To the Moon and Back

06.15.14

The Moon’s Been Lying About Its Age

New research shows Earth and the Moon are older than we thought—to the tune of 60 million years. But is it really all that much?

Earth and the Moon are just a tiny bit older than we thought, according to a new study announced at the Goldschmidt Geochemistry Conference in Sacramento, California, last week. Just a smidge more ancient. Specifically, Earth is 60 million years older, give or take 20 million.

Sixty million years is a pretty long span of time… from our point of view, at least. After all, modern humans evolved less than a million years ago, and the last of the non-avian dinosaurs (since birds are dinosaurs, they ain’t all extinct) died out 65 million years ago. However, 60 million years is only about 1 percent of the age of the solar system: a measurable amount, but not a huge difference in the scheme of things.

To put it another way: before this recalibration, scientists estimated that Earth finished forming about 100 million years after the solar system was born. The new data suggest the Earth-shattering kaboom that made the Moon happened between 20 million and 60 million years after the oldest rocks in the solar system formed. 

If it’s such a slight difference compared with the 4.5 billion years of Earth’s history, those 60 million years might not seem that important. However, a lot can happen in that amount of time—and an older Earth compresses the amount of time for it to form and cool down, making our planet’s earliest years an even more interesting time.

We may never find the absolute age of Earth, but between the age of the oldest rocks and the new xenon data, we’re gradually piecing together the earliest history of our planet.

Rewind back to the time before there was an Earth. The Sun and solar system formed from a cloud of gas and dust. Small lumps within that cloud grew bigger, slowly gathering up much of the matter to make the planets, moons, and other large bodies. But some small chunks of rock were left behind: small asteroids and meteoroids, which are the most pristine remnants of the earliest moments of the solar system.

By measuring the age of hard crystals inside meteorites—meteoroids that fell to Earth—scientists determined the oldest rocks in the solar system formed about 4,568,000,000 years ago, a number we usually round to 4.6 billion years. Earth must be younger than that: It’s the end product of accumulation of matter from the primordial cloud, and therefore finished growing after meteoroids did.

How much younger is harder to gauge. When Earth first formed, its surface was molten, so there are no rocks for us to study from that era. To complicate matters, a Mars-sized protoplanet collided with Earth in early times, breaking off a big fragment that became the Moon. For that reason, we’ll likely never know the exact age of Earth: just knowing it must be younger than the oldest solar system rocks, while being older than its own surface.

The oldest surviving rocks on Earth and the Moon solidified after all this chaos. To date, the earliest crystal found is 4.36 billion years old, so the Moonsplat was earlier. So, geologists have to look to other dating methods, ones that can measure into the period when rocks were still molten.

Since the Moon-birthing impact stripped Earth of whatever atmosphere it had, a new one formed during the cooling period; some of that air got trapped in the rocks as they hardened. Since the chemical composition of the early atmosphere was very different than what it is today, researchers use the bubbles of trapped gas to study what early Earth was like.

They can also use it to find the age Earth stopped forming, or at least the moment at which things calmed down after the Moon-making collision. That’s the heart of the new study, by geochemists Guillaume Avice and Bernard Marty from the University of Lorraine in Nancy, France. They measured the amount of different isotopes of xenon trapped in quartz crystals. Two isotopes of a single chemical element differ in the number of neutrons, which can make the difference over whether that atom is stable or not.

Xenon isn’t common in the modern Earth atmosphere, but it used to be more so; over time, much of it escaped into space. By comparing the isotope ratio of xenon trapped in 3.4 billion-year-old rocks in Australia with 2.7 billion-year-old minerals, the researchers could use the rate of decay and the rate of xenon escape to calculate when the atmosphere formed.

We may never find the absolute age of Earth, but between the age of the oldest rocks and the new xenon data, we’re gradually piecing together the earliest history of our planet. These results bring us closer to knowing when the Moon formed, which itself is vital information for understanding the beginnings of the Solar System and the formation of planets.

What’s 60 million years, after all? Maybe a lot. Time will tell.