Popular Science Monthly/Volume 24/December 1883/Surface Characters of the Planet Mars
SCHIAPARELLI continued his observations of the topography of the planet Mars during its last opposition, i. e., from October 26, 1881, to the end of February, 1882, and his results were communicated in a preliminary report early in March to the Accademia dei Lincei, of Rome.
Owing to the prevailing weather, his observations were restricted to fifty—days from the end of December to the beginning of February. Among these, sixteen evenings were remarkably favorable, so much so that the greatest magnifying powers could be used.
It was therefore possible, notwithstanding the fact that the apparent diameter of Mars was not over 16" (against 19" in 1877), to obtain results which surpass all previous endeavors. Beginning with the white polar spots, Schiaparelli first mentions that the northern polar spot was always more or less visible. During the months of November and December it appeared separated into several branches or masses, as was also the case in 1879. In the latter half of January these branches began to amalgamate and form a regular, continuous, and uniform calotte, the diameter of which reached about 50° at the beginning of February, and then decreased in a distinctly noticeable manner; while, On the contrary, the southern polar spot remained invisible during the entire period of the observations, even in January and February, when the south pole entered the field of view 2°. From this, in connection with the experience gained in 1879 relative to the visibility of the spot, he concludes that eight months after the southern solstice it had not yet attained a diameter of 20°—a diameter which, according to the observations during the previous opposition, it generally attained to a few weeks before this solstice.
During the course of the observations, various white or whitish spots made their appearance at the southern edge of the planet, greatly resembling the polar spot, but after exact examination and measurement proved to be one or the other of the well-known southern islands of the planet, which appeared white around their edges in consideration of a property peculiar to these localities.
The dark portion (ocean?) which surrounds these islands did not seem to possess this property; and, in order to explain how the polar spot, during the southern winter on Mars, can occupy a part of this locality, it becomes necessary to make the assumption that at such times this part undergoes such changes that it is enabled to appear of a bright white color.
Similar white or whitish spots were observed at intervals at other points of the yellow surface of the planet; some of the better determined points, which had already been noticed in 1877 and 1879, were also visible on this occasion, while others remained invisible. A number of white spots were observed, which, however, were only temporary, particularly in the neighborhood of the northern polar calotte. Emanating from this position, there often would be noticed white inclined stripes passing toward the equator of the planet; the arrangement of these seemed to be dependent upon the rotation of Mars—other positions near the edge of the planet likewise presented a whitish appearance.
A general dimming of the white spots which hid the configuration of the planet was observed on the 18th of January, between the meridians of 40° and 120°. It extended only over the yellow-portions, which are supposed to be continents, and often covered the canals, but completely avoided the darker portions, which represent the oceans and larger lakes. It was not a contiguous covering, but consisting of white or whitish spots, which were irregularly distributed.
The atmosphere of Mars appears to have been more transparent than during 1877. Not only the luminous and the opaque zone of the rim were smaller, but in some parts of the planet the contrast between the light and shade was more distinctly visible with an inclined illumination, and so it was possible to more readily distinguish objects at the edge of the planet than at the center.
During November the north pole advanced some 7° to 8° within the circle of the visible hemisphere; but the hope of being enabled to examine the surface in the vicinity of this pole was unrealized on account of the unfavorable weather. For this reason the limit of the chart of 1881-'82 does exceed 60° north latitude, and, hence, does not extend much beyond the portions explored in 1879; but the parts lying between 30° and 60° northern latitude could be more closely examined. On this occasion also the lower end of the chart is limited by a series of dark stripes which appear to be connected with the northern ocean. The peculiar character of the surface of Mars can not, however, be well explained until after the next opposition. It was impossible to explore the southern ocean with exactness beyond 50° south, although all of the islands which had previously been recognized were observed as white spots similar to the polar snow. All of the smaller seas which branch off from the equator were very distinct in their configuration. The continents and the interior lakes between the bright equatorial zone and the south ocean could be drawn with the greatest accuracy. A few changes in the appearance of particular portions as compared with their shape in 1879 were noticed, and as hundreds of thousands of square kilometres of surface, which were formerly light, had in the mean while become dark, so on the other hand many of the sections which previously were dark now became luminous. These changes prove that the darkening principle which produces them is due to something which is movable and extends over the surface of the planet (for instance, water or some other liquid), or perhaps something capable of being transmitted from place to place (such as vegetation).
