Popular Science Monthly/Volume 10/March 1877/Is the Moon a Dead Planet?
IT is not a little curious that the great body which for ages has been proverbial for its changeableness should have at last come to be looked upon as the most unchangeable of bodies. When the earth was regarded as constituting the universe, and the heavenly bodies as mere exhalations from it, the moon was, of course, believed to be nothing but a meteor—a great lantern hung in the sky to illuminate and rule the terrestrial night; but, when modern astronomy had established the idea that the earth is but a moving planet, and the planets themselves great orbs like our own globe, speculation inevitably arose in regard to their condition. It was then concluded that the moon may be like the earth, with its oceans, plains, mountains, atmosphere, vegetation, and inhabitants; and this idea long prevailed as a part of the great doctrine of the plurality of worlds. But an opposite opinion at length grew up among astronomers, which has been greatly strengthened in recent years. This change of view has been largely ascribed to the celebrated astronomer Mädler, who made a very forcible statement of the differences and contrasts between the condition of the moon and that of the earth, and pointed out that the current view that the moon may be a copy of the earth is impossible. These views crept into astronomical text-books, and gradually led to the conviction that the moon is a sort of played-out or defunct planet, destitute of air and life—a mere mass of rocks and cinders, cold, lifeless, and unchangeable.
But although this view is still current among astronomers in general, there is a class of astronomers (selenographers, as they are called) who have studied the lunar surface with long and profound attention, and to whom we are indebted for our present knowledge of our satellite, who hold a different view. They agree in the belief that many processes of actual lunar change are in progress, and they have detected the existence of a lunar atmosphere. This conflict of opinion is said to be due to the fact that the labors of selenographers are accessible to most astronomers, and are hence but imperfectly known. The inquiry, we are told, is one of great difficulty, requiring a thorough acquaintance with several branches of science. The results arrived at by prolonged, minute, and careful observation of the details of the lunar formation are rejected because the proofs on which they rest are not well understood.
The most prominent instance of supposed lunar change on the surface of the moon is that of the crater Linné. On the northwest quadrant of the moon, near the centre of a level tract about 430 miles in diameter, there is a bright crater called Bessel, nearly 14 miles in diameter, with a circular wall rising 4,000 feet above the interior, and about 1,600 feet above the surrounding plain. Scattered over this plain are a few small craters, some 22 miles in diameter, with walls about 300 feet high. Near its eastern centre an eminent selenographer named Lohrman placed a distinct, bright crater about five miles in diameter, which he described as being, after Bessel, the most conspicuous object on this great tract of level ground. Ten years later, our greatest selenographer, Baron von Mädler, confirmed Lohrman's observations, and made this crater a subject of special study, naming it Linné. In the drawings of Schmidt, who was about this time making lunar observations of this particular part of the moon, Linné is shown as a deep crater corresponding with the descriptions of Lohrman and Mädler.
In October, 1866, when Linné was in a position to be most conspicuous, Schmidt was startled by finding no trace of the deep, wide crater, but only a faint cloud marking about five miles in diameter. Schmidt at once announced the circumstance, and nearly every astronomer in Europe turned his attention to the spot. But Linné has never since been seen of the size and character given it by Lohrman, Mädler, and Schmidt. This large crater unquestionably no longer exists. Powerful telescopes reveal, in its place, a white, cloudy marking containing a small crater-cone with an opening scarcely one-twentieth the size of the former crater.
The reason why astronomers will not admit the reality of any change in Linné is, first, a strong prejudice against the possibility of such change; and, second, the fact that Schröter, of Lillienthal, the earliest of the great selenographers, in one of his first drawings, made November 5, 1788, with low powers, does not draw Linné as a crater. Near its place he draws a white spot on a ridge marked V, and a larger spot marked G. Schmidt took this white spot to be Linné, a view strongly urged by Huggins, and accepted as correct by astronomers. They say, as Schröter's drawing is not unlike the present appearance of Linné, Lohrman, Mädler, and Schmidt, must have been mistaken as to what they thought they saw. Again, in a map made by Lahire in the seventeenth century, no trace of Linné can be found. It may be said, however, that all the maps of this period omit numerous craters far larger than Linné. They were principally full-moon drawings, where Linné would not be visible as a crater. Riccioli's map, however, shows Linné as a distinct crater. But the present crater on the site of Linné could not possibly have been seen by Riccioli with the optical means at his command.
In every other instance of discrepancy between the drawings of Schröter, and Beer, and Mädler, Schröter's are rejected, while in this particular case one of Schröter's earliest drawings, made with imperfect instruments, is brought forward to prove the incorrectness of his great successors.
