Finding a lunar meteorite's home.

Scientists have for the first time pinpointed the source of a meteorite that came from the moon. By measuring the rock's age, the researchers have precisely dated the rock's lunar home, the Imbrium impact basin, which is the youngest of the moon's large impact craters. That crater's age may have special significance because Imbrium's formation marks the end of an era in which space debris walloped the inner solar system. Researchers have long argued that life on Earth couldn't have evolved until after that bombardment.

Found in the desert of Oman in 2002, the 7-ounce stone has a greenish hue mixed with bits of white. That appearance is typical of lunar rocks, but this meteorite turns out to be exceptional.

Edwin Gnos of the University of Bern in Switzerland and his colleagues found that relative to other rocks from the moon, the stone contains high concentrations of uranium, thorium, and potassium. The ratios of these elements to each other match those of only one group of lunar rocks, located in the Imbrium impact basin on the moon's near side.

Gnos' team determined the meteorite's age by radioactive dating. The group describes the turbulent history of the meteorite, dubbed Sayh al Uhaymir (SaU) 169, in the July 30 Science.

According to the researchers, the 1,160-kilometer-wide Imbrium basin, along with the main rock in the meteorite, formed when an asteroid struck the moon 3.909 billion years ago. Two other impacts--2.8 billion years ago and 200 million years ago--brought the material close to the moon's surface. Finally, less than 340,000 years ago, another impact dislodged SaU 169 from the moon. The meteorite struck Earth 9,700 years ago.

Suffering at least five impacts in nearly 4 billion years, "SaU 169 is a rock which demonstrates impressively how rocks can travel, like a ping-pong-ball, from one body to another," the researchers note.--R.C.