The Earth and moon have two Trojan points, one leading ahead of the moon, known as the L-4 point of the system, and one trailing behind, its L-5 point.
The researchers computed that this second moon could have stayed at a Trojan point for tens of millions of years. Eventually, however, this Trojan moon’s orbit would have destabilized once our moon’s orbit expanded far enough away from Earth.
The resulting collision would have been relatively slow at 4,500 to 6,700 miles per hour (7,200 to 10,800 kph), leading its matter to splatter itself across our moon as a thick extra layer of solid crust tens of miles thick instead of forming a crater.
A number of explanations have been proposed for the far side’s highlands, including one suggesting that gravitational forces were the culprits rather than an impact from Francis Nimmo at the University of California, Santa Cruz, and his colleagues. Nimmo said that for now there is not enough data to say which of the proposals offers the best explanation for this lunar contrast. “As further spacecraft data and, hopefully, lunar samples are obtained, which of these two hypotheses is more nearly correct will become clear,” Nimmo said in a statement.