National Geographic Daily News
An illustration of the moon being formed.

This illustration of the moon being formed appears on the July cover of National Geographic magazine.

Illustration by Dana Berry, National Geographic

Robert Irion

for National Geographic

Published July 9, 2013

It has taken centuries for scientists to settle on a creation story for our moon, the most popular of which is depicted on the July cover of National Geographic magazine. But as I learned at a recent lunch with Erik Asphaug, a planetary scientist at Arizona State University, the debate is still far from finished.

Before the giant impact model gained traction nearly four decades ago, three other models were in contention. One said the moon condensed from the same whirling cloud of dust that created Earth. But this "binary" model couldn't explain why the moon, far from being a smaller twin of Earth, is much less dense than our planet, with no iron core.

A second model held that the young molten Earth spun so rapidly that it split apart, flinging a giant blob of magma into space. But Earth's spin today and the moon's orbit don't fit the pattern predicted by the "fission" model.

In the third model, Earth's gravity lassoed the moon as it wandered through from some distant part of the solar system. This "capture" scenario was appealing until the Apollo astronauts brought their moon rocks back home. The minerals in them turned out to be similar to those in Earth's mantle—not exotic at all.

The giant impact model avoided all these problems. When it came along in the 1970s, the model fit an emerging view of how the solar system as a whole had formed. In that view, gaseous and rocky protoplanets grew within a disk around the young sun, competing for space, for tens of millions of years. Collisions were inevitable.

As Earth got bigger, it absorbed several Mercury-size or Mars-size objects. The final major blow was an impact so fierce that it left a permanent reminder in orbit around us. According to the impact model, the moon coalesced mostly from the shattered debris of the impactor, a rocky protoplanet similar to Earth. Because the impactor's own iron core sank into Earth's core, the moon is all rock.

Giant Impact, Big Questions

It's a tidy picture, and it has become conventional scientific wisdom. "Five years ago, this article would have said the moon's story was figured out," Asphaug said. "But it's not."

Today, he explained, scientists generally agree that something smashed into Earth to give birth to the moon—but new evidence has cast doubt on details of the giant impact model.

Ongoing analysis of moon rocks, for instance, has shown that the moon and Earth's mantle are not just similar—they're nearly identical. Elements such as oxygen, silicon, and titanium come in several varieties, or isotopes. The blends of these isotopes match up so closely that it seems the moon must have formed almost entirely from fragments of Earth, rather than from the impactor.

One way around this problem is to resurrect the old fissioning Earth idea, but this time assisted by impacts. Repeated small impacts could have spun the growing Earth like a child's merry-go-round, Asphaug said, until it was rotating once every two hours—a breakneck speed for a rocky body that big. "It would have looked oblong," Asphaug said, twirling an egg-shaped roll on the lunch table.

A furiously spinning egg-shaped planet would have been under such strain that a single smaller impact—perhaps by a body just one-tenth the size of Mars—could have made it "explode," Asphaug went on. (A colleague of his calls this the Pinto model, a reference to the 1970s cars that allegedly tended to burst into flame when rear-ended.) Most of the material splashed into orbit would have been from Earth, thus creating a moon with the right chemical blend.

Another possibility is a "hit-and-run" collision by a bigger but speedier impactor that kept going after it blasted huge chunks of Earth's mantle into space; the moon would have formed from those pieces. Or maybe the moon did form from the impactor, but it was then coated with a thick layer of Earth material, which had remained in orbit for at least a century after the impact as an ultrahot disk of rocky vapor.

A second challenge for the giant impact model is to explain why the far side of the moon is so much more mountainous and thick-crusted than the side we see. Asphaug has proposed that Earth briefly had a second moonlet, which plastered itself onto the far side of its larger companion.

"The barn door is wide open, and now we have lots of ideas," Asphaug said. "There probably will be another 'aha' moment in five years or so."

But for now, the moon holds on to its mystery.

Robert Irion directs the science writing program at the University of California, Santa Cruz.

Murphy Walker
Murphy Walker

Since the Earth was a molten hell-hole in its early stages of development, and was prone to massive, violent volcanic activity, could it be that during one such massive explosion the Earth ejected a large amount of its magma into space? 

kathleen sisco
kathleen sisco

I suspect that the Earth itself is the sole parent as 2013 science has revealed;  then how did it happen? 

The Earth core may be periodically heated to the point that it suffers 'blow outs' of earth gases.  This early blow out may have been so powerful that a significant portion of the lithopshere separated and became the moon.  This would explain why the moon's crust is light, and has no sideorphile elements.  It would explain why the Earth's crust is thin. 

Chakrabarti Chinmoy
Chakrabarti Chinmoy

Wonderful photograph as all National Geographic photos are.  The write up completes the picture.

Samuel Hock
Samuel Hock

The Sumerians some 5000 thousand years ago already knew of the Moon having been created by a gaint impact. It is mentioned in what we call their "mythology" but we are starting to understand that their could be many truths  hiden in mythology which our science is only beginning to catch up to.

Alex Hopson
Alex Hopson

If such a large impact occurred, would there not be evidence of:

1. the foreign body that impacted the earth aside from it being completely absorbed into the earth's core ie planet deforming impact depression; 

2. a large, gashing hole down to the earth's mantle or core as a result of the exit material leaving the earth, roughly equivalent to the mass of the moon; 

3. variation in the earth's rotation around the sun that could be mathematically verified?

Looking for responses from greater minds than mine!

