Staring at the moon's mantle.

If you want to know how a planet formed or to understand the chemical composition of a moon, you can't stop at the crust. You've got to get under their skin.

The crust coating Earth's moon accounts for only a few percent of its mass and extends no more than 100 kilometers beneath the surface. In contrast, the mantle-the layer of rocky, silicate-rich material that lies directly under the crust-extends thousands of kilometers deep and accounts for about half the moon's mass.

The challenge is to find exposed chunks of the mantle. At present, excavating mantle material by drilling a 100-km hole through the moon's crust isn't possible. But for years planetary scientists have held onto the hope that nature has already done the dr illing for them. A new study suggests their hunch was right.

Researchers focused their search on the moon's South Pole-Aitken basin, a mammoth depression 12 km deep and 2,500 km across-a quarter of the moon's circumference.

The asteroid or comet that slammed into the moon's South Pole to create this gigantic crater most likely penetrated into the mantle. The crater walls then may have collapsed to form today's shallower depression.

After analyzing images collected at different wavelengths by the Clementine spacecraft, a low-cost mission that orbited the moon for 71 days in 1994 (SN: 11/18/95, p. 324), researchers say they have almost certainly detected the chemical signature of mant le material in the Aitken basin. Paul G. Lucey of the Hawaii Institute of Geophysics and Planetology in Honolulu and his colleagues find that the crater floor, sampled in several places, has a slightly higher abundance of titanium and a significantly high er abundance of iron than the lunar crust does. The team concludes that the bottom of the South Pole-Aitken basin is most likely a mixture containing about equal proportions of upper mantle and lower crust.

Although the Apollo missions of the late 1960s and early 1970s brought back to Earth numerous moon rocks, "a sample [from the mantle] has not been found before," says Lucey.

He cautions that the evidence does not yet rule out a less likely alternative-that the projectile which struck the moon's South Pole didn't penetrate the mantle and that the chemical signature detected by Clementine reflects some previously unseen composi tion of crust. That interpretation would mean that "cratering models need drastic revision, [that] our understanding of lunar impactors is really lousy," says Lucey.

Carle M. Pieters of Brown University in Providence, R.I., says that Clementine's high resolution provides the best data so far on the basin. But she adds that the craft can only infer, not directly detect, abundances of elements such as titanium and iron. By directly measuring abundances of several elements, a sensitive spectrometer aboard the Lunar Prospector, a small craft scheduled for launch next year, should settle the question of whether the floor of the south polar crater truly contains mantle mate rial, Lucey says.

If the craft confirms the existence of mantle material in the basin, planetary scientists would love to get their hands on a sample. "We'd learn more about the moon from a single sample [of mantle] at the South Pole than all the samples [of crust] ever ob tained from the Apollo missions," says Lucey.