The Moon Is Older Than Previously Thought, Formation Occurred 4.51 Billion Years Ago

The moon was formed by a violent, head-on collision between the early Earth and a “planetary embryo” called Theia, a UCLA-led team of geochemists and colleagues reported in 2016.

The newest research would mean that the moon formed “only” about 60 million years after the birth of the solar system — an important point because it would provide critical information for astronomers and planetary scientists who seek to understand the early evolution of the Earth and our solar system.

That has been a difficult task, Barboni said, because “whatever was there before the giant impact has been erased.” While scientists cannot know what occurred before the collision with Theia, these findings are important because they will help scientists continue to piece together major events that followed it.

It’s usually difficult to determine the age of moon rocks because most of them contain a patchwork of fragments of multiple other rocks. But Barboni was able to analyze eight zircons in pristine condition. Specifically, she examined how the uranium they contained had decayed to lead (in a lab at Princeton University) and how the lutetium they contained had decayed to an element called hafnium (using a mass spectrometer at UCLA). The researchers analyzed those elements together to determine the moon’s age.

“Zircons are nature’s best clocks,” said Kevin McKeegan, a UCLA professor of geochemistry and cosmochemistry, and a co-author of the study. “They are the best mineral in preserving geological history and revealing where they originated.”

The Earth’s collision with Theia created a liquefied moon, which then solidified. Scientists believe most of the moon’s surface was covered with magma right after its formation. The uranium-lead measurements reveal when the zircons first appeared in the moon’s initial magma ocean, which later cooled down and formed the moon’s mantle and crust; the lutetium-hafnium measurements reveal when its magma formed, which happened earlier.

“Mélanie was very clever in figuring out the moon’s real age dates back to its pre-history before it solidified, not to its solidification,” said Edward Young, a UCLA professor of geochemistry and cosmochemistry and a co-author of the study.

Previous studies concluded the moon’s age based on moon rocks that had been contaminated by multiple collisions. McKeegan said those rocks indicated the date of some other events, “but not the age of the moon.”

The UCLA researchers are continuing to study zircons brought back by the Apollo astronauts to study the early history of the moon.

Stuart Wolpert