Except for the black sky in the background, the photograph might have been mistaken for a composite of the scenic grandeur of Grand Canyon and the barren desolation of the Badlands of South Dakota. But when it was flashed unexpectedly onto a screen at a meeting of the American Institute of Aeronautics and Astronautics in Boston last week, sophisticated space scientists and engineers recognized the terrain immediately. It was a spectacular closeup shot of lunar landscape. That photograph of the moon's Crater of Copernicus, said NASA Scientist Martin Swetnick, is "one of the great pictures of the century."
Taken by the high-resolution camera aboard Lunar Orbiter 2 from a point only 28.4 miles above the moon's surface, and about 150 miles south of Copernicus, the picture gave scientists a fresh slant on one of the moon's most prominent craters. For the first time they could, in effect, peer over the rim of Copernicus and get a close-in look at its walls, floor and central mountains—areas they had seen through earth-based telescopes, but only from directly above. The new look may have already shed new light on the processes that formed the pockmarked surface of the moon.
Stupendous Collision. To University of Arizona Astronomer Gerard Kuiper, one of the world's leading lunar experts, Orbiter's photograph seemed to confirm his theory that the 1,000-ft.-high mountains in the center of Copernicus were partially formed by volcanic activity. Scattered over their slopes, he says, are humps similar to the cinder cones found on major terrestrial volcanoes. The picture also clearly shows that the floor of the crater is remarkably flat. To Kuiper, this indicates that the subsurface was once in a fluid or plastic state, and that it solidified, causing the crater floor to level off.
These clues strengthen Kuiper's belief that Copernicus was formed by the impact of a comet, one of three or four that have hit the visible side of the moon during its 4½-billion-year lifetime. He estimates that the comet weighed a million million tons, had a nucleus ten miles in diameter, and crashed into the moon at a speed of 35 miles per second. The explosion produced by the stupendous collision was intensified by the comet's high content of ice expanding into steam on impact. The resulting blast produced a crater 60 miles across and at least two miles deep; it hurled pulverized debris hundreds of miles in all directions.
The heat of impact and the resulting steam penetrated deep into the moon and formed a pool of molten material that later solidified as the crater floor. The hot lunar material and huge chunks of rubble floating in it, says Kuiper, created the volcanic structures that can be seen in Orbiter's picture.
Housekeeping Pictures. Other scientists had other interpretations. Cornell University Astronomer Thomas Gold, who believes that Copernicus was formed, like most other lunar craters, by the impact of a meteorite, theorizes that its smooth floor consists of compacted dustlike material that is continually being knocked off crater walls by micrometeorites. U.S. Geological Survey Geologists John McCaulay and Richard Eggleton were fascinated by the apparent presence of erosion channels on the far wall of the crater. They suggest that the channels may have been formed by solid particles flowing down the crater wall.