It will require long study of this region with powerful telescopes to determine the nature of the change undergone by Linné. From numerous observations the explanation agreeing best with the present condition of the surface is, that the walls of the old crater have collapsed and fallen into the interior. In this way the interior would be almost entirely filled up, leaving a rough, cone-like crater in the centre. Under favorable conditions, with a powerful telescope, the surface immediately around the small crater appears rough and irregular. Round the border of the old crater are numerous mounds and blocks, and on the east, one or two peaks or low hills, seeming like portions of the old wall. The difficulty of making these observations is very great, and they are only possible in the finest atmospheric conditions.
Proctor has tried to show that the changes in Linné are variations of tint due to differences of illumination. But no selenographer will admit that any alteration whatever in illumination could make an object where Linné is placed, look at one period like a considerable and deep crater, and at another as a small, scarcely visible crater.
The facts about Linné may be therefore summed up very briefly. According to three or more independent selenographers, the most experienced that science has seen, the object named Linné was a conspicuous crater of large diameter and great depth. Now, in its place all that exists is a tract of uneven ground, containing a small, scarcely visible, insignificant, crater-like object, It is impossible that one could ever be systematically taken for the other. It is inconceivable how our three greatest selenographers could have systematically and independently made the same blunder, and that one blunder only. For in no other case do we find any error of this nature. Their description must, therefore, be held to truly describe the nature of the formation at their epoch (1820-'45). The object is no longer of the same size and description. A real physical change on the moon's surface must therefore have occurred at this point. This, then, is the conclusion that selenographers as a body have arrived at; yet, despite the strong evidence on which it rests, it is not generally recognized by astronomers.
The next instance of change on the surface of the moon is that of the crater named Messier by Beer and Mädler. In its equatorial region, on the westernmost of the great lunar plains, close to one another, are two small crater-plains about nine miles in diameter, surrounded by very low ridges and mounds and crater-like depressions. These two formations, named Messier and Messier A, were discovered and described by Schröter, who regarded Messier as slightly the larger. Beer and Mädler examined these formations most carefully on more than three hundred distinct occasions between 1829 and 1837. They declared that the two crater-plains were exactly alike in every particular. Both were circular and of the same size, with bright, grayish white walls and a yellowish-gray interior. The walls were of the same height, with wall-peaks situated in the same relative position. In diameter, form, height of walls above the surrounding surface, and depth and color of the interior of the walls, Beer and Mädler declared that they were completely alike. Some years after this, a slight dissimilarity between the forms of the two craters was noticed, and in November, 1855, the Rev. T. W. Webb, one of the best living lunar observers, discovered that the eastern crater-plain, Messier A, appeared the larger of the two.
In March of the following year he observed that not only was Messier the smaller, but that it was elliptical. He confirmed these observations on repeated occasions, and in 1857 made drawings showing Messier A unchanged, while Messier had an elliptical form, with a long diameter of about 10 or 11 miles and a short diameter of about 72 to 8 miles. The matter attracted little further attention until 1870 to 1875. During these years Messier and Messier A were studied with the aid of powerful telescopes, and during the past year the long diameter of Messier appears to be 12.2 miles and the short diameter 6.9. The difference between the form and dimensions of these two formations is now obvious in the smallest astronomical telescope. It is inconceivable that Beer and Mädler could have failed to recognize these differences with their fine Fraunhofer equatorial, with which, on hundreds of different occasions, they carefully scrutinized them in search of differences.
This slow squeezing out of shape of an immense crater-plain is a change that seems to defy explanation. Nothing analogous now exists on the surface of the earth, and it is not surprising that there should be a strong reluctance to admit that such a change has occurred. A careful examination of Messier and its neighborhood, however, suggests that, instead of a bodily compression of the entire crater, where has been a gradual sliding of the north and south walls into the interior, and a pushing of the entire western wall outward and westward down an incline existing there. Stenographers could point to a hundred cases where a like circumstance has occurred. As far as is at present known, this explanation accords with the condition of the formations around Messier, but further observations with powerful telescopes are indispensable. Unfortunately, it is a difficult matter to study, for on only one or two days in the year is Messier likely to be found in a proper condition to be observed.
Other instances of change in the lunar surface have been detected by selenographers, but the evidence on which they rest is not so overpowering as is needed to induce their acceptance.
