Tiny crystals extracted from lunar dust brought to Earth by Apollo 17 astronauts over 50 years ago have revealed that the moon is around 40 million years older than believed until now, scientists announced on Monday.

The scientists’ study that involved atom-by-atom analysis of the crystals using a technique called atom probe tomography has pushed back the age of the moon by 40 million years to 4.46 billion years.

Scientists have known for more than three decades that the moon was formed when a Mars-sized object collided with Earth during the early stages of the solar system’s formation. The moon is the largest chunk of Earth that tore away in the collision. But when exactly this happened has remained unclear.

“These crystals are the oldest known solids that formed after the giant impact,” said Philipp Heck — a specialist in cosmochemistry and planetary sciences at the Field Museum and professor at the University of Chicago, US — who led the study.

When the Mars-sized object hit the Earth and formed the moon, heat from the impact would have melted the rock that eventually cooled to become the moon’s surface. “When the surface was molten, (these) crystals couldn’t form and survive. Any crystals on the moon’s surface must have formed after this lunar magma ocean cooled,” Heck said in a media release.

The atom probe tomography analysis showed that many of the atoms inside the crystals had undergone radioactive decay, a natural process.