The Theia-ory: Moon is Earth?

by Nurul Anisa Najiha Mispan and Nur Khalisa Mohd Padzli

A Glance through The Giant-Impact Theory

News flash: The moon is NOT cheese. But hey, it might be Earth. In this article, we will unwrap how Theia is responsible for the moon’s formation and dive into its mechanism, evidence, and alternative hypotheses. But before that, who is Theia? Theia is a Mars-sized protoplanet that struck the Earth and led to the birth of our natural satellite. (1)(2)

This collision is explained through the Giant Impact Theory, also known as the Theia Impact or the Big Splash. (2)(3)

In 1974, Dr William K. Hartmann and Donald R. Davis resurfaced a long-disregarded idea by Professor Daly of Harvard University that centrifugal forces were not the cause of the moon’s creation. Daly insisted the Moon resulted from an impact by another celestial body, opposing the first theory made by George Darwin in 1898 which we will talk about later on. (4)(25)

Mechanisms of The Theia Theory

Figure 1: NASA’s supercomputer simulation of The Giant Impact; YouTube. (5)

Nasa’s latest supercomputer simulation displays how the Moon might have formed in less than a day. Theia smashed into baby Earth (also named Gaia), ejecting molten debris that twirled round and round and then eventually formed the Moon. (5)

Figure 2: Immediate Origin of the Moon as a Post-impact Satellite; Jacob Kegerries, NASA Ames Research Centre. (6)(24)

As seen in the diagram above, a large body (green) ejected from the crash, carried the moon (purple) and due to momentum, projected the moon forward into a wide and stable orbit. Because of its giant mass (green), this bigger body falls back to merge with the Earth whilst the Moon stays moving in its orbit. (6)

Evidence

Peas from a pod

Now that you get the idea of Theia Theory, what’s the account of the Moon being accused to be alike to the Earth? Lunar rocks brought by the Apollo missions have contributed to this finding. It turns out the Moon and Earth possess similar chemical isotopes. (7)

For instance, oxygen (O) isotopes found in both Moon and Earth are significantly similar and believe it or not, it is not the only element. Along the list, we have other elements like Tungsten (W) and Chromium (Cr) which have the same isotopes in both planetary bodies. (26)

Figure 3: The precise oxygen isotopes data for Earth (dashed line), Moon, Mars, and meteorites. When the value falls on the dashed line, it means the oxygen isotopes of the body are exactly like the Earth. (8)

On Earth, oxygen-16 is the most abundant isotope compared to oxygen-17 and oxygen-18. When a ratio is made between oxygen-17 to oxygen-16 versus oxygen-18 to oxygen-16 in both the Earth and the Moon, they have the same range value. However, as for comparison made for meteorites and on Mars, the ratio range value differs. It shows that the distribution of isotopes, in this case, oxygen isotopes are not the same throughout the universe. This similarity between these two bodies further strengthens the idea that the Moon is made up of Earth which is obviously caused by the impact of Earth and Theia. (8)

Figure 4: This remarkable discovery of similarity in oxygen isotopes in Earth and Moon is pioneered by Edward Young, a geologist from UCLA (middle). (16)

Junjun Zhang and colleagues at the University of Chicago (USA) and the University of Bern (Switzerland) have also found another isotopic element to support this evidence which is Titanium. Titanium-50 to Titanium-47 shows the identicality of Earth and its satellite. Unlike what many speculate, the impact between Theia and the Earth does not cause Moon to derive from Theia’s composition. This is proven through refractory elements like Titanium. (9)

Not only that, planetary scientist Kevin Righter of the Astromaterials Research and Exploration Science Division (ARES) at NASA’s Johnson Space Center in Houston, Texas managed to build a model to compare the concentration of 14 different siderophiles (iron-loving elements) where he found elements from Theia do not correspond well to his calculated values of Moon yet, Earth did. (10)

Merry-go-round

The angular momentum inside the Earth-Moon system plays a role in supporting this theory. The rotational angular momentum (3.41 × 1041 g·cm2/s) is significantly higher compared to the momentum of other planets such as Mars and Venus. Researchers suggest that this excessive momentum is obtained from the extra energy of a head-on impact during the collision between Earth and another planet of half its size. (13)

On a second note, both directions of the Earth’s spin and the Moon’s orbit are anti-clockwise. Although plenty of other planets with moons have the same relationship, moving in the same orientation still perfects the idea that the Earth and Moon were formed from the same accretion disk which resulted after the impact. An accretion disk is a flattened structure made up of particles around a central body. (14)(15)

Figure 5: Simple model of Giant Impact Hypothesis; Citronade — Own work, CC BY-SA 4.0. (22)

Author’s note: Did you know? Triton is the largest of Neptune’s 13 moons and is known for its orbit that moves opposite to the direction of Neptune’s rotation.

