Chemistry says Moon is proto-Earth's mantle, relocated

September 12, 2016, Washington University in St. Louis
Two recent models for the formation of the moon, one that allows exchange through a silicate atmosphere (top), and another that creates a more thoroughly mixed sphere of a supercritical fluid (bottom), lead to different predictions for potassium isotope ratios in lunar and terrestrial rocks (right). Credit: Kun Wang

Measurements of an element in Earth and Moon rocks have just disproved the leading hypotheses for the origin of the Moon.

Tiny differences in the segregation of the isotopes of potassium between the Moon and Earth were hidden below the detection limits of analytical techniques until recently. But in 2015, Washington University in St. Louis geochemist Kun Wang, then the Harvard Origins of Life Initiative Prize postdoctoral fellow, and Stein Jacobsen, professor of geochemistry at Harvard University, developed a technique for analyzing these isotopes that can hit precisions 10 times better than the best previous method .

Wang and Jacobsen now report isotopic differences between lunar and terrestrial rocks that provide the first experimental evidence that can discriminate between the two leading models for the Moon's origin. In one model, a low-energy impact leaves the proto-Earth and Moon shrouded in a silicate atmosphere; in the other, a much more violent impact vaporizes the impactor and most of the proto-Earth, expanding to form an enormous superfluid disk out of which the Moon eventually crystallizes.

The isotopic study, which supports the high-energy model, is published in the advance online edition of Nature Sep.12, 2016. "Our results provide the first hard evidence that the impact really did (largely) vaporize Earth," said Wang, assistant professor in Earth and Planetary Sciences in Arts & Sciences.

An isotopic crisis

In the mid-1970s, two groups of astrophysicists independently proposed that the Moon was formed by a grazing collision between a Mars-sized body and the proto-Earth. The giant impact hypothesis, which explains many observations, such as the large size of the Moon relative to the Earth and the rotation rates of the Earth and Moon, eventually became the leading hypothesis for the Moon's origin.

In 2001, however, a team of scientists reported that the isotopic compositions of a variety of elements in terrestrial and are nearly identical. Analyses of samples brought back from the Apollo missions in the 1970s showed that the Moon has the same abundances of the three stable isotopes of oxygen as the Earth.

This was very strange. Numerical simulations of the impact predicted that most of the material (60-80 percent) that coalesced into the Moon came from the impactor rather than from Earth. But planetary bodies that formed in different parts of the solar system generally have different isotopic compositions, so different that the isotopic signatures serve as "fingerprints" for planets and meteorites from the same body.

The probability that the impactor just happened to have the same isotopic signature as the Earth was vanishingly small.

So the giant impact hypothesis had a major problem. It could match many physical characteristics of the Earth-Moon system but not their geochemistry. The isotopic composition studies had created an "isotopic crisis" for the hypothesis.

At first, scientists thought more precise measurements might resolve the crisis. But more accurate measurements of oxygen isotopes published in 2016 only confirmed that the isotopic compositions are not distinguishable. "These are the most precise measurements we can make, and they're still identical," Wang said.

A slap, a slug or a wallop?

"So people decided to change the giant impact hypothesis," Wang said. "The goal was to find a way to make the Moon mostly from the Earth rather than mostly from the impactor. There are many new models—everyone is trying to come up with one—but two have been very influential."

In the original giant impact model, the impact melted a part of the Earth and the entire impactor, flinging some of the melt outward, like clay from a potter's wheel.

A model proposed in 2007 adds a silicate vapor atmosphere around the Earth and the lunar disk (the magma disk that is the residue of the impactor). The idea is that the silicate vapor allows exchange between the Earth, the vapor, and the material in the disk, before the Moon condenses from the melted disk.

"They're trying to explain the isotopic similarities by addition of this atmosphere," Wang said, "but they still start from a low-energy impact like the original model."

But exchanging material through an atmosphere is really slow, Wang said. You'd never have enough time for the material to mix thoroughly before it started to fall back to Earth.

So another model, proposed in 2015, assumes the impact was extremely violent, so violent that the impactor and Earth's mantle vaporized and mixed together to form a dense melt/vapor mantle atmosphere that expanded to fill a space more than 500 times bigger than today's Earth. As this atmosphere cooled, the Moon condensed from it.

