Pioneering scientists from Durham University’s Institute for Computational Cosmology used the most detailed supercomputer simulations yet to reveal an alternative explanation for the Moon’s origin, 4.5 billion years ago. It revealed that a giant impact between Earth and a Mars-sized body could immediately place a Moon-like body into orbit around Earth.
In their search for scenarios that could explain the present-day Earth-Moon system, the researchers simulated hundreds of different impacts at high resolution, varying the angle and speed of the collision as well as the masses and spins of the two colliding bodies. These calculations were performed using the SWIFT open-source simulation code, run on the DiRAC Memory Intensive service (“COSMA”), hosted by Durham University on behalf of the DiRAC High-Performance Computing facility.
The extra computational power revealed that lower-resolution simulations can miss out on crucial aspects of large-scale collisions. With high-resolution simulations, researchers can discover features that weren’t accessible in previous studies. Only the high-resolution simulations produced the Moon-like satellite, and the extra detail revealed how its outer layers contained more material originating from the Earth.
If much of the Moon formed immediately after the giant impact, then this could also mean that less became molten during formation than in the traditional theories where the Moon grew within a debris disk around Earth. Depending on the details of the subsequent solidification, these theories should predict different internal structures for the Moon.
Co-author of the study, Vincent Eke, said: “This formation route could help explain the similarity in isotopic composition between the lunar rocks returned by the Apollo astronauts and Earth’s mantle. There may also be observable consequences for the thickness of the lunar crust, which would allow us to pin down further the type of collision that took place.”
Moreover, they discovered that even when a satellite passes so close to the Earth that it might be expected to be torn apart by the “tidal forces” from Earth’s gravity, the satellite can actually survive. In fact, it can also be pushed onto a wider orbit, safe from future destruction.
A range of new possibilities
Jacob Kegerreis, lead researcher of the study, said: “This opens up a whole new range of possible starting places for the Moon’s evolution. We went into this project not knowing exactly what the outcomes of these very high-resolution simulations would be. So, on top of the big eye-opener that standard resolutions can give you wrong answers, it was extra exciting that the new results could include a tantalizingly Moon-like satellite in orbit.”
The Moon is thought to have formed after a collision between the young Earth and a Mars-sized object, called Theia, 4.5 billion years ago. Most theories construct the Moon by a gradual accumulation of the debris from this impact. However, this has been challenged by measurements of lunar rocks showing their composition is like that of Earth’s mantle, while the impact produces debris that mostly comes from Theia.
This immediate-satellite scenario opens up new possibilities for the initial lunar orbit as well as the predicted composition and internal structure of the Moon. This could help to explain unsolved mysteries like the Moon’s tilted orbit away from Earth’s equator; or could produce an early Moon that is not fully molten, which some scientists propose could be a better match for its thin crust.
The many upcoming lunar missions should reveal new clues about what kind of giant impact led to the Moon, which in turn will tell us about the history of Earth itself.
The research team included scientists at NASA Ames Research Centre and the University of Glasgow, UK, and their simulation findings have been published in the Astrophysical Journal Letters.
Reference: “Immediate Origin of the Moon as a Post-impact Satellite” by J. A. Kegerreis, S. Ruiz-Bonilla, V. R. Eke, R. J. Massey, T. D. Sandnes and L. F. A. Teodoro, 4 October 2022, Astrophysical Journal Letters.
The research was partly supported by a DiRAC Director’s Discretionary Time award and a Science and Technology Facilities Council (STFC) grant.