National Geographic : 1969 Feb
How did earth get its moon? Just as experts debate the causes of the moon's configurations, they dispute its origin. Three theories predominate. 1. In the "sister" hypothesis (top), earth and moon formed at the same time from a vast cloud of cosmic matter that con densed into the bodies of our solar sys tem. 2. The "daughter" theory con tends that the earth once rotated so rapidly that it became blimp-shaped and tore in two. The smaller blob of matter entered into orbit as the moon. 3. The "spouse" theory holds that the moon came from elsewhere in the solar system. Sweeping too near, it was snared by earth's gravity and "mar ried"-locked in orbit. Nuggets of moon matter-so Dr. Dean Chapman claims of these tek tites, gathered from Southeast Asia, the Philippines, and Australia. The glassy blobs indicate by their shapes and surface sculpture that they have passed through earth's atmosphere. Dr. Chapman, an aerodynamics spe cialist at Ames Research Center, be lieves with a number of colleagues that tektites are lunar fragments hurled into space during meteorite collisions with the moon. After he acquired a number of huge tektites for his collection, a fellow scientist spoofed him by painting a super-tektite for his wall. PAINTINGSBYDAVISMELTZER© N.G.S. Mr. Gault told me as he prepared his gun for a demonstration. "That's nearly ten times the speed of a military rifle bullet, and it simulates the effects of a small meteorite striking the moon. We reproduce the lunar environment as closely as we can by putting the target in a vacuum and by dropping the target just fast enough at the precise moment of impact to simulate the moon's low gravity." Hydrogen gas compressed by a powder ex plosion propels the missile for maximum ve locity, but for our experiment we used a simple powder charge. We watched through a heavy quartz window as the pellet smashed into a bed of fine-grained sand, leaving a rimmed crater many times its own size. "That rim is the significant thing," said Mr. Gault. "When we fire the gun at rocks, we can knock out a rough hole, but we never get a rim like that, and we don't get small craters that look like that. But with granular material, or with such material over a hard rock layer, we can duplicate all the different 222 kinds of small craters we see on the moon. "With this help," Mr. Gault went on, "sci entists can calculate the depths of the fine sur face material from the photographs, either by the shapes of the craters or by determining how deep craters have to be before the im pacts begin to excavate subsurface rock." Large craters, of course, have broad "ejecta aprons" of rocks and boulders. These have probably come from much deeper than the present crater bottoms, however. The very large craters were originally as much as six or seven miles deep, and the largest of the circular maria-Imbrium may have been 50 miles deep for a very brief time. Many scientists think that the bottoms of these vast chasms rose to compensate for the loss of material. In addition, volcanic ma terial has partially filled many of the pits. As Dr. John O'Keefe of NASA's Goddard Space Flight Center at Greenbelt, Maryland (page 213), puts it, "The moon just won't accept an insult like that. It reacts!"