long one of astronomy's most puzzling mysteries -- has emerged from a supercomputer simulation of what could happen to a rotating blob of liquid resembling a molten Earth that may have existed billions of years ago.

According to the computer model, a spinning liquid Earth first flings out streams of molten material that coalesce into a huge, spinning doughnut encircling the Earth. Then the densest part of the doughnut contracts into the moon while the rest scatters into space.

The hypothesis was developed by Richard H. Durisen, an Indiana University astronomer, and reported informally to the National Science Foundation, which funds his work.

Several other hypotheses have been proposed over the years but all have problems. One idea is that the moon and Earth coalesced independently from a single cloud of gas and dust orbiting the early sun. If this were the case, both should have about the same mix of elements. The Earth, however, is rich in iron while the moon has little.

Another is that the moon formed elsewhere and passed close enough to be captured by Earth's gravity. Its orbital characteristics, however, are unlikely to have been produced this way.

Durisen's computer model overcomes both problems. Most of the iron in the molten Earth sinks to the core before the iron-poor outer layer is flung out to form the doughnut. And the orbital characteristics of the doughnut-born moon are those of the real moon