Why the Moon's New Birthday Means the Earth Is Older Than We Thought

The challenge of estimating the Earth’s age has forced geochemists to rely on some more creative methods.

Jun 13 2014, 2:20pm
Image: Ralph Bernstein/NASA

You're likely familiar with the theory of how the Moon formed: a stray body smashed into our young Earth, heating the planet and flinging debris into its orbit. That debris coalesced and formed the Moon.

The impact theory still holds, but a team of geochemists from the University of Lorraine in Nancy, France has refined the date, finding that the Moon is about 60 million years older than we thought. As it turns out, that also means the Earth is 60 million years older than previously thought, which is a particularly cool finding considering just how hard it is to estimate the age of our planet.

There aren’t easily accessible ancient rock layers dating back to the Earth’s early formation that field geologists can walk up to observe. The challenge of estimating the Earth’s age has forced geochemists to rely on some more creative methods.

One method has been to study and measure the changes in the proportions of different gases left over from the Earth’s earliest days. Knowing the rate at which specific isotopes decay, scientists can work backwards to determine the age of the Earth.

One of the gases scientists use as a benchmark when discussing the age of the Earth is xenon, a noble gas known for having a large number of isotopes. French researchers Guillaume Avice and Bernard Marty analyzed xenon gas locked in 3.4 billion year old quartz stone found in South Africa and 2.7 billion year old quartz stone in Australia. 

The composition of gases changed throughout the Earth’s history, so differences in isotopes correspond to different phases of and major events in the Earth’s life. They’re basically gaseous time capsules. Xenon gas is a marker of the Earth’s atmosphere’s formation, a time in the planet’s history researchers believe this other body smashed into it, forming the Moon.

By comparing the current isotopic ratios of xenon with that locked in the rocks, Avice and Marty used this the ancient gas residue data to refine dating techniques.

Previously, the Earth' s atmosphere was estimated to have formed about 100 million years after the solar system’s formation. But because scientists don’t think the atmosphere would have survived the Moon-creating impact, the Moon and the atmosphere are assumed to be roughly the same age. According to the latest xenon study results, the impact, and therefore the Earth’s atmosphere’s formation, happened about 40 million years after the solar system formed, making both the Earth and Moon 60 million years older than we thought.

But even this result isn’t perfect. As Avice said, "it is not possible to give an exact date for the formation of the Earth.” But it’s a good way to estimate, and this latest work may be inexact but it’s getting closer to the real number.

And remember, sixty million years might sound like a huge difference, but we have to keep in mind that the age of the Universe, which scientists currently believe to be about 13.8 billion years old.

Still, the ramifications of this finding are far reaching. “These differences set time boundaries on how the planets evolved, especially through the major collisions in deep time which shaped the solar system,” said Avice.