348 NEWS FROM JUPITER. BY RICHARD A. PROCTOR, B.A. (CAMBRIDGE), THE HE planet Jupiter has passed during the last year through a singular process of change. The planet has not, indeed, assumed a new appearance, but has gradually resumed its normal aspect after three or four years, during which the mid zone of Jupiter has been aglow with a peculiar ruddy light. The zone is now of a creamy-white colour, its ordinary hue. We have, in fact, reached the close of a period of disturbance, and have received a definite answer to questions which had arisen as to the reality of the change described by observers. Many astronomers of repute were disposed to believe that the peculiarities recently observed were merely due to the instruments with which the planet has been observed-not, indeed, to any fault in those instruments, but, in fact, to their good qualities in showing colour. A considerable number of the earlier accounts of Jupiter's change of aspect came from observers who used the comparatively modern form of telescope known as the silvered-glass reflectors, and it is well known that these instruments are particularly well suited for the study of colour-changes. Nevertheless, observations made with the ordinary refracting telescope were not wanting; and it had begun to be recognised that Jupiter really had altered remarkably in appearance, even before that gradual process of change which, by restoring his usual aspect, enabled every telescopist to assure himself that there had been no illusion in the earlier observations. I propose now to discuss certain considerations which appear to me to indicate the nature and probable meaning of the phenomena which have recently been observed in Jupiter. It seems to me that these phenomena are full of interest, whether considered in themselves or in connection with those circumstances on which I had been led to base the theory that Jupiter is a planet altogether unlike our earth in condition, and certainly unfit to be the abode of living creatures. I would first direct special attention to the facts which have been ascertained respecting the atmosphere of Jupiter. It does not appear to have been noticed, as a remarkable circumstance, that Jupiter should have an atmosphere recognisable from our distant station. Yet, in reality, this circumstance is not only most remarkable, but is positively inexplicable on any theory by which Jupiter is regarded as a world resembling our own. It is certain that, except by the effects produced when clouds form and dissipate, our terrestrial atmosphere could not be recognised at Jupiter's distance with any telescopic power yet applied. But no one who has studied Jupiter with adequate means can for a moment fail to recognise the fact that the signs of an atmosphere indicate much more than the mere formation and dissipation of clouds. I speak here after a careful study of the planet during the late opposition, with a very fine reflecting telescope by Browning, very generously placed at my disposal by Lord Lindsay; and I feel satisfied that no one can study Jupiter for many hours (on a single night) without becoming convinced that the cloudmasses seen on his disc have a depth comparable with their length and breadth. Now the depth of terrestrial cloud-masses would at Jupiter's distance be an absolutely evanescent quantity. The span of his disc represents about 84,000 miles, and his satellites, which look little more than points in ordinary telescopes, are all more than 2,000 miles in diameter. I am satisfied that anyone who has carefully studied the behaviour of Jupiter's cloud-belts will find it difficult to believe that their depth is less than the twentieth part of the diameter of the least satellite. Conceive, however, what the depth of an atmosphere would be in which cloud-masses a hundred miles deep were floating! It may be asked, however, in what sense such an atmosphere would be inexplicable, or, at least, irreconcilable with the theory that Jupiter is a world like our earth. Such an atmo sphere would be in strict proportion, it might be urged, to the giant bulk of the planet, and such relative agreement seems more natural than would be a perfect correspondence between the depth of the atmosphere on Jupiter and the depth of our earth's atmosphere. But it must not be forgotten that the atmosphere of Jupiter is attracted by the mass of the planet; and some rather remarkable consequences follow when we pay attention to this consideration. Of course a great deal must be assumed in an inquiry of the sort. Since, however, we are discussing the question whether there can be any resemblance between Jupiter and our earth, we may safely (so far as our inquiry is concerned) proceed on the assumption that the atmosphere of Jupiter does not differ greatly in constitution from that of our earth. We may further assume that at the upper part of the cloud-layers we see, the atmospheric pressure is not inferior to that of our atmosphere at a height of seven miles above the sea-level, or one-fourth of the pressure at our sea-level. Combining these assumptions with the conclusion just mentioned, that the cloudlayers are at least 100 miles in depth, we are led to the following singular result as to the pressure of the Jovian atmosphere at the bottom of the cloud-layer :-The atmosphere of any planet doubles in pressure with descent through equal distances, these distances depending on the power of gravity at the planet's surface. In the case of our earth, the pressure is doubled with descent through about 3 miles; but gravity on Jupiter is more than 21 times as great as gravity on our earth, and descent through 1 mile would double the pressure in the case of a Jovian atmosphere. Now 100 miles contain this distance (1 mile) more than seventy-one times; and we must therefore double the pressure at the upper part of the cloud-layer seventyone successive times to obtain the pressure at the lower part. Two doublings raise the pressure to that at our sea-level; and the remaining sixty-nine doublings would result in a pressure exceeding that at our sea-level so many times that the number representing the proportion contains twenty-one figures.