The Moon offered an active battleground of contention. The central argument for interpretation of the Apollo measurements with seismometers placed on its surface by the astronauts is illustrated by [a 1985 "Fluffy" cartoon by W. B. Park in] Figure 19. [Not shown. "Fluffy", the mascot of several critics of the small comets who insist that the Moon should "ring like a bell"--even if the objects are fluffy small comets--is shown falling through the air like a rock. The caption notes that it doesn't matter whether Fluffy lands on his feet or not.] That is, no matter what the impacting object on the Moon was, rock or cat or small comet, the disturbances recorded by the seismometers would be the same. Although amusing, this position is not reasonable when one considers the different results of a hand-thrown rock or snowball, each weighing one pound, for example. It was expected that the Moon would be "ringing like a bell" from the large number of meteors which were causing the fireballs in the Earth's atmosphere. Surprisingly the Moon was silent, except for a relatively rare meteor event or the thermal groans of the surface.

The ambiguity presented by the numerous fireballs in our atmosphere and the "silence of the Moon" can be resolved with the existence of small comets. The lunar surface is covered by a dust layer with thicknesses typically in the range of feet. This soil layer is called the regolith. The lunar seismometers could easily detect the impact of a stony object because it penetrated through the soil layer to the bedrock and "rang" the Moon's interior. On the other hand, the impact of a fluffy small comet would produce shallow craters in the lunar soil, Fig 20. Apollo astronaut walking on the Moon not sufficiently deep to penetrate to the bedrock. Because of this shallow penetration and the structural fragility of the small comets only weak disturbances would be caused by the small comet impacts. Indeed the lunar surface is known to be saturated with shallow craters with diameters of tens of feet which would be expected from numerous impacts by small comets. A photograph of such a shallow crater is shown in Figure 20 [right] at the right-hand side of the Apollo astronaut.

With the Polar spacecraft confirmations of the existence of small comets in three independent ways there was a new flurry of activity intended to show that their presence was contrary to the lunar cratering record. This evidence was based upon the comparison of Apollo images of the lunar surface in 1972 and images of the same area of the Moon with the Clementine spacecraft in 1994. The comparison of these images separated in time by 22 years in order to search for new impacts on the surface of the Moon due to small comets was a great idea. Unfortunately the work failed as to be noted by the images shown on the cover of Geophysical Fig 21. Apollo and Clementine images of the Moon Research Letters of December 15, 1997 and reproduced here as Figure 21[left]. At the top of the figure is shown the high-resolution Apollo image. A scale bar of 1 kilometer, or 3300 feet, in the bottom image is valid for both images. The dimensions of the shallow craters from the impact of the small comets are expected to be in the range of a few tens of feet at most. The Apollo images are magnificent and do record craters of these dimensions at their limits. The Clementine image of the same area of the Moon is shown in the lower panel of Figure 21 [left]. The resolution of this image is so poor that it takes some examination to convince the viewer that the same surface area of the Moon is being examined. To detect craters of the size associated with small comet impacts is hopeless. To hypothesize huge bright spots as the cometary impacts defies reasonable considerations of the actual impacts. It is disappointing that the Clementine images were too blurry for the comparison. Otherwise the results would have been very exciting.

For many years after the original publication of the small comet papers in 1986 a frequent objection to their presence in the vicinity of Earth was based upon reports on the surface of the Moon during the Apollo missions that there was a remarkable absence of water or water snow. The critics claimed that the frequent impacts of the small comets on the Fig 22. Lunar Prospector Image, Water on Moon lunar surface should have left tell-tale signs of water. Of course, even though the Moon's gravity is relatively low in the sense that it cannot prevent the high-speed water outflow from the small comet impacts from escaping into space, there should be at least a little water ice or snow which is trapped in the surface crevasses and other shadowed areas of the lunar surface. The lunar surface is cold but any water ice or snow is quickly vaporized by direct sunlight. Last year a remarkable search for lunar water was reported by the Lunar Prospector spacecraft which is currently orbiting the Moon. A sensor on board this spacecraft was capable of remotely sensing the presence of hydrogen-bearing substances. The most likely substance with hydrogen is water. One of the first maps of this water on the lunar surface is shown in Figure 22 [right]. The view is the northern polar region of the Moon. The color blue indicates more water and red is lesser quantities. There are many tons of water in the polar region. This fascinating survey continues with the spacecraft orbit being adjusted to achieve lower altitudes and hence to obtain views with better resolution. The Moon is not dry. [Next Page]


Presidential Lecture, Part 1