New study favors violent mash-up Moon hypothesis

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Artist's impression of a Mars-sized planetoid smashing into proto-Earth(Credit: Dana Berry/SwRI)

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A new study that analyzed isotopic differences in potassium contained in terrestrial and lunar rock samples is helping to shed light on the formation process that led to the creation of Earth's Moon. The research could help reveal the nature of a cataclysmic collision between Earth and a Mars-sized body, which is believed to have taken place in the distant past.

Elements in rocks that originated in different parts of the solar system are known to boast distinctive – albeit minute – differences in the compositions of their isotopes. By observing this sort of isotopic fingerprint, scientists are capable of determining where a sample of material from outer space originated.

Researchers have attempted to apply this technique to elements in lunar rocks, including tungsten and potassium, in order to gain insights regarding the formation process that sculpted Earth's Moon. The earliest isotopic analysis carried out by researchers discovered that many elements in Moon rocks bore a nearly identical isotopic fingerprint to their terrestrial cousins.

The revelation that the Moon appeared to be created largely from the same material as Earth's mantle led scientists to abandon the previously held hypothesis that Earth's companion had formed as a result of a grazing collision between the proto-Earth and a Mars-sized body, nicknamed Thea.

Numerical simulations of this scenario suggest that isotopes found in elements in terrestrial and lunar samples would differ significantly, as around 60 – 80 percent of the satellite's material would belong to the impactor, which originated in a different part of the Solar System.

Since the isotopes found in lunar and terrestrial samples were nearly identical, this could not be the case. Scientists were then tasked with developing new hypotheses that would explain the creation of our Moon predominantly from the material of the proto-Earth.

Whilst many hypotheses were proposed in the aftermath of the discovery, two in particular rose to prominence. The first Moon-creation model suggests that the impactor struck proto-Earth in a relatively low-energy collision, melting a part of our young planet's mantle, and flinging it out into orbit like water escaping a spinning ball. This created a disk of molten debris.

Illustration of the two leading formation models for Earth's Moon(Credit: Kun Wang)

The team behind the hypothesis proposed that, as a by-product of the collision, both proto-Earth and the impactor would be enveloped in a silicate vapor atmosphere, which would act to facilitate the transfer of molten material to the impactor. Whilst the model could theoretically account for the isotopic similarities between terrestrial and lunar rocks, the transfer of magma debris to the proto-Moon would be incredibly slow – too slow for the mixed material to fall back to Earth according to Kun Wang, one of the authors of the study.

The second hypothesis is predicated on a high-impact collision between proto-Earth and Thea. Under this model, the force of the impact would have vaporized both the impactor and the majority of Earth's mantle, creating a vast atmosphere composed of supercritical fluid that extended across a region of space 500 times the volume of present day Earth. Over time, Earth's Moon would have formed out of the superfluid atmosphere. This model would allow for a greater mixing of materials from the two celestial bodies in a shorter time frame.

The new study observed differences between potassium isotopes present in samples of moon and terrestrial rock. The seven lunar rocks analyzed by the team were returned to Earth over the course of a number of Apollo-era missions. These rocks were compared with eight terrestrial samples that were deemed to be representative of Earth's geochemical make up.

Wang, a geochemist at Washington University, St. Louis, along with study co-author Stein Jacobsen, a professor of geochemistry at Harvard University, Massachusetts, leveraged a method for isotopic analysis that, developed by the two in 2015, boasts a precision 10 times greater than any previous technique.

The team discovered that the lunar rocks contained more of the heavy isotope potassium-4 than their Earthly cousins. If the Moon had formed as per the silicate atmosphere model, exactly the opposite would have been predicted. Therefore the findings support the hypothesis that the Moon was created in a catastrophic collision, which vaporized much of Earth's mantle.

So, next time you see the Moon, try and look past its placid beauty to the incredible violence of its past.

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