We are fairly confident about the idea that the Moon formed when a Mars sized object hit the young Earth over 4 Billion years ago. The resulting debris cloud condensed to form the Moon, giving the Earth a surprisingly large Moon for its size. Next to Charon and Pluto, the Earth’s Moon is the largest relative to the size of its planet. It’s also the third largest of all moons in the Solar System, next to Ganymede and Titan, #1 and #2 respectively. but there is one problem with this theory of the Moons formation: The Earth and Moon are made of exactly the same material, or said another way, they have the same isotopic ‘fingerprint.’ The Physics of the Moons size and orbit make sense but it is expected that during such a massive collision, the leftover material from the impactor, dubbed ‘Theia,’ would become part of the Moon.
A team of scientists from the University of Maryland are the first to reconcile this issue with the Moon’s formation, by looking at a particular isotope of Tungsten present in both the Moon and Earth. They found that based on the amounts of this isotope, the original impact of Theia was so powerful that its material thoroughly mixed with the Earth’s before condensing to form the Moon. There are still other possibilities that lead to the Earth and Moon having the same fingerprint. Theia could have been made of material similar to that of the Earth, or the Moon could have been made of material from early Earth, though this type of impact is unusual.
The main idea of this new study is that after the Earth and Moon formed, containing the same materials, they continued to collect new material from the early solar system, which contained less of the isotope Tungsten-182 than the amount already present. Since the Earth was larger it collected more of this new material than the Moon, and it means that the Earth should have a relatively lower amount of Tungsten-182 than the Moon. Its like having two exact same tanks of water, and you add the same amount of fruit juice to each tank. After the fruit juice spreads out, you add more water to both tanks, but a much larger amount to the ‘Earth’ tank. In the end the ‘Earth’ tank will be more diluted and will have less juice per litre than the ‘Moon’ tank.
This is exactly what the team observed. The relative amount of Tungsten-182 in the Moon rocks was larger than that of Earth rocks, and the difference was exactly the amount expected if the Earth and Moon started out with the same relative amount. This finding supports the idea that the impact was large enough to thoroughly mix the material of Theia and young Earth, and it largely rules out the possibility that Theia started out with a similar composition to that of young Earth.
That is epic science!