Our nearest neighbor, the moon, is still something of a mystery to us. For decades, scientists have wondered why it appears so lopsided, with dark volcanic plains on the near side (the side we see) and rugged, cratered mountains and a thicker crust on the far side. Now we might be closer to knowing why.

Analysis of lunar soil and rock brought back from the far side by China’s Chang’e-6 mission suggests that a massive impact long ago changed the moon’s interior.

The samples were collected from the South Pole-Aitken basin, a massive impact crater covering nearly one-quarter of the moon’s surface. Because it is so deep, researchers from the Chinese Academy of Sciences wanted to see whether the impact had reached the moon’s mantle and changed its chemistry.