Selenology: A Primordial Moon

No one was more disappointed than Nobel Laureate Harold Urey, 76, when the 55 Ibs. of lunar samples brought back by the Apollo 11 astronauts turned out to be igneous or heat-formed rock, possibly of volcanic origin. Long a champion of a “cold” moon—the theory that it has never had a molten core like the earth’s—the University of California chemist sadly admitted that he could have been wrong. The moon, he conceded in the face of the rocks, might be hot, or geologically active, after all. “Poor old fellow,” said one of NASA’s younger geologists several weeks ago, “his ideas are impossibly wrong.”

Last week, as the first phase of the rock analysis neared completion in Houston’s Lunar Receiving Lab, it was Urey’s young critics who seemed to have been wrong. Though they were correct in saying that the samples gathered up in the Sea of Tranquillity had once been molten rock, they appear to have been far off the mark in estimating their age. The rocks were not several hundred million years old, as many geologists had speculated, but at least 3.1 billion years old.

Those added years had startling geological implications. They meant that the moon’s maria, or seas, were not created by relatively recent—and possibly continuing—volcanic activity. Instead, the maria had probably survived largely intact since early in the moon’s life. Because the relatively uncratered maria are probably the last major features to have been formed on the lunar surface, the moon’s appearance has remained essentially unchanged for billions of years. “It’s something, isn’t it?” Urey reflected last week. “Rocks sticking up above the surface . . . perhaps they haven’t changed much since they were formed.”

Geologically Akin. The scientists had erred in other ways. In the first exciting days after the lunar specimens arrived in Houston, they had suggested that the moon and the earth were closely akin in geological evolution and structure, and that the moon was made of earthlike layers. Now more careful study is showing that these initial ideas have almost as many holes as the moon itself. Not only have the rocks sprung such chemical surprises as an unusually high content of titanium, but the moon’s seismic activity is also not what it had seemed to be.

At first, the seismometer left behind at Tranquillity Base radioed back several signals that were interpreted in some quarters as distinct moonquakes, a hint that the moon—like the earth —was stratified and geologically alive. Now, says Geophysicist Gary Latham of Columbia University, investigators think that the patterns may have been caused spuriously by the seismometer itself. Yet, even while it seemed to be working well, says Latham, the seismometer detected only infrequent, relatively small lunar rumbles. He accounts for that odd seismic behavior by speculating that the moon contains a large amount of cold, fragmented material that would diffuse any shock waves.

Not surprisingly, one of the few jubilant scientists in Houston last week was Geochemist Oliver Schaeffer, who led the team that calculated the age of the lunar material. He used potassium-argon dating, a method based on the rate at which radioactive potassium decays into argon (it takes 1.3 billion years for half the potassium to decay); as time passes, the ratio between the potassium and argon in a specimen changes at a known rate, thus revealing the approximate age of the sample. If there is any error at all, Schaeffer explains, he has underestimated the age of the rocks, because some argon may have been lost.

In addition to the argon that resulted from potassium decay, Schaeffer found an abundance of solar argon—and of helium and neon—that has collected during eons of bombardment by the sun. These particles were lodged only in the surface of the rocks, where they had remained undisturbed for hundreds of millions of years—more evidence that the moon’s exterior has not undergone any recent upheavals.

Hot Earth. Though the Lunar Receiving Lab’s examination will continue until the rocks are released from quarantine this month, it has not yet answered any of the basic questions about the moon’s origin. But if the moon is actually proved never to have had a molten interior (the maria melting could have been caused by meteor impacts), scientists would be hard put to sustain one of the theories of the moon’s creation: that it was torn, cataclysmically, from a hot earth. On the other hand, a cold moon does not upset either of the two other major moon-origin theories: that it was formed from scattered cosmic debris whirling around the earth, or that it was a planetary interloper captured when it strayed into the earth’s gravitational field.

Urey himself believes that if the moon does indeed prove to be “cold,” its virtually intact primordial surface may provide not only important clues about the moon’s beginnings but also about the origin of the solar system. The most ancient rocks found on earth are 3.3 billion years old, or more than a billion years younger than the planet itself, and the moon rocks from the Sea of Tranquillity are about the same age. Nonetheless, Urey and other cold-moon proponents think that when men reach the lunar highlands, which are generally considered to be older than the maria, they will find material as old as 4.5 billion years—or almost as old as the solar system itself.