Explaining the moon's two-faced appearance.

The far side of the moon looks unfinished, as though its geological development stalled early on. Although vast volcanic eruptions resurfaced much of the moon's near side more than 2.5 billion years ago, the far face shows little repaving.

Using data collected last year by the Clementine spacecraft, two planetary geologists now proffer an explanation for the moon's unequal volcanic history. The thicker lunar crust on the far side inhibited large eruptions there, suggest R. Aileen Yingst and James W. Head of Brown University in Providence, R.I. They discussed their work at the annual meeting of the Geological Society of America in New Orleans last week.

Yingst and Head focused their study on two giant basins carved by impacts on the far side. The basins contain "ponds" of erupted basalt much smaller than the vast seas of rock, or maria, on the near side.

Most of the ponds appear in regions where the crust was relatively thin, less than 50 kilometers deep. In the 2,500-km-wide South Pole--Aitken Basin, for example, Yingst and Head found that 94 percent of the erupted rock occurs in areas of thin crust.

This trend supports a theory, proposed previously by Head, that explains how eruptions occurred during the moon's volcanic phase. Because the lunar crust has a low density, molten basalt would not have been buoyant enough to rise passively through the crust. Instead, the basalt got trapped beneath the crust in reservoirs until enough pressure accumulated to force the molten rock upward. According to this theory, basalt reached the surface most easily through areas of thinner crust on the near side.

Maria T. Zuber of the Massachusetts Institute of Technology questions the theory, however. "That doesn't explain the whole story, because some of the thinnest crust on the moon is on the far side," says Zuber, who used Clementine data to map lunar crustal thickness. Zuber notes that the South Pole--Aitken Basin on the far side has regions of extremely thin crust. Even so, this area lacks the vast basaltic maria that sit atop areas of thicker crust on the near side.

To explain the volcanic deficit on the far side, some scientists have suggested a deeper reason. The lunar interior beneath the far side may have had limited quantities of radioactive elements, making it colder than rock under the near side.