Astronomers have long been puzzled by the differences between the side of the moon that always faces Earth—the near side—and the side that always faces away, the far side. The topography of the near side is relatively low and flat, while that of the far side is high and mountainous with a much thicker crust.
According to a new computer model, this discrepancy can be explained if a smaller “companion moon” collided with our moon’s far side early in its history. Such a collision would have left the far side splattered with especially hard rocky material that now forms the current lunar highlands.
For the theory to work, the smaller moon must have crashed into the larger one at about 4,400 miles (7,081 kilometers) an hour.
“This is the slowest possible collision the two massive bodies could have if they fell into each other’s gravity,” explained study co-author Erik Asphaug, a planetary scientist at the University of California, Santa Cruz (UCSC).
At this relatively slow speed, the far-side collision wouldn’t have been energetic enough to melt rock or carve out a crater. But it would have been forceful enough to plaster material from the smaller moon onto the larger moon.
“It’s like a car crash, where you have crumpled bumpers but you don’t melt the cars as they’re colliding,” Asphaug said. “This is the same kind of phenomenon.”