The Moon: considered as a planet, a world, and a satellite.

by James Nasmyth; James Carpenter

PLATE VIII.
COPERNICUS.

Aqueous vapour being denied us, we must look in other directions for an ejective force. Of the nature of the lunar materials we can know nothing, and we might therefore assume anything; some have had recourse to the supposition of expansive vapours given off by some volatile component of the said material while in a state of fusion, or generated by chemical combinations. Professor Dana refers to sulphur as probably an important element in the moon’s geology, suggesting this substance because of the part which it appears to play in the volcanic or igneous operations of our globe, and on account of its presence in cosmical meteors that have come within range of our analysis. Any matter sublimated by heat in the substrata of the moon would be condensed upon reaching the cold surrounding space, and would be deposited in a state of fine powder, or otherwise in a solid form. Maedler has attributed the highly reflective portions of some parts of the surface, such as the bright streams that radiate from some of the craters, Copernicus and Tycho for instance, to the vitrification of the surface matter by gaseous currents. But in suppositions like these we must remember that the probability of truth diminishes as the free ground for speculation widens. It does not appear clear how expansive vapours could have lain dormant till the moon assumed a solid crust, as all such would doubtless make their escape before any shell was formed, and at an epoch when there was ample facility for their expansion.

While we are not insensible of the value of an expansive vapour explanation, if it could be based on anything beyond mere conjecture, we are disposed to attach greater weight to that afforded by the principle sketched in our third chapter, viz., of expansion upon solidification. We gave, as we think, ample proof that molten matter of volcanic nature, when about passing to the solid state, increases its bulk to a considerable degree, and we suggested that the lunar globe at one period of its history must have been, what our earth is now, a solid shell encompassing a molten nucleus; and further, that this last, in approaching its solid condition, expanded and burst open or rent its confining crust. At first sight it may seem that we are ascribing too great a degree of energy to the expansive force which molten substances exhibit in passing to the solid condition, seeing that in general such forces are slow and gradual in their action; but this anomaly disappears when we consider the vast bulk of the so expanding matter, and the comparatively small amount that in its expansion it had to displace. It is true that there are individual mountains on the moon covering many square miles of surface, that as much as a thousand cubic miles of material may have been thrown up at a single eruption; but what is this compared to the entire bulk of the moon itself? A grain of mustard-seed upon a globe three feet in diameter represents the scale of the loftiest of terrestrial mountains; a similar grain upon a globe one foot in diameter, would indicate the proportion of the largest upon the moon. A model of our satellite with the elevations to scale would show nothing more than a little roughness, or superficial blistering. Turn for a moment to our map ([Plate IV].), upon which the shadows give information as to the heights of the various irregularities, and suppose it to represent the actual size of some sphere whose surface has been broken up by reactions of some kind of the interior upon the exterior—suppose it to have been a globe of fragile material filled with some viscous substance, and that this has expanded, cracked its shell, oozed out in the process of solidification, and solidified: the irregularity of surface which the small sphere, roughened by the out-leaking matter, would present, would not be less than that exhibited in the map under notice. When we say that a lunar crater has a diameter of 30 miles, we raise astonishment that such a structure could result from an eruption by the expansive force of solidifying matter; but when we reflect that this diameter is less than the two-hundredth part of the circumference of the moon, we need have no difficulty in regarding the upheaval as the result of a force slight in comparison to the bulk of the material giving rise to it. We have upon the moon evidence of volcanic eruptions being the final result of most extensive dislocations of surface, such as could only be produced by some widely diffused uplifting force. We allude to the frequent occurrence of chains of craters lying in a nearly straight line, and of craters situated at the converging point of visible lines of surface disturbance. Our map will exhibit many examples of both cases. An examination of the upper portion (the southern hemisphere of the moon) will reveal abundant instances of the linear arrangement, three, four, five or even more crateral circles will be found to lie with their centres upon the same great-circle track, proving almost undoubtedly a connexion between them so far as the original disturbing force which produced them is concerned. Again, in the craters Tycho (30), Copernicus (147), Kepler (146), and Proclus (162), we see instances of the situation of a volcanic outburst at an obvious focus of disturbance. These manifest an up-thrusting force covering a large sub-surface area, and escaping at the point of least resistance. Such an extent of action almost precludes the gaseous explanation, but it is compatible with the expansion on consolidation theory, since it is reasonable to suppose that in the process of consolidation the viscous nucleus would manifest its increase of bulk over considerable areas, disturbing the superimposed crust either in one long crack, out of the wider opening parts of which the expanded material would find its escape, or “starring” it with numerous cracks, from the converging point of which the confined matter would be ejected in greatest abundance and, if ejected there with great energy and violence, would result in the formation of a volcanic crater.

The actual process by which a lunar crater would be formed would differ from that pertaining to a terrestrial crater only to the extent of the different conditions of the two globes. We can scarcely accept Scrope’s term “basal wrecks” (of volcanic mountains that have had the summits blown away) as applicable to the craters of the moon, for the reason that the lunar globe does not offer us any instance of a mountain comparable in extent to the great craters and whose summit has not been blown away. Scrope’s definition implies a double, or divided process of formation: first the building up of a vast conical hill and then the decapitation and “evisceration” of it at some later period. There are grounds for this inferred double action among the terrestrial volcanoes, since both the perfect cone and its summitless counterpart are numerously exemplified. But upon the moon we have no perfect cone of great size, we have no exception whereby the rule can be proved. It is against probability, supposing every lunar crater to have once been a mountain, that in every case the mountain’s summit should have been blown away; and we are therefore compelled to consider that the moon’s volcanic craters were formed by one continuous outburst, and that their “evisceration” was a part of the original formative process. We do not, however, include the central cone in this consideration: that may be reasonably ascribed to a secondary action or perhaps, better, to a weaker or modified phase of the original and only eruption.

Fig. 20.

Fig. 21.