A Cataclysmic Collision Formed the Moon, but Killed Theia

The Moon is the first object in space that fascinates we Earthlings. The Sun might be more prominent, but you can’t stare at the Sun without ocular damage. Anyone can gaze at the Moon, with or without binoculars or a telescope, and wonder where it came from and what it all means.

New evidence from a team at UCLA is clarifying the story of the Moon’s origins. According to this research, the Moon was formed as a result of a massive collision between Earth and a “planet embryo” about the size of Mars called Theia. This collision happened about 100 million years after the Earth was formed. Published on January 29th in the journal Science, this new geological evidence strengthens the case for the collision model.

The researchers compared Earth rocks with rocks retrieved from the Moon over the years. (Over 380kg of rocks have been brought back to Earth.) They found that these samples—collected on Apollo missions 12, 15, and 17—had the same chemical composition as seven rocks collected from Earth’s mantle, in Hawaii and Arizona. The key to the comparison lies in the nature of the oxygen atoms in the rocks.

Oxygen is a highly reactive element. It is easily combined with other elements, and is the most common element in the Earth’s crust. There are several different oxygen isotopes present in the Earth’s crust, and on other bodies in the solar system. The amount of each isotope present on each body is the “fingerprint” that makes the formation of each body different.

But the team at UCLA has shown that Earth and the Moon share the same cocktail of oxygen isotopes. They have the same fingerprint. This means that somehow, someway, their formation is linked. It can’t be pure coincidence. Says Edward Young, lead author of the new study, “We don’t see any difference between the Earth’s and the Moon’s oxygen isotopes; they’re indistinguishable.”

So how did this happen? How do Earth and the Moon share the same oxygen fingerprint? Enter Theia, an embryonic planet that got in the way of Earth’s orbit around the Sun. And as the research shows, this collision had to be more than a glancing blow. The collision had to be direct and cataclysmic.

This video shows how the collision would have played out.

A glancing blow would mean that the Moon would be mostly made of Theia, and would therefore have a different oxygen isotope fingerprint than Earth. But the fact that the Earth and Moon are indistinguishable from each other means that Theia had to have been destroyed, or rather, had to become part of both the Earth and the Moon.

“Theia was thoroughly mixed into the Earth and the Moon, and evenly dispersed between them. This explains why we don’t see a different signature of Theia in the Moon versus Earth,” said Young.

If this collision had not taken place, our Solar System would look very different, with an additional rocky planet in the inner regions. We also would have no Moon, which would have changed the evolution of life on Earth.

This collision theory, called the Theia Impact, or the Big Splash, has been around since 2012. But in 2014, a team of German researchers reported in Science that the Earth and Moon have different oxygen isotope ratios, which threw the collision formation theory into doubt. These new results confirm that it was a cataclysmic collision that gave birth to the Moon, and changed our Solar System forever.

5 Replies to “A Cataclysmic Collision Formed the Moon, but Killed Theia”

  1. In response to the last paragraph, I find it hard to accept that this has confirmed anything when the German study not 2 years prior says otherwise. Was there something wrong with the other study that voided their findings or changed the interpretation of their results? We can’t say this new study confirms anything just because it’s newer.

    1. That’s the think with true science. It has to be replicated a number of times to finally get results. It can’t be tainted with politics and desires.

      I truly think we don’t have a sample size large enough of moon rocks to prove or disprove any theory of the collision between Theia and the Proto-Earth. You want a true interpretation of that event, you will need an extremely detailed simulation of what would happen with various collisions of two bodies. You’ll need the orbits of those bodies and everything. The simulation should account for varying masses and have very different outcomes with those masses and orbits. There should be only one scenario that leads to our Earth and Moon with our current masses and orbits. Even if this simulation takes months to run, it still needs to be run over several scenarios. See you in a decade after the simulations are ran enough.

      Oh, and let’s not forget the possibility of Proto-Earth already having a satellite orbiting it prior to the collision. Maybe that satellite captured the ejecta. Would explain the imbalance of the moon’s makeup with a denser portion pointing towards Earth now.

  2. Interesting that you say the theory has been aound since 2012. I heard proposed as a possiblle account back in high school In the late 50’s. I think it had been around a while then. TGhey just hadn’t named the theoretical Theia.

    1. The reference to 2012 was a reference to the comparison on isotopes from Earth and Moon rocks published that year. The Giant Collision theory has been in wide circulation since the late 70’s, although of course I cannot say that it was not discussed in the 50”s. For that matter, for all either of us know, it may have been discussed in the 19th Century. The real question is when it was first published in peer review journals.

  3. Two bodies collide and two bodies emerge. The video has the smaller body completely disintegrate to the core and then jerked back as if on a leash to whip around and smash in again in less than one orbit. The integrity of the mars sized object needs to be respected in the collision. A lot of what was Theia may still be part of the Moon.

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