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Earth’s Moon is Made up of Smaller Moons

The Moon is made up of smaller moons having merged after colliding into each other, says a new study published in the journal Nature Geoscience.

Earth’s Last Moon

Next time you’re admiring the moon, remember that you might actually be looking at many smaller ones. Our satellite could be the product of mini moons that came together to form the final masterpiece that has inspired artists from all over the world throughout time. Or so goes a new theory put forward by a team of researchers, led by Weizmann Institute’s Professor Raluca Rufo; they believe that this moon of ours would not be our first at all, but rather, our last one.

Many Moons Ago…

Conventionally, in the world of science, the moon is regarded as a single piece of mass that was formed from one massive impact between Earth and a Mars-like planet (named Theia) back in the day. The new hypothesis, on the other hand, mentions many more collisions, and accordingly, proto-Earth – a name given to our planet in its early developmental stages – once had several moons.

From Moonlets to Big Moon

Proto-Earth got moulded through impacts from other heavenly bodies, events that would have contributed additional material to the planet—this would have continued until it became as big as it is now. These series of collisions allegedly resulted into disks of debris around it that would later aggregate to form moonlets, which would have then been pulled to each other to form the moon as we know it.

Graphic depicting the changes that resulted into the moon. Other bodies colliding into the Earth resulted in a disk of debris around the young planet; these pieces then came together to form mini moons (moonlets); the latter were ultimately attracted to each other to form one bigger moon. Photo credits: R. Rufo et al, via Nature Geoscience.

To test the plausibility of the existence of the moonlets, the team carried out 800 simulations of possible impacts on the ancient Earth, thereby coming up with a new model that supports their hypothesis.

“Our model suggests that the ancient Earth once hosted a series of moons, each one formed from a different collision with the proto-Earth,” says co-author Professor Hagai Perets. “It’s likely that such moonlets were later ejected, or collided with Earth or with each other to form bigger moons.”

Changing Orbits

The moonlets would come together, shifting from their original orbits due to their mutual gravitational attraction. It might be argued that two moons could not have been near each other because proximity is not possible due to the Earth’s tidal forces expected to push its moons outwards (a phenomenon that is currently happening, as our Moon has now been shown to migrate away from us by around 1 cm every year) such that a pre-existing moon would have been far away by the time another one forms. But, the researchers explaining the the gravity exerted by the moons would lead to their meeting, surpassing other forces. As co-author Professor Perets points out, “a previously formed moon could therefore already exist when another moon-forming giant impact occurs”.

“It’s likely that small moons formed through the process could cross orbits, collide and merge,” says Professor Rufo. “A long series of such moon-moon collisions could gradually build-up a bigger moon—the Moon we see today”.

Furthermore, the researchers’ model is in line with current scientific theories of Earth’s formation, thereby not discrediting the mini-moons-amalgamation theory. Rather, the latter could even account for the reasons why the moon’s chemical make-up bears close resemblance to the Earth’s. Other scientists, not involved in the study, think that Rufo’s hypothesis is plausible.

“I think this is a real contender in with the other moon-forming scenarios,” says planetary scientist Robin Canup from the Southwest Research Institute in Boulder, US. “This out-of-the-box idea isn’t any less probable – and it might be more probable – than the other existing scenarios.”


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