How did the earth obtain the moon, a natural satellite? This is a long-debated issue, including the giant celestial impact theory, which believes that the moon was formed by the collision of the early Earth and a Mars-sized rocky body called Theia, and has become one of the many explanations. the frontrunner. But the details of how this happens are unclear, and scientists are still trying to explain many of the observations. Now, a new study published in the journal Nature Geoscience has solved one of the biggest mysteries surrounding the impact theory: What exactly happened? Namely why assuming Theia existed, the moon ended up being almost identical to the Earth, and not Theia. According to impact theory, Theia was a celestial body roughly the size of Mars or smaller that hit the "growing" Earth 4.5 billion years ago. The collision generated enough heat to create a magma ocean and ejected large amounts of debris into orbit around the Earth, which later merged to form the moon, a theory that explains how and how fast the Earth and Moon spin each other.

The Earth-Moon is tidally locked, meaning the Moon always shows the same side to Earth as it rotates around the Earth. That's why China achieved such an achievement when it landed the Chang'e-4 spacecraft on the far side of the moon in 2019, which is that direct communication with the far side of the moon has never been possible from the earth. The Moon and Earth are nearly identical in composition, except that the Moon has less iron and lighter elements like hydrogen, which is needed to create water. The giant impact theory explains why, and the heavy element iron would be retained on Earth. The heat generated during impacts and ejections into space would boil away lighter elements.

While the rest of Earth and Theia's material would have mixed together, computer models recreated the events that led to the formation of the Moon. The model that best matched all observations suggested that the Moon should have been made of material from Theia around 80 material composition. So why is the moon suspiciously similar to Earth? One explanation is that Theia and early Earth had the same ingredients from the beginning. But this seems unlikely, as every documented planetary body in our solar system has its own unique composition. The slight difference reflects the distance from the Sun, where the celestial body formed.

An alternative explanation is that the two bodies mixed much more thoroughly than expected, leaving a less clear Theia signature on the moon. But this is also unlikely, as it would require a much larger impact than what actually occurs. The new study resolves this dilemma by showing that the Earth and moon are not as similar as previously thought. Researchers have made very precise observations of the distribution of oxygen isotopes in rocks brought back from the moon by the Apollo astronauts. In chemistry, the nucleus of any element is made up of particles called protons and neutrons; isotopes of an element have the same number of protons in the nucleus as regular versions, but a different number of neutrons.

In this case, the oxygen isotope O-18, which has 8 protons and 10 neutrons, is slightly less dense than the more common O-16 (which has 8 protons and 8 neutrons) heavier. Research shows that the oxygen isotope compositions of the Earth and the Moon are slightly different, and their profiles are not the same after all. What's more, the difference increases when looking at rocks in the lunar mantle, a layer below the surface or crust that is lighter in oxygen isotopes than Earth's. This is important because the Earth's crust is where mixed debris might end up, and the deeper layers would contain more fragments of Theia.

So Theia is different from Earth, and the Moon is different from Earth, but these results also tell us a lot more about Theia itself. One might expect more heavier isotopes to be closer to the sun due to gravity. Theia must have had more lighter oxygen isotopes than Earth, suggesting it may have formed farther from the sun than Earth. With the results of this study, the giant impact theory has crossed another hurdle in explaining the formation of the moon, and in the process, we're also learning a lot more about Theia itself.