The Moon is much older than we thought  

A crescent moon over mountains.

The Moon’s place in our sky is a given. But, our understanding of when it began to illuminate the night may have been incomplete. Image credit: Benjamin Voros via Unsplash.


Though we know that the Moon was formed not long after the 4.54-billion-year-old Earth the precise time of its origin is a subject of constant research. A new study suggests the Moon may have actually formed 4.46 billion years ago, 40 million years earlier than we thought. 

The leading theory for the Moon’s formation is the “giant impact hypothesis”. It suggests that Theia, a Mars-sized planet, collided with a young, growing Earth 4.5 billion years ago. This launched huge cloud of debris and molten rock into space, which coalesced through gravity and formed the Moon. At this stage, its surface was completely molten, and is referred to as the Lunar Magma Ocean.  

The leading theory for the Moon’s formation is the “giant impact hypothesis”. It suggests that Theia, a Mars-sized planet, collided with a young, growing Earth 4.5 billion years ago.

As the Moon’s magma surface cooled and solidified over time, crystals called zircons formed. They are chemically inert and made of silicon, and have been able to survive on the Moon for billions of years. They are the oldest known solids formed after the Giant Impact. 

It was these zircon crystals that were brought back to Earth in lunar rock samples by the Apollo 17 mission in 1972. Our understanding of the crystal has allowed us to analyse these samples to determine their age and date the Moon’s earliest moments. 

When forming, the zircon mineral incorporates uranium atoms into its crystal structure but excludes lead. Over time, since uranium is radioactive, it will decay into lead at a well-characterised rate, such that older zircon crystal samples contain more lead. The ratio of lead to uranium in a sample can thus be used to determine the sample’s age.  

The new study, published in the journal Geochemical Perspectives Letters, re-analysed a lunar rock sample using a different technique. Led by Jennika Greer, a global team of scientists working with the lab of Philipp Heck at the University of Chicago used Atom Probe Tomography (APT), which uses a beam of charged particles to sharpen a piece of the lunar sample into a point using a focused ion beam microscope. UV lasers are then used to evaporate atoms from the surface of the point, which are then accelerated into a mass spectrometer. How fast the atoms move in the spectrometer tells us how heavy they are, indicating what they’re made of, and enabled the researchers to determine the sample’s uranium-to-lead ratio.  

APT also allowed the researchers to closely look at the makeup and position of the lead atoms in the sample, which demonstrates whether they resulted from radioactive decay or were just there by chance. This has allowed them to improve upon a 2021 study, where mass spectrometry was also used but couldn’t differentiate between these two types of lead sample involved.  

This sample was dated at 4.46 billion years old. The Moon must be at least as old as this zircon, so this data outlines its minimum age. This means we can suggest a narrower age range of the Moon, of around 50 million years, when the Moon would have formed and solidified. Heck even suggests that some lunar samples  from the Apollo 17 mission could be older than this sample, meaning we may be able to anchor the Moon’s formation more confidently in time.  

Heck even suggests that some lunar samples  from the Apollo 17 mission could be older than this sample, meaning we may be able to anchor the Moon’s formation more confidently in time. 

What do these discoveries mean? 

Life on Earth is unequivocally tied to the Moon, as the planet and its satellite have developed together. The theorised Giant Impact caused the complete reshaping of Earth’s surface, and the early Moon likely exhibited intense gravitational effects on Earth. Additionally, this collision likely stabilised Earth’s self-rotational axis, which is required to maintain stable climatic conditions, providing a more favourable environment for the evolution and continuation of life. 

Just as the gravitational force from the Moon helped shape Earth as we see it today, so did life itself. Internal heat from Earth’s initial formation powered volcanos, which released gases into Earth’s atmosphere. Primitive life developed in these extreme conditions, but through their slow evolution, they began to transform Earth’s surface into something more familiar. Oxygen produced by the very first photosynthesisers increased over time, allowing for the evolution of more oxygen-dependent organisms. This turned Earth’s pre-historic, more toxic atmosphere into something more hospitable for the life of today. With these stable climatic conditions, more and more complex life was able to evolve, shaping the Earth, just as heat and gravity—and the Moon—did in its early history. 

The Moon influences Earth even today, for instance, controlling our tides. This occurs since the gravitational pull of the Moon creates tidal bulges on Earth, where areas of water on Earth become raised. The movement of Earth beneath these tidal bulges, as it rotates on its axis, causes sea levels to rise and fall. These tides have proved important in the evolution of life on Earth, by forming dynamic environments, facilitating the mixing of nutrients, and enabling the establishment of ecosystems.  

‘When you know how old something is, you can better understand what has happened to it in its history.’

Jennika Greer

Jennika Greer, lead author of this new study, argued that ‘when you know how old something is, you can better understand what has happened to it in its history’. Furthermore, it might also help us to understand Earth better. Getting to a more accurate time of the Moon’s formation means we can better approximate the timing of events during the formation of early Earth. As well as helping us understand how other moons might form around planets beyond our Solar System, it might equally allow us to better understand this planet we call home.