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 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.
The new study is based on 259 computer simulations of how the solar
system evolved from a primordial disk of planetary embryos swirling
around the sun. The programs simulate the crashes and mergers of the
small bodies until they meld into the rocky planets that exist
today.
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.
The previous assessment was based on measuring the naturally
occurring radioactive decay of telltale atoms inside lunar rocks.
The same process, however, also led to findings that the impact
happened between 50 million and 100 million years after the solar
system's formation.
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"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.
The results also open another even bigger mystery about why some
planets, like Mars, form relatively quickly, while others, like
Earth and possibly Venus, take far longer.
Analysis of Martian meteorites and the computer simulations indicate
Mars was finished in just a few million years.
There are no known Venus meteorites, and spacecraft so far have not
been dispatched to either Mars or Venus to collect samples.
"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.
(Editing by Marguerita Choy)
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