Cast your mind back to when the Earth was a baby. The solar system was a brutal nursery. Giant fragments of rock swirled chaotically around a fiery young sun, regularly bombarding small planets. The Earth was formed during this period, aptly named Hades, and without this steady rain of fire that builds the bones of our planet, we wouldn’t be here at all.
And neither does the moon.
Towards the end of this period, about 4.5 billion years ago, a Mars-sized protoplanet named Theia slammed into Earth in a collision that is believed to have released 100 million times more energy than the asteroid that ended the dinosaurs. The impact destroyed Theia, threw a titanic plume of material into orbit – and gave birth to our moon.
This giant impact scenario is the leading theory of how the moon formed because it fits much of what we observe about Earth and the Moon today. But scientists are still debating the details. Early simulations of the impact, for example, suggested that the moon would be made mostly of material from Theia, but analysis of lunar rocks shows that the geochemical composition of Earth and the Moon is nearly identical.
Now, however, a new high-resolution simulation, described in a recent paper by NASA Ames scientists and researchers at Durham University, may help resolve the discrepancy.
According to the paper, the results in a number of possible impact scenarios more closely match observations, including the moon’s orbit and composition. But perhaps most surprisingly, where previous work suggested that the moon would take months or years to form, the new simulation suggests that our satellite was formed and put into orbit in a matter of hours.
In the simulation, shown in the video below, Theia hits Earth with a glancing blow. An arc of material, originating from both the Divine and the Earth, enters orbit and forms two bodies. The largest of these, doomed to fall back to Earth, catapults the smallest, the moon, into a stable orbit. If the initial collision took place at midnight, the moon would have formed by morning.
This is not the first attempt to better fit our observations into the origin story of the moon’s giant impact.
Scientists have proposed and are simulating a number of theories to explain the moon’s geochemical composition. These include higher energy or multiple impacts, a hit and run, or the possibility of an earlier impact when the Earth was still covered by a magma ocean. These are still possible, though each has its own set of challenges.
Here, the team took a different approach, suggesting that perhaps the problem isn’t the theory but the simulation of it. Earlier simulations used hundreds of thousands or millions of “particles”—you can think of them as idealized digital stand-ins for pieces of Earth and Divine, each obeying the laws of physics upon collision. The latest simulation, on the other hand, uses hundreds of millions of particles, each about 8.5 miles (14 kilometers) wide.
It is the highest resolution digital representation of the formation of the Moon.
The analysis brought the mechanics of large impacts into focus in a way that before, less detailed simulations simply could not. And along the way, the work threw a new, potentially simpler theory into the hat: That the moon formed quickly, in a single step. The team found that this scenario could create a moon like ours, from orbit to composition.
But while the new project is enticing, further strengthening it will require more high-resolution simulations and, importantly, future missions that collect more samples from the moon itself.
Whatever scientists find, the moon’s formation story has far-reaching implications. Its fate is closely tied to Earth’s, from tides to plate tectonics and the rise and evolution of life itself. If we find that our moon is extreme—as it appears to be in our solar system, at least—perhaps the chances of life arising and surviving long distances elsewhere are slimmer. We just don’t know yet.
That’s why it’s important to create and study simulations like this.
“The more we learn about how the moon formed, the more we discover about the evolution of our own Earth,” Vincent Eke, a researcher at Durham University and co-author of the paper, said in a statement. “Their stories are intertwined – and could be repeated in the stories of other planets changed by similar or very different collisions.”
Image credit: NASA Ames Research Center