Although more than 99 percent of the meteorites found here on Earth are pieces of asteroids, geologists have identified around 30 meteorites from the moon, and a similar number from Mars. In a report published today in Science, Edwin Gnos of Bern University in Switzerland and his colleagues analyzed a half-pound lunar meteorite called Sayh al Uhaymir 169 (SaU 169), which turned up in the Omani desert in 2002.
SaU 169 contains evidence of four meteoric events that melted, as well as dislodged, the rock while it was on the moon. Based on the ratio of lead isotopes in its tiny crystal formations, the scientists date the first impact at 3.9 billion years ago, which they contend corresponds to the big meteoric crash that formed the 600-kilometer-wide Imbrium Basin. "Many people think we already know the age of the Imbrium basin, 3.85 billion years, from Apollo 15 samples," comments Randy L. Korotev of Washington University. If the older age given by SaU 169 is correct, theories about how the moon's surface was shaped may need revision.
One of the remarkable things about SaU 169 is its very high concentration of radioactive thorium. By assuming that SaU 169 came from one of the most radioactive spots in gamma-ray maps, Gnos and his team were able to limit the origin of the meteorite to a few sites on the moon's surface. They narrowed the candidates down to one, the Lalande Crater area, by matching the ages of known craters to the dates of the second and third impacts. The last impact, which launched the meteor into space, occurred less than 340,000 years ago. SaU 169 thus presumably spent a relatively short time in outer space before Earth's gravity pulled it in. --Michael Schirber