More than fifty years ago, Apollo 17 astronauts scooped up a small rock from the lunar surface, unaware that the fragment would carry a mystery big enough to puzzle scientists for decades. Cataloged as sample 76535, this unassuming piece of the Moon was unlike many others collected during humanity’s last mission to the lunar surface. It was forged nearly 50 kilometers beneath the crust, yet when researchers examined it, they found something unusual: the rock bore almost no scars from the violent forces that should have flung it upward.
That contradiction raised a profound question. How could a rock travel from deep underground to the surface without being shattered or reshaped by shock? For decades, the leading explanation pointed to one of the most catastrophic events in the Moon’s history—the impact that formed the enormous South Pole–Aitken Basin, the largest crater in the solar system. But that hypothesis carried its own complications, demanding multiple impacts and improbable journeys for the rock to end up at the Apollo 17 landing site.
Now, new research led by planetary scientist Evan Bjonnes at Lawrence Livermore National Laboratory suggests a more elegant answer. Using advanced simulations, his team has traced the rock’s story not to the far side of the Moon but to a much closer event—the creation of the Serenitatis Basin on the near side. Their findings not only solve a decades-old puzzle but also ripple outward, altering our understanding of the Moon’s timeline and even reshaping how we view Earth’s own ancient history.
A Time Capsule from the Early Solar System
Sample 76535 is more than just a fragment of lunar crust; it is a time capsule. By analyzing its chemistry and texture, scientists determined it crystallized around 4.25 billion years ago, during the infancy of the solar system. At that time, the Moon was still cooling, its surface scarred repeatedly by enormous impacts as stray debris from planetary formation swept through the inner solar system.
The absence of shock features on the sample suggested it had not been catapulted by a catastrophic, high-energy event. Instead, Bjonnes and his team proposed that the rock’s journey was far more subtle. In their computer models, the immense impact that carved out Serenitatis Basin triggered a process known as crater collapse. During this stage, deep layers of the crust can rebound upward, bringing material from tens of kilometers below to near the surface—without the destructive forces expected from an explosive excavation.
The scenario fits perfectly. Serenitatis is one of the largest and oldest visible basins on the Moon’s near side, and its formation could have placed deep-seated rocks like sample 76535 within reach of Apollo 17 astronauts billions of years later.
“This rock may be small, but it carries a huge story about the Moon’s early history,” Bjonnes explained. “It’s like holding a 4.25-billion-year-old diary entry from the Moon’s crust.”
Redrawing the Lunar Timeline
If the Serenitatis impact occurred 4.25 billion years ago, as this research indicates, it predates previous estimates by nearly 300 million years. That revision is not a small adjustment—it forces scientists to rethink the tempo of lunar bombardment and, by extension, the history of the entire inner solar system.
For decades, planetary scientists have used the Moon as a stand-in for Earth’s lost geological record. Unlike Earth, where plate tectonics, volcanism, and erosion erase ancient surfaces, the Moon has preserved its scars. Every crater, every basin is a window into the solar system’s violent youth. By dating lunar impacts, scientists build a timeline that extends to Earth, Mars, and Mercury.
Pushing Serenitatis further back in time suggests that the era of giant impacts may have peaked earlier than once believed. If so, Earth’s own surface—where life would eventually emerge—may have endured its most intense bombardment during an even earlier window. That shift changes how we imagine the planet’s environment when oceans were forming and the first conditions for habitability were coming into place.
“By pushing Serenitatis back in time, we’re shifting the entire timeline of when big impacts happened across the solar system,” Bjonnes said. “That has ripple effects for understanding Earth’s early environment too.”
Apollo’s Legacy, Still Alive
Perhaps the most remarkable part of this discovery is that it comes from a rock collected more than half a century ago. Apollo missions may feel like history, but their legacy continues to shape modern science. The samples stored carefully in laboratories still hold secrets waiting to be unlocked by new generations of scientists armed with tools undreamed of in the 1970s.
In this case, computer simulations of giant impacts offered the missing key. By digitally recreating how lunar basins form, Bjonnes and his team could watch in detail how rocks like sample 76535 might have been lifted upward. The results show that the Moon’s history can be written not only in stone but also in code.
“It’s amazing that more than half a century later, Apollo samples are still revealing brand-new insights,” Bjonnes reflected. “They continue to provide valuable new clues about the Moon’s past.”
Looking Ahead: Lessons for Future Exploration
The study carries practical implications for future lunar missions. As humanity prepares to return to the Moon with Artemis astronauts and robotic explorers, scientists are paying close attention to which regions may hold the most revealing samples. Bjonnes suggests that rocks appearing “out of place” on the surface could be particularly valuable. If crater collapse processes are widespread, many basins may have delivered deep crustal material upward, waiting to be discovered.
Collecting such samples could provide not just a richer picture of the Moon but also sharper insights into the evolution of Earth and the other rocky planets. The story of a single rock has already redrawn timelines stretching across the solar system. Future discoveries may do even more.
A Small Rock with a Vast Story
In the end, sample 76535 reminds us that even the smallest fragments can carry enormous meaning. A pebble collected half a century ago has whispered a story that spans billions of years, from the chaos of the early solar system to the laboratories of today. It shows us that history is not just written in grand events but also hidden in quiet survivors—rocks that traveled gently upward, preserved against all odds.
The Moon, silent and scarred, continues to teach us. And through its ancient rocks, it not only reveals its own past but also illuminates the story of Earth, reminding us that our histories are forever intertwined.
More information: Evan Bjonnes et al, Evidence for an Early Formation of Serenitatis Basin at 4.25 Ga Shifts Lunar Chronology, Geophysical Research Letters (2025). DOI: 10.1029/2025gl116654
