The theory would explain why the far side of the Moon has a thick, mountainous crust while the near side is flat – a question physicists have never been able to agree on.
It could also solve the riddle of why the side of the Moon facing Earth is rich in minerals like potassium and phosphorus, and the other side is not, researchers said.
Dr Martin Jutzi from the University of Bern in Switzerland, who led the research, said it is likely that both moons were created at the same time, when an object the size of Mars crashed into Earth and broke up.
A computer model suggests that after about 50 million years the smaller moon would have hit our Moon, which was about three times larger.
Because of the slow speed at which the collision happened, the smaller moon would have broken up and "splatted" itself against our Moon's crust in mountainous piles rather than leaving a crater.
The clouds of rock would have settled onto the far hemisphere regardless of where the impact happened because the moon is not perfectly round, according to the study published in the Nature journal.
This process, known as accretion, would have pushed the Moon's sea of magma across to the side facing Earth, helping to explain why it is richer in minerals.
Prof Erik Asphaug, of the University of California, Santa Cruz (UCSC), who co-authored the study, said: "Impact modellers try to explain everything with collisions. In this case, it requires an odd collision: being slow, it does not form a crater, but splats material onto one side."
Dr Jutzi added: "The little moon falls down on the big Moon but the speed is low because the gravity of the Moon is not too strong. Most of the small moon gets accreted, and a very small part gets re-ejected and is probably still in orbit around the moon."
Last year Prof Francis Nimmo, also of UCSC, suggested an alternate theory that tidal forces were responsible for the formation of highlands on the Moon's far hemisphere.
Read more at The Telegraph
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