The moon's Procellarum basin—the dark part seen from Earth sometimes said to resemble a "man in the moon"—was likely formed when an asteroid the size of Austria slammed into the lunar surface some 3.9 million years ago, a team of Japanese scientists has concluded. The team examined mineral mapping and light reflection data from the Selene moon orbiters, and concluded that the basin contains a mineral that was probably generated when rocks melted when the roughly 187-mile-wide asteroid hit, the Asahi Shimbun reports.
"The latest study explains why the moon's two sides are so different," one professor says. "It helps unravel the mystery of the moon's history." The side of the moon facing Earth is full of flat dark "seas," while the dark side is thicker and more mountainous. (Read more moon stories.)