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The moon formed inside a cloud of Earth's vaporized rock

"Our model starts with a collision that forms a synestia," said researcher Simon Lock.

By Brooks Hays
The moon formed inside a cloud of Earth's vaporized rock
A new theory suggests the moon formed inside a donut-shaped cloud of vaporized rock. Photo by Sarah Stewart/UC Davis

Feb. 28 (UPI) -- Scientists have a new theory for how Earth got its moon. According to their analysis, the moon was born inside Earth before Earth was a solid planet.

At the time of the moon's formation, Earth was a spinning, donut-shaped cloud of vaporized rock -- what's known as a synestia.

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Scientists detailed their new theory in a paper published this week in the Journal of Geophysical Research: Planets.

"The new work explains features of the Moon that are hard to resolve with current ideas," Sarah Stewart, professor of Earth and planetary sciences at the University of California, Davis, said in a news release. "The moon is chemically almost the same as the Earth, but with some differences. This is the first model that can match the pattern of the moon's composition."

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The most popular theory for the moon's formation holds that the satellite was birthed by a collision between Earth and a Mars-size rock called Theia. Rock and metal fragments sprayed into space by the collision coalesced to form the moon.

The new model suggests both the Earth and moon formed from the same giant collision. Scientists hypothesize that early in the infancy of the solar system, a pair of planet-size bodies collided, creating a synestia, a cloud of vaporized rock.

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As the rock vapors condense into the liquid form, the cloud shrinks and then trasforms into a molten planet. Scientists at UC Davis and Harvard University believe a glob of vaporized rock escaped during impact, expelled beyond the pull of the condensing synestia. As a result, the seed for what would become the moon began orbiting the newly condensed planet.

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"Our model starts with a collision that forms a synestia," said Simon Lock, a graduate student at Harvard. "The moon forms inside the vaporized Earth at temperatures of four to six thousand degrees Fahrenheit and pressures of tens of atmospheres."

Scientists say one of the advantages of their model is that it is versatile, as a synestia can form a variety of ways. The Theia theory relies on a collision of just the right size at just the right time.

Because the Earth and moon formed from mostly the same vaporized material, the theory explains the two bodies' similar composition. However, the theory also explains why the moon is slightly different, as it would have lost the most easily vaporized materials during its expulsion from the synestia.

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