Not one of the old dark lines which have been called "canals" was missing, and causes which in all probability were due to the sun produced numerous phenomena, which in former oppositions were only suspected. That brilliant, light-red color mixed with white, which in 1877 occupied the whole of the equatorial zone and a large part of it in 1879, was found in 1882 to be entirely absent. Undefined shadows began to form in this luminous veil surrounded by stains of an orange-yellow color; these shadows become darker by degrees, concentrate themselves and absorb bodies by changing into groups of more or less black lines; at the same time the orange color extends, and finally, with but little exception, covers the whole of the so-called continental zone.
The large areas of the so-called "Alcionia" ocean and gulf, which in 1879 appeared to belong to the "ocean," resolved themselves into complicated bunches of definite lines. Finally, one could see what we have every reason to believe is the true aspect of the planet. Besides this, we noticed the peculiar and unexpected phenomena of the doubling of the canals, which will probably tend to considerably alter the present views of the physical characters of the planet. This doubling is clearly not an optical effect, dependent upon the increased optic power, as is the case in the double stars; nor is it produced by the longitudinal division of a canal. It takes place under the following circumstances: To the right or left of an existing line, without any change in its direction or position, another parallel line is produced which differs from the first in appearance and direction only in exceptional cases. Between the lines so produced, the distance varied from 12° to 6° (350 to 700 kilometres). Among certain of the lines doubling could only be suspected, but not observable at the small distance (5°) separating them. Sometimes a line was darker or broader at two or more points, and the accompanying line would also show this peculiar feature. The length of each pair may differ considerably, and vary from 15° to 80°. Some were of a reddish-brown color, somewhat darker than the ground from which they could be distinguished; others, generally the finer ones, were very dark. The broader ones formed true bands, the sides of which were perfectly parallel. They followed (as far as could be judged without exact measurements) the direction of the large circles of the planet, and only in a few cases were they bent off toward the side. No irregularities could be observed among them with the magnifying (417) power used. Certain of them show such great regularity that they might be designated as a series of parallel lines drawn by the aid of a ruler. In some cases, several pairs would combine, one behind the other, and form a double polygonal line; with very definitely marked angles such a series would occupy a great extent. This phenomenon of doubling appears to be connected with certain epochs—and it takes place almost simultaneously over the entire surface of the planet, covered by the bright portions (continents?). Not a trace of these was observed in 1877 during the weeks which followed the southern solstice of the planet. A single isolated instance was noticed in 1879 on the 26th of December. The appearance of this doubling was the more surprising, as a careful examination on December 23d and 24th gave no cause for suspecting any such change. During the last opposition, a reappearance of this phenomenon was impatiently looked for, but it did not show itself for two months, and then later than was expected; at first indistinct and dim, but becoming more distinct on the following day. This was one month after the autumnal equinox of Mars. The doubling continued to be visible until after the end of February. On the 11th of January another doubling had already made its appearance, but was not further noticed because the canals which doubled were very irregular. Great, therefore, was our surprise to find that, on the 19th of January, a canal which passed through the center showed two straight parallel lines, which, on repeated examination, were found to be true phenomena. From this date the number of canals appearing doubled increased; even on the 24th of February when the apparent diameter of Mars had been reduced to less than 10°, the doubling of the canals could be distinguished. In an aggregate (exclusive of a few cases which could not be configured on account of the insufficient power of the telescope to define such delicate cases), some twenty cases of doubling were noticed, seventeen of which occurred in the course of one month, i. e., from January 19th to February 19th—the mean of the time corresponding to about the end of the second month after the autumnal equinox of the planet. In addition to these there were probably others which made their appearance; but, unfortunately, the unfavorable weather and the increasing distance of the planet prevented a successful following up of the further development of these highly important phenomena. In a few cases it was possible to determine some premonitory signs of the doubling.
On January 13th a very light and indefinite shadow began to spread itself parallel to the canal known as "Ganges"; on the 18th and 19th these portions were covered with white spots, on the 20th the Ganges appeared to be composed of two lines, but the phenomenon was still doubtful; on the 21st the doubling was distinct and remained so until February 23d. Similar observations were made on other lines.
Everything leads to the conclusion that we have here a periodical phenomenon, which is probably connected with the seasons of Mars. If this be the case, we may hope to extend these observations during the next opposition, when we shall be able to see the seasons of the planet advanced about eighty days. This opposition will take place January 1, 1884. The position of Mars on this date will be identical with that on the 13th of February, 1882, and the apparent diameter will be about 12·9", that is, pretty near the mean diameter which the planet had during the finding of the above-described doublings. Therefore there is reason to hope that these phenomena may again be determined and confirmed by other observers. The desire to obtain such information has been the main object of the foregoing communication.