We found the culprit

To reinforce the validity of the Big Splash Theory, seismologists are studying two continent-sized layers of rocks beneath West Africa and the Pacific Ocean. They found that earthquakes abruptly slow down as they pass through these layers. This means the layers are denser and chemically different. (17)

A geodynamic researcher at Arizona State University, Qian Yuan suggests that these distinct layers are remnants of Theia buried inside the Earth based on isotopic evidence and the age of these rocks, which fit the timeline of The Theia Impact Theory. (17)(21)

Figure 6: Qian Yuan’s YouTube explanation (21)

Doubts and Answers

“Based on how solar systems are speculated to be made (from explosions and collisions), other planets such as Venus must have also undergone similar collisions thus, creating a moon similar to Earth’s. But why does Venus not have a moon?”

Well, maybe a second collision took place and knocked Venus’s moon out of existence or at least, out of its orbit.

“If a such collision with Theia occurred, wouldn’t the impact of it melt most of Earth and formed a magma ocean?”

Exactly, it did. In fact, at the time the earth was forming, the inner solar system was extremely hot to begin with. Energy from the collision made the earth even hotter. The molten remained on the surface molten from the impact for thousands of years. Until the earth was bombarded with asteroids. These asteroids came from the asteroid belt and fell out of their orbit due to the closest planet’s gravity, Jupiter. But what is interesting is that these asteroids contain frozen water particles. As they crashed into the earth, they smashed apart releasing this water that cools down the surface molten of the Earth. With explosion after explosion, the ocean is eventually formed. (18)

Alternative Hypotheses

Earth vomited out The Moon.

After spinning endlessly, Earth might have expelled parts of it and created the moon. (25) This is a theory brought by George Darwin, son of Charles Darwin (famous for his Theory of Evolution), in the Fission Theory of 1879. Sun’s gravity pulled a part of Earth shaping a bulge on the molten form Proto Earth, also known as a solar tidal bulge. To overcome gravity, a centrifugal force led to an expulsion of this bulge, and you know what happens next. (19) The Moon is created.

Figure 7: Fission Theory; Parkview Astronomy (23)

However, this theory does not explain how the earth’s angular momentum could ever be significant enough to throw out an object sized such as the moon.

The Moon and Earth formed separately but together?

The Accretion Theory or others might say The Condensation Theory, neither names matter, since it is wrong anyway, suggests that the Earth and the Moon accreted individually as they orbit a black hole during the solar system formation.

Though, this would not explain why The Moon orbits around Earth instead of the Sun just like any other planet that is formed by accretion. It also does not explain the similarity in composition between these two space blobs.

Unsolved theft of the Moon.

Other scholars say the Earth stole the Moon. As an Earth stan, I must disagree.

This notion is called The Capture Theory. It states that the Moon formed elsewhere in the solar system and eventually got caught up in Earth’s gravitational field. Some even say that our dearest Moon was originally Venus’s. [No way right!?] (20)

Still, Capture Theory once again does not take into mind the comparable materials of the Earth and the Moon brought back by NASA’s Apollo mission. The materials found on Earth and Moon are so similar to each other. Not only that, but most pulling processes relating to gravity would end up in a collision, especially for a Moon this big, so what could have allowed the moon to politely slow down and end up orbiting Earth without smashing it into pieces instead? [Sounds invalid to me]

Summary

To summarise, based on the overall ratio of evidence against doubts, it is safe to say that we strongly agree with the Theia Theory. As a matter of fact, even NASA refers to the Big Splash as the “leading” theory of The Moon’s formation. (27)

Scientifically speaking, however, this theory is yet to be proven entirely, and again, NASA’s handy-dandy scientists are still pending in their research for more proof to seal this idea once and for all. In the meantime, how far would you believe The Giant Impact Theory?

References

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