An international team of geologists has measured the precise age and chemical composition of a meteorite from the Moon's Imbrium impact basin, sometimes called the right eye of the "man in the Moon". The results provide the best estimate yet of when the Moon and the Earth stopped being bombarded by large rocks and could therefore help scientists calculate with more certainty when life on Earth began (E Gnos et al. 2004 Science 305 657).
The meteorite, called Sayh al Uhaymir (SaU) 169, weighs about 200 grams and was found in Oman in January 2002. It is one of only 30 such lunar meteorites to be found. Edwin Gnos at the Institut für Geologie in Bern in Switzerland and colleagues from the US, UK, Germany and Sweden analyzed its thorium, uranium and potassium composition and found that it almost certainly comes from the Imbrium crater.
Gnos and co-workers were then able to pinpoint the origin of the meteorite within the Imbrium crater by comparing the chemical composition of the lunar soil attached to the meteorite with data from NASA’s Clementine and Lunar Prospector missions — from 1994 and 1998 respectively. Using electron microprobe analysis, Raman and gamma-ray spectroscopy, X-ray tomography and data from satellite images, they found that the meteorite was launched from a smaller crater near the so-called Lalande impact crater, which itself lies south of the Imbrium basin.
The researchers also used ion-probe dating techniques and mass spectrometry of the mineral zircon to calculate that the meteorite is around 3.9 billion years old, and were able to do this with unprecedented precision. In addition, by measuring the ratio of carbon-14 to carbon-12 within the meteorite (which originated from carbon dioxide within the Earth’s atmosphere), they calculated that the meteorite arrived on Earth about 9700 years ago.
According to Gnos, the improved dating of the age of the Imbrium basin will lead to a better understanding of how different layers in the lunar soil have evolved over time. “This is important, not only for the Moon, but also for related crater age dating on Mars and the entire inner solar system because such age estimations are based on data from the Moon,” he says.
