Popular Science Monthly/Volume 31/August 1887/Changes in the Aspect of Mars
|CHANGES IN THE ASPECT OF MARS.|
THE planet Mars has for a long time signalized itself to observers by the remarkable traits of its constitution. In consequence of its relative nearness, the telescope has been able to furnish us with a number of data respecting its physical geography and its meteorology; and it has been a very rich source of results concerning the philosophy of the solar system and the physical universe in general.
It is well known that Mars displays some bright spots, and others dark, of which we have every reason to consider the former to be continents, the latter seas. Toward the poles appear large white zones, varying in size at different times, which are caps of ice, susceptible of occasional breakings-up like our icebergs. In the thin and transparent atmosphere we can distinguish clouds, currents, and sometimes whirlwinds quite like the cyclones that rage among us.
Besides these intimate analogies with the earth, the study of Mars reveals especial features, some of which are most satisfactorily explained by considerations of comparative geology. With the tenuity of the atmosphere is associated a much smaller extension of the seas, and the relative repartition of land and water is very different from what prevails on the earth. Astronomers observe, as one of the most remarkable peculiarities of the surface of this planetary neighbor of ours, a large number of long and narrow passages and seas like bottle-necks. In our globe the oceans are of three times the surface of the continents; and Europe, Asia, and Africa form together a single island, while another island is formed by the union of the two Americas. But, on Mars, an almost complete equality exists between the surfaces occupied by the continents and by the seas. Further, they are mingled with one another in such a complicated manner that a traveler might visit nearly all the quarters of the planet, either by land or by boat, without having to leave the element on which he began his journey.
This much assumed, it should be recollected that Mars is older in the planetary series than the earth; that is, having been individualized at a more ancient period, and having a smaller volume, it has reached a more advanced stage in the sidereal evolution. Hence the planet represents now, in its great lines and independent of its individual characteristics, a condition which the earth will ultimately attain. One of the effects of the secular cooling of the earth is to determine the progressive absorption of the waters of the ocean by the successively consolidated rocky masses. Hence a striking comparison might be made between the present Martial seas and the terrestrial oceans after we shall have supposed they have been in a more or less great part absorbed. The results of innumerable soundings have permitted the construction of bathymetric charts of our oceans; and thirteen years ago I described the form of the Atlantic Ocean at four thousand metres below the present surface as "bottle-necked." If, then, we suppose the water of the Atlantic to become absorbed in the profound masses at this moment in process of solidification, in such a way that the level of that ocean shall be depressed by four thousand metres, we shall have at the same time a much smaller surface covered by water, and a narrow and elongated form of the seas, or exactly the conditions which Mars presents. At the same time that the water is thus drunken up, the air also will be undergoing absorption. All the rocks are aerated. "We know what trouble we have in driving the air from even the most compact rock of which we wish to obtain the density with precision. Since the different mineral masses become aerated while they are becoming moist, and consequently, while they are cooling, the atmospheric strata should undergo a progressive decrease. It is, therefore, natural that the atmosphere of Mars should be much thinner than that of the earth; and that is an excellent condition for the telescopic study of the planet.
For the earth, geology furnishes a kind of indirect confirmation of this progressing absorption of the atmosphere. The results of the experiments of physicists, of Mr. Tyndall in particular, go to show that a slight augmentation in the thickness of our atmosphere or in the proportion of vapor it contains, would suffice to cause the solar heat to be stored in larger quantity and wasted more slowly; that is, in short, would make what we call climates disappear—a warm and nearly equable temperature prevailing over all the earth. Now, one of the most remarkable characteristics of the ancient geological periods is just this absence of climate, which is indicated by the uniformity of fauna and flora over the whole planet; and this confirms our opinion that the atmosphere once formed a much thicker bed than it does now.
While there thus exist traits in common between Mars and the earth, a strange motive of interest lies in the existence on the surface of the former globe of very important details of structure which are without analogy with us. M. Schiaparelli first perceived, in 1877, in the continents of Mars, which had been till then very large and without solution of continuity, a system of dark channels, often very slender, which divided the surface into a multitude of lands isolated and separated from one another like the meshes of a net. Notwithstanding the tenuity of these channels, they are not less than one hundred and twenty kilometres in breadth, while some of them are fully forty-eight hundred kilometres long. These results were at first received with incredulity by astronomers, who were afterward, however, constrained to recognize their rigorous exactness. The works of the distinguished director of the observatory at Milan upon this subject, of which the last one, relative to the opposition of 1879-'80, constitutes a quarto memoir of one hundred and nine pages and six plates, are very interesting.
But it is since this magnificent publication appeared that the author, on the occasion of the opposition of Mars of 1881-'82, was a witness of some marvelous changes which are fully described in a memoir that has not yet appeared, but which M. Schiaparelli has kindly sent me. It appears from these observations, and from others which were made between 1884 and 1886, that Mars is at this moment the theatre of phenomena of stupendous grandeur which will be adequate in a few years to impress profound changes in its aspect. The views taken by M. Schiaparelli show that a number of the channels previously described are doubling, or are being paralleled by similar ones having the same dimensions and directions. The appearance on the new map of the hemisphere is nearly the same, Dr. Terby, an eminent areographer, of Louvain, suggests, as would be produced in the former one by covering it with a double-refracting crystal. To this phenomenon, which has no analogy, M. Schiaparelli has given the name of the gemination of canals; and he has prepared a full memoir respecting it, which will shortly appear.
Although they were met at first with incredulity, these astonishing discoveries have received and are receiving constant confirmation from the observations of such men as Boeddicker and Burton in Ireland, M. Perrotin of the observatory at Nice, and his collaborators, MM. Trepied, Thollon, and Gautier. Other observers, like Messrs. Green, Knobel, and Denning, have not been so fortunate in verifying his facts, but their researches, published in the memoirs of the London Astronomical Society, and in the "Monthly Notices," are full of interest. It adds to the mystery that the gemination seems to be made gradually, though rapidly, and with progressive accentuation. Thus, the canal Nilus, at the junction of the eightieth meridian and parallel 20° north, is paralleled by another canal, which is very faintly marked and hardly visible in the older map. In the new map the two canals have a nearly equal intensity.
By careful comparison of Schroeter's designs, made a century ago, and Herschel's earlier ones, M. Terby has discovered analogous modifications on the Martial surface. Some of them are local enlargements of some of the seas, like that called Kaiser, and other changes in the details of configurations which had been supposed to be fixed. Of similar character is the memoir of M. Van de Sand Baghuyzen, in the "Annals of the Observatory of Leyden," in which the author interprets the designs of Schroeter and finds in them a trace of many of M. Schiaparelli's details. Père Lamey has also made many observations of Mars, which have led him to some original conclusions worthy of investigation.—Translated for the Popular Science Monthly from La Nature.