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Moon is 4.47-B years old, U.S. researchers say

WASHINGTON, April 17 — The Moon is actually about 4. 47 billion years old, U.S. researchers analyzing stony meteorites which they found contain ancient fragments from the Moon-forming giant impact said Thursday.

The idea that the Moon formed following a collision between a Mars-sized planet and the early Earth is now widely accepted, but the precise timeline of the event is still uncertain.

Ages of the most ancient lunar samples returned by the Apollo astronauts are still being debated, because of disagreements about the isotope systems used for dating.

A team led by Bill Bottke of Southwest Research Institute used numerical simulations to show that the giant impact likely created a disk near Earth that eventually coalesced to form the Moon, while ejecting huge amounts of debris completely out of the Earth- Moon system.

They argued that numerous kilometer-sized fragments from that event traveled out to the main asteroid belt between Mars and Jupiter and struck asteroids at much higher speeds than typical main belt collisions, heating the surface and leaving behind a permanent record of the giant impact.

Subsequent, less violent collisions between asteroids have since ejected some shocked remnants back to Earth in the form of fist-sized meteorites,they reported in the U.S. journal Science.

"By modeling their temporal evolution, and fitting the results to ancient impact heating signatures in stony meteorites, we infer that the Moon formed about 4.47 billion years ago, which is in agreement with previous estimates," they wrote.

The team also suggested that the moon-forming giant impact was the biggest and most recent known collision in the inner solar system.

Intriguingly, some debris may have also returned to hit the Earth and Moon after remaining in solar orbit over timescales ranging from tens of thousands of years to 400 million years.

"The importance of giant impact ejecta returning to strike the Moon could also play an intriguing role in the earliest phase of lunar bombardment," said Bottke. "This research is helping to refine our time scales for 'what happened when' on other worlds in the Solar System." (PNA/Xinhua)

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