A small rock fragment, weighing just a few ounces and collected by Apollo 17 astronauts more than 50 years ago, is forcing a major revision of the moon’s violent youth. The unassuming sample, known as 76535, has long puzzled scientists with its pristine condition despite an origin deep within the lunar crust. New research, leveraging powerful computer simulations, has now resolved this paradox, revealing a new history for the sample and pushing back the timeline for major impacts on the moon by 300 million years.
This revised timeline has significant implications not just for the moon, but for our understanding of the entire inner solar system’s early history. Because the Earth’s own geological record from that era has been erased by tectonic activity, scientists rely on the moon’s battered surface to calibrate the history of asteroid and comet bombardments. By re-dating a key lunar impact, the new study recalibrates our understanding of the conditions on an early Earth, potentially affecting theories about how and when life could have emerged. The findings, published in the journal Geophysical Research Letters, demonstrate the enduring scientific value of the samples returned by the Apollo missions a half-century ago.
A Decades-Old Lunar Puzzle
When Apollo 17 astronaut Harrison Schmitt collected sample 76535 in 1972, its importance was not immediately obvious. Chemical and textural analysis later revealed a startling biography: the rock had formed at a depth of nearly 50 kilometers, or over 31 miles, deep inside the lunar crust. What made this so perplexing was the rock’s condition. It showed almost no signs of the intense, violent shocks that would be expected from an event powerful enough to excavate material from such a depth and hurl it to the surface.
For decades, the leading theory proposed that only the most colossal of lunar impacts could have been responsible for dredging up the rock. Scientists largely credited its journey to the formation of the South Pole–Aitken Basin, the largest and one of the oldest impact craters on the moon. This massive feature, located on the lunar far side, seemed like the only candidate capable of such a deep excavation. However, the lack of shock damage in sample 76535 remained a nagging inconsistency in this explanation, a mystery waiting for better tools to solve.
Simulations Reveal a New Origin
That solution arrived in the form of advanced computer simulations run by a team led by planetary scientist Evan Bjonnes of Lawrence Livermore National Laboratory. The researchers modeled various large impact events, and their results pointed to a different culprit: the impact that formed the Serenitatis Basin, a large, well-known feature on the near side of the moon. The models showed that this impact could have brought the deep-crust sample to the surface through a surprisingly gentle mechanism.
The key, according to the simulations, lies in the later stages of crater formation. After the initial violent impact, the transient crater collapses, a process that can draw material from deep below and lift it toward the surface without subjecting it to the extreme pressures of the initial blast. This “rebound” effect could have transported sample 76535 kilometers upward, eventually placing it near the location where the Apollo 17 crew would later find it, far from the impact site itself.
Recalibrating Solar System History
Pinpointing the Serenitatis impact as the origin of sample 76535 has done more than just solve a long-standing geological puzzle; it has reset a critical clock in planetary science. The new research dates the Serenitatis event to approximately 4.25 billion years ago, which is about 300 million years earlier than previous estimates. This single adjustment creates a domino effect, pushing back the entire timeline of heavy bombardment in the moon’s early history.
This revised lunar chronology directly impacts our understanding of early Earth. “By pushing Serenitatis back in time, we’re shifting the entire timeline of when big impacts happened across the solar system,” Bjonnes explained. “That has ripple effects for understanding Earth’s early environment too.” Since the moon serves as our primary record for the bombardment that both it and Earth endured, this new date suggests a different tempo and timing for the massive impacts that shaped our own planet’s formative years.
Insights from Lunar Zircons
Further complicating and enriching the story of the early moon is separate, recent research focusing on a different mineral from the Apollo samples: zircon. Scientists studying zircons from samples collected during the Apollo 14, 15, and 17 missions have gained new insights into the moon’s primordial magma ocean. The early moon is thought to have been covered in a global sea of molten rock, and these zircon crystals hold clues to how long that ocean took to cool and solidify.
The analysis of these zircons suggests the magma ocean may have been short-lived. More significantly, this research points to a major cataclysmic impact that occurred about 4.33 billion years ago, remelting a portion of the already solid lunar surface. Many researchers believe this event was the formation of the South Pole-Aitken basin. Together, the zircon data and the new analysis of sample 76535 paint a complex picture of a chaotic early moon, where multiple enormous impacts were reshaping the surface in a relatively short period.
The Enduring Legacy of Apollo
The continued revelations from lunar samples highlight the incredible foresight of the Apollo program. “It’s amazing that more than half a century later, Apollo samples are still revealing brand-new insights,” Bjonnes stated. “They continue to provide valuable new clues about the Moon’s past.” In total, the six Apollo landing missions brought back 842 pounds of lunar material, a scientific collection whose value only grows as analytical techniques become more sophisticated.
This research also provides practical guidance for future lunar exploration, including NASA’s Artemis program. Bjonnes suggested that astronauts should be trained to look for rocks that appear “out of place” on the surface. As the crater collapse mechanism could have lifted deep crustal rocks at numerous basins across the moon, such geologically unusual samples could be relatively accessible. These rocks, like their famous predecessor 76535, could hold the keys to filling in the remaining gaps in the moon’s—and Earth’s—earliest history.