Wild simulations of cosmic collisions show the Moon forming in just hours
It’s believed that the Moon formed billions of years ago, from debris from a cosmic collision with Earth. New high-resolution simulations not only illustrate the idea in stunning detail, but reveal that the Moon’s birth might have taken mere hours.
The leading hypothesis for the Moon’s origins says it appeared around 4.5 billion years ago, very soon after Earth itself formed. The inciting incident is said to have been a collision between our early homeworld and a hypothetical, Mars-sized protoplanet named Theia. This mammoth impact would have sent huge amounts of material from both worlds into orbit, where some of it congealed to form what we now know as the Moon.
This story explains some features of the Moon, including its mass and orbit, but leave some major plot holes that are hard to fill. For one, previous simulations of the impact and aftermath indicate that the Moon should be mostly composed of material from Theia, but isotopic studies of lunar rocks brought back by the Apollo missions show that it’s very similar to Earth material.
For the new study, scientists at NASA and the University of Durham used a supercomputer to run hundreds of high-resolution simulations of the impact, changing variables like impact angle, speeds, masses, planetary spins, and other factors. The end results are some enthralling animations of the impact that show new dynamics that other simulations had missed.
The video looks like a cosmic lava lamp, as two orange blobs approach and collide, spraying long strings of material out into orbit around the proto-Earth. Soon this ejected material forms two other blobs, a larger one closer to Earth and a smaller one farther out. The closer blob slowly falls back to Earth, but not before its gravity shunts the smaller blob outwards into a stable, wide orbit. And there you have it – a Moon is born.
The most intriguing implication of this simulation is that the whole process takes only a few hours. One day the Earth didn’t have a Moon, and the next it did. It was previously thought that debris from the crash would have taken months or years to accumulate into the Moon we know today.
The team says that this scenario shows that most of the material that formed the Moon originated from the Earth’s mantle, which explains the similar isotope composition. It also neatly explains a few other mysteries, including why the Moon’s orbit is tilted away from the Earth’s equator, and why it seems to have a relatively thin crust.
Of course, more study will be needed to find the model that fits best, and some of the strongest evidence would likely come from analyzing more Moon rocks taken from different parts of the surface, and deeper down in the crust. With humans poised to return as imminently as 2024, we might soon have more answers.
The research was published in the Astrophysical Journal Letters.