Tungsten-182 than the moon.

Sure enough, when comparing rocks from the moon and Earth, Walker and his team found that the moon has a slightly higher proportion of Tungsten-182. The key, however, is how much.

"The small, but significant, difference in the Tungsten isotopic composition between Earth and the moon perfectly corresponds to the different amounts of material gathered by Earth and the moon post-impact," Walker said. "This means that, right after the moon formed, it had exactly the same isotopic composition as Earth's mantle."

This finding supports the idea that the mass of material created by the impact, which later formed the moon, must have mixed together thoroughly before the moon coalesced and cooled. This would explain both the overall similarities in isotopic fingerprints and the slight differences in Tungsten-182.

It also largely rules out the idea that the Mars-sized body was of similar composition, or that the moon formed from material contained in the pre-impact Earth. In both cases, it would be highly unlikely to see such a perfect correlation between Tungsten-182 and the amounts of material gathered by the moon and Earth post-impact.

"This result brings us one step closer to understanding the close familial relationship between Earth and the ," Walker said. "We still need to work out the details, but it's clear that our early solar system was a very violent place."

Explore further: New isotopic evidence supporting moon formation via Earth collision with planet-sized body

More information: "Tungsten isotopic evidence for disproportional late accretion to the earth and moon," Mathieu Touboul, Igor Puchtel and Richard Walker, was published on April 8, 2015, in the Advance Online edition of the journal Nature. nature.com/articles/doi:10.1038/nature14355

Related: The origin of the Moon and its composition

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10 comments

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verkle
Apr 08, 2015
This comment has been removed by a moderator.
Vietvet
4.1 / 5 (9) Apr 08, 2015
@verkle

You know it's a sin to lie?
ian_miller
5 / 5 (6) Apr 09, 2015
Theia impacted between 30 -100 My after Earth formed. Where did this dust come from then? Both bodies would be expected to acquire the same amount of dust, if there were dust, but the Moon would leave its dust on the surface, while Earth reprocesses its surface regularly. The fact that the earth has less of this dust remnants on its surface appears to me to be exactly what would be expected irrespective of how the Moon formed.
Torbjorn_Larsson_OM
5 / 5 (3) Apr 09, 2015
This is why I love science!

By pure coincidence, the same number of Nature contains model work that resolves the "started out alike" scenario. Taking account for _all_ involved planetesimals, and noting that their compositions will differ depending from where on the disk they originate, the likelihood that the impactors are similar is ~1/3. As a comparison, that Mercury is in orbital resonance instead of tidally locked is a similar likelihood outcome.

Another group will need to replicate the tungsten results, now that the analysis methods are finally up to the task. But coincidentally they are are validated by matching highly siderophile element (HSE) behavior during late veneer accretion, as well with the find last year of small Earth-Moon differences in oxygen isotopes. [ http://www.bbc.co...32219494 ]

[tbctd]
Torbjorn_Larsson_OM
5 / 5 (3) Apr 09, 2015
[ctd]

So we can be fairly certain that:

1) The Tellus-Theia collision spawned Earth-Moon
2) The HSE element differences are mainly a result of late veneer
3) Earth (and Moon) emerged ~ 90 million years after the protoplanetary disk formed, as dated by the HSE clock of late veneer accretion.

@ian_miller: When I read your comment, I was reminded about this:

Theia contributed ~ 10 % of mass, and the earlier planet was remelted and differentiated another round. The geologists prefer to label the earlier planet Tellus, and time Earth's formation from the impact event. The interregnum before the Hadean would be the Chaotian. "... the new time scale will enable geologists studying the early history of the planet to write more accurate papers ..." [ http://www.popsci...r-system ]

It isn't officially decided yet. FWIW et cetera.
Baruch
1 / 5 (3) Apr 09, 2015
Somehow I'm a bit confused here.
Either these scientists didn't read the research that shows that water is locked into the very heart of the rocks in the moon and hence had to be present at its original formation or else they are simply ignoring that research completely.

If the moon formed according to the impact theory, then surely ALL the water present at the time should have been vaporised and vanished in the heat released by said impact?

Yet, here we have clear physical, real observation showing that there is abundant water locked up in the very heart of the moon - a total contradiction of what should have happened according to the impact theory.

So which is more plausible? The direct observation and measurement or the unsubstantiated impact theory which bears no observational support?
Baruch
1 / 5 (3) Apr 09, 2015
@VietVet
@verkle
You know it's a sin to lie?


Somehow playing the man instead of answering the question he posed smacks of an inability to reason on an objective basis.

I happen to concur with the gist of his question - which physical theory explains how two bodies can collide with the total annihilation of the one and then reform into two separate, orbiting orbs?

I've seen a previous model that calculates just how this could have happened, unfortunately that model requires miraculous coincidences of angles of approach, speed and mass.
It might be easier for me to burrow throw the centre of the earth in a month than for those coincidences to happen.

bobbysius
5 / 5 (1) Apr 09, 2015
"This is surprising, because the Mars-sized body that created the moon is expected to have been very different. So the conundrum is that Earth and the moon shouldn't be as similar as they are."

I don't understand why Theia is expected to have been so isotopically different. Formation theories for Theia show it would have formed at the L4 or L5 point in Earth's orbit and thus coalesced out of the same region of the solar nebula. Yes, its gravity would less than that of the proto-Earth, but that would really only affect the abundances of lighter elements like hydrogen and helium. Why then is this even a "conundrum"?
whisperin_pines
not rated yet Apr 12, 2015
A conclusion drawn from the examination of TWO moon rocks?
Jaeherys
5 / 5 (1) Apr 13, 2015
If the moon formed according to the impact theory, then surely ALL the water present at the time should have been vaporised and vanished in the heat released by said impact?


I would not think that to be the case. I would suspect that water bound to other molecules like rocks and minerals would be unbound upon impact and ejected into orbit along with all other matter. This matter would then reform those previous associations as crystals as matter condensed. All types of molecules should be more or less ejected into orbit based on their initial velocities, those that exceed this and leave Earth's orbit should be in about the same ratio as those that remain. Thus, there should still be about the same ratio of water in these rocks as there was initially, assuming both bodies had the same amount of water.

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