Earth's largest lunar meteorite announced.

Earth's largest lunar meteorite announced

A meteorite found seven months ago in Antarctica now appears to rank as the largest piece of the moon yet recovered on Earth. Weighing 663 grams, just under a pound and a half, it and a related sample raise to eight the number of lunar rocks identified among the thousands of meteorites collected and cataloged on Earth.

Before 1982, scientists did not even know whether meteorite impacts could actually knock rock fragments free of the moon's gravity and send them to Earth. However, a tiny chunk weighing little more than an ounce and found in Antarctica late in the previous year by a U.S. team reminded some researchers of samples brought back from the moon by the Apollo astronauts. When various scientists analyzed bits of that meteorite, they agreed nearly unanimously that it indeed had come from the moon. Since then, Japanese researchers, who also go meteorite-hunting at the bottom of the world, have identified five more Antarctic rocks as coming from the moon.

Confirming the existence of lunar meteorites proved of particular interest, in part because some researchers suspect that certain other meteorites found on Earth come from Mars. As yet, no one has confirmed a Martian origin for any of these rocks, and the likelihood of meteorites reaching Earth from Mars remains speculative (see story, p.53).

Scientists with a U.S. team located the latest lunar addition (designated MAC88105) along with about 1,000 other meteorites on an icy expanse of Antarctica's MacAlpine Hills. Among them, says Marilyn M. Lindstrom of NASA's Johnson Space Center in Houston, was a smaller fragment (MAC88104) weighing just over 2 ounces, which is probably a broken-off chip of the record-setting rock. According to Brian Mason of the Smithsonian Institution in Washington, D.C., the evidence that the two finds came from the moon includes their iron-manganese ratio, high aluminum oxide percentage, low amounts of sodium and potassium, and the composition of the glassy matrix that holds the smaller fragments together.

Each chunk is a form of rock called a breccia, consisting of small particles of various rocks and minerals crushed and recombined by impacts on the lunar surface. Most of their outer surfaces are dark gray and pitted, as though bits had been removed by temperature changes and other effects of weathering. A thin gray-green "fusion crust," probably formed by the heat of the meteorites' descent through Earth's atmosphere, covers about 30 percent of their surfaces.