SUPPOSED CHANGES IN THE MOON.
In this Magazine for August last I considered the moon's multitudinous small craters with special reference to the theory that some among those small craters may have been produced by the downfall of aerolithic or meteoric masses upon the moon's once plastic surface. Whether it be considered probable that this is really the case or not with regard to actually existent lunar craters, it cannot be doubted that during one period of the moon's history, a period probably lasting many millions of years, many crater-shaped depressions must have been produced in this way. As I showed in that essay, it is absolutely certain that thousands of meteoric masses, large enough to form visible depressions where they fell, must have fallen during the moon's plastic era. It is certain also that that era must have been very long-lasting. Nevertheless, it remains possible (many will consider it extremely probable, if not absolutely certain) that during sequent periods all such traces were removed. There is certainly nothing in the aspect of the present lunar craters, even the smallest and most numerous, to preclude the possibility that they, like the larger ones, were the results of purely volcanic action; and to many minds it seems preferable to adopt one general theory respecting all such objects as may be classed in a regular series, than to consider that some members of the series are to be explained in one way and others in a different way. We can form a series extending without break or interruption from the largest lunar craters, more than a hundred miles in diameter, to the smallest visible craters, less than a quarter of a mile across, or even to far smaller craters, if increase of telescopic power should reveal such. And therefore many object to adopt any theory in explanation of the smaller craters (or some of them) which could manifestly not be extended to the largest. Albeit we must remember that certainly if any small craters had been formed during the plastic era by meteoric downfall, and had remained unchanged after the moon solidified, it would now be quite impossible to distinguish these from craters formed in the ordinary manner.
While we thus recognise the possibility, at any rate, that multitudes of small lunar craters, say from a quarter of a mile to two miles in diameter, may have been formed by falling meteoric masses hundreds of millions of years ago, and may have remained unchanged even until now, we perceive that on the moon later processes must have formed many small craters, precisely as such small craters have been formed on our own earth. I consider, at the close of the essay above mentioned, the two stages of the moon's development which must have followed the period during which her surface was wholly or in great part plastic. First, there was the stage during which the crust contracted more rapidly than the nucleus, and was rent from time to time as though the nucleus were expanding within it. Secondly, there came the era when the nucleus, having retained a greater share of heat, began to cool, and therefore to contract more quickly than the crust, so that the crust became wrinkled or corrugated, as it followed up (so to speak) the retreating nucleus.
It would be in the later part of this second great era that the moon (if ever) would have resembled the earth. The forms of volcanic activity still existing on the earth seem most probably referable to the gradual contraction of the nucleus, and the steady resulting contraction of the rocky crust. As Mallet and Dana have shown, the heat resulting from the contraction, or in reality from the slow downfall of the crust, is amply sufficient to account for the whole observed volcanian energy of the earth. It has indeed been objected, that if this theory (which is considered more fully in my "Pleasant Ways in Science") were correct, we ought to find volcanoes occurring indifferently, or at any rate volcanic phenomena of various kinds so occurring, in all parts of the earth's surface, and not prevalent in special regions and scarcely ever noticed elsewhere. But this objection is based on erroneous ideas as to the length of time necessary for the development of subterranean changes, and also as to the extent of regions which at present find in certain volcanic craters a sufficient outlet for their subterranean fires. It is natural that, if a region of wide extent has at any time been relieved at some point, that spot should long afterwards remain as an outlet, a sort of safety-valve, which, by yielding somewhat more quickly than any neighbouring part of the crust, would save the whole region from destructive earthquakes; and though in the course of time a crater which had acted such a part would cease to do so, yet the period required for such a change would be very long indeed compared with those periods by which men ordinarily measure time. Moreover, it by no means follows that every part of the earth's crust would even require an outlet for heat developed beneath it. Over wide tracts of the earth's surface the rate of contraction may be such, or may be so related to the thickness of the crust, that the heat developed can find ready escape by conduction to the surface, and by radiation thence into space. Nay, from the part which water is known to play in producing volcanic phenomena, it may well be that in every region where water does not find its way in large quantities to the parts in which the subterranean heat is great, no volcanic action results. Mallet, following other experienced vulcanologists, lays down the law, "Without water there can be no volcano;" so that the neighbourhood of large oceans, as well as special conditions of the crust, must be regarded as probably essential to the existence of such outlets as Vesuvius, Etna, Hecla, and the rest.
