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Last Updated: Friday, 30 July, 2004, 17:54 GMT 18:54 UK
Moon rock's 'biography' revealed
Sayh al Uhaymir 169, Science
Lunar meteorites tell scientists about the chemistry of the Moon's surface
The violent history of a meteorite from the Moon has been charted for the first time, Science magazine reports.

Analyses of the rock's geology show it endured three giant impacts, before finally being hit so hard that it was catapulted into space.

The rock's composition also allowed scientists to pinpoint its place of origin - the Moon's Imbrium Basin.

Never before have scientists been able to describe a lunar meteorite's travel itinerary in such fine detail.

Little fragments

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.

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.

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.

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.

Sayh al Uhaymir 169, Science
The Earth's gravitational pull finally overcame the little rock, dumping it in Oman
Therefore, when a lunar meteorite turns up with a different chemistry, it could have come from any number of places.

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.

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.

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.

For two years Dr Gnos's team carefully analysed SaU 169, allowing them to paint a vivid picture of its tumultuous past.

Largest crater

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.

We used isotopic methods to date all these different stages of the rock
Edwin Gnos, University of Bern
In liquid form it probably splashed to the rim of the crater, where it sat buried in rubble for the next billion years.

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.

Finally a fourth impact, around 340,000 years ago, sent the rock ricocheting into space.

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.

"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.

"And we could attribute all the stages to specific craters where these events most likely happened."

Far side

SaU 169 tells a story of its dramatic history because each impact caused a change within the rock, which the researchers could detect.

"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.

Oman, Science
Geologists discovered the little meteorite in the desert of Oman in 2002
"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."

SaU 169 is undoubtedly valuable because it allowed scientists to map its journey with such unprecedented detail.

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.

"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."




SEE ALSO:
Scientists find new Moon mineral
27 Apr 04  |  Science/Nature
Doubts resurface about lunar ice
12 Nov 03  |  Science/Nature
The legacy of Apollo
14 Dec 02  |  Science/Nature


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