The moon is a chunk – not a clumped ball of sand

British astronomers have a new theory about the origin of the moon. According to the new theory, the moon did not form from clumping debris left over from a violent collision between Earth and a smaller planet, the common idea for moon formation. On the contrary, the moon would have detached at a single blow and would have been put into orbit around the earth following this same collision.

To reach this conclusion, astronomers simulated the formation of the moon about 4.5 billion years ago on a supercomputer in greater detail than before. They published their results in Letters from the Astrophysical Journal.

The composition of a moon formed according to the new theory is more consistent with measurements of the composition of lunar rocks than the composition of a conventionally formed moon.

But Wim van Westrenen, a moon researcher at the Free University of Amsterdam and not involved in the study, has reservations about the new theory. “Although the chemical composition is now more similar, the distribution of these elements in the moon is probably incorrect here. According to the new simulation, the composition of the interior of the moon must be very different from that of the exterior. This is not what we observe in the lunar samples.We see that the young moon has been very hot and therefore must be well mixed.

Tossed around the earth

The moon is believed to have formed about 4.5 billion years ago when young Earth collided with Theia. It is a hypothetical small planet the size of Mars. According to the common idea, the sand would have been projected around the earth like a disc. It would have clumped together later. In a few thousand years, the moon was formed.

If the moon did indeed form this way, the moon should have been made up of much more than half of Theia’s material, computer simulations show. But from lunar samples brought back to Earth by astronauts from the Apollo missions, it appears that the moon appears to be made up of 99% material from Earth.

The English astronomers simulated the collision between Theia and Earth with higher resolution than previous models and came up with a new theory that better matches the observed composition. According to the new theory, there was also a collision between Theia and Earth 4.5 billion years ago. But instead of creating sand first, the moon was snapped open on impact and orbited within hours. According to the simulation, such a moon would be made up of 60% terrestrial material.

Van Westrenen: “That’s a lot more ground material than in the old simulations, which matches the observations better – but it’s still not the 99% we need. Here too, additional tricks are eventually needed to explain the difference between simulations and observations, just like with the old theory. However, it is exciting to see that with detailed simulations of the moon’s history, we are seeing things that are different from those of older models. That’s to say.”