This new idea, reported in the journal Nature, could explain a long standing puzzle about the differences between the near and far sides of the lunar surface.
The near side is relatively low and flat with many large dark basalt mare, while the far side is high and mountainous, with thicker crust.
The work, based on computer simulations undertaken by planetary scientists Erik Asphaug and Martin Jutzi from the University of California, Santa Cruz, claims the lunar far side highlands, are the solid remains of a collision with a smaller companion moon.
It builds on the giant impact model for the origin of the moon, in which a Mars-sized planet called Theia collided with the proto-Earth early in the solar system’s history. The impact ejected debris into Earth orbit, which eventually coalesced to form the moon.
The new work by Asphaug and Jutzi suggests this giant impact also created another, smaller body, initially sharing an orbit with the moon. Eventually they collided, with the smaller one coating one side of the Moon with an extra layer of solid crust tens of miles thick.
The researchers hypothesize that the companion moon was initially trapped in a gravitationally stable “Trojan point.”
It became destabilized after the moon’s orbit expanded away from the Earth, something it’s still doing today at a rate of about three centimeters (1.8 inches) per year.
Asphaug and Jotzi believe the impact would have made the moon lopsided and reoriented so one side faces Earth.
Their model also shows the impact squishing a molten subsurface layer over to the opposite (Earth-facing) side of the moon.
Sarah Maddison of Melbourne’s Swinburne University said, “While it’s not proof that this is what’s happened, from their models, they seem to explain quite a few things including the dichotomy in the composition of the moon’s crust.”
Maddison says the Trojan orbit also makes the idea of a low-speed impact feasible.
“Because the two moons are both in the same orbit around Earth, they’re traveling at similar speeds,” she said.
According to Maddison, last week’s discovery of the first Trojan asteroid orbiting with the Earth around the sun helps strengthen the idea.