Popular Science Monthly/Volume 22/February 1883/The Formation of Lunar Craters
M. JULES BERGERON says, in a paper communicated to the French Academy of Sciences, and republished in "La Nature": "I have noticed that when gases or vapors pass through a mass having the consistency of paste, they leave behind them funnel-shaped holes. Struck by the analogy which these holes present with the craters of the moon, I have endeavored to reproduce the phenomena on a larger scale. To simplify my experiments as much as possible, I used alloys, melting at relatively low temperatures, taking first Wood's alloy, of seven parts of bismuth, two of cadmium, two of tin, and two of lead, which melts at about 158° Fahr. Having introduced into the mass, melted in the salt-water bath, a current of warm air by means of a tin pipe, I allowed it to cool slowly while the inflation of warm air was still continued. The ebullition which took place reached all the parts—which were beginning to solidify and form a pellicle—over a considerable surface; and there was formed before me a large circle, around which the edges gradually rose under the continued inflation, till it began to assume the appearance of a crater. At the same time, the metallic mass becoming thicker as the cooling went on, while it was still blown out by the air, could no longer drive the solid particles away from itself, and rose above the crater in such a way as to form a cone, which grew visibly more prominent. The crater also became more hollow, with its inner walls more inclined than the outer walls, and I had before me a formation strikingly analogous to the craters of the moon. The same phenomena were observed, whatever alloy I employed.
"Similar processes have possibly taken place on the moon. Instead of gas, the reliefs may have been produced by the vapors, which rose freely from the body while it was in a fluid state; but the superficial part of the planet being cooled much more rapidly than the interior, the latter, still fluid, continued to emit vapors after the surface had become quite thick. The vapors found their way along the superficial envelope, and came out only at particular points, where, doubtless, the process of solidification was least nearly accomplished. The vapors may subsequently have been condensed, or absorbed, by the substances constituting the rock of the moon.
"As my first experiments were made in a capsule, the objection might be made that the circular form of the crater was produced under the influence of the shape of the walls of the vessel. To obviate such criticism, I employed a rectangular basin, in which I melted an alloy of four parts of lead, four of tin, and one of bismuth. The phenomena were produced as in the former case; but I found that the aspect of the mass after the formation of the crater varied according to what metal was used. With Wood's alloy, which is very fusible, the projections that fell on the edge of the crater flowed away, and left no trace of their passage. With the second alloy the projections
all continued visible, and gave a rent aspect to the crater. Moreover, since the warm air was not hot enough to melt the metal, the projections might eventually overhang the bottom, as appears in Fig. 1.
"The second experiment was marked by a very interesting incident. Two concentric circular areas were noticed, the one nearer to the center being the higher. This was due to an interruption to the passage of the air during the formation of the crater. The edges of Copernicus, Archimedes, and several other lunar craters are marked by analogous features.
"A formation like a dike appears to rise in the center of a considerable number of craters on the moon. I have been able to produce something analogous to this also, a representation of which is visible in Fig. 2. After I had ceased blowing in air, a last bubble was formed, which uplifted the mass, but could not project it above the edge of the crater. The lunar dikes are very probably formed in this way, by the action of the gas, at the end of the active period of the craters."