We Were Wrong About Where The Moon Came From

The leading lunar formation theory says the impact melted a part of the Earth and the entire impactor (Theia).

Both the Earth and the magma that formed the Moon would have been extremely heated in the wake of the impact.

Because the Earth is so much larger than the Moon, it would have sent more potassium to the Moon than vice versa.

Kun Wang from Washington University in St Louis and Stein Jacobsen at Harvard – examined minuscule amounts of potassium in moon and Earth rocks and found minute differences – possible only if their raw materials were thoroughly mixed in a superheated fog before they coalesced.

Other scientists remain skeptical of Wang’s findings, claiming the lunar samples he studied do not accurately reflect the Moon’s potassium levels. Their research was published in Nature Geoscience yesterday. “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 Arts & Sciences.

Simulation after simulation of the impact predicted that most of the material (60 to 80 percent) that formed the Moon would have come from the impactor, rather than from Earth, and it was extremely unlikely that Earth and Theia had the same chemical make-up.

This is according to new analyses on the chemical composition of seven samples brought back during the Apollo missions, which could now disprove a leading theory on the origin of the moon.

This was very odd.

The collision was wild enough that most of the moon was once part of the still-forming Earth. Isotopes can act as geologic fingerprints, because prior work has suggested that planetary bodies that formed in different parts of the solar system generally have different isotopic compositions.

There could also be a chance that the space object that was also involved in the alleged collision might have had the same isotopic signature as Earth’s. The giant impact model explained a variety of the moon’s physical characteristics, including its size and rotation. The isotopic composition studies had created an “isotopic crisis” for the hypothesis.

Until recently, scientists suggested more precise isotopic measurements might reveal small differences, but 2015 testing quashed the notion.

Wang and his colleagues will now continue to study the Apollo lunar samples and try to gather even more clues from them.

A slap, a slug or a wallop?

One hypothesis suggested a silicate vapor atmosphere spread about the impacted Earth and vaporized the impactor, allowing the exchange of materials between the Earth and its impactor.

Roughly 4.5 billion years ago, a massive Mars-sized object slammed into Earth, vaporizing the early planet and spurring the creation of the moon, according to a new study.

While no one’s come out to dispute the claims outright, the onus is now on Wang and his team to make their hypothesis more convincing and weighted in evidence than the one we’ve been carrying around for nearly 50 years.

Two recent models for the formation of the moon, one that allows exchange through a silicate atmosphere (top), and another that creates a more thoroughly mixed sphere of a supercritical fluid (bottom), lead to different predictions for potassium isotope ratios in lunar and terrestrial rocks (right).

The only way to explain this higher abundance of potassium-41 in the Moon’s composition is to accept that there was a more violent impact, says Wang.

The two models Wan discusses are one whereby a low-energy impact leaves the proto-Earth and Moon cocooned in a silicate atmosphere and the other involves a much more violent impact.

As that vapor began to cool following the high-energy collision, the melt that condensed out of it was made up of the heavier isotopes, such as potassium-41. But the researchers weren’t finding signs of that in the samples; instead, chemical analyses on the samples were returning isotopic compound readings that were almost identical. The force of the impact vaporized the smaller and most of the larger, resulting in a very big atmosphere 500 times the current size of current Earth, filled with a super-hot fluid mantle.