It’s only in the past 50 years that scientists have had a peek at the whole of the moon. Because the moon doesn’t spin on its axis, the harvest moon that we saw and sang about (or barked about, in some cases), was the exact same moon, showing the exact same face and the exact same features. Then the space program began orbiting the moon, and researchers discovered that the terrain on the dark side was very different from what human have seen for hundreds of thousands of years.
The moon as we know it is a series of low, lava-lined planes pockmarked by craters. The far side is covered by rugged, mountainous highlands. Why is that? scientists wondered. What could have caused such a sharp disparity between the two sides of a single body in space?
Erik Asphaug, a planetary scientist at UCSC, believes he has an answer. About 4 billion years ago, long before people started staring at the moon—long before there was even life on this planet—he posits that the Earth had two moons: the moon as we know it and a second, smaller moon that tailed behind it, following a similar orbit. He believes that the larger moon’s gravity pulled the smaller moon in, and the two collided. “At a low enough impact speed,” he explains, “you actually deposit material.” Explaining the phenomenon in more terrestrial terms, he says it’s “like you literally threw a cow pie on the ground and there it is.”
The theory also explains other lunar oddities, like the relative abundance of rocks infused with KREEP—potassium (K), rare earth elements (REE) and phosphorus (P)—on the near side of the moon despite their dearth on the far side. “The momentum of that impact squashes the KREEP onto the other hemisphere,” he says.
Today, there’s a moon in the sky called the moon, but if Asphaug and his collaborator Martin Jutzi of the University of Bern in Switzerland are correct, long ago there may have been two.
Read more at Scientific American and Nature.
