More than fifty years ago, Apollo 17 astronauts scooped up a small rock during their mission to the Moon.
That fragment, labeled sample 76535, has puzzled scientists ever since.
Despite forming nearly 50 kilometers beneath the lunar surface, the rock shows almost no evidence of the intense shocks that should occur when material from such depths is blasted upward.
For decades, researchers debated how it could have reached the surface in such a pristine state.
The leading idea was that the Moon’s largest impact crater, the South Pole–Aitken Basin, hurled it from the far side of the Moon to the Apollo 17 landing site.
But that theory had complications. Moving the rock across the lunar surface would have required at least one more impact event, and it would be almost impossible for it to travel so far without bearing scars of violent shock.
Now, new research from Lawrence Livermore National Laboratory offers a much simpler solution.
Led by planetary scientist Evan Bjonnes, the team used advanced computer simulations to revisit the question.
They discovered that another enormous collision—the one that created the Serenitatis Basin on the Moon’s near side—could have lifted the rock to the surface in a far less destructive way.
The key lies in what happens after a giant impact. According to Bjonnes and his colleagues, during the later “collapse” stage of crater formation, material from tens of kilometers down can be drawn upward.
Instead of being violently ejected, deep rocks can ride the rising crust more gently, preserving their internal structure.
In simulations, the Serenitatis impact moved material from depths similar to that of sample 76535 up to just a few kilometers from the surface.
From there, smaller processes could have placed the rock in the exact spot where Apollo astronauts eventually picked it up.
This new model not only solves the decades-old mystery of how the rock arrived at the surface, but it also reshapes the Moon’s timeline.
The research suggests the Serenitatis impact occurred about 4.25 billion years ago—around 300 million years earlier than previous estimates. That shift has profound consequences. If Serenitatis is older, other major lunar basins may also need to be redated.
The Moon plays a critical role in reconstructing Earth’s history. Because our planet’s oldest surface layers have been erased by tectonics and erosion, scientists often use the Moon as a reference to estimate the impact history of the entire inner solar system.
Moving Serenitatis back in time effectively recalibrates that history, suggesting that Earth and its neighboring planets endured heavy bombardment earlier than we thought.
“This rock may be small, but it carries a huge story about the Moon’s early history,” Bjonnes said. “It’s like a time capsule from 4.25 billion years ago.”
The study also highlights the lasting value of Apollo samples. Even after half a century, these rocks continue to reveal new secrets thanks to modern technology.
For Bjonnes, the research also offers a practical lesson for future explorers.
Astronauts studying the Moon’s large basins should look closely at unusual or “out-of-place” rocks on the surface. Similar deep-origin samples may be waiting to tell more stories about the earliest days of the Moon—and by extension, the Earth.
Source: Lawrence Livermore National Laboratory.
