18 October 2012

Giant collision formed the Moon

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The Moon formed when a Mars-sized space rock smashed into a young, fast-spinning Earth, according to new research that adds weight to a long-held theory.
Moon NASA

Colour image of the Moon taken by NASA's Galileo spacecraft in 1990. New research confirms the giant-impact theory, that the Moon formed when a Mars-sized object collided with Earth more than 4.5 billion years ago. Credit: NASA / JPL

SYDNEY: The Moon formed when a Mars-sized space rock smashed into a young, fast-spinning Earth, according to new research that adds weight to a long-held theory.

New research has emerged explaining how the Moon formed after a giant impact between our Earth and a giant body named Theia – in Greek mythology, the mother of the moon, Selene. The collision melted and vapourised Theia and much of the early Earth’s mantle, and the rock vapour condensed to form the Moon.

The new research is based on computer simulations and studies of lunar soil brought back from the Apollo missions. The findings are published in three papers across the journals Science Express and Nature.

Robin Canup, a planetary scientist at the Southwest Research Institute in the United States, and author of one of the Science Express papers, said the findings identify “two new types of planetary collisions capable of producing the Earth and Moon”.

“Wholesale melting events”

First proposed by astronomers back in 1975, the giant-impact theory suggested the Moon was formed from leftover debris after a collision between the Earth and a Mars-sized object more than four-and-a-half billion years ago.

“Previous giant impact models predicted that the Moon would be formed primarily from the impactor,” said Sarah Stewart, a planetary scientist at Harvard University in the U.S. and co-author of one of the Science Express papers. “However, we expect the impactor to have different isotopes than the early Earth,” she added.

The Moon rocks, geochemists discovered, were in fact chemically similar to Earth rocks but short on volatiles (easily evaporated elements, such as sodium, potassium, zinc and lead). The giant impact theory explains this depletion.

“You require some kind of wholesale melting event of the Moon to provide the heat necessary to evaporate the zinc,” explained James Day of the Scripps Institution of Oceanography in California, co-author of the Nature paper.

Fast-spinning Earth

To explain the shared composition of the Earth and Moon, Canup used a computer simulation that treated the colliding bodies as 300,000 discrete particles, each assigned a different chemical composition, during a simulated impact.

The results, Canup said, show “that an impact can produce a disk and Earth that share the same composition. Prior impact models could not do this.” Canup added that she ran more than 100 simulations to ensure the best results.

Stewart and co-author Matija Ćuk, also from Harvard University, simulated the giant impact event using a much faster-spinning Earth – with a two to three hour Earth day – and found that it could produce enough matter to form the Moon.

“It is much easier for a giant impact to launch material into orbit around the Earth if the early Earth was spinning quickly,” explained Stewart.

The Earth slowed to its present 24-hour day when the Moon fell into orbit around it, the researchers said.

Promising direction, but more tests needed

Steven Tingay, director of the Curtin Institute of Radio Astronomy in Western Australia, said the findings enhance the existing theories about the way that the Earth and Moon formed, but that there was still no clear answer.

“I think [the] results are very interesting [but] neither are likely to provide the final word on how the Moon formed via an impact with the Earth,” he said.

Canup said more tests are needed to better understand the way planets are formed.

“The ultimate likelihood of each impact scenario will need to be assessed by improved models of terrestrial planet formation,” she said.

“Hopefully at some point, there will be differences between the two sets of predictions that lead to one model or another or neither being supported by the observational evidence,” added Tingay. “In my view, that is likely to be some time in the making, but it is great that these papers point in some promising directions.”

More information:
Science Express
Nature
With Agençe France-Presse
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