The Development of Ideas About the Moon's Surface

From the time of the invention of the telescope to the first Moon landings, the characteristic surface of the Moon was the subject of speculation. In 1892 geologist G. K. Gilber interpreted detailed observations of the Moon as signifying that the lunar craters are the result of meteorite impacts. He performed experiments by firing lead shot (small balls of lead) into clay, and made craters that strongly resembled those on the Moon. But firing lead shot into clay does not accurately mimic the tremendous energy released during a large impact, so experimental investigations were of limited value. Specifically, experimental investigations led to the belief that the impactor's momentum would stretch a crater into an ellipse (an oval) whereas, in fact, the energy derived from the impactor's momentum is released symmetrically and a primary impact will make a circular crater irrespective of the angle at which it hits. The elliptical craters observed on the Moon are "secondaries" created by debris from primary events. Initially Gilbert argued that a depression one kilometer wide near Winslow in Arizona was an impact crater, but after a magnetic study failed to find the nickel-iron meteorite that he thought should lie beneath its floor, he concluded that the pit must be a volcanic crater created by an explosive release of steam. Impact craters, it appeared, were unique to the lunar surface.

Gilbert was the first to recognize that the mountain rings that enclose the circular maria (the mostly flat, large areas on the Moon resembling seas) are the edges of craters representing catastrophic impacts . It was clear that a basin large enough to contain Mare Imbrium, one of the Moon's largest maria, could not be of volcanic origin. Furthermore, he pointed out a radial pattern in the surrounding terrain that had evidently been gouged by debris thrown out on very low-angle trajectories.

Then, in 1935, geologists suggested that several large circular structures on Earth might mark terrestrial impacts. They called them astroblemes (star wounds). After expressing his belief that lunar craters were also the result of meteorite impacts, geophysicist Robert Dietz discovered shatter cones (rare features only known to form in the bedrock beneath large meteorite impact craters) in a large crater in Ontario, Canada. Evidently, the Moon was not unique in having been battered by cosmic debris.

Ralph Baldwin drew all the evidence together in a book published in 1949 and effectively proved the case for the Moon's craters being due to impact. A "crater curve" plotting the frequency of craters of different sizes displayed the distribution typical of those seen in studies of holes made during bombing. It was evidence that the dynamic process was the same; volcanism would not have produced craters with such a characteristic distribution of sizes.

In the early 1960s, after proving that Meteor Crater (the depression near Winslow, Arizona) was really an impact, Gene Shoemaker demonstrated that the history of the lunar surface could be studied by using the conventional stratigraphic analysis techniques employed by geologists. This enabled geologists to "steal" the Moon fromastronomers. Their first task concerned the relationship of the circular maria to the enclosing rings of mountains. Photogeologists had interpreted what seemed to be small volcanic domes and vents, flow fronts, lava tubes, and compressional ridges as indicating that the maria were lava flows. Low-angle illumination established that the maria had very shallow regional slopes, indicating they were formed by lava of extremely low-viscosity. The flows had erupted from fissures and flooded all the low-lying terrain they could reach, creating enormous plains. High points on features that had not been completely submerged could be seen protruding through the flows. It was possible to see where the flows had encircled the surrounding rough terrain.

The nearest terrestrial equivalent seemed to be the flood basalts that erupted from fissures and then buried their sources beneath vast plateaus of lava. However, eruptions required the Moon's interior to have once been hot enough to melt rock, and not everyone agreed that this was the case. Nevertheless, excursions to the Moon and samples taken confirmed that the craters had been caused by impacts and that the maria were indeed seas of molten lava from within the Moon.