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February 01, 2016

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Head-on crash produced our moon, study says

Feb. 1, 2016
Courtesy of UCLA
and World Science staff

The Moon formed thanks to a head-on crash be­tween the early Earth and a “plan­e­tary em­bryo” called The­ia about 100 mil­lion years af­ter Earth formed, a new study claims.

Ac­cord­ing to the stu­dy, the im­pact left two bod­ies in its af­termath, Earth and Moon, with bits of the shat­tered The­ia mixed pret­ty evenly in­to both, ex­plain­ing why they’re chem­ic­ally very sim­i­lar.

Artist's conception of the Earth-Theia colli­sion. (© Will­iam K. Hart­mann)


Sci­en­tists al­ready knew about the high-speed col­li­sion, but many thought it more of a pow­er­ful side-swipe than a head-on im­pact. 

But in the new work, pub­lished Jan. 29 in the jour­nal Sci­ence, sci­en­tists ar­gue that the side-swipe would­n’t have led to such a si­m­i­lar make­up be­tween Earth and Moon.

The sci­en­tists an­a­lyzed se­ven moon rocks gath­ered by the Apol­lo 12, 15 and 17 mis­sions. They com­pared these to six vol­can­ic rocks from a deep lay­er of Earth called the mantle—five found in Ha­waii and one from Ar­i­zo­na. 

A chem­ical sig­na­ture in ox­y­gen atoms from these rocks, which take up half their weight, pro­vid­ed the key clues, the sci­en­tists said. Al­most all of Earth’s ox­y­gen is in a form called O-16, but there al­so are small amounts of heav­i­er vari­ants, or iso­topes, called O-17 and O-18. Earth, Mars and oth­er plan­e­tary bod­ies in our so­lar sys­tem each has a un­ique ra­tio among the iso­topes—each one a dis­tinc­tive “fin­ger­print” of that body.

In 2014, Ger­man sci­en­tists re­ported in Sci­ence that the moon al­so has its own un­ique ra­tio, un­like Earth’s. But the new study claims the op­po­site. The ra­tios are “indis­tin­guish­able,” said ge­o­chem­ist Ed­ward Young of the Un­ivers­ity of Cal­i­for­nia Los An­ge­les, the lead au­thor, adding that his re­search team used state-of-the-art tech­nol­o­gy and tech­niques.

Had Earth and The­ia col­lid­ed glanc­ingly, Young said, most of Theia would have gone into the Mo­on, giving it a un­ique ox­y­gen iso­tope ratio. But a head-on col­li­sion would probably have pro­duced what we see. “The­ia was thor­oughly mixed in­to both the Earth and the moon, and evenly dis­persed be­tween them,” Young said.

The­ia was grow­ing, and probably would have grown into a plan­et had the crash not ob­lit­er­ated it as an in­de­pend­ent ob­ject, Young said. He and some oth­er sci­en­tists think The­ia was roughly Earth-sized; oth­ers be­lieve it was smaller, per­haps more Mars-sized.

An in­ter­est­ing ques­tion is wheth­er the col­li­sion re­moved any wa­ter that the early Earth may have con­tained, Young and col­leagues said. Af­ter the im­pact—per­haps tens of mil­lions of year lat­er—s­mall as­ter­oids likely hit the Earth, in­clud­ing ones pos­sibly rich in wa­ter, Young said. Col­li­sions of grow­ing bod­ies oc­curred very of­ten back then, he added, though Mars avoided large ones.

Ac­cord­ing to Young and col­leagues, the in­i­tial idea for the head-on crash sce­nar­i­o came in 2012 from Matija Cuk, now a re­search sci­ent­ist with the Moun­tain View, Calif.-based SETI In­sti­tute, and Sar­ah Stew­art, now a pro­fes­sor at Un­ivers­ity of Cal­i­for­nia Da­vis; and sep­a­rate­ly, dur­ing the same year, by Rob­in Canup of the South­west Re­search In­sti­tute.


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The Moon formed thanks to a head-on crash between the early Earth and a “planetary embryo” called Theia about 100 million years after Earth formed, a new study claims. According to the study, the impact left two bodies in its aftermath, Earth and Moon, with bits of the shattered Theia mixed pretty evenly into both, explaining why both are chemically very similar. Scientists already knew about the high-speed collision, but many thought it more of a powerful side-swipe than a head-on impact. But in the new work, published Jan. 29 in the journal Science, scientists argue that the side-swipe wouldn’t have led to such a similar makeup between Earth and Moon. The scientists analyzed seven moon rocks gathered by the Apollo 12, 15 and 17 missions. They compared these to six volcanic rocks from a deep layer of Earth called the mantle—five found in Hawaii and one from Arizona. A chemical signature in oxygen atoms from these rocks, which take up half their weight, provided the key clues, the scientists said. Almost all of Earth’s oxygen is in a form called O-16, but there also are small amounts of heavier variants, or isotopes, called O-17 and O-18. Earth, Mars and other planetary bodies in our solar system each has a unique ratio among the isotopes—each one a distinctive “fingerprint” of that body. In 2014, a team of German scientists reported in Science that the moon also has its own unique ratio, unlike Earth’s. But the new study claims the opposite. The ratios are “indistinguishable,” said geochemist Edward Young of the University of California Los Angeles, the lead author, adding that his research team used state-of-the-art technology and techniques. Had Earth and Theia collided in a glancing blow, Young said, the vast majority of the moon would have been made mainly of Theia, and the Earth and moon should have different oxygen isotopes. But a head-on collision would probably have produced what we see. “Theia was thoroughly mixed into both the Earth and the moon, and evenly dispersed between them,” Young said. “This explains why we don’t see a different signature of Theia in the moon versus the Earth.” Theia was growing, and probably would have eventually become a planet had the crash not totally destroyed it as an independent world, Young said. Young and some other scientists think Theia was roughly Earth-sized; others believe it was smaller, perhaps more Mars-sized. An interesting question is whether the collision removed any water that the early Earth may have contained, Young and colleagues said. After the impact—perhaps tens of millions of year later—small asteroids likely hit the Earth, including ones possibly rich in water, Young said. Collisions of growing bodies occurred very often back then, he added, though Mars avoided large ones. According to Young and colleagues, the initial idea for the head-on crash scenario came in 2012 from Matija Cuk, now a research scientist with the Mountain View, Calif.-based SETI Institute, and Sarah Stewart, now a professor at University of California Davis; and separately, during the same year, by Robin Canup of the Southwest Research Institute.