Oct 20, 2021 I Paul Seaburn

It May Have Taken Two Planetary Collisions With Earth to Make the Moon

One of the more commonly accepted theories among space scientists speculating on how the Earth’s moon was formed is that a young planet about the size of Mars – now called Theia -- collided with our young planet, breaking itself  into two pieces with one hurling into space to become our satellite while the rest melded into Earth. That’s a pretty clean collision-and-result for two space bodies of that size, and a new study proposes that the birth of the Moon was more like a space demolition derby, with Theia bouncing off of Earth and taking a damaged trip around the solar system racetrack before coming back for a second knockout collision that created the Moon … and may have been part of the reason why Earth has a Moon and Venus doesn’t.

“In the canonical model of Moon formation, a Mars-sized protoplanet "Theia" collides with proto-Earth at close to their mutual escape velocity vesc and a common impact angle ∼45°. The "graze-and-merge" collision strands a fraction of Theia's mantle into orbit, while Earth accretes most of Theia and its momentum. Simulations show that this produces a hot, high angular momentum, silicate-dominated protolunar system, in substantial agreement with lunar geology, geochemistry, and dynamics.”

The study, published in The Planetary Science Journal, is technical and math-laden. Fortunately, Sky and Telescope brings it down a few pay grades. The accepted Theia/Earth collision model worked well – until astronauts brought back Moon rocks showing that the Moon’s composition is much more like Earth than a foreign planet. Creating other models by adjusting Theia’s impact speed either destroyed both planets or didn’t create enough velocity to do both the merge and kick-out. Erik Asphaug, lead author and planetary science professor in the School of Earth and Space Exploration at University of Arizona, come up with a model that was more like a hit-and-run accident – Theia hit Earth at a high speed, glanced off and went back out at a lower speed. The second time it came around – making this more like a hit-and-run-and-come-back-to-check-the-damage – Theia was traveling slow enough to knock out the chunk that became the Moon and then meld into Earth.

“I first thought maybe there was a mistake.”

Asphaug’s model also showed Earth passing other hit-and-run planetoids and space rocks to Venus, which absorbed then rather than passing them to the next in line. That explains why Venus’ composition resembles that of outer solar system material. It also may explain why Venus has no Moon -- either it never got hit by another planet or the amount of impacts it received from both space and Earth’s deflections obliterated it.

Venus 570x342

“Getting samples from Venus is the key to answering all these questions.”

This is all still theoretical and Asphaug admits they can only be resolved with a trip to Venus to gather rocks and return them to Earth.

Wouldn’t that be a more beneficial use of Jeff Bezos’ Blue Origin than sending actors into near space?

Paul Seaburn

Paul Seaburn is the editor at Mysterious Universe and its most prolific writer. He’s written for TV shows such as "The Tonight Show", "Politically Incorrect" and an award-winning children’s program. He's been published in “The New York Times" and "Huffington Post” and has co-authored numerous collections of trivia, puzzles and humor. His “What in the World!” podcast is a fun look at the latest weird and paranormal news, strange sports stories and odd trivia. Paul likes to add a bit of humor to each MU post he crafts. After all, the mysterious doesn't always have to be serious.

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