So much premised, let us enquire whether it is antecedently likely that in the moon volcanic action may still be in progress, and afterwards consider the recent announcement of a lunar disturbance, which, if really volcanic, certainly indicates volcanic action far more intense than any which is at present taking place in our own earth. I have already, I may remark, considered the evidence respecting this new lunar crater which some suppose to have been formed during the last two years. But I am not here going over the same ground as in my former paper ("Contemporary Review" for August, 1878). Moreover, since that paper was written, new evidence has been obtained, and I am now able to speak with considerable confidence about points which were in some degree doubtful three months ago.
Let us consider, in the first place, what is the moon's probable age, not in years, but in development. Here we have only probable evidence to guide us, evidence chiefly derived from the analogy of our own earth. At least, we have only such evidence when we are enquiring into the moon's age as a preliminary to the consideration of her actual aspect and its meaning. No doubt many features revealed by telescopic scrutiny are full of significance in this respect. No one who has ever looked at the moon, indeed, with a telescope of great power has failed to be struck by the appearance of deadness which her surface presents, or to be impressed (at a first view, in any case), with the idea that he is looking at a world whose period of life must be set in a very remote antiquity. But we must not take such considerations into account in discussing the a priori probabilities that the moon is a very aged world. Thus we have only evidence from analogy to guide us in this part of our enquiry. I note the point at starting, because the indicative mood is so much more convenient than the conditional, that I may frequently in this part of my enquiry use the former where the actual nature of the evidence would only justify the latter. Let it be understood that the force of the reasoning here depends entirely on the weight we are disposed to allow to arguments from analogy.
Assuming the planets and satellites of the solar system to be formed in some such manner as Laplace suggested in his "Nebular Hypothesis," the moon, as an orb travelling round the earth, must be regarded as very much older than she is, even in years. Even if we accept the theory of accretion which has been recently suggested as better according with known facts, it would still follow that probably the moon had existence, as a globe of matter nearly of her present size, long before the earth had gathered in the major portion of her substance. Necessarily, therefore, if we assume as far more probable than either theory that the earth and moon attained their present condition by combined processes of condensation and accretion, we should infer that the moon is far the older of the two bodies in years.
But if we even suppose that the earth and moon began their career as companion planets at about the same epoch, we should still have reason to believe that these planets, equal though they were in age so far as mere years are concerned, must be very unequally advanced so far as development is concerned, and must therefore in that respect be of very unequal age.
It was, I believe, Sir Isaac Newton who first called attention to the circumstance that the larger a planet is, the longer will be the various stages of its existence. He used the same reasoning which was afterwards urged by Buffon, and suggested an experiment which Buffon was the first to carry out. If two globes of iron, of unequal size, be heated to the same degree, and then left to cool side by side, it will be found that the larger glows with a ruddy light after the smaller has become quite dark, and that the larger remains intensely hot long after the smaller has become cool enough to be handled. The reason of the difference is very readily recognised. Indeed, Newton perceived that there would be such a difference before the matter had been experimentally tested. The quantity of heat in the unequal globes is proportional to the volume, the substance of each being the same. The heat is emitted from the surface, and at a rate depending on the extent of surface. But the volume of the larger exceeds that of the smaller in greater degree than the surface of the larger exceeds the surface of the other. Suppose, for instance, the larger has a diameter twice as great as that of the smaller, its surface is four times as great as that of the smaller, its volume eight times as great. Having, then, eight times as much heat as the smaller at the beginning, and parting with that heat only four times as fast as the smaller, the supply necessarily lasts twice as long; or, more exactly, each stage in the cooling of the larger lasts twice as long as the corresponding stage in the cooling of the smaller. We see that the duration of the heat is greater for the larger in the same degree that the diameter is greater. And we should have obtained the same result whatever diameters we had considered. Suppose, for instance, we heat two globes of iron, one an inch in diameter, the other seven inches, to a white heat. The surface of the larger is forty-nine times that of the smaller, and thus it gives out at the beginning, and at each corresponding stage of cooling, forty-nine times as much heat as the smaller. But it possesses at the beginning three hundred and forty-three (seven times seven times seven) times as much heat. Consequently, the supply will last seven times as long, precisely as a stock of three hundred and forty-three thousand pounds, expended forty-nine times as fast as a stock of one thousand pounds only, would last seven times as long. In every case we find that the duration of the heat-emission for globes of the same material equally heated at the outset is proportional to their diameters.