How Did the Moon Form?

nasa's apollo missions provided crucial answers about the moon's formation

By Jonny Lupsha, Wondrium Staff Writer

Where did Earth’s Moon come from and how did it form? The nearest celestial body shines big, bright, and round in the sky, but how did it get there? Humanity’s renewed interest in the Moon raises questions.

Two people observing the moon
By studying collected rocks from the Apollo 11 mission, scientists learned about the Moon’s formation. Photo by Frame Stock Footage / Shutterstock

Earth’s Moon is the biggest, brightest object in the night sky. Silver and perfectly round, it hangs in space to be studied and revered. However, despite the hundreds of thousands of years we’ve been looking up at the Moon, nobody on Earth knew where it actually came from, until after the Apollo 11 mission sent a crew to its surface, where they made history—and collected rocks to help solve an age-old mystery.

Fifty years after the last man walked on the Moon, NASA has recovered the unmanned Artemis I capsule, marking the beginning of our next phase of lunar exploration. In his video series The Origin and Evolution of Earth: From the Big Bang to the Future of Human Existence, Dr. Robert M. Hazen, the Clarence J. Robinson Professor of Earth Sciences at George Mason University, divulges the origins of the Moon.

What Did the Apollo Missions Accomplish?

“Answers to the mystery of the Moon’s origins had to await data from the Moon rocks,” Dr. Hazen said. “The six Apollo landings between July of 1969 and December of 1972 featured Moon walks by 12 NASA astronauts, starting with Neil Armstrong and Buzz Aldrin. These missions resulted in a phenomenal collection of more than 840 lbs. of Moon rocks and soil samples from six different sites.”

Additionally, a number of lunar measurements were taken on the Apollo missions, from seismic surveys to gravity readings.

The Apollo missions were historical for all of humanity and for the Americans and Soviets locked in the Space Race. They were a boon for the military-industrial complex, inspired innovations like minicomputers, and they also made a tremendous impact on Earth scientists.

“We had nothing to tell us for sure of what the Moon is made, nor when, nor where,” Dr. Hazen said. “We had no real way of knowing for sure how the Moon was formed. All that changed with the first batch of returned lunar samples: We could, for the first time in human history, literally touch the Moon.”

How Was the Moon Formed?

Theories about the Moon’s formation abounded, but Apollo’s lunar samples proved them all wrong. The prevailing theory of the Moon’s formation is that about 4.53 billion years ago, two, planet-sized objects ended up in gravitational competition for Earth’s current orbit. One was Earth, the other Theia. Theia was likely larger than Mars and about one-third of the Earth’s mass.

“But there’s a rule of astrophysics: No two planets can share the same orbit,” Dr. Hazen said. “Eventually they will collide, and the larger planet always wins. So it was with Earth and Theia.”

This theory is known as “The Big Thwack.” Some scientists believe it all happened at once, while others think Theia was bounced off Earth a bit and then gravity brought it back for the big finale. Both planets damaged the other, with Theia being obliterated into dust while pulverizing some of Earth’s crust and mantle, knocking pieces into space.

“Some material escaped to deep space, but most of the savaged remains were retained in orbit by Earth’s unyielding gravitational grip,” Dr. Hazen said. “From this roiling cloud, the dense metal from the cores of both worlds comingled and sank to form a new, larger core for Earth.”

Meanwhile, up in space, Theia’s rocky debris and some of what it took from Earth coalesced due to gravity, with bigger chunks absorbing smaller ones—in a microcosm of the original clumping that formed the planets in our solar system. The Moon may have formed its current size in just a few years.

The Origin and Evolution of Earth: From the Big Bang to the Future of Human Existence is now available to stream on Wondrium.

Edited by Angela Shoemaker, Wondrium Daily