The leading theory for how the Moon formed was that it was the product of a catastrophic collision between Earth and another planet the size of Mars 4.5 billion years ago, known as the giant impact hypothesis. The impact was enough to throw a huge amount of material into orbit that led to its formation, but the full effect was not clear until now.
In a paper, published in Nature, Kun Wang and Stein Jacobsen from Harvard University discuss how our planet and natural satellite have a chemical signature that points to a devastating formation that threw the mantle of the proto-Earth into space, hinting at a more powerful impact than other theories suggest.
“Our results provide the first hard evidence that the impact really did (largely) vaporize Earth,” said Wang, assistant professor in Earth and Planetary Sciences, in a statement.
Samples from the Apollo missions in the 60s and 70s had indicated that both Earth and the Moon were formed from the same materials, which suggested that our satellite didn’t come from the material of the impactor. And this new study backs up the theory that the Moon is, essentially, made from pieces of Earth.
“The goal was to find a way to make the Moon mostly from the Earth rather than mostly from the impactor,” said Wang. “There are many new models – everyone is trying to come up with one – but two have been very influential.”
One hypothesis suggests that a low energy impact created a disk of debris around Earth, as well as a large but low density silicate vapor atmosphere. This atmosphere and the debris then condensed over time into the Moon. The other idea instead requires a high energy impact that pulverized Earth and relocated the mantle to outer space, creating a dense cloud of molten rock around Earth that was 500 times bigger than our planet.
This mantle-atmosphere would have been a supercritical fluid, where both liquid and gas phases of rock co-existed. The processes that happen within such a fluid are very particular, and they can leave specific traces in lunar rocks.
To distinguish between the two ideas, the researchers then actively looked for a potential indication of the superfluid phase. They analyzed the presence of potassium isotopes (atoms with a different number of neutrons) in Moon and Earth rocks, and discovered that lunar rocks have more heavy potassium.
In a tenuous silicate atmosphere, the heavy potassium would have rained back to Earth, which doesn’t match with what was observed. The detected enrichment of lunar rocks suggests that the process was quick and likely happened under high pressure, implying a powerful impact.