For decades, scientists have pieced together the moon’s history using samples from its near side, collected during NASA’s Apollo missions. But then in May 2024, China’s Chang’e 6 mission touched down on the mysterious far side of the moon. By June, it returned to Earth with precious cargo: 1,935.3 grams of lunar material, the first-ever samples from the moon’s hidden half.

Now, a team of Chinese scientists has reported what they found after analyzing these samples. Their findings confirm what was suspected for a long time: the early moon was once entirely engulfed in a molten “magma ocean.” This global sea of liquid rock, which likely lasted for tens to hundreds of millions of years, cooled and crystallized to form the moon’s crust and mantle. 

A Molten Moon

The theory that a global magma ocean once coated the Moon in its early history is known as the lunar magma ocean (LMO) model. It describes how the debris from the impact of Theia (a Mars-sized planet that smashed into early Earth) coalesced to form the moon. The energy from this massive event caused the newly formed moon to be extremely hot with a molten surface.

As the magma ocean began to cool, minerals with different densities are thought to have crystallized out of the molten material, with denser minerals like olivine and pyroxene sinking to form the moon’s mantle, while lighter minerals, such anorthosite, floated to the surface and formed the moon’s crust.

The idea of a lunar magma ocean was first proposed after the Apollo missions, but it was based solely on samples from the moon’s near side.

“Without samples from the far side, it was like solving a puzzle with half the pieces missing,” said Liu Dunyi, a senior researcher at the Chinese Academy of Geological Sciences.

The Chang’e 6 samples have now filled in the gaps. The team analyzed basalt fragments from the far side and found striking similarities to low-titanium basalts from the near side. Both must have formed in a magma ocean, suggesting that this molten phase was a global phenomenon covering the entire moon.

But the samples collected at the South Pole-Aitken Basin also revealed something unexpected. The ratio of uranium to lead isotopes in the far-side basalts differed from those on the near side. This divergence, the researchers argue, is no coincidence. It points to a massive asteroid impact that reshaped the moon’s mantle around the Chang’e landing site billions of years ago.

A Fiery History

The South Pole-Aitken Basin, where Chang’e 6 landed, is no ordinary crater. It’s the largest and oldest impact basin in the inner solar system, stretching 2,500 kilometers wide and plunging 13 kilometers deep. Formed around 4.3 billion years ago, this colossal scar is a reminder of the violent chaos of the early solar system.

The impact of a huge asteroid likely altered the moon’s mantle, changing its chemical and physical properties. “In other words, the moon was once covered by a global magma ocean, but later bombardments of asteroids caused different evolution processes on the near and far sides,” explained Long Tao, a senior researcher on the team.

These insights are important. Earth’s surface has been reshaped by tectonic activity, erasing much of its early impact record, whereas the moon is like a geological museum, with a billion-year-old history still on display. “Studying the moon’s impact history helps us understand Earth’s own past, which has been obscured by tectonic activities,” Long added.

As scientists continue to analyze the Chang’e 6 samples, they may uncover even more secrets about the moon’s origins — and, by extension, the history of our solar system.

The findings appeared in the journal Science.