Updated: March 10, 2020 3:28:15 pm
There are multiple theories which try to explain the formation of the Moon, but the giant-impact hypothesis is the most reliable on. It claims that around 4.5 billion years ago, something the size of Mars called Theia collided with a newly formed Earth. The colossal impact is believed to have resulted in the object fusing with Earth and primed it for life while breaking off a large chunk that went on to become the Moon.
While this theory continues to become what favoured model for explaining the formation of the Moon for years, there was one big spanner stuck in the works. However, a new study, published in the journal Nature Geoscience, claims to have found traces of Theia in the Moon– putting in place the lost piece of the puzzle.
“This model was capable of accounting for the then-recent observations from samples returned by the Apollo missions, which included the Moon’s low iron content relative to Earth, depletion in volatiles and enrichment in refractory elements, while avoiding most of the pitfalls of previous lunar origin theories,” researchers from the University of New Mexico wrote in their paper.
What was the problem with the giant-impact hypothesis?
Models made by scientists predicted that around 70 to 90 per cent of the Moon should have been made up of mooshed and reformed Theia, which means that the oxygen isotopes of Moon would have been different from Earth’s oxygen isotopes. However, oxygen isotopes in lunar samples were very similar to terrestrial oxygen isotopes while being very different from oxygen isotopes on other Solar System objects.
One possible explanation was that Earth and Theia had similar compositions, to begin with— the odds of which are actually extremely small. Another explanation was that everything got completely mixed during the impact– which, according to simulations, isn’t very likely.
Over the years, researchers published several papers trying to explain the similarity of Earth and Moon’s oxygen isotopes, which doesn’t sit well with the giant-impactor hypothesis. These theories included the idea of Theia fusing with Earth, creation of a cloud of dust that went on to become Earth and Moon, Theia and Earth forming really close to each other, and more.
Theia is buried deep inside Moon
Planetary scientist Erick Cano and team reanalysed the lunar samples. They acquired a range of samples from different rock types gathered on the Moon including both high and low titanium basalts from the lunar maria; anorthosites from the highlands, and norites from the depths, brought upwards during a process called lunar mantle overturn; and volcanic glass.
They modified a standard isotope analysis technique to produce high-precision oxygen isotope measurements to find out that oxygen isotopic composition varied depending on the type of rock tested.
“We show that the method of averaging together lunar isotope data while ignoring lithological differences does not give an accurate picture of the differences between the Earth and Moon,” they wrote in the paper.
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The researchers found that the deeper the rock sample’s origins, the heavier the oxygen isotopes, compared to that of Earth’s. They said that this difference could be explained if only the outer surface of the Moon was pulverised and mixed during the impact, resulting in the similarity with Earth, but deep inside the Moon, the Theia chunk remained relatively intact leaving its oxygen isotopes closer to their original state.
“Clearly, Theia’s distinct oxygen isotope composition was not completely lost through homogenisation during the giant impact,” the researchers concluded. “This result thereby eliminates the necessity for giant-impact models to include a mechanism for complete oxygen isotope homogenisation between the two bodies and provides a foundation for future modelling of the impact and lunar formation.”
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