* I say would result in such a pressure, because in reality there are limits beyond which atmospheric pressure cannot be increased without changing the compressed air into the liquid form. What those limits are we do not know, for no pressure yet applied has changed common air, or either of its chief constituent gases, into the liquid form, or even produced any trace of a tendency to assume that form. But it is easily shown that there must be a limit to the increase of pressure which air will sustain without liquefying. For the density of * The problem is like the well-known one relating to the price of a horse, where one farthing was to be paid for the first nail of 24 in the shoes, a halfpenny for the next, a penny for the third, two pence for the fourth, and so on. It may be interesting to some of my readers to learn, that if we want to know roughly the proportion in which the first number is increased by any given number of doublings, we have only to multiply the number of doublings by ths, and add 1 to the integral part of the result, to give the number of digits in the number representing the required proportions. Thus multiplying 24 byths gives 7 (neglecting fractions); and therefore the number of farthings in the horse problem is represented by an array of 8 digits. any gas changes proportionately to the increase of pressure, until the gas is approaching the state when it is about to turn liquid. Now air at the sea-level has a density equal to less than the 900th part of the density of water; so that if the pressure at the sea-level were increased 900 times, either the density would not increase proportionally, which would show that the gas was approaching the density of liquefaction, or else the gas would be denser than water, which must be regarded as utterly impossible. Or if any one is disposed, for the sake of argument, to assume that a gas (at ordinary, temperatures) may be as dense as water, then we need proceed but a few steps farther, increasing the pressure about 18,000 times instead of 900 times, to have the density of platinum instead of that of water, and no one is likely to maintain that our air could exist in the gaseous form with a density equalling that of the densest of the elements. We are still an enormous way behind the number of twenty-one figures mentioned above; and in fact, if we supposed the pressure and density to increase continually to the extent implied by the number of twenty-one figures, we should have a density exceeding that of platinum more than ten thousand millions of millions of times! Of course this supposition is utterly monstrous, and I have merely indicated it to show how difficulties crowd around us in any attempt to show that a resemblance exists between the condition of Jupiter and that of our earth. The assumptions I made were sufficiently moderate, be it noticed, since I simply regarded (i.) the air of Jupiter as composed like our own; (ii.) the pressure at the upper part of his cloud-layer as not less than the pressure far above the highest of our terrestrial cumulus clouds (with which alone the clouds of Jupiter are comparable); and (iii.) the depth of his cloud-layer as about 100 miles. The first two assumptions cannot fairly be departed from to any considerable extent, without adopting the conclusion that the atmosphere of Jupiter is quite unlike that of our earth, which is precisely what I desire to maintain. The third is, of course, open to attack, though I apprehend that no one who has observed Jupiter with a good telescope will question its justice. But it is not at all essential to the argument that the assumed depth of the Jovian atmosphere should be even nearly so great. We do not need a third of our array of twenty-one figures, or even a seventh part, since no one who has studied the experimental researches made into the condition of gases and vapours can for a moment suppose that an atmosphere like ours could remain gaseous, except at an enormously high temperature, at a pressure of two or three hundred atmospheres. Such a pressure would be obtained, retaining our first two assumptions, at a depth of about fourteen miles below the upper part of the cloud-layer. This is about the 6,000th part of the diameter of Jupiter; and if any student of astronomy can believe that that wonderfully complex and changeful cloud envelope which surrounds Jupiter has a thickness of less than the 6,000th part of the planet's diameter, I would recommend as a corrective the careful study of the planet for an hour or two with a powerful telescope, combined with the consideration that the thickness of a spider's web across the telescopic field of view would suffice to hide a breadth of twenty miles on Jupiter's disc. But we are not by any means limited to the reasoning here indicated, convincing as that reasoning should be to all who have studied the aspect of Jupiter with adequate telescopic power. We have in Jupiter's mean density an argument of irresistible force against the only view which enables us even hypothetically to escape from the conclusions just indicated. Let it be granted, for the sake of argument, that Jupiter's cloud layer is less than fourteen miles in depth, so that we are freed for the moment from the inference that at the lower part of the atmosphere there is either an intense heat or else a density and pressure incompatible with the gaseous condition. We cannot, in this case, strike off more than twenty-eight miles from the planet's apparent diameter to obtain the real diameter of his solid globe-solid, at least, if we are to maintain the theory of his resemblance to our earth. This leaves his real diameter appreciably the same as his apparent diameter, and as a result we have the mean density of his solid globe equal to a fourth of the earth's mean density, precisely as when we leave his atmosphere out of the question. Now I apprehend that the time has long since passed when we can seriously proceed at this stage to say, as it was the fashion to say in textbooks of astronomy, "therefore the substance of which Jupiter is composed must be of less specific gravity than oak and other heavy woods." We know that Brewster gravely reasoned that the solid materials of Jupiter might be of the nature of pumicestone, so that with oceans resembling ours a certain latitude was allowed for increase of density in Jupiter's interior. But in the presence of the teachings of spectroscopic analysis, few would now care to maintain, as probable, so preposterous a theory as this. Everything that has hitherto been learned respecting the constitution of the heavenly bodies, renders it quite unlikely that the elementary constitution of Jupiter differs from that of our earth. Again, it was formerly customary to speak of the possibility that Jupiter and Saturn might be hollow globes, mere shells, composed of materials as heavy as terrestrial elements. But whatever opinion we may form as to the possibility that a great intensity of heat may vaporise a |