Remnants of ancient catastrophe show the moon is 40 million years older than we thought

The moon’s violent birth may have happened 40 million years earlier than scientists previously thought.

That finding is based on remnants of molten crystals recovered from the moon’s surface, according to a study published Monday in Geochemical Perspectives Letters.

These slivers of radioactive zircon are almost infinitesimally small — so tiny they must be evaluated atom by atom.

But they contain the record of an ancient cataclysm of planetary proportions, scientists from Chicago’s Field Museum wrote.

More than 4 billion years ago, a rogue planet the size of modern Mars slammed into the young Earth, ejecting untold billions of tons of debris into Earth’s orbit.

For all the destruction it unleashed, that collision made Earth as we know it possible, scientists wrote. As the moon condensed from the drifting clouds of dust and molten rock in Earth’s orbit, it began to stabilize the planet below.

For example, the gravitational pull of the moon ultimately gave us the tides, the 24-hour day and a more stable rotational axis, allowing both a consistent climate and cycle of day and night.

For geochemists, this makes the birth of the moon “an anchor point for so many questions about the Earth,” said lead author Jennika Greer, who is now at the University of Glasgow.

“When you know how old something is, you can better understand what has happened to it in its history,” Greer added.

The key to determining the moon’s age rests with tiny pieces of crystalline dust found amid samples brought back by the last crewed mission to the lunar surface.

Amid the dust brought down by members of the 1972 Apollo 17 mission, Field Museum scientists found microscopic bits of zircon — a material that condensed on the surface of the newly formed moon like sugar crystals from cooling water.

That condensation must have occurred at a specific stage in the moon’s early formation, explained senior author Philipp Heck, who runs the meteorite research program at the Field. 

Until the primordial, molten moon had cooled into its current form, “zircon crystals couldn’t form and survive. So any crystals on the Moon’s surface must have formed after this lunar magma ocean cooled,” Heck said.

Had the crystals formed any earlier, Heck noted, “they would have been melted and their chemical signatures would be erased.”

But while those crystals provide a qualitative trace of the moon’s earliest days, connecting that to a number is far more complicated.

Luckily, the zircon crystals themselves function as tiny clocks — or, as the scientists called them, hourglasses.

“In an hourglass, sand flows from one glass bulb to another, with the passage of time indicated by the accumulation of sand in the lower bulb,” Heck said.

Similarly, over time unstable radioactive elements like uranium decay into stable ones like lead — a process that occurs at a well-understood rate.

The Field team sharpened their lunar samples to an incredibly fine point using a focused ion beam microscope, which uses a stream of charged particles to take pictures of tiny objects — or change them.

“[It’s] almost like a very fancy pencil sharpener,” Greer said. “Then, we use UV lasers to evaporate atoms from the surface of that tip. The atoms travel through a mass spectrometer, and how fast they move tells us how heavy they are, which in turn tells us what they’re made of.”

By discovering the ratio of radioactive “parent” atoms to stable “daughter” atoms within those samples then gives scientists a highly accurate clock with which they can determine that those atoms began to separate at least 4.46 billion years ago — meaning that the moon must be at least that old.

That contrasts with previous findings that the moon was about 4.42 billion years old. The 40 million year difference is only a small fraction of the moon’s total age — a change of less than 1 percent. 

But it is more dramatic when compared to the age of the solar system when the moon — and its effects on the new Earth — began to form. The findings allowed scientists to date the moon’s formation to within the first 110 million years of the solar system’s existence.

“It’s amazing being able to have proof that the rock you’re holding is the oldest bit of the Moon we’ve found so far,” Greer said.

“When you know how old something is, you can better understand what has happened to it in its history,” she added.