The thorough mixing of this atmosphere explains the identical isotope composition of the Earth and Moon, Wang said. The mantle atmosphere was a "supercritical fluid," without distinct liquid and gas phases. Supercritical fluids can flow through solids like a gas and dissolve materials like a liquid.

Why potassium is decisive

The Nature paper reports high-precision potassium isotopic data for a representative sample of lunar and terrestrial rocks. Potassium has three stable isotopes, but only two of them, potassium-41 and potassium-39, are abundant enough to be measured with sufficient precision for this study.

Wang and Jacobsen examined seven lunar rock samples from different lunar missions and compared their potassium isotope ratios to those of eight terrestrial rocks representative of Earth's mantle. They found that the lunar rocks were enriched by about 0.4 parts per thousand in the heavier isotope of potassium, potassium-41.

The only high-temperature process that could separate the potassium isotopes in this way, said Wang, is incomplete condensation of the potassium from the vapor phase during the Moon's formation. Compared to the lighter isotope, the heavier isotope would preferentially fall out of the vapor and condense.

Calculations show, however, that if this process happened in an absolute vacuum, it would lead to an enrichment of heavy potassium isotopes in lunar samples of about 100 parts per thousand, much higher than the value Wang and Jacobsen found. But higher pressure would suppress fractionation, Wang said. For this reason, he and his colleague predict the Moon condensed in a pressure of more than 10 bar, or roughly 10 times the sea level atmospheric pressure on Earth.

Their finding that the lunar rocks are enriched in the heavier potassium isotope does not favor the silicate atmosphere model, which predicts lunar rocks will contain less of the heavier isotope than terrestrial rocks, the opposite of what the scientists found.

Instead it supports the mantle atmosphere model that predicts lunar rocks will contain more of the heavier isotope than .

Silent for billions of years, the potassium isotopes have finally found a voice, and they have quite a tale to tell.

Explore further: A dash of water on the lunar rocks

More information: Kun Wang et al, Potassium isotopic evidence for a high-energy giant impact origin of the Moon, Nature (2016). DOI: 10.1038/nature19341

Related Stories

A dash of water on the lunar rocks

July 7, 2016

Ever since Apollo astronauts walked the lunar surface in 1969 and brought rocks back for laboratory analysis, it has been clear that lunar rocks are missing chemical components that boil off at relatively low temperature, ...

Water hidden in the Moon may have proto-Earth origin

September 10, 2013

Water found in ancient Moon rocks might have actually originated from the proto-Earth and even survived the Moon-forming event. Latest research into the amount of water within lunar rocks returned during the Apollo missions ...

Recommended for you

Nanoscale Lamb wave-driven motors in nonliquid environments

March 19, 2019

Light driven movement is challenging in nonliquid environments as micro-sized objects can experience strong dry adhesion to contact surfaces and resist movement. In a recent study, Jinsheng Lu and co-workers at the College ...

OSIRIS-REx reveals asteroid Bennu has big surprises

March 19, 2019

A NASA spacecraft that will return a sample of a near-Earth asteroid named Bennu to Earth in 2023 made the first-ever close-up observations of particle plumes erupting from an asteroid's surface. Bennu also revealed itself ...

The powerful meteor that no one saw (except satellites)

March 19, 2019

At precisely 11:48 am on December 18, 2018, a large space rock heading straight for Earth at a speed of 19 miles per second exploded into a vast ball of fire as it entered the atmosphere, 15.9 miles above the Bering Sea.

Levitating objects with light

March 19, 2019

Researchers at Caltech have designed a way to levitate and propel objects using only light, by creating specific nanoscale patterning on the objects' surfaces.

28 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

cray74
4.1 / 5 (9) Sep 12, 2016
Per the Verge's coverage, which had a counterpoint from lunar geologist Munir Humayun, Wang's team's selection of lunar breccias and basalts was a poor one for assessing potassium isotopes. The breccias are made from meteorite impacts and thus rather contaminated, while the cooling basalts are known to off-gas potassium and thus may appear depleted in K41. Since the isotopic difference is about 0.4 parts per thousand, contamination and depletion can easily obscure true isotopic differences between the Earth and moon.

As Humayun said, "I'm very pleased overall with what [Wang et al] have done, I just wish they had used better samples," and "Even a small amount of potassium loss could give rise to a measurable effect. I can't say these concerns kill this theory, but you have to work past these concerns before you can claim you have the potassium composition measured."
FredJose
1.4 / 5 (21) Sep 12, 2016
Silent for billions of years, the potassium isotopes have finally found a voice, and they have quite a tale to tell.

