Three meteorites from the moon: how many launchings to earth?

It was less than two years ago that scientists finally decided that they knew for the first time where a single one of the thousands of meteorities collected on earth had actually come from. Even at a glance, it looked to some geologists like a piece of the moon, but researchers were uncertain whether any natural process--such as the impact of another meteorite -- could really blast a rock fragment free of the gravity of so massive a body. It was only the extraordinary agreement of a host of scientific analyses (SN: 3/26/83, p. 196) that led to the final, firm conclusion.

Then, only 15 months later, as though the one startling example had somehow broken the ice, two more cases were identified (SN: 8/4/84, p. 70). Some researchers had already speculated that if one lunar chunk could be tossed all the way to earth, it had probably happened often. But still unknown is how often a sufficiently strong ejection from the lunar surface actually takes place.

Since last fall, therefore, a consortium of scientists has been working to find out whether the three meteorites from the moon represent scattered debris from a single, titanic event, or from three separate occurrences. But unlike the case of the first such chunk, whose lunar identity simply became more and more likely from the very first hints, the question of whether the other two are related to it--to be the key topic of a symposium in Tokyo next month--is looking anything but easy.

All three meteorites were recovered in Antarctica,one from the Allen Hills and the other two from the Yamato Mountains. "From our data," report Michael Lipschutz, Jane Dennison and Patrick Kaczaral of Prudue University in Lafayette, Ind., "Yamato 791197 and Allen Hills 81005 [the two samples receiving the most attention so far] did not come from the same lunar region, hence were lunched from the moon in different impact events." If that conclusion, based on measurements of "volatile" elements that vaporize at relatively low temperatures, is borne out, it would be a vote of sorts for ejections from the moon being at least less rare than total flukes. It could also interest researchers wondering whether certain other meteorities may have come from Mars, though more gravity must be overcome there.

But according to Roman Schmitt of Oregon State University in Corvallis, who has been working with J.C. Laul of Battelle Northwest in Richland, Wash., on largely "refractory" or higher-temperature trace elements, the two lunar meteorites are "twins"--chips, one might infer after additional studies, off "the same old block."

Two rocks, both thrown all the way from the moon, differ markedly by one standard, yet are called "twins" by another. Could the differing volatile abundances be due to some kind of contamination that affected the now volatile-rich Yamato rock but not the one from the Allen Hills? It is possible, but unlikely, Lipschutz says, since two samples of Yamato 791197 vary by about 10-fold in seven different elements, an imporbable amount of diversity within a given rock if the cause were some outside contaminant. Nor would it probably make much difference, he says, if the Yamato and Allen Hills meteorites were subjected to different amounts of weathering. (Other meteorites whose relative amounts of weathering have been well determined, he points out, show no corresponding difference in composition.)

Furthermore, there is a third lunar meteorites -- Yamato 82192 -- which may turn out to be more decisively different from the other two than they are from each other. The Allen Hills rock and Yamato 791197 are both black with "inclusions" of white, while Yamato 82192 is "pink, with a big beige, splotch," Lipschutz says. Most of the U.S. researchers have yet to receive their samples of that one for study, but Kunihiko Nishiizumi of the University of California at San Diego notes that studies in Japan are indicating its composition to be "extremely diferent" from the other two.