A titanic collision could have formed the moon within hours, new simulation shows

Scientists have been struggling for decades to answer a question an inquisitive child might ask: Where did the moon come from?

Now, the most high resolution computer simulation ever made shows it could have been formed in a matter of hours following a collision between a proto-Earth and a Mars-sized orbital intruder.

Researchers from NASA and the Institute for Computational Cosmology’s Planetary Giant Impact group and Durham University in the U.K. developed the simulation. 

Theories about the origin of the moon have varied greatly over time, spurred by the fact that the moon is an unusual planetary satellite. No other rocky planet in our solar system has such a large moon.

One earlier idea suggested the moon was formed elsewhere in the solar system, wandered close to the Earth and was captured by our planet’s gravity.

Another notion was that the Earth and moon peacefully formed together from the protoplanetary disk of dust and rocks that surrounded our Sun more than four billion years ago.

Then there is the fission idea, which proposes the primitive molten Earth was spinning so fast, part of its surface was flung off to become the moon. Some suggested the chunk that formed the moon came from what eventually became the Pacific Ocean.

Many of those early theories were cast into doubt over time. When the rocks brought back from the moon by the Apollo astronauts during the moon missions from 1969 to 1973 were analyzed, they were found to have a similar chemical and isotopic composition to the Earth. If the moon formed somewhere else in the solar system and was captured, it would most likely have a different composition.

The physics of gravitational capture and simultaneous formation proved to be unworkable with an object as large as our moon, and there is no evidence that the Earth was spinning super fast in the distant past.

The giant impact

This similarity between the composition of Earth and moon led to the giant impact model. This theory suggests that during the early days of planetary formation, a smaller proto-Earth was struck by another body, now referred to as Theia, that was roughly the size of Mars.

The collision would have been colossal.

The violent impact would have destroyed Theia and mixed material from both worlds was thrown out into space, some returning to Earth and some eventually becoming the moon. 

Earlier computer simulations showed how the debris temporarily turned the Earth into a ringed planet like Saturn. Over decades, or hundreds of years, material in the ring eventually coalesced into the moon. But exactly how that scenario played out had not been well understood. Now, this new, more detailed simulation shows the ring theory might be wrong and suggests the moon could have been born in a matter of hours.

WATCH: Video Simulation of the moon’s formation from NASA’s Ames Research Center

Several parameters were tested, such as the angle of attack, speed of the impact and whether Theia was spinning or not. The scenario this detailed model predicts that results in the moon as it is today, starts with Theia hitting the Earth with a glancing blow, blending together with the Earth.

After that, the “splash” from the impact sends a giant blob into orbit that wobbled like jelly, with a smaller blob breaking off. The larger piece was drawn back into the Earth by gravity, while the smaller was thrown far enough away to remain in orbit and become the moon.

It is hard to believe that the peaceful orb shining in our night skies had such a violent beginning. But that is the history of all planets and moons in our solar system as they banged and crashed into each other inside a giant primordial cloud of gas and dust.

Rings of dust and gas have been spotted around other stars in our galaxy, with evidence of collisions happening within. One can only imagine the violence taking place there and strange new worlds that might emerge.