The potassium isotopes are still absolutely silent. It's the scientists rather enthusiastic interpretation that is creating all the noise.
Now of course we just need to sort out those tiny little gotchas that have crept in:
1. The moon is 1/4 the size of the earth.
2. It's just far away enough to blot out the sun in a total solar eclipse and which it just so happens is 400 times bigger and 400 times further away.
3. The moon is traversing in the ecliptic plane - just perfectly set up for the above.
4. The moon still has water. And had a much stronger magnetic field than earth.
5. The orbital plane and size of the moon just happens to balance out the planetary wobble to make life on earth possible.
6. It also just happened to be the exact distance away to have inspired the equations of gravity.
Coincidence, too much????? Play lotto instead?
shavera
4.6 / 5 (21) Sep 12, 2016
1. No it's not. It's 27.24% the size of Earth (comparing nominal radii). If your argument is based on this being some magic ratio made by some 'creator' then your creator has a weird taste for odd ratios.

2. Neat astronomical coincidence. But not always precisely true. Sometimes the moon is further away in its orbit and there's a 'ring of fire' eclipse. Sometimes it's closer and there's a total eclipse.

3. No it isn't. That's why we don't have an eclipse every single new and full moon. The general fact that things are *mostly* within the same plane is angular momentum at work.

4. So what?

5. Anthropic principle: Had our planet not been capable of supporting life, we wouldn't be here to remark on its capacity to support life.

6. Wut? No seriously, wut? 'the equations of gravity' were used to explain all sorts of planetary and satellite orbits. The moon isn't particularly unique in this regard.
wduckss
1.3 / 5 (14) Sep 12, 2016
Observe part of the whole and make conclusions has never brought a positive result (it is a the construction with a lot of "if". When this we apply to the whole we see that is wrong.

"Why there are differences in the structure of the objects in our system" https://www.acade...r_system
tblakely1357
2.7 / 5 (7) Sep 12, 2016
Many now say that a large moon is crucial in stabilizing earth's climate. Supposedly, if not for our large moon, it would be very unlikely that complex lifeforms could evolve. If true, then earth analogs must be very rare. Factor in the need for a strong magnetic field, the right type of star and being in the 'Goldilocks' zone of a galaxy makes other 'earths' even rarer.

I wonder how many other earth analogs are in this galaxy. Is it even possible to make an estimate without a lot of wild ass guessing?
Tektrix
4.4 / 5 (7) Sep 12, 2016
Observe part of the whole and make conclusions has never brought a positive result . . .


Crystallographers disagree.
barakn
4.2 / 5 (20) Sep 12, 2016
4. The moon still has water. And had a much stronger magnetic field than earth.
FredJose is a liar. The lunar magnetic field is far weaker than earth's and is not even a dipole. Most of the water is found in high-latitude lunar craters, meaning it was added from external sources after the moon's surface had already solidified.
Zzzzzzzz
4.3 / 5 (12) Sep 12, 2016
barakn - everyone knows FredJose is a liar, except FredJose
Zorcon
5 / 5 (4) Sep 12, 2016
"low-energy impact" in a manner of speaking, lol.
baudrunner
1 / 5 (8) Sep 12, 2016
Just about everybody has always thought that a collision was responsible for the moon's creation. It is even described in the Enuma Elish, if Sitchin's work pans out, and I think that it does. What nobody really knows is, at what time in Earth's geological history did this collision take place? We should really know that, before we start drawing additional conclusions.
Zorcon
4.2 / 5 (10) Sep 12, 2016
Just about everybody has always thought that a collision was responsible for the moon's creation.


It's an old hypothesis, but until the Apollo missions returned samples, there were two viable competing ones - capture, and formed-in place.

What nobody really knows is, at what time in Earth's geological history did this collision take place?


Any such collision pretty much reset Earth's geological records to zero (i.e., vaporized the previous geology). So at least we have a lower limit - the oldest rocks found on Earth or Moon With one caveat: it is possible (likely, in fact) that solid pieces were ejected into Earth-crossing solar orbits and later recaptured... possibly after Earth & Moon had cooled enough to preserve them. A good place to look would be on the Moon.

Zorcon
3.7 / 5 (11) Sep 12, 2016
Putting up a Moon just to prove is an extremely passive aggressive way to say "All your base are belong to us". Why did this god of yours not put his signature everywhere?


