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24 Hours of Chaos: The Day The Moon Was Made

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Rebecca
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« on: March 25, 2007, 04:33:19 pm »

Case is not airtight

The new model is a significant improvement over previous efforts, which treated gravity as an overall issue or worked with no more than 3,000 computational lumps. But it is just one step toward a fuller understanding of what really happened.

Jay Melosh, a University of Arizona researcher who is known for his work in modeling asteroid impacts, told SPACE.com the new model is an incremental step rather than a trailblazing one. And there are outstanding questions about some assumptions made.

"Their case is not airtight," Melosh said.

In a review of the work that also appears in Nature, Melosh argues that the real promise is in how computers are becoming powerful enough to handle the complicated scenario of a such a colossal impact.

"Not only does such a collision involve all the details of shock physics, melting and vaporization, but the mutual interactions of all those hot fluids squirting around in space have to be taken into account," Melosh writes.

He says that as with any attempt to model the Moon formation, the results hinge on an incomplete understanding of how the energy, density and pressure would affect the material of which the Earth and Moon are composed.

And Canup acknowledges that there is not, and never will be, direct physical remains of the Moon-forming impacter. The ensuing drama was so hot, and the characters so well-mixed, that there are no ancient layered deposits to provide clues, as are found by people like Melosh who study smaller and more recent asteroid impacts.

But Melosh said the prospects for better models are promising. In fact, he is working with Canup and Asphaug on ways to refine the new model to better account for the shock and fluid dynamics. And he figures others will soon use the improved computing power and more capable software packages to produce their own scenarios.

"More studies of this kind will be published in the not-too-distant future," Melosh said.

Other ways the Moon might form

The new study strengthens just one theory of how the Moon might have formed. Other scientists have suggested that the Moon developed elsewhere in the solar system and was captured by Earth. An even more remote possibility is that the Earth and Moon condensed together out of the material that formed the solar system.

Another idea is that gravitational interactions between the Earth, the Sun, and other developing planets simply tore Earth apart and the Moon formed from this debris.

But the majority of researchers prefer the impact theory. And though a similar impact would be extremely unlikely today, it was a fairly common occurrence back when the solar system was forming.

"The last stages of planetary accumulation were very violent," Melosh said. "An event of this type within 100 million years of the birth of the solar system is not rare at all."

The Moon is not the only result of this chaos. In fact, the present spacing between planets "evolved by a sort of natural selection involving the demise of intervening objects whose orbits were not so stable," Melosh said.
http://www.space.com/scienceastronomy/solarsystem/moon_making_010815-2.html
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