In a new paper published yesterday, researchers at the Southwest Research Institute (SwRI) detailed the results of a new computer model that gives new evidence to support the 'giant impact hypothesis' — which states that the Moon was created by a massive object striking the Earth during its formation.
The idea that the Moon originated from the Earth was first hypothesized by English astronomer George Darwin back in 1898. He thought that the Moon had spun off from the molten Earth at a time when the planet was rotating much faster, and although his idea that the Moon was gradually getting further away from the Earth was later confirmed by experiments, he could never make the math work out to have the Moon originate from the Earth's surface. In 1946, Canadian geologist Reginald Aldworth Daly — who was also an early supporter of the continental drift theory — proposed the idea that a massive impact explained the Moon's origin better. Daly's idea was largely ignored by the scientific community, but saw fresh light when it was discussed at a 1974 scientific conference, and then published in a 1975 paper in the scientific journal Icarus.
The 'giant impact hypothesis' says that the Earth was struck by a protoplanet — named Theia after the Greek goddess who was mother of Selene, the goddess of the Moon — that had about one-tenth the mass of Earth (roughly the mass of Mars), and the two protoplanets merged to form the planet we stand on today. The impact would have also thrown off a large amount of matter in the process — most of which would have been from Theia — and this matter quickly coalesced to form the Moon. Previous attempts to model this fell short of confirming the hypothesis though, because they found that the compositions of the Earth and Moon should be very different from each other — due to the different compositions of proto-Earth and Theia. However, evidence from examination of the Moon and rocks brought back by the Apollo astronauts showed that the Earth and Moon are very similar.
This new simulation, developed by Dr. Robin M. Canup at SwRI, instead assumes that Theia and Earth were much closer in size, with Theia and proto-Earth being about 45% and 55% of Earth's current mass, respectively. The model used 'smoothed-particle hydrodynamics' — a method of modeling fluids — to simulate the collision, tracking the thermal and gravitational interactions of 300,000 discrete particles with time. The results showed that after the initial impact, both protoplanets would have impacted again and completely merged to form a single larger planet, surrounded by a scattered disk of debris. The different compositions of the two protoplanets would have been mixed far better in this scenario, and the orbiting disk of debris would end up with nearly the exact same composition as the new Earth's mantle. This orbiting disk would have then coalesced to form the moon, and accounts for the similarities between the composition of the Earth and the Moon.
The model did suffer from one problem though, in that it caused the resulting Earth-Moon system to spin far too quickly. However, another paper published by Dr. Matija Ćuk of the SETI Institute and Dr. Sarah T. Stewart of Harvard University showed that the system could have slowed down very soon after impact, due to interactions between the Sun and the new Moon called 'evection resonance'.
"By allowing for a much higher initial angular momentum for the Earth-Moon system, the Ćuk and Stewart work allows for impacts that for the first time can directly produce an appropriately massive disk with a composition equal to that of the planet's mantle," said Canup, according to Science Daily. The research of Ćuk and Stewart also expanded the potential scenarios by showing that a result similar to Canup's — with a debris disk that matched the composition of the planet's mantle — could be produced by impacting a much smaller, faster-moving object onto a large target that is rotating rapidly due to a previous impact.
In a related story, another group of researchers lead by Frédéric Moynier, a professor at Washington University in St. Louis, MO, have published a paper confirming long-sought-after evidence that the Moon experienced a type of sorting by mass called 'isotopic fractionation' that was expected if the Moon was formed due to a large impact with proto-Earth. Isotopic fractionation is when elements are 'enriched' to become heavier versions of themselves — most often used to describe nuclear materials such as uranium. Moynier and his team found a slightly greater amount of a heavier isotope of zinc in Moon rocks, which can be explained by the heavier zinc atoms condensing out of the debris disk earlier than lighter zinc atoms, and the resulting sorting showing up in rocks on the lunar surface.
Although the researchers admitted that a better understanding of planetary formation and 'evection resonance' would be needed to figure out exactly how the moon came to be, their combined efforts have shed some much-needed light on a hypothesis that has been stuck in scientific limbo for 34 years, while providing some much-needed vindication to one of our own.
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