Probably to separate the infidels from the True Pastafarians. The FSM works in mysterious ways.
ddaye
5 / 5 (6) Sep 12, 2016
Just about everybody has always thought that a collision was responsible for the moon's creation.


This isn't right, it was a new idea after the Apollo samples were studied. One common explanation I learned earlier, which predated the acceptance of plate tectonics, was that the moon was ripped out of the crust and left the modern Pacific Ocean basin. Others included separate formation and capture of the passing Moon by the Earth, and the two forming together.
shavera
5 / 5 (4) Sep 13, 2016
can someone first come up with a physical simulation that something the size of the moon could really form from 2 bodies colliding?


When they say "model" up above, I'm pretty sure they are including a whole host of simulations as part of that model.
Nik_2213
5 / 5 (2) Sep 13, 2016
This is remarkable work; still, they really, really need material from beyond the debris field of 'Late Heavy Bombardment' which may have mildly or grossly contaminated these samples.

Perhaps the Chinese would be kind enough to oblige ? They seem the only likely candidates for multiple sample-return missions in next decade...

FWIW, the 1:400/1:400 average is numerology. It's no more significant than Mars' average day (Sol ?) being a fair match to our tide times. We also get annular eclipses. And the average Earth/Moon separation is steadily increasing due to tidal dissipation. The Earth's average rotation is gradually slowing. Both amply documented by fossil shells' growth rings !! The Moon used to be much, MUCH closer, with much more frequent total eclipses. Given umpteen (*) more million years, we'll only get 'annulars'...

*) future tidal dissipation estimates are beholden to minutiae of plate tectonics opening and closing broad, shallow sea-ways...
torbjorn_b_g_larsson
4 / 5 (4) Sep 13, 2016
"a counterpoint from lunar geologist Munir Humayun, Wang's team's selection of lunar breccias and basalts was a poor one for assessing potassium isotopes"

Thanks, I thought it would have all sorts of problems. If anything, many geologists have indications that the Earth mantle is poorly mixed, which is the opposite of the model proposed here.

@Bart: "can someone first come up with a physical simulation that something the size of the moon could really form from 2 bodies colliding?"

Of course, it has been done for years, or the giant impact hypothesis would have been abandoned now that we can do these types of simulations. [Say, https://www.youtu..._JBQtH9o ]
torbjorn_b_g_larsson
4 / 5 (4) Sep 13, 2016
@tblakely: "Many now say that a large moon is crucial in stabilizing earth's climate."

Not many among the experts.

It was shown years ago that the early simulations had an error, and with the model physics corrected simulations of Mars show that it has long enough stable periods for interesting evolution. (I.e. > 0.5 Gyrs, the time taken for complex life to evolve on Earth.)

Repeating a misconception many times does not make it fact.
Nik_2213
5 / 5 (1) Sep 13, 2016
I suppose one take on the 'Large, rare Moon' thing is that, given so many exo-planetary gas giants, there'll be many tidally stirred mega-moons in 'goldilocks zone'...

Who'd look at *our* system as a possible habitat for life after noting that Jupiter's too far out ??
baudrunner
1 / 5 (1) Sep 13, 2016
Any such collision pretty much reset Earth's geological records to zero (i.e., vaporized the previous geology).
Not necessarily. Whether the collision occurred when Earth was in a molten state in its early history, or long after it had cooled, matters in the final analysis.
1five3
1 / 5 (5) Sep 13, 2016
@Phys1:

Putting up a Moon just to prove is an extremely passive aggressive way to say "All your base are belong to us". Why did this god of yours not put his signature everywhere: "Jahweh/Odin/Zeus/Jeebus Fecit Anno 0".
In the clouds, on shells, on the backside of pebbles, on the human forehead, why, on the surface of the Moon.


Everything you have or will ever observe or come to understand carries the same signature, whether you recognize, remember, or acknowledge what "it" is. And everything you ever do or say shows less about the universe than it does about what you understand and believe about yourself. Knowing this, the most important question to ask yourself is why you don't know what "you" are, where you came from, and where you are going.
Maggnus
3.4 / 5 (5) Sep 13, 2016
@Phys1:

Putting up a Moon just to prove is an extremely passive aggressive way to say "All your base are belong to us". Why did this god of yours not put his signature everywhere: "Jahweh/Odin/Zeus/Jeebus Fecit Anno 0".
In the clouds, on shells, on the backside of pebbles, on the human forehead, why, on the surface of the Moon.


