Page:The Moon (Pickering).djvu/41

 CHAPTER IV

theories have been advanced at different times to explain the origin of the lunar craters. Thus some have thought them the scars of huge bubbles that had burst, others that they were formed out of ice, while still others supposed that they were the scars left by the fall of enormous meteorites. The most natural theory is that they were formed very much like some of the craters that we find upon our Earth.

Let us examine these various theories a little more in detail. It is true that in volcanic regions where the lava is very viscous huge bubbles many feet in diameter sometimes form. When they burst they sometimes leave scars not unlike the lunar craters, but they do not have a central peak, such as is common upon the Moon, nor is it possible to conceive of bubbles one hundred or more miles in diameter.

The ice theory, first suggested by S. E. Peal, supposes the site of each crater to have been originally occupied by a pool of water, kept warm from below. The water evaporated and was deposited as snow in the region immediately surrounding it, thus building up the crater walls. This theory does not account for the central peak, nor for the rough and jagged character of the crater walls. Since the craters must be very ancient, if the walls were composed of ice they would long before this have flattened out in the processes of glacial flow. To accomplish this upon the Earth would require only a few years. On account of the lessened force of gravity, it would take just six times as long to do it upon the Moon.

The meteoric theory was first suggested by Proctor in 1873. Meteors of the necessary size, however, that approached with planetary velocities, would on striking the Moon generate so much heat that it would not only completely melt the meteor, but also the crater walls that it might form, besides a considerable area of the surrounding country. Moreover, many of the apertures would be elongated where the meteor struck the surface at a considerable angle. We have already seen that when the Moon first separated from the Earth it could not at once have taken its present form, but must have been more or less fragmentary. Gilbert, in 1893, showed that these fragments, which would 21