Push into Space

(5 of 9)

Dutch-born Astronomer Gerard Kuiper (rhymes with hyper), head of the University of Chicago's Yerkes and McDonald observatories, thinks the moon was formed at the same time as the earth (5½ billion years ago), but at first it revolved only about 20,000 miles from the earth's surface. Beyond it were a lot of smaller satellites arranged in a disk somewhat like the rings of modern Saturn.

This situation did not last. When the earth acquired oceans, the great tides aroused in them by the nearby moon made the earth rotate more slowly. This made the moon spiral outward. As it moved, it crashed into the lesser satellites, each of them blasting an impact pit in its surface. The bigger pits punched through the moon's crust and were filled with lava from the molten interior. The biggest satellite of all, about 100 miles in diameter, hit the present site of the lunar plain called Mare Imbrium—the right eye of the "man in the moon."

Crunchy Snow. After this climactic event Astronomer Kuiper thinks the moon led an increasingly peaceful life. It picked up the rest of the small satellites, which made the fresh-looking pits on its surface. Cosmic rays and other high-speed particles bombarded its surface, riddling the material with microscopic holes. This beaten-up stuff is only an inch or so thick, says Kuiper, and it is not dust. He thinks it would feel underfoot "like crunchy snow."

Nobel Prizewinner Harold Urey of the University of California at La Jolla, another leading moon authority, agrees with Kuiper about there being lava on much of the moon's surface, but he does not think that it welled out of a molten interior. Instead, he contends, it was formed on the spot by the energy of great meteors that hit the moon and melted both themselves and the local lunar rock. He thinks that the present surface material may be something like sand or gravel.

Radiation Erosion. The newest and most radical moon theory was developed by British Cosmologist Thomas Gold, now at Harvard. Professor Gold agrees that the moon was pockmarked long ago by large meteors, and it may have been built up entirely by such accretion. But he does not think that the smooth, dark areas that are called maria (seas), because early astronomers thought they were exactly that, are filled with lava. He thinks that they are low places full of fine dust that was removed by a kind of erosion from the moon's highlands. In some places it may be more than a mile deep.

There is no water on the moon, so Gold's erosion cannot be like the kind that wears down earth's mountains. He thinks that the chief eroding agent is high-energy radiation from the sun helped by cosmic rays and meteorites. They slowly chewed a flour-fine dust from the moon's exposed rocks and kept it stirred up so that it gradually flowed into low places like the interiors of old craters and the maria.