By Ron Cowen of Nature magazine
A chemical analysis of lunar rocks may force scientists to revise the leading theory for the Moon's formation: that the satellite was born when a Mars-sized body smacked into the infant Earth some 4.5 billion years ago.
If that were the case, the Moon ought to bear the chemical signature of both Earth and its proposed 'second' parent. But a study published today in Nature Geoscience suggests that the Moon's isotopic composition reflects only Earth's contribution.
Junjun Zhang at the University of Chicago in Illinois and her colleagues used a mass spectrometer to make the most precise measurement so far of the relative abundance of titanium-50 and titanium-47 in Moon rocks gathered by the Apollo missions in the 1970s. The authors report that the lunar ratio of the two isotopes is identical to that found in Earth's mantle, within about 4 parts per million.
This presents a conundrum for the lunar-formation model, Zhang says, because any Mars-sized body that might have collided with the fledgling Earth is believed to have been chemically distinct. Studies of meteorites -- the modern stand-ins for planet-sized bodies that once roamed the Solar System -- indicate that such objects had an isotopic titanium abundance that could have deviated from the terrestrial value by as much as 600 parts per million. And because simulations suggest that the second body contributed more than 40% of the Moon's bulk, the lunar isotopic ratio shouldn't mirror the terrestrial value so closely.
Vapor trail
Zhang and colleagues' chemical analysis is not the first to challenge the theory. Researchers have long known that the isotopic ratio of oxygen in Moon rocks bears the same signature as Earth's mantle. But because oxygen is easily vaporized in a collision, it could have been readily exchanged between Earth and the cloud of vapor and magma that was produced by the impact and coalesced to form the moon, allowing both bodies to reach the same isotopic abundance. Titanium does not vaporize as easily and it would have been more difficult -- although not impossible -- for both bodies to have reached the same ratio, notes Zhang.
"That's why this is not just another similarity between Earth and the Moon," but a finding to be reckoned with, notes planetary scientist Robin Canup of the Southwest Research Institute in Boulder, Colorado.
Other models that merit consideration, Zhang says, include the fission model, according to which the Moon was spun out of Earth's mantle early on, when the planet's centrifugal force might have exceeded its gravitational force.
But Canup says that although the collision model may need revision, it need not be abandoned. She has modeled a collision between Earth and a renegade protoplanet about twice the mass of Mars -- heavier than previously considered. A more massive second body would have substantially altered Earth's original isotopic composition, leading to a newborn Moon and evolving Earth that are more similar than in previous simulations.
Zhang agrees there are still ways for the collision model to work. If the fledgling Moon had cooled more slowly than assumed, there could have been enough time for an exchange of titanium isotopes between the cloud of vapor and magma and the Earth. In their most recent simulation of the moon's formation after a giant impact, Canup and her colleague Julien Salmon found evidence of a longer formation time. They presented the findings at the Lunar and Planetary Science Conference in The Woodlands, Texas, on 22 March.
"Our study cannot provide a definite answer to the origin of the Moon yet," says Zhang. "The message we hope to convey is that isotopic homogeneity between the Earth and Moon is a fundamental new constraint on the evolution of the Earth-Moon system."
This article is reproduced with permission from the magazine Nature. The article was first published on March 25, 2012.
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3 Comments
Add CommentMy understanding is that Theia formed at the L4 or L5 Lagrange points from the same material that formed the earth. i.e. material with exactly the same isotopic ratio. It grew to the point that it was perturbed out of L4 or L5 by Jupiter into a horse shoe orbit that eventually collided with the earth at low velocity.
Reply | Report Abuse | Link to thisI thought the giant impact hypothesis requires a very low collision velocity.
This could be a common occurrence.
Doesn't Venus's spin rate indicate it to was hit at low velocity by a large impactor during formation (just a different angle of impact). Ditto the Borealis basin on Mars.
In light of this :
Reply | Report Abuse | Link to thisplease critic the following theory which I find fascinating. He seems to have put together great evidence from genomic evolution, fossil records as well a astrophysical calulations to support it.
(Yes, it is as revolutionary as the idea of floating continents once was.)
it's long and detailed includeing examination of many factors but this part is the astrophysics part:
http://web.archive.org/web/20100615055048/http://www.brojon.org/frontpage/WHAT_REALLY_KILLED_THE_DINOSAURS.html [archive.org]
"CHAPTER FIVE:"
Summary:
Earth was spinning faster 65 millions yrs ago (yes only 65 million) and actually slung off moon material from equator in a non violent event not requiring moltenization of moon.
Explains light surface crust match in moon material and earth surface crust.
He claims conservation of momentum is conserved when calculating current spin and speed of earth's rotation and masses are correct.
many more facts addressed in chapters 1 through 4 including animal fossil reocrds etc in full article which is fascianting ifyou are interested in early plant genomics etc which he claims also support his theory.
yes, I am aware he is saying this happened during dinosaur epoch. In fact it's key to existanceof dinosaurs and larger insects which he says could only have existed in low gravity envronment of earth's equator when spinning in 7hrs causing near zero g at equator.
Laugh AFTER reading his detailed genome and plant evolutionary supporting evidence in fossil record.
(many more facts addressed in chapters 1 through 4 including animal fossil reocrds etc in full article which is fascianting ifyou are interested in early plant genomics etc which he claims also support his theory.)
The Author (of the alternative slung peice of earth's equator formed the moon theory) writes: "If the earth spins at 24 hours per day, as it does today, then the force of gravity at the equator is about the same as the force of gravity at the north and south poles. If the earth spins faster, in about a 12 hour revolution day, then the force of gravity at the equator is around half of what it would be at the poles. And if the earth spins around in about 7 hours, then the force of gravity becomes rather close to zero at the equator, while the force at the poles remains the same. "
Reply | Report Abuse | Link to thisAlso:
"A little further calculation can show that almost all of the moons in our solar system were ejected from their parent planet by this very same process. It is a natural part of the aging and evolution of planetary systems. As a rotating planet cools, it spins faster and faster, becoming oblate and unstable. It releases a moon from the oblate surface, just like Tara Lipinski ( an ice skater ) putting out her arms to slow down her spin, decreasing the planet's rotation rate and it becomes stable again. This may be repeated with the larger planets. After a long time, the larger planets continue to cool and shrink and release another moon. The larger the planet the more moons it produces to become stable."
http://web.archive.org/web/20100615055048/http://www.brojon.org/frontpage/WHAT_REALLY_KILLED_THE_DINOSAURS.html