How did the moon form? A supercomputer may have just found the answer

The moon could have formed immediately after a cataclysmic impact that tore off a chunk of Earth and hurled it into space, a new study has suggested.

Since the mid-1970s, astronomers have thought that the moon could have been made by a collision between Earth and an ancient Mars-size protoplanet called Theia; the colossal impact would have created an enormous debris field from which our lunar companion slowly formed over thousands of years.

But a new hypothesis, based on supercomputer simulations made at a higher resolution than ever before, suggests that the moon's formation might not have been a slow and gradual process after all, but one that instead took place within just a few hours. The scientists published their findings October 4 in the journal The Astrophysical Journal Letters.

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"What we have learnt is that it is very hard to predict how much resolution you need to simulate these violent and complex collisions reliably — you simply have to keep testing until you find that increasing the resolution even further stops making a difference to the answer you get," Jacob Kegerreis, a computational cosmologist at Durham University in England, told Live Science.

Scientists got their first clues about the moon's creation after the return of the Apollo 11 mission in July 1969, when NASA astronauts Neil Armstrong and Buzz Aldrin brought 47.6 pounds (21.6 kilograms) of lunar rock and dust back to Earth. The samples dated to around 4.5 billion years ago, placing the moon's creation in the turbulent period roughly 150 million years after the formation of the solar system. 

Other clues point to our largest natural satellite being birthed by a violent collision between Earth and a hypothetical planet, which scientists named after the mythic Greek titan Theia — the mother of Selene, goddess of the moon. This evidence includes similarities in the composition of lunar and Earth rocks; Earth's spin and the moon's orbit having similar orientations; the high combined angular momentum of the two bodies; and the existence of debris disks elsewhere in our solar system. 

But exactly how the cosmic collision played out is up for debate. The conventional hypothesis suggests that as Theia crashed into Earth, the planet-busting impact shattered Theia into millions of pieces, reducing it to floating rubble. Theia's broken remains, along with some vaporized rocks and gas ripped from our young planet's mantle, slowly mingled into a disk around which the molten sphere of the moon coalesced and cooled over millions of years. 

Yet some parts of the picture remain elusive. One outstanding question is why, if the moon is mostly made out of Theia, do many of its rocks bear striking similarities to those found on Earth? Some scientists have suggested that more of Earth's vaporized rocks went into creating the moon than Theia's pulverized remnants did, but this idea presents its own problems, such as why other models suggest that a moon made mostly of disintegrated Earth rocks would have a vastly different orbit than the one we see today. 

To investigate different possible scenarios for moon formation following the collision, the new study's authors turned to a computer program called SPH With Inter-dependent Fine-grained Tasking (SWIFT), which is designed to closely simulate the complex and ever-changing web of gravitational and hydrodynamic forces that act upon large amounts of matter. Doing so accurately is no simple computational task, so the scientists used a supercomputer to run the program: a system nicknamed COSMA (short for "cosmology machine") at Durham University's Distributed Research Utilising Advanced Computing facility (DiRAC). 

By using COSMA to simulate hundreds of Earth-Theia collisions with different angles, spins and speeds, the lunar sleuths were able to model the aftermath of the astronomical crack-up at higher resolutions than ever before. Resolutions in these simulations are set by the number of particles the simulation uses. According to Kegerreis, for gigantic impacts the standard simulation resolution is usually between 100,000 and 1 million particles, but in the new study he and his fellow researchers were able to model up to 100 million particles.

"With a higher resolution we can study more detail — much like how a larger telescope lets you take higher resolution images of distant planets or galaxies to discover new details," Kegerreis said. 

"Secondly, perhaps even more importantly, using too low a resolution in a simulation can give you misleading or even simply wrong answers," he added. "You might imagine that if you build a model car out of toy blocks to simulate how the car might break in a crash, then if you use only a few dozen blocks, it might just split perfectly down the middle. But with a few thousand or million, then you might start to get it crumpling and breaking in a more realistic way." 

The higher-resolution simulation left the researchers with a moon which formed in a matter of hours from the ejected chunks of Earth and the shattered pieces of Theia, offering single-stage formation theory that provides a clean and elegant answer to the moon's visible properties, such as its wide, tilted orbit; its partially molten interior; and its thin crust. 

However, the researchers will have to examine rock and dust samples excavated from deep beneath the moon's surface — an objective of NASA's future Artemis missions — before they can confirm how mixed its mantle could be.

"Even more samples from the surface of the moon could be extremely helpful for making new and more confident discoveries about the moon's composition and evolution, which we can then trace back to model simulations like ours," Kegerreis said. "Missions and studies like these and many others steadily help us to rule out more possibilities and narrow in on the actual history of both the moon and Earth, and to learn more about how planets form throughout and beyond our solar system."

Such investigations could also shed light on how Earth took shape and became a life-harboring planet.

"The more we learn about how the Moon came to be, the more we discover about the evolution of our own Earth," study co-author Vincent Eke, an associate professor of Physics at Durham University, said in a statement. "Their histories are intertwined — and could be echoed in the stories of other planets changed by similar or very different collisions."