How old is our Moon? Spoiler alert: it is not as young as it looks
The Moon is our closest neighbor in space and it is the only planetary body outside of Earth where humans have set foot. As a result, we know a lot about the Moon. We know that the Moon is less than a third of the size of Earth and that every 27 days, the Moon rotates around Earth and spins at the same time. This means that the same portion of the Moon is always facing Earth, and the side that isn’t is often referred to as the dark side of the moon.
Nevertheless, much about the Moon remains shrouded in mystery, including its age. So far, it has been estimated to be about 4.35 billion years ‘young’. However, according to a new study, the Moon may have been subject to a remelting event that reset the age of almost all lunar rocks and has tricked scientists into thinking our Earth’s constant companion is younger than it really is.
The Moon is our Earth’s loyal companion in space and a popular scientific object. Credit: Lorenzo Di Cola/Imago
How did the Moon form?
Our Earth formed at the same time as the other Solar System planets, around 4.54 billion years ago, but initially it was not accompanied by a Moon. According to the prevailing theory, the Moon formed after a Mars-sized planet called Theia smashed into young Earth. The collision ejected enormous amounts of magma that solidified and collapsed, ultimately creating our Moon.
While the collision theory of the Moon’s formation is the most popular (and with the greatest evidence), there are also competing theories including the idea that during Earth’s early existence, because of its fast rotation, some material broke away and began to orbit in space and eventually formed the Moon.
Calculating the age of the Moon
Scientist-astronaut Harrison H. Schmitt, Apollo 17 lunar module pilot, collects lunar rake samples. Samples like these have enabled scientists to work out how old the Moon is. Credit: NASA
Irrespective of the different theories surrounding its formation, scientists initially estimated the Moon’s age at around 4.35 billion years which means it came into existence at about 250 million years after the formation of our solar system. This estimate was based on analyses of lunar rock samples from NASA’s Apollo missions.
However, a recent reanalysis of Apollo samples has revealed tiny structures, called zircon crystals, within some of the rocks, which are much older than the rest of the sample material. One study in 2017 suggested that some of these crystals could be up to 4.51 billion years old which would make the Moon only around 30 million years younger than Earth. Another study in 2023 proved that other crystals date back to at least 4.46 billion years ago.
The great lunar ‘reset’
The latest study, published in December 2024, offers a way to explain the up to 150-million-year-discrepancy between the original estimate and the new findings. Computer modeling and analysis of previous research suggest the 4.35-billion-year-old rock samples collected during the Apollo missions may actually not date back to the Moon’s formation but instead to a later event in which the Moon temporarily heated up, causing its surface to melt and then crystallize.
A Moon rock known as Big Bertha was collected by the Apollo 14 crew. Weighing around 9 kg, it is one of the largest collected during the Apollo missions. Credit: NASA
In the new study, the scientists suggest the melting event could have been triggered by a ‘disorganization’ in the Moon’s orbit around Earth as the Moon got caught in a gravitational tug of war between our planet and the Sun. This disorganization probably happened when the Moon’s elliptical orbits began to get more distant from Earth—which is still happening today. As the Moon moves, it is squeezed and stretched by Earth’s gravity, resulting in what is known as tidal heating events—one of which most likely happened 4.35 billion years ago.
According to the scientists in this study, the extreme heating likely re-melted the moon’s surface, effectively ‘resetting all the clocks’ in lunar rocks and thus hiding the Moon’s real age.
The remelting event also explains why there are fewer impact craters on the Moon than would be expected because any craters from before the event would have been erased by the melting, effectively giving it a global facelift.
Conclusion
While the new study supports the idea that the Moon is older than previously thought, the Moon’s exact date of birth remains a mystery—and could still elude us for a while. The researchers estimate that the Moon could be anywhere between 4.43 billion and 4.53 billion years old.
Although the difference between the original estimate of 4.35 billion years and the new maximum estimate of 4.53 billion years may seem relatively small when it comes to timescales in the universe, nailing down what occurred in those chaotic early days of our Solar System is key to understanding how the planets in our cosmic neighborhood came into existence.