Everything you have or will ever observe or come to understand carries the same signature, whether you recognize, remember, or acknowledge what "it" is. And everything you ever do or say shows less about the universe than it does about what you understand and believe about yourself. Knowing this, the most important question to ask yourself is why you don't know what "you" are, where you came from, and where you are going.

And if my aunt had balls, she would be my uncle.
enteroctopus
3.3 / 5 (3) Sep 14, 2016
Many now say that a large moon is crucial in stabilizing earth's climate. Supposedly, if not for our large moon, it would be very unlikely that complex lifeforms could evolve. If true, then earth analogs must be very rare. Factor in the need for a strong magnetic field, the right type of star and being in the 'Goldilocks' zone of a galaxy makes other 'earths' even rarer...

I favor an "early earth" vs Rare Earth, which is just a variation, I suppose. Why isn't the galaxy absolutely teeming with intelligent civilizations? Well, it may well be 100 billion years from now. We're still only 13.7 by into the existence of the known universe, and in this time favorability of life has increased dramatically.

It stands to reason that favorability will continue to increase, and cooler stars which are magnitudes more abundant mature and stabilize. We aren't special, we're early to the party.
Whydening Gyre
5 / 5 (1) Sep 14, 2016
It stands to reason that favorability will continue to increase, and cooler stars which are magnitudes more abundant mature and stabilize. We aren't special, we're early to the party.

Nope. We're right in the middle..:-)
Whydening Gyre
5 / 5 (1) Sep 14, 2016
I favor an "early earth" vs Rare Earth, which is just a variation, I suppose.

Rare Earth was a better name for a 60's band, tho...
Why isn't the galaxy absolutely teeming with intelligent civilizations?

It more than than likely is - Gaia has mapped a billion stars and only (roughly) 2% into the bigger picture...
Well, it may well be 100 billion years from now.

or it was 100bn years ago... (or it is now)
We're still only 13.7 by into the existence of the known universe,

That we know of so far...
and in this time favorability of life has increased dramatically.

only in some locations. Other locations? Not so favourable...:-)

TopJimmy
4 / 5 (4) Sep 14, 2016
re: FredJose

Now of course we just need to sort out those tiny little gotchas that have crept in:
1. The moon is 1/4 the size of the earth.
2. It's just far away enough to blot out the sun in a total solar eclipse and which it just so happens is 400 times bigger and 400 times further away.
3. The moon is traversing in the ecliptic plane - just perfectly set up for the above.


Hahahahaha, he took this directly from Ancient Aliens. Love watching the show, don't buy any of it. Apparently he does.
StudentofSpiritualTeaching
1 / 5 (2) Sep 17, 2016
Interesting work. But it is a pity that all research is for the time being focused on the 2 collision scenarios. My own working assumption is that the true story is one of Earth catching the moon several million years ago. Curious about that story?

Here the earliest summary from 1975:
http://www.future...Report_5

and in 1986 the prediction of our scientists in nearer future getting fooled by amazing isotopic similarities:
http://www.future...port_207
LuisDineman
1 / 5 (1) Sep 17, 2016
New findings just keep confirming my thoughts :)
Assuming that the earth was very hot right from the beginning, and progressively cooled down, the moon would not have been formed.
Instead, if the cooling was preceded by an influx of heat energy, for example from multiple asteroid impacts while still accreting from initial cloud of matter, some of the still undifferentiated mantle and crust would have ended up in space either by slow evaporation in the form of volcanism, or convective mantle plum ejections (that would look like sun's coronal mass ejections). Once the bombardment ends, and the cooling begins, the heavy elements in the atmosphere would either fall back to earth, or form rings and satellites in space. This of course depends on factors such as composition and mass of the planet and its proximity to the sun.
The point still is that you should not look for some special cause of moon formation. It is straight forward thermodynamics.
TheGhostofOtto1923
5 / 5 (1) Sep 18, 2016
mostly in craters at the poles
""The Moon, in fact, has water in all sorts of places; not just locked up in minerals, but scattered throughout the broken-up surface, and, potentially, in blocks or sheets of ice at depth." The results from the Chandrayaan mission are also "offering a wide array of watery signals."

Water water everywhere

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.