The moon formed at least 4.51 billion years ago, no more than 60 million years after the formation of the solar system, researchers report online January 11 in Science Advances. This update to the moon’s age is in line with some previous estimates (SN Online: 4/17/15), although some argue the moon formed 150 million to 200 million years after the solar system’s birth.
A precise age is important for understanding how Earth evolved and how the solar system behaved in its formative years, says study coauthor Melanie Barboni, a geologist at UCLA. “If we want to understand other solar systems,” she says, “the first thing we have to do is understand ours.”
A run-in between Earth and something roughly the size of Mars is thought to be responsible for the creation of the moon. To nail down when this happened, Barboni and colleagues examined fragments of the mineral zircon brought back from the moon by the Apollo 14 astronauts. Relative amounts of uranium and lead as well as abundances of hafnium isotopes and the element lutetium provided radioactive decay clocks that record when the early moon’s global ocean of magma solidified. Hafnium and lutetium help determine when a crust formed over the moon’s liquid mantle while the radioactive decay of uranium to lead pinpoints when the zircon crystallized.
Previous analysis of the same zircon fragments revealed a similar age (within 68 million years after the formation of the solar system), but came with larger uncertainties. New techniques for uranium-lead dating and for understanding how the bombardment of the lunar surface by cosmic rays alters hafnium led to the improved age estimate.