- The violent history of a meteorite from the Moon has
been charted for the first time, Science magazine reports.
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- Analyses of the rock's geology show it endured three
giant impacts, before finally being hit so hard that it was catapulted
into space.
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- The rock's composition also allowed scientists to pinpoint
its place of origin - the Moon's Imbrium Basin.
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- Never before have scientists been able to describe a
lunar meteorite's travel itinerary in such fine detail.
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- Little fragments
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- One of the only ways to understand the chemistry of the
Moon's surface is by analysing lunar meteorites - little fragments of rock
that are flung into space during great impacts before occasionally winding
up on Earth.
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- But they are pretty rare - a lunar meteorite is a special
discovery. Only 30 of them have ever been found in deserts around the globe.
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- Lunar meteorites are useful for building up a data base
of the range of rocks on the Moon. But it is hard to work out which rocks
come from where, because Moon rocks do not often betray their exact place
of origin.
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- The problem is that humans have just visited a tiny area
of the moon - so they only know for sure what type of rocks are found in
these specific locations.
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- Therefore, when a lunar meteorite turns up with a different
chemistry, it could have come from any number of places.
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- Now, Edwin Gnos of the University of Bern in Switzerland
and his colleagues have found a rock with the same chemistry as Moon rocks
brought back by Apollo astronauts visiting the Imbrium Basin.
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- So they suspect this meteorite must have come from the
same area. For the very first time, they believe they have pinpointed a
Moon rock's origin.
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- Dr Gnos and two other geologists discovered the little
meteorite in the desert of Oman in 2002. They named the rock - which is
no bigger than a clenched fist - Sayh al Uhaymir (SaU) 169.
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- For two years Dr Gnos's team carefully analysed SaU 169,
allowing them to paint a vivid picture of its tumultuous past.
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- Largest crater
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- Their results suggest that, about 3.8 billion years ago,
the rock was melted by the giant impact that created the Imbrium Basin,
one of the largest craters on the Moon.
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- In liquid form it probably splashed to the rim of the
crater, where it sat buried in rubble for the next billion years.
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- Then, 2.8 billion years ago another impact exposed the
rock again, before a third impact about 200 million years ago flung it
tens of kilometres across the surface of the Moon.
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- Finally a fourth impact, around 340,000 years ago, sent
the rock ricocheting into space.
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- For several thousand years it quietly orbited the Earth,
before the planet's gravitational pull eventually overcame the little rock,
dumping it in Oman about 10,000 years ago.
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- "We used isotopic methods to date all these different
stages of the rock," Dr Gnos told BBC News Online. "And we also
used chemistry to link that with data known from the Apollo mission - and
also from satellite mapping of the lunar surface.
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- "And we could attribute all the stages to specific
craters where these events most likely happened."
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- Far side
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- SaU 169 tells a story of its dramatic history because
each impact caused a change within the rock, which the researchers could
detect.
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- "With each impact there is a resetting of various
parameters within the rock," explained Richard Greenwood from the
Open University, UK. "The first thing you want to date is when the
rock actually formed.
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- "Then there would be impacts that would reset various
things within the rock, which you can date as well - so you can get a chronology
of events."
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- SaU 169 is undoubtedly valuable because it allowed scientists
to map its journey with such unprecedented detail.
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- But less "forthcoming" Moon rocks are also
very important, even though scientists can only hope for a fuzzy idea of
where they came from. These meteorites can tell us about places on the
Moon man has never been, and may never go.
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- "Some meteorites come from the far side of the Moon,"
said Dr Greenwood. "And that is a good thing because otherwise we
could never sample that area of the Moon."
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- © BBC MMIV http://NEWS.BBC.CO.UK/2/hi/science/nature/3939369.stm
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