Did the moon form inside a ‘spinning donut’ Earth?

(Credit: Getty images)

The moon may have formed inside the Earth when our planet was a seething, spinning cloud of vaporized rock, called a synestia, a new explanation for the moon’s origin suggests.

Researchers say the new model, described in the Journal of Geophysical Research–Planets, resolves several problems in lunar formation.

“The new work explains features of the moon that are hard to resolve with current ideas,” says Sarah Stewart, professor of earth and planetary sciences at the University of California, Davis.

“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,” she explains.

Hear Sarah Stewart discuss synestias:

Current models of lunar formation suggest that the moon formed as a result of a glancing blow between the early Earth and a Mars-size body, commonly called Theia. According to the model, the collision between Earth and Theia threw molten rock and metal into orbit that collided together to make the moon.

The new theory relies instead on a synestia, a new type of planetary object that forms when a collision between planet-sized objects results in a rapidly spinning mass of molten and vaporized rock with part of the body in orbit around itself. The whole object puffs out into a giant donut of vaporized rock.

Synestias likely don’t last long—perhaps only hundreds of years. They shrink rapidly as they radiate heat, causing rock vapor to condense into liquid, finally collapsing into a molten planet.

This artist’s rendering (based on NASA rendering) shows the hot, molten moon emerging from a synestia, a giant spinning donut of vaporized rock that formed when planet-sized objects collided. The synestia is in the process of condensing to form the Earth. (Credit: Sarah Stewart/UC Davis)

“Our model starts with a collision that forms a synestia,” says Simon Lock, graduate student at Harvard University and a visiting student at UC Davis. “The moon forms inside the vaporized Earth at temperatures of four to six thousand degrees Fahrenheit and pressures of tens of atmospheres.”

An advantage of the new model, Lock says, is that there are multiple ways to form a suitable synestia—it doesn’t have to rely on a collision with the right sized object happening in exactly the right way.

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Once the Earth-synestia formed, chunks of molten rock injected into orbit during the impact formed the seed for the moon. Vaporized silicate rock condensed at the surface of the synestia and rained onto the proto-moon, while the Earth-synestia itself gradually shrank.

Eventually, the moon would have emerged from the clouds of the synestia trailing its own atmosphere of rock vapor. The moon inherited its composition from the Earth, but because it formed at high temperatures it lost the easily vaporized elements, explaining the moon’s distinct composition.

Other researchers are from Harvard, the University of Bristol, and the SETI Institute. NASA, the US Department of Energy, and the UK’s Natural Environment Research Council funded the work.

Source: UC Davis