The remnants of a second moon that orbited the Earth billions of years ago may be splattered across the far side of our moon, scientists claim.
The two moons are believed to have been created at the same time and followed a similar path to the moon we’re familiar with today, but after tens of millions of years of peaceful co-existence, the two appear to have crunched together in a gentle collision that left the smaller, just a third of the size, spread across the larger like a cosmic pancake.
Researchers put forward the idea after computer simulations found that a collision with a second, sibling moon in Earth's early history might solve the longstanding puzzle of why the two faces of the moon differ so dramatically.
While the near side, which always faces the Earth, is low-lying and relatively flat, the far side is high and mountainous, with a crust tens of kilometres thicker.
The idea builds on what planetary scientists call the "big impact" model of the moon, in which a planet the size of Mars slammed into the Earth in the early days of the solar system and knocked out a vast shower of rocky debris, which later coalesced as the moon.
"The impact produced a disc of debris around the Earth and from this disc we got the moon, but there is no reason why only one moon would be formed," Martin Jutzi at the University of Bern in Switzerland told the Guardian.
Jutzi and his colleague, Erik Asphaug at the University of California in Santa Cruz, U.S. decided to simulate what might happen if a second moon was created from the rock and dust that fell into orbit around the Earth.
Computer models showed that a sister moon roughly 1,200km in diameter could have accompanied the larger moon around the Earth for tens of millions of years.
But as the two moons' orbit moved further away from Earth, the balance of forces became unstable and they collided.
A high-speed impact would have punched a giant crater into the moon and kicked a shower of rock into space, but if the two bodies met at less than three kilometres a second, the smaller moon would have splatted onto the surface of the larger and stayed there.
"A slower collision doesn't produce such intense shockwaves and causes much less damage than a high-velocity collision," Jutzi said.
"It's kind of a gentle collision that doesn't form a big crater. The smaller moon gets more or less pancaked onto the larger moon."
If Jutzi is right, the impact thickened the moon's crust on the far side, creating the highlands and forcing subsurface magma to the opposite side.
"It wouldn't matter where the impact happens, because after the collision, the moon would reorient itself so that the material left from the impact was on the far side," Jutzi said.