British astronomers have a new theory for 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 the moon’s formation. On the contrary, the moon would have broken off as a whole in one fell swoop and been put into orbit around the earth as a result of that same collision.
To reach that conclusion, the astronomers simulated the moon’s formation about 4.5 billion years ago on a supercomputer with more detail than was previously possible. They published their results in The Astrophysical Journal Letters.
The composition of a moon formed according to the new theory is more consistent with measurements of the composition of moon rocks than the composition of a moon formed in the conventional way.
But Wim van Westrenen, 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 those elements in the moon is probably incorrect here. According to the new simulation, the composition of the inside of the moon must be very different from that of the outside. That’s not what we observe in lunar samples. We see that the young moon has been very hot and must therefore be well mixed.”
Tossed around the earth
The moon would have formed about 4.5 billion years ago when the young Earth collided with Theia. That’s a hypothetical smaller planet the size of Mars. According to the current idea, grit would have been thrown around the earth like a disk. This would have clumped together later. Within a few thousand years, the moon was formed.
If the moon did indeed form that 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 99 percent composed of Earth-derived material.
The English astronomers simulated the collision between Theia and Earth in higher resolution than previous models and came up with a new theory that better fits 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 grit first, the moon was snapped off on impact and kicked into orbit within hours. According to the simulation, such a moon would consist of 60 percent earth material.
Van Westrenen: “That is considerably more terrestrial material than in the old simulations, which corresponds better with the observations – but it is not yet the 99 percent that we need. Here, too, additional tricks are ultimately required to explain the difference between simulations and observations, just like with the old theory. However, it is exciting that with detailed simulations of the history of the moon we see different things than in old models. Yes.”