A Hot New Idea About the Moon

For more than a century, astronomers have debated the question of how the Moon formed. The most popular theory currently en vogue states that, long ago, the Earth was struck by a body roughly the size of Mars, releasing material that entered orbit around our fledgling planet, which cooled to form our planetary companion.

Computer models suggest, however, that such an event would produce a Moon with a composition similar to that of the impactor. However, we know our planetary companion has a makeup much like our home world. A team of astrophysicists in Japan, working with colleagues at Yale University, believe they have solved that dilemma. They found that if the impacting body were solid, while our world were still covered in magma, then the collision would have formed a moon similar to the one we observe today.

“In our model, about 80% of the moon is made of proto-Earth materials. In most of the previous models, about 80% of the moon is made of the impactor. This is a big difference,” explains Yale geophysicist Shun-ichiro Karato.

Roughly 50 million years after the Earth began to coalesce from a cloud of gas and dust surrounding the Sun, the surface of our world was covered in hot, liquefied rock. When the impactor struck the Earth with a glancing blow, magma on the surface of our world would have heated faster than the solid material of the impacting body, causing the magma to expand in volume as it entered orbit around the Earth.

Previously, astronomers and geologists have been forced to devise unconventional scenarios of the impact in order to explain the geology of samples returned from the Moon by the Apollo astronauts. This new finding could explain the observed geology without the need for a complex impact.

“[A] giant, solid impactor hit the proto-Earth while it was covered with a magma ocean, under the conventional collision conditions… [B]ecause of the large difference in shock heating between silicate melts and solids (rocks), a substantial fraction of the ejected, Moon-forming material is derived from the magma ocean,” researchers described in Nature Geoscience.

Initially, the Moon rotated much faster than it does today, as did the Earth. However, because the Moon is not perfectly spherical, bulging slightly at its equator, a torque built up between the pair of bodies, slowing the spin of the Moon like an ice skater slows as she extends her arms during a twirl on the ice. Once the spin of the Moon slowed to the same rate as it took to orbit the Earth, the bulge was in line with our planet, and one face of the lunar surface eternally faced our world.

“The Earth would be a very different place if the moon did not exist. Not only did the Earth slow down the Moon’s rotation, but the Moon is slowing down the rotation rate of the Earth. Since the moon’s formation, the Earth has been slowing its rotation due to the friction of the tides caused by the moon, and in reaction to this exchange of energy, the moon has been moving farther away from the Earth.,” The Solar System Exploration Research Virtual Institute explains.

A look at how the Moon evolved over time, following its formation. Credit: NASA Goddard.

“The moon will guide you through the night with her brightness, but she will always dwell in the darkness, in order to be seen.”

― Shannon L. Alder

Still, the Moon lacks some of the volatile ingredients of the Earth, notably potassium, sodium, and copper which are relatively common on our home planet. Understanding these differences between the bodies could prove vital as we search for the complete story of what happened during the formation of our planetary companion.

The next time you look up to the Moon, remember it all may have started in a giant pool of magma.

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James Maynard is the author of two books, and thousands of articles about space and science. E-mail: thecosmiccompanion@gmail.com

The Cosmic Companion

Exploring the wonders of the Cosmos, one mystery at a time