Moon emerges from cloud of vaporized Earth rock: study

Updated 2018-03-01 11:09:02 Xinhua

A new theory about the origin of the moon has suggested that the moon was not spun out of the aftermath of a collision, but emerged from a seething, donut-shaped cloud of vaporized rock inside the Earth.

The study, published Wednesday in the Journal of Geophysical Research: Planets, showed that a new type of planetary object called "synestia", 10 times the size of the Earth, formed when a collision between planet-sized objects resulted in a rapidly spinning mass of molten and vaporized rock with part of the body in orbit around itself.

Simon Lock from Harvard University and Sarah Stewart from University of California, Davis first put forward the new theory in 2017 and gave detailed explanations in the new study.

According to researchers, it begins with a "seed," a small amount of liquid rock that gathers just off the center of the donut-like structure.

As the structure cools, vaporized rock condenses and rains down toward the center of the synestia. Some of the rain runs into the moon, causing it to grow.

"The rate of rain fall is about 10 times that of a hurricane on Earth," Lock said, adding that the whole process happens remarkably fast, with the moon emerging from the synestia in just a few tens of years, and the Earth forming about 1,000 years later.

Current models of lunar formation suggested 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 the Earth and Theia threw molten rock and metal into orbit that collided together to make the moon.

"Getting enough mass into orbit in the canonical scenario is actually very difficult, and there's a very narrow range of collisions that might be able to do it," Lock said. "There's only a couple of degree window of impact angles and a very narrow range of sizes...and even then some impacts still don't work."

Steward cited evidences that the isotopic "fingerprint" for both the Earth and moon are nearly identical, suggesting both came from the same source, but in the canonical story, the moon formed mostly from the remnants of just one of the two bodies that collided.

However, tests have also shown that the moon is far less abundant in many volatile elements such as potassium, sodium and copper that are relatively common on the Earth.

"There hasn't been a good explanation for this," Lock said. "People have proposed various hypotheses for how the moon could have wound up with fewer volatiles, but no one has been able to quantitatively match the moon's composition."

Researchers assumed that the lack of volatile elements on the moon can be explained by the fact that the moon formed, surrounded and pressured by tens of atmospheres of vapor and at high temperature.

"This is a basic model," Lock said. "We've done calculations of each of the processes that go into forming the moon and shown that the model could work, but there are various aspects of our theory that will need more interrogation."

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