How did the Moon form? The most popular theory is that a Mars-sized object called Theia collided with a young planet Earth about 4.31 billion years ago. The impact sent a massive amount of debris into space, which eventually recombined into the Moon. The hole in both the Earth and this argument is that in 1969, and for a few years after, astronauts brought back moon rocks that were analyzed and found to be nearly identical to Earth rocks. There was nothing in them that indicated the existence of a Earth-bashing, Moon-forming Theia. If you believe that the moon missions really happened, do you believe this proves the Theia thing didn’t?
According to a new study in Nature Geoscience, researchers in Israel developed a simulation model that replaced one big impact with many smaller ones smashing into Earth over millions of years. The model showed those collisions kicking out small amounts of debris that fused into mini-moons or “moonlets.” About 20 of these moonlets eventually moved into an orbit far enough away from Earth that allowed their own gravitational forces to pull them together into a fusion that became the satellite we know today.
This sounds plausible, but wouldn’t this mean rocks from different areas of the Moon would have different compositions depending on where and when they were knocked off the face of the Earth? Gareth Collins, a planetary scientist at Imperial College London who reviewed the study, explains why the isotopic signatures (material compositions) of the Moon are the same as Earth’s no matter where a sample come from.
The Moon becomes a blend of multiple compositional signatures, rather than two, and so the effect of each impactor on bulk composition is reduced. It is rather like mixing colours: the more distinct colours you add, the less change each new one makes until the result is dark brown.
The model shows the multiple impact theory creating the Moon over ten percent of the time, while the Theia impact only happens in 1 to 2 percent of the simulations. While that sounds conclusive, it doesn’t close the case on the mysterious origin of the Moon. That, the researchers agree, would require trips back to the Moon for more samples from more areas and from deeper digs into the lunar surface.
China’s unmanned Chang’e 5 mission in 2017 and Chang’e 6 mission in 20201 are the only ones scheduled to visit the Moon and return with samples. Is becoming the first nation to prove the origin of the Moon enough of an incentive to stimulate a new space race?