Hovering alongside Earth for millennia, the moon has been a familiar sight in the night sky—a silent witness to the unfolding of human history. On August 1, 1971, some of that history was written on the moon itself, when Apollo 15 astronauts David Scott and James Irwin stumbled upon a remarkably ancient chunk of lunar crust that would provide crucial clues about the moon's origins.
"Guess what we just found," Scott radioed to mission control in Houston, Texas. "Guess what we just found! I think we found what we came for."
The Moon Started as a Chunk of Earth 4.5 Billion Years Ago
That sample, nicknamed the Genesis Rock, ended up overturning then-prevailing theories of the moon's formation—that it had been seized by Earth or birthed alongside it—while unveiling crucial evidence that pointed to a now widely accepted theory.
The giant impact hypothesis says that approximately 4.5 billion years ago, a Mars-sized object, often referred to as Theia, collided with a fledgling Earth. In doing so, it flung massive amounts of Earth's molten rock into space, which gradually coalesced under gravity to form the moon we recognize today.
The crash was so cataclysmic that it also corrected Earth's previously extreme axial tilt. That, in turn, helped regulate our planet's climate and tides—critical factors that set the stage for life's eventual emergence.
"It reset everything on Earth," says Philipp Heck, a researcher at the University of Chicago and the Field Museum of Natural History, who has studied ancient lunar samples. "If life had already started to evolve on Earth, it would have been completely reset by this giant impact."
Estimates of the Moon's True Age Have Shifted
The moon's formation is inextricably linked to Earth's early history, including pinpointing when its earliest forms of life emerged. But the exact timing of this pivotal event remains unclear. No rocks from this time have survived on Earth due to its active, surface-recycling geology.
Most of the lunar meteorites that fell to Earth were found to be roughly 4.35 billion years old. Nearly all of the 900 pounds of moon samples hauled back to Earth during the Apollo era were also dated to around the same age. They are believed to have crystallized from an ocean of magma on the moon after the giant impact.
In late 2023, a team of scientists including Heck dated telltale zircon crystals (highly durable minerals) in the oldest-yet Apollo sample to 4.51 billion years, pushing back previous estimates of the moon's age by 40 million years. Such a stretch of time is a blink of an eye in the 4.5 billion-year history of Earth and the moon, Heck says, "but it is a lot for the evolution of life—40 million years ago, humans didn't exist on this planet."
Studies of lunar rocks largely suggest the moon formed about 200 million years after the solar system began. However, simulations of the solar system's evolution predict that the moon must have formed tens of millions of years earlier still, because by 200 million years, much of the primordial material ferrying the potential to collide with Earth would have already been swept up into forming planets. This would have made the conditions for a giant impact improbable.
That the moon-forming impact occurred more than 150 million years after the solar system started “is very, very unlikely,” says Maxime Maurice, a researcher at the Dynamic Meteorology Laboratory in France.
The Moon Remelt Theory
A study published in December 2024 supports the earlier formation of the moon, while offering an explanation for the younger-dating moon rocks: Early gravitational interactions with Earth caused the moon to briefly heat up so much it partially remelted, producing new rocks. This volcanic facelift reset isotopic ages of older rocks, making them appear younger than they actually were. This could explain why nearly all Apollo samples are dated to 4.35 billion years.
"Our story is a way of allowing both groups to be right," says study lead author Francis Nimmo, who is a planetary scientist at the University of California, Santa Cruz. Such a melting event would also have buried or otherwise erased all ancient impact basins on the moon, Nimmo says, which could explain why there are fewer observed lunar basins than expected for an object its age.
Mysteries About the Moon Persist
A key challenge in determining the moon's true age is that much of it remains unstudied. That includes its pristine far side, which has been less affected by geological processes compared to the Earth-facing side and could help resolve lingering discrepancies in age estimates.
Lunar rocks returned to Earth by Apollo astronauts, including those from the last batch opened in 2019 to mark the 50th anniversary of the Apollo 11 moon landing mission, continue to offer valuable but limited snapshots of the moon's history.
Maurice, who previously attempted to pin down the moon's age, likens the effort to gradually scraping bits of dust off a dust-covered painting.
"At some point, we hope that we will get the whole picture," he says.
