The Moon may be 40 million years older than previously believed (Credit: Public Domain)

About 4.5 billion years ago, a Mars-sized planet smashed into the newly-formed Earth. The collision ejected a large amount of planetary debris into space. Scientists believe that due to the energy from the impact, the debris was initially molten. It solidified over time as the material cooled and eventually became the Moon. However, how quickly that happened is up for debate.

Previous research estimated that the Moon formed about 4.42 billion years ago. However, a recent study asserts that our satellite is 4.46 billion years old. This is 40 million years older than previously thought.

Apollo 17 astronaut Harrison Schmitt collects moon rocks and rock chips with a lunar rake (Credit: Public Domain)

The team, led by Jennika Greer at the University of Glasgow, Scotland, arrived at this conclusion after studying lunar rock samples. They were brought back in 1972 as part of the Apollo 17 mission. It was the last time astronauts went to the Moon.

More specifically, the scientists sought out the zircon crystals present in the rocks. The mineral is believed to be the first to have crystallized when the Earth's molten debris cooled.

The researchers began by using ultraviolet lasers to evaporate atoms from the zircon crystals. The mineral contains radioactive uranium, which turns to lead over time. The process is called radioactive decay. The proportion of the uranium to lead atoms indicated that the zircon crystals were 4.46 billion years old. The scientists believe the mineral's age provides a more accurate estimate of when the Moon formed. The findings were published in the journal Geochemical Perspectives Letters in June 2023.

A lunar zircon grain under a microscope (Credit: Jennika Greer/ University of Glasgow/ CC-BY-SA-2.0)

"When the surface was molten like that, zircon crystals couldn't form and survive. So any crystals on the Moon's surface must have formed after this lunar magma ocean cooled," says study co-author Philipp Heck at the University of Chicago. "Because we know how old these crystals are, they serve as an anchor for the lunar chronology."