Seamus MacNicol
Seamus MacNicol

Immanuel Velikovsky's 1950 Worlds in Collision was the original source of the impactor concept. Please give him credit. He was vilified for many years for his radical ideas.

Bert McDirt
Bert McDirt

This work supports claims from Zechariah Sitchin's work.  The work is based on translations of Sumerian texts which suggest that, among other things, the moon was formed when earth collided with an exoplanet.  Sitchin was long derided as a fraud and a fool; as this theory gains "scientific" credibility, let us not forget the pioneers that first proposed this explanation.

Rudy Breuning
Rudy Breuning

Exit Theory

A large very dense iron object impacts Earth and, though captured by Earth's core, creates "exit debris"- i.e. material is blown out from side opposite impact, much like  an "exit wound".

This theory accounts for the physical evidence.

Rudy Breuning

Michael Banis
Michael Banis

@Яолпзь Ргщисе 

To not question what is in front of you is not expanding your mind and I am sorry for you.

@Luke Edward

Thank you I feel your attempt wasted everyone has an opinion some are uneducated and childish but they are just going to except it by the spoon full because looking at the world threw someone else's thoughts, ideals, and believes is easier then creating your own.

Яолпзь Ргщисе
Яолпзь Ргщисе

The Word of God is very powerful.. ungodly scientist never realize it, but not all of them.. 

John 1:1 - In the beginning was the Word, and the Word was with God, and the Word was God•••••

God is powerful creator that cannot be underestimated.. 

John 1:3 - Through him all things were made; without him nothing was made that has been made•••••

God stays to be good and cast away the bad.. Truth will stand..

Praise the Lord and God bless u all..

Dino Castillo
Dino Castillo

Es definitivo, mi teoría es que: la luna entró en nuestro campo gravitacional y a) golpeo ligeramente y de manera angular la tierra, formando el golfo de México que salpicó toda esa tierra formando las islas del Caribe; rebotando lo suficiente para ser atrapada por nuestro campo magnético hasta el día de hoy que gira a nuestro alrededor. O b) golpeó en el Océano Pacífico generando una gran ola que golpeó las costas de Sudamérica creando la Cordillera de los Andes, en cuya parte baja separó América del Sur de la Antártica, creando unas cuantas islas. En ambos casos generó el giro de 90° del eje terráqueo cambiando completamente los polos, colocando la zona tropical en los nuevos polos; y desacelerando el planeta por lo que la fuerza gravitacional que posiblemente era de 4 m/seg2, pasó a lo que hoy conocemos de 9.81 m/s2; lo que hizo que los dinosaurios pesen enormemente, se cansen por habérseles multiplicado el peso, dejen de alimentarse por su difícil andar y mueran; a más del cambio abrupto de temperatura que destruyó la vegetación propia de esa zona antes del cambio. DINO EDUARDO ESTEBAN CASTILLO LUCIO, GUAYQUIL, ECUADOR, SUDAMÉRICA.

Hy3percrav3 Threadbare
Hy3percrav3 Threadbare

@Яолпзь РгщисеThis isn't really helpful.

How did God (Assuming She exists) create the Moon and the Universe" That is the question. That is what Science is about. 

Appreciating Creation and getting closer to the Creator by studying the Universe and Its workings. At least that is the Jesuit Philosophy.

Just spewing New Testament is Being lazy. Isn't there a quote in Solomon that says "Knowledge is Power"?

Luke Edward
Luke Edward

@Яолпзь Ргщисе

Unfortunately for you, your statement explains nothing and has no proof whatsoever. You are just repeating the words "fed" to your brain since you were born in an attempt to explain in an easy way the surrounding world without thoroughly analyzing it. At least you could question the problem and try to find some aswers that seem rational and not resort to a set of simple and not credible phrases form a book full of fary tales.

T. Venegas
T. Venegas

@Dino Castillo

Translate from google: Just wanted to know what you said.

It is definite, my theory is that: the moon came into our gravitational field already) hit angularly slightly and the ground, forming the Gulf of Mexico that all the land forming splashed Caribbean islands; bouncing enough to be trapped by our magnetic field until today that revolves around us. Or b) hit in the Pacific Ocean generating a large wave hit the coasts of South America creating the Andes, whose lower part separated South America from Antarctica, creating a few islands. In both cases led to the 90 ° rotation of the shaft completely changing the poles globe, placing the tropics in new poles, and slowing the planet so that the gravitational force was 4 m/sec2 possibly, came to what is now know of 9.81 m/s2, what made the dinosaurs weigh greatly, get tired having been multiplied by the weight, stop to feed its hard to go and die, to over abrupt temperature change that destroyed the vegetation of that area before of change. DINO CASTILLO EDUARDO ESTEBAN LUCIO, Guayquil, ECUADOR, SOUTH AMERICA.

roy valenzuela
roy valenzuela

@Luke Edward @Яолпзь Ргщисе NO, No, No Luke. You speak in terms of absolutes ..."and not resort to a set of simple and not credible phrases form a book full of fairy tales"....Just ask the king of the fairy tale/ it in the privacy of your room where your peers can't see ya...what do you have to loose?...If not now than when life is shot to hell.

Timothy Steele
Timothy Steele

@T. Venegas @Dino Castillo Both of those "ideas" if you wanna even call them that, have been discounted and or already have explanations for their existence. Scratch your Gulf of Mexico stuff, and the bouncing part. and the part about Antarctica, The continental drifts have been explained already. Move along no thinking actually taking place here. Nothing to see, nothing to learn.

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