The Moon Just Got 100 Million Years Younger

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The moon is quite a bit younger than scientists had previously believed, new research suggests.

A new map of lunar craters pop with color.
NASA/Goddard Space Flight Center/DLR/ASU

The leading theory of how the moon formed holds that it was created when a mysterious planet — one the size of Mars or larger — slammed into Earth about 4.56 billion years ago, just after the solar system came together. But new analyses of lunar rocks suggest that the moon, which likely coalesced from the debris blasted into space by this monster impact, is actually between 4.4 billion and 4.45 billion years old.

The finding, which would make the moon 100 million years younger than previously thought, could reshape scientists' understanding of the early Earth as well as its natural satellite, researchers said.

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"There are several important implications of this late moon formation that have not yet been worked out," Richard Carlson, of the Carnegie Institution for Science in Washington, D.C., said in a statement.

"For example, if the Earth was already differentiated prior to the giant impact, would the impact have blown off the primordial atmosphere that formed from this earlier epoch of Earth history?" added Carlson, who is presenting the new results Monday (Sept. 23) in London at a meeting organized by the Royal Society called "Origin of the Moon."

Scientists know the solar system's age (4.568 billion years) quite well. And they can pin down the formation times of relatively small bodies such as asteroids precisely, too, by noting when these objects underwent extensive melting — a consequence, in part, of the heat generated by the collision and fusion of these objects' building-block "planetesimals."

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For example, analysis of meteorites that came from the asteroid Vesta and eventually rained down on Earth reveals that the 330-mile-wide (530 kilometers) space rock is 4.565 billion years old. Vesta cooled relatively quickly and is too small to have retained enough internal heat to drive further melting or volcanism, Carlson explained.

But it's tougher to nail down the age of larger solar-system bodies, he said.

"Ask the same question of the Earth or moon, and you don't get a very precise answer," Carlson said. "Earth likely took longer to grow to full size compared to a small asteroid like Vesta, and every step in its growth tends to erase, or at least cloud, the memory of earlier events."

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Scientists keep getting better and better estimates, however, as they refine their techniques and technology improves. And those estimates are pushing the moon's formation date farther forward in time.

The moon is thought to have harbored a global ocean of molten rock shortly after its dramatic formation. Currently, the most precisely determined age for the lunar rocks that arose from that ocean is 4.360 billion years, the researchers said.

And here on Earth, scientists have found signs in several locations of a major melting event that occurred around 4.45 billion years ago. So, evidence is building that the catastrophic collision that formed the moon and reshaped Earth occurred around that time, rather than 100 million years or so before, the researchers said.

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