The New Moon

Nearly five years after Neil Armstrong and Buzz Aldrin landed on the Sea of Tranquility, many questions about the moon remain unanswered. Indeed, some scientists feel that they may never completely learn the origin and history of the earth's immediate neighbor. Yet as a result of painstaking analysis of the 838 Ibs. of lunar rocks and the wealth of data collected by six Apollo crews, a fundamental understanding of the moon as well as of the early years of the earth is now finally beginning to emerge.

Scientists generally agree that the moon was created some 4.6 billion years ago, during the birth throes of the solar system. In fact, one of the most significant findings disclosed at last month's fifth annual lunar-science conference in Houston was a report by scientists from the California Institute of Technology that a rock collected by the Apollo 17 astronauts during the final moon voyage apparently traces back to that awesome event. It first cooled off and solidified some 4.6 billion years ago, making it older than any specimen previously found on the moon or earth.

Tiny Pebbles. At the time of the formation of the rock, the moon had just been created, possibly by the accretion of debris in orbit around the earth. As the debris--drawn by lunar gravity and ranging in size from tiny pebbles to huge boulders many miles across--crashed into the enlarging moon, it eventually generated enough heat to turn the lunar surface into a sea of molten lava. Slowly, as the bombardment lessened, the lava cooled and hardened into a crust that was then cratered by the impact of the remaining debris. When the rain of rocks eventually ended some 3.9 billion years ago, the moon's surface was covered by great craters and basins. Other changes were still to come. Deep within the moon, heat from the slow decay of radioactive elements like uranium and thorium gradually built up, melting the once solid interior rock. Finally, between 3 billion and 4 billion years ago, lava broke through the hard outer crust in a great spasm of volcanic activity, possibly forming some new craters and flooding the great low-lying basins to create the moon's maria.

The young earth probably under went a similar period of bombardment and volcanic eruptions. But about 4 billion years ago, the histories of the earth and moon veered sharply apart. While the weak lunar gravity could not prevent the volcanic gases from escaping into space, the more massive earth held on to its volcanic vapors; it also was better able to retain its internal heat. The atmosphere and seas were formed, creating conditions for the first stirrings of life. Eventually, winds, water and mountain-building eradicated or covered virtually all geological traces of the planet's violent beginnings. In contrast, the moon has remained largely unchanged since its last burst of volcanism, disturbed only by an occasional meteorite or a periodic moonquake (caused by the gravitational tug of the earth or sun as the moon's lopsided orbit occasionally brings the moon closer to them).

Holy Grail. That, at least, is the picture that emerges from the lunar rocks. But one overwhelming question remains: If the moon was indeed formed from rocky chunks of matter in orbit around the earth, how did the debris originate? Scientists have devised ingenious scenarios to answer that question. At the Houston conference Geochemist John A. Wood of the Smithsonian Astrophysical Observatory in Cambridge, Mass., suggested that thousands of chunks of debris shooting through the primordial solar system were trapped near the earth and broken up by gravitational forces; but many of their heavier components, notably iron, were thrown back into space. That would explain the paucity of such materials in the moon rocks. After innumerable collisions, these fragments eventually formed a lunar-sized sphere.

A different approach was put forth by Geochemist A.E. Ringwood of the Australian National University in Can berra. He speculates that the ring of debris around the young earth was not captured from afar but formed out of hot gases left over from the formation of the earth itself. As such gases condensed, laboratory experiments have shown, they would have formed solids similar in composition to the moon. Yet each of these intriguing ideas has its strong critics. As Ringwood dryly concedes: "There is no consensus regarding the origin of the moon."

If the post-Apollo studies have shown anything at all, it is that there is, as yet, no overall explanation for the origin of the planets -- or their moons. Says Caltech Geophysicist Gerald J. Wasserburg: "Before Apollo, there was one magic recipe in the solar nebula by which all the planets, including the moon and earth, were formed. No one thinks that way any more. The Holy Grail is gone, and we're beginning to face the real problems of planetary formation for the first time."

This file is automatically generated by a robot program, so viewer discretion is required.