New 'geologic clock' resets date for moon's formation

By Reuters  • 

Earth’s moon started forming up to 65 million years later than some previous estimates, according to a study released today that uses a new way to calculate the birthday of the 4.47 billion-year-old planet’s only natural satellite.

The mega-asteroid that smashed into Earth, launching debris that later became the moon, happened about 95 million years after the birth of the solar system, research in this week’s issue of the journal Nature showed.

The finding disputes, with a 99.9 percent degree of accuracy, some previous estimates that the moon-forming impact occurred as early as 30 million to 40 million years after the solar system’s formation, some 4.58 billion years ago.


By that geologic clock, Earth’s last big chuck came from a Mars-sized body that hit about 95 million years after the solar system’s formation, the study showed.

“We think that the thing that hit Earth and ended up forming the moon, the lion’s share of it stayed on Earth. A small fraction of its mass and some material from Earth was pushed off into space to form the moon,” astronomer John Chambers, with the Carnegie Institution for Science in Washington DC, said in an interview.

“That was probably the last big event,” he added.


“Our new method is independent of radiometric techniques and so we break through the controversy,” lead researcher Seth Jacobson, with Cote d’Azur Observatory in Nice, France, wrote in an email.




“Discovering that the moon-forming impact occurred late is surprising because we know from Martian meteorites that Mars formed relatively quickly. How this discrepancy arises is another big question for the future,” Jacobson said.