Study finds that moon is 100 million years younger than we thought

Scott Sutherland

Although scientists know quite a bit about our moon, and even have an excellent idea of how it formed, one detail about it has been difficult to pin down, and that's its exact age. Previous estimates put it as roughly the same age as Earth, but a new study suggests that it's younger than that, by around 100 million years.

Billions of years ago, something at least as big as the planet Mars slammed into the young Earth. In the aftermath of that impact, what was left of Earth and that other planet-sized object merged into what would become the Earth we know today, while the debris the impact threw into orbit eventually coalesced into the moon. This origin story for the moon is known as the 'giant impact hypothesis', and studies have suggested that it happened just as the Earth was fully formed, about 4.5 billion years ago.

However, Richard Carlson, a planetary scientist at the Carnegie Institution for Science, has presented new research at the 'Origin of the Moon' meeting of the Royal Society of London today. Analysis of rocks from the moon using newer technology indicates that the impact actually happened around 4.4 billion years ago — 100 million years later than previously thought.

At that time, Earth had already gone through a process known as 'differentiation', where denser materials would have sunk towards the core and lighter materials rose to the surface. It's also quite likely that the planet had liquid water on the surface and a thick primordial atmosphere as well. According to Carlson, all the implications of such a late development of the moon haven't been worked out yet, but the impact may have blown off Earth's primordial atmosphere.

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With more studies showing abundant supplies of water on the moon, both in surface minerals and potentially deeper down, this idea of a younger moon may offer clues to that as well. If the Mars-sized impact happened after liquid water began to collect on Earth's surface, much of that water would have ended up in space after the event, where it may have coalesced along with the minerals that formed the moon.

(Image courtesy: NASA/JPL-Caltech)

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