Where did the moon come from?

Earth was in a massive collision with a giant object about the size of Mars about 4.5 billion years ago. The theory is that one of the massive chunks of debris ended up sticking around which is how the moon was born.


There are still unanswered questions remaining surrounding the creation of the moon.

These include how big the impactor was, what speed and angle it was travelling before it hit Earth and whether the collision happened with one or several other bodies.

A team of researchers armed with a supercomputer has come up with new tools for scientists to investigate the consequences of giant impacts and the new insights could shed light on the creation of the moon.

Jacob Kegerreis, a researcher, said, “We think that these giant impacts of proto-planets colliding are a common path of planet formation in our solar system. And one piece of the puzzle that is interesting to look at, is how much atmosphere these collisions directly remove.”

Studies have already shown that in the collision that is thought to have created the moon the earth lost between 10% and 60% of its atmosphere. The how and why is less understood, because so far simulations of the impact have failed to account for the many parameters that determine atmospheric erosion during a collision.


Kegerreis and his team set out with the objective to find out how atmospheres behave when rocky planets undergo a huge collision such as the one that resulted in the formation of the moon.

“A few different projects have touched on this in the past, but they did not explore the huge range of possible parameters. Giant impacts can happen at different angles, speeds, masses and so on. We were trying to explore that whole parameter space, and the diversity of impacts, to understand how much atmosphere these collisions directly remove,” Kegerreis said.


The researchers ran about 300 simulations of different giant impacts, altering parameters like speed, angle, impactor mass and composition to study the consequences of different types of collisions.

The test scenarios included masses three times the earth’s mass down to a few percent of the planet’s mass, head on and grazing angles and speeds ranging from 10 to 30 km/s.


The simulations were run on an open-source simulation code and carried out on the COSMA supercomputer at the University of Durham that provides high-performance computing for science and technology research in the UK.

Kegerreis said that the compute power enabled the team to effectively run 3D high-resolution scenarios of different colliding planets at unprecedented speeds.

“Once we came up with a list of 300 or so scenarios that we thought were most interesting, it was basically a matter of equation-solving. Nothing stops us from doing this by hand, of course, but that would have taken an incredibly long time,” Kegerreis said.


The results showed the various effects that a giant impact can have on a planet’s atmosphere based on the different factors that the researchers input in each of their scenarios. The researchers came up with a series of correlations that could be used by scientists to validate the hypotheses they may have about the giant impact that formed the satellite planet.

Kegerreis said, “The moon-forming impact has been studied a lot, and there are five or six different plausible theories about it. We have not found out exactly how the moon was formed, but thanks to the wide range of scenarios we looked at, we got a solid handle on the effect of the parameters that were at play in forming the moon.”


According to the study, what is known as the canonical moon-forming impact whereby the earth would have collided with a Mars-sized, low velocity and oblique impactor would have resulted in a loss of 10% of atmosphere. A more head-on impact at slightly greater speeds on the other hand would have removed much more atmosphere from the earth.

There were also other correlations that came out of the study that were previously unknown beyond the moon-forming impact. Giant impacts between young planets and massive objects were shown to potentially add significant atmosphere to a planet if the impactor also has a lot of atmosphere.

In some cases, changing even only one of the variables led to the complete obliteration of the impacted planet.

Kegeirris and his team used the results from the study to develop a new prediction tool called a scaling law which can anticipate the atmospheric loss that can result from different giant impacts. The scaling law can be applied to any type of collision that involves planets with Earth’s like thin atmospheres.

The researchers hope that the tool will speed up research in the field of giant impacts and how those huge collisions can have lasting effects on the later stages of planet formation. Kegeirris is excited at the prospect of applying the technology to study the evolution of exo-planet atmospheres.

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