For decades, scientists have puzzled over why the two sides of the moon look so different.
The “near side”, which faces Earth, is covered in dark, flat volcanic plains, while the “far side” is a rugged, mountainous landscape with little volcanic activity.
New research published on Tuesday in the journal Proceedings of the National Academy of Sciences suggests one possibility lies in a cataclysmic space crash billions of years ago that literally baked the far side of the moon, stripping away the ingredients needed for volcanoes to form.
This mystery is known to experts as the lunar dichotomy. To the naked eye, the side we see from Earth looks like a “man in the moon” because of its smooth lava flows, but the far side has a much thicker crust and very few of those smooth spots. To solve this mystery, researchers from the Institute of Geology and Geophysics of the Chinese Academy of Sciences analyzed tiny samples — about the weight of a few grains of salt — brought back by the Chang’e 6 mission. This was a historic mission because it collected the first-ever rocks from the moon’s far side, specifically from a massive crater called the South Pole-Aitken basin.
The team, led by professor Tian Hengci, looked at a specific chemical element called potassium. Potassium comes in different weights, known as isotopes. When something incredibly hot happens — like a massive asteroid hitting the moon — the lighter versions of potassium evaporate into space more easily than the heavier ones.
The researchers discovered that the samples from the far side were packed with heavy potassium isotopes, which act as a chemical fingerprint. It indicates that when a giant object slammed into the moon to create the South Pole-Aitken basin, the heat was so intense that it caused “volatile” elements — chemicals like potassium (lighter versions), zinc, and sulfur that turn into gas easily — to vaporize into space.
This loss of chemicals had a permanent effect on the moon’s geology because volatile elements help lower the melting point of rocks deep inside a planet. Without them, the moon’s interior stays stiff.
Because the far side lost some of these easy to melt elements during that ancient crash, it struggled to produce the magma needed for volcanic eruptions. This explains why the near side stayed volcanically active for longer, while the far side became a dormant, mountainous wasteland.
“The findings suggest that asteroid impacts do more than just leave a dent on the surface; they can fundamentally change the chemistry of a planet’s guts,” Tian said.
“While other theories suggest the moon’s unevenness was caused by Earth’s gravity or uneven radioactive heating, this new evidence highlights the profound influence of giant space crashes in shaping the worlds of our solar system,” he added.