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The meteorite, designated Sayh al Uhaymir (SaU) 169, is the first to be precisely located to Mare Imbrium, the youngest of the large meteorite craters on the Moon. The crater formed from a collision 3.9 billion years ago, a key date for lunar and even terrestrial stratigraphy, because life on Earth would have evolved only after the heavy meteorite bombardment had ended.
Image: Peter Vollenweider and University of Arizona. SaU 169 before laboratory tests and the location of Imbrium basin.
Geologists who found the meteorite and scientists from across the world including UK experts analysed the unique stone and reported their work in the 30 July issue of Science. Swiss geologist Edwin Gnos is first author of the article titled "Pinpointing the Source of Lunar Meteorite: Implications for the Evolution of the Moon."
Gnos and colleagues found the 206-gram meteorite in Oman on 16 January 2002. The geologists were on a joint meteorite search expedition sponsored by the Government of Oman, the Natural History Museum of Berne and the University of Berne, Switzerland. "The desert in Oman is the new place to find meteorites," said A.J. Tim Jull of the University of Arizona, USA. He analysed beryllium and carbon isotopes that told how long the meteorite was in space after it was launched from the Moon and how long ago it fell to Earth at Oman.
Scientists who've acquired the special permits needed to search for meteorites in Oman and North Africa during the past half-dozen years have been amply rewarded. Seven of the 30 known lunar meteorites have been found in Oman, seventeen have found in Antarctica, five in North Africa and one in Australia.
The team recognized that the meteorite was of lunar or Martian origin because it wasn't magnetic. Meteorites from planetary bodies don't contain metal. This rock was greenish coloured and contained white angular feldspar inclusions typical of lunar rocks. But when they tested it with a Geiger counter, they found it was no typical lunar rock. They found it contained high levels of radioactive uranium, thorium and potassium. Gamma ray-spectroscopy lab tests told them that the ratios between these elements fit only one enigmatic group of lunar rocks called "KREEP," the acronym of K for potassium, REE for rare earth elements, and P for phosphate.
"At that moment, it was clear that the rock had something to do with the large Imbrium impact basin, the right eye of the man in the Moon," said Gnos. The Imbrium impact basin on the lunar nearside is the only area where KREEP rocks are found. KREEP rocks are known both from samples returned by the Apollo missions and by NASA's Lunar Prospector Orbiter radioactivity survey in 1998-99.
The scientists conducted a battery of laboratory tests to piece together a detailed history of the meteorite, they summarised:
At 3.909 billion years ago, an asteroid collided with the Moon, forming the 1 160 kilometre diameter Imbrium impact basin. Crushed and molten rocks mix and solidify to form the main rock type in meteorite SaU 169.
At 2.8 billion years ago, a meteorite hit the Moon, forming the 25 kilometre diameter Lalande crater south of the Imbrium basin. The impact blasts material, including the main rock type in SaU 169, and deposits it as an ejecta blanket around the crater. The ejecta there mixes with other lunar soil.
At 200 million years ago, a another impact brings the rock that will become a meteorite to within a half-meter of the lunar surface.
At less than 340 000 years ago, another impact hits the Moon, producing a crater a few kilometres in diameter and ejects SaU 169 from the Moon.
NASA images and identified a young, three-kilometre diameter crater 70 kilometres north-northeast of Lalande as the meteorite's likely launch site.
Measured beryllium 10 in SaU 169 determined the meteorite's Moon-to-Earth transit time at around 300 000 years.
Measured carbon 14 in SaU 169, shows the meteorite fell in present-day Oman around 9 700 years ago.
The team concluded on their website "Without the Apollo and Lunar sampling programs, and especially the huge advance in knowledge of the Moon acquired during investigations in the last 20 to 30 years, we would only be able to tell that SaU 169 is an exceptional lunar rock. Without background information from such missions as Clementine and Lunar Prospector, we could never have linked ages and chemical data with lunar surface information. SaU is a rock which demonstrates impressively how rocks can travel, like a ping-pong-ball, from one body to another."
More info: SaU 169
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