Ryan North and Carlos Gómez have once again delved into the realm of astrophysics in Fantastic Four #25. Marvel’s First Family have been transported to an alternate timeline and an Earth that never had a Moon. Reed Richards gives and excellent synopsis of the Giant Impact Hypothesis, which is the prevailing hypothesis for the formation of our Moon.
There are five main hypotheses for how our Moon formed: Fission Theory, Capture Theory, Condensation Theory, Colliding Planetesimals Theory, and Ejected Ring Theory (we’ll ignore the misuse of the word theory here). Fission Theory suggests that the Moon was formed by a piece of Earth, possibly from the Pacific Ocean basin, that separated early in the planet’s existence, due to the speed of its rotation. Capture Theory posits that the Earth’s gravity captured an asteroid as it passed by, much the same way Jupiter acquired many of its moons. Condensation Theory advocates for the idea that the Earth and the Moon formed together during the early years of our solar system.
The currently accepted Giant Impact Hypothesis has pieces of both the Colliding Planetesimals and Ejected Ring Theories. This idea is that early in the formation of our solar system, the proto-Earth known as Gaia had a neighboring protoplanet, known as Theia. Theia was approximately the size of Mars, though some say it could have been much larger, and it formed in the L4 or L5 Lagrange Point on a similar orbit to Gaia. Gravitational perturbations from Venus or an impact from a large asteroid may have changed Theia’s orbit enough to put it on a collision course with Gaia.
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From here scientists are split on a couple of crucial details. Originally it was believed that Theia hit Gaia with a glancing blow, which resulted in the current tilt and spin of the Earth, but did not account for the chemical composition similarities of the Earth and the Moon (many of the same minerals are found on both). Research from 2012 has shown that the compositional similarities of the Earth and the Moon are supported by a more direct impact of the two protoplanets.
Another point of contention is how quickly the Moon formed after the collision. Traditional thought is that the Moon took months or years to accrete from the debris produced by the collision of Gaia and Theia. A newer hypothesis published in Astrophysical Journal Letters suggests that the collision resulted in a satellite composed of a mixture of the mantles of both protoplanets, which was caught in Earth’s orbit mere hours after the collision. This hypothesis additionally claims that a secondary satellite composed of mostly mantle material from Theia was also created and pulled back into the mantle of Gaia, as part of the Earth’s accretion.
Possible evidence of this new hypothesis can be found in the Earth’s mantle as two continent-sized anomalies below Africa and the Pacific Ocean. These anomalies have low seismic velocities and a higher iron oxide content than the surrounding material, which is very similar what we’ve analyzed from the Moon. Simulations have shown that a direct collision of Gaia and Theia could have resulted in mantle material from Theia being injected into the core of Gaia, while the material ejected from the two bodies subsequently formed the Moon, and these two subcrustal blobs.
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While this is the currently accepted hypothesis for the formation of our Moon, there are still questions the Giant Impact Hypothesis can’t answer, so there’s plenty of research that has to be done to fully understand the origins of our only natural satellite.
AIPT Science is co-presented by AIPT and the New York City Skeptics.
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