Periodical variations in the color and brightness of lunar regions, such as would result from processes of vegetation, were first noticed by Beer and Mädler, but they regarded the absence of masses of water upon the moon as a fatal objection to this explanation. The variation in the floor of Plato is one of the most interesting of these changes. Plato is a circular plain 60 miles in diameter, surrounded by a belt of highlands from 3,000 to 3,500 feet in height. These highlands at sunrise are a pale yellowish gray, gradually changing to grayish white. At sunrise the interior of Plato appears of a cold gray, but, as the sun rises higher above the horizon of Plato, and the solar rays fall more perpendicularly on this region, the whole surface grows rapidly brighter, until, about two days after sunrise, the interior of the formation attains its brightest tint. It is then a cold, light yellow gray, often approaching a pale yellow, in fact, and brighter than the surface of the Mare Imbrium on the north, while the surrounding highlands are a bright grayish white, tinted here and there with gray. Judging from what occurs in any of the numerous other formations resembling Plato, this may be considered the normal tint, inasmuch as those other formations which present exactly the same phenomena up to this time, and which, under similar conditions, present exactly the same appearance, retain this tint unaltered until near sunset. After, however, the second day, the floor of Plato commences to undergo a most extraordinary and anomalous change, which renders it unique on the moon, for, instead of growing lighter, the interior commences to become darker. Four days after sunrise it is materially darker than the northern Mare, and a cold gray in tint, while the surrounding highlands are a bright white in color, tinted with gray; the appearance they retain until the thirteenth day after sunrise, growing a little, though not very much, brighter toward full moon. Two days later the floor of Plato has become a dark gray; at full moon it is deep steel-gray; and, about two days after full, reaches its darkest tint, a very deep steel-gray, almost approaching a black color. Under these conditions, it is one of the very darkest portions of the entire lunar surface, though, seven days prior, it was one of the lightest portions of the surface of its kind. After this, it gradually lightens in tint, but much slower, and never reaches so light a tint.
This extraordinary periodical change in the tint of the floor of Plato has hitherto received no explanation, but its existence has been put beyond the pale of doubt, and Mr. Birt has, at the instance of a British Association Committee, carefully discussed a numerous series of observations, made in the years 1869, 1870, and 1871, by six or seven independent observers.
Proctor has attempted to show that these appearances are the effect of contrast. Thus, at sunrise, the floor of Plato is thrown against a dark background, due to the sombre, barely-illuminated surrounding regions; while at full moon it has for a background the brilliantly illuminated surrounding highlands, and should look much darker. But if the great darkening observed to occur in the tint of the interior of Plato is merely apparent, and only what must occur when a darkish walled plain is surrounded by a bright background, or, rather, bright environs—and this is all Mr. Proctor ascribes to it—it must be a perfectly normal occurrence, and the same must take place in every similarly-placed formation, unless that has something anomalous about it to prevent this taking place. All selenographers could instance a number of such walled plains where no such darkening occurs. Are we, then, to assume that these plains possess anomalously constituted interiors, and that only Plato, of all the lunar formations, exhibits the normal phenomena? This is, of course, entirely inadmissible, and selenographers are thoroughly aware that the effects of contrast alluded to by Mr. Proctor are entirely incapable of bringing about such an immense darkening in tint as is apparent in the case of the floor of Plato.
The reason of this singular darkening in the floor of Plato is not yet understood. Selenographers believe that it results from an actual change due to the heating action of the solar rays. But the further elucidation of the subject requires special observations with special appliances. Thus, as in so many selenographical problems, patient observation establishes the existence of certain phenomena, but the elucidation of the meaning of the phenomena established is checked for want of special observations that are never made. For these, selenographers have to appeal to those astronomers devoted to what has been termed astronomical physics, but they are too much engaged on more fascinating subjects to give attention to such inquiries.
The instances here dealt with will show that selenographers are not without strong evidence in favor of the opinion that has long been unanimously held by them, that processes of change are still actively at work upon the moon. It must not, however, be supposed that the above are the sole instances which they have recognized, because this is by no means true, only the difficulties in establishing others have led to their omission here. Dealing with a subject such as selenography, it is only those who are familiar with all its details who properly appreciate the evidence in favor of or against its problems. The difficulties in the way of making the true bearings of selenographical questions properly understood are greater than might be imagined; for even the very elementary fact that volcanic changes such as are now active on the earth would not be recognizable on the moon, in the present state of our acquaintance with the configuration of its surface, is not generally understood. When, however, the attention of astronomers is more generally directed to the study of the lunar surface, Science will he greatly the gainer, as it is there that the past and future of our earth is to be learned.