CHAPTER VII. MARS.

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Mars, though considerably smaller than the Earth, is commonly regarded as the planet which, taken all in all, bears most resemblance to the Earth, though only one-fourth its size. Under circumstances which have already been briefly alluded to in Chapter I., Mars exhibits from time to time a slight phase, but nothing approaching in amount the phases presented by the two inferior planets, Mercury and Venus. When in opposition to the Sun, that is to say when on the meridian at midnight, it has a truly circular disc; but between opposition and its two positions of quadrature it is gibbous. At the minimum phase, which is at each quadrature, E. or W. as the case may be, the planet resembles the Moon 3 days from its “full.” These phases are an indication that Mars shines by the reflected light of the Sun. It is a remarkable tribute to Galileo’s powers of observation that with his trumpery telescope, only a few inches long, he should have been able to suspect the existence of a Martial phase. Writing to a friend in 1610 he says:—“I dare not affirm that I can observe the phases of Mars; however, if I mistake not, I think I already perceive that he is not perfectly round.”

Fig. 12.—Four views of Mars differing 90° in longitude (Barnard).

The period in which Mars performs its journey round the Sun (called the sidereal period) is about 687 days; but owing to the Earth’s motion we are more concerned with what is called the planet’s synodical period of 780 days than with its sidereal period of 687 days. The synodical period is the interval between two successive conjunctions or oppositions of the planet as regards the Earth, and 780 days being twice 365 and 50 days over, it follows that we have an opportunity of seeing the planet at its best about every 2 years; and this is one of the reasons why Mars has been so much and so thoroughly studied as regards its physical appearance. Of course Mars is not equally well seen every 2 years, because it may so happen at a given opposition that it may be at its nearest to the Sun (perihelion), and the Earth at its farthest from the Sun (aphelion), in which case the actual distance between the two bodies will be the greatest possible. What is therefore wanted is for the planet to be nearest to the Sun and nearest to the Earth at the same time, under which circumstances it shines with a brilliancy rivalling Jupiter. This favourable combination occurs once in 7 synodical revolutions, or about every 15 years. The most favourable oppositions occur at the end of August, and the least favourable at the end of February. The next very favourable opposition will not occur until 1909. Mars may approach to within about 35 millions of miles from the Earth at a favourable opposition, whilst under extreme circumstances the other way it may be no nearer than 61 millions of miles at opposition.

Mars in opposition is a very conspicuous object in the Heavens, shining with a fiery red light which has always been regarded as a peculiar attribute of the planet, so much so that its name, or epithet, in many languages conveys the idea of “fiery” or “blazing.” It is recorded that in August 1719 its brilliancy was such as to cause a panic amongst the public.

Telescopically examined, Mars is always found to exhibit patches of shade of various sizes and shapes, and, on the whole, fairly permanent from year to year. During the last few years in particular these markings have been subjected to very careful scrutiny and measurement at the hands of numerous observers of skill and experience, and armed in many cases with very powerful telescopes. The conjoint effect of the observations obtained has been largely to augment our knowledge of the planet’s geography, or (to use the proper term) “areography.” Before describing the minutest details recorded and pencilled by the best observers, it will be best to speak of the leading general features which are within the grasp of comparatively small telescopes—say, refractors of 6 inches and reflectors of 12 inches in aperture. The first thing which presents itself as very obvious on the disc of Mars, is the fact that certain portions are ruddy, whilst others are greenish in hue. It is generally assumed that the red areas represent land and the green areas water. On this subject Sir John Herschel’s remarks, penned about half a century ago, may be said still to stand good. He ascribes the ruddy colour to “an ochrey tinge in the general soil, like what the red sandstone districts on the Earth may possibly offer to the inhabitants of Mars, only more decided.” The propriety of this thought will be best appreciated by a reader who has travelled through parts of North Gloucestershire, and seen a succession of ploughed fields in that locality. The deep red colour of the soil is in many places very conspicuous. It has often been remarked that the redness of Mars is much more noticeable with the naked eye than with a telescope; and Arago carried this idea one step further in suggesting that the higher the optical power the less the colour. This, however, might naturally be expected.

The most prominent surface marking on Mars is that known as the “Kaiser Sea,” sometimes called the “V-mark” from its resemblance to that letter, though a leg of mutton would be quite as good a simile. East of the Kaiser Sea and a little north of the planet’s equator is a well-defined dark streak known as “Herschel II. Strait”; whilst on the west side is another shaded area which has been called “Flammarion Sea.” These three features are so very conspicuous, that, provided the hemisphere in which they are situated is fairly in front of the observer, his telescope, if it will show anything on Mars, will show these. The white patches seen on certain occasions at Mars’s N. pole and close to its S. pole form another special feature of interest connected with this planet. It admits of no doubt whatever that these are immense masses of snow and ice which undergo at stated intervals changes analogous to the changes which we know happen in the great fields of ice situated in the regions of the Earth surrounding the Earth’s two poles. Not only do these white patches look like snow, but if attention is paid to the changes they undergo and the epochs at which the changes take place there will be found abundant confirmation of this theory, for these patches decrease in size when brought under the Sun’s influence on the approach of summer and increase again in size when the summer is over and winter draws near. In the second half of 1892 the Southern Pole was in full view, and during especially July and August the diminution of the snow area from week to week was very evident. Schiaparelli, who observed it with great attention during that season, noted at the commencement of the season that the snow reached at the first as far as latitude 70° and formed a polar cap some 1200 miles in diameter. Its subsequent decrease, however, was so marked that two or three months later the diameter of the snow patch had dwindled to no more than 180 miles, and became indeed still smaller at a later period. The summer solstice on Mars occurred on October 13, 1892, which was therefore the epoch of midsummer for Mars’s southern hemisphere. Whilst these changes were taking place in the southern hemisphere, no doubt changes of the reverse character were going on in the northern hemisphere, but they were not visible from the Earth because the North Pole was situated in that hemisphere of Mars which was turned away from the Earth. In previous years, however, the North Pole being turned towards the Earth its snow was also seen to undergo the same sort of change; in other words, was seen to melt. This happened, and was seen in 1882, 1884, and 1886. These observations of the alternate increase and decrease of the polar snow on Mars may be viewed with telescopes of moderate power, but of course it is more interesting and profitable to watch them with a large telescope. The fact (for it is an undoubted fact) that the north polar snow is concentric with the planet’s axis whilst the southern polar patch is eccentric to the extent of about 180 miles from the southern pole is one which has not yet received a satisfactory explanation. If both patches were eccentric so as to be exactly opposite to one another an explanation would be much more easy for we might say that the poles of rotation lay in one direction and the poles of cold in another.

I have spoken on a previous page of three specially conspicuous shadings of Mars, and other similar shadings to the number perhaps of a couple of dozen were generally recognised by astronomers (having been mapped and named) down to about the year 1877. In that year the astronomical world was startled by the announcement that Schiaparelli of Milan, an able and competent observer, had discovered that those shaded areas which all previous astronomers had regarded as continents or vast tracts of land, were in reality islands, that is to say, so far, that the continents in question were cut up by innumerable channels intersecting one another at various angles. When this discovery was announced, and older observations and drawings came to be examined, it was found, or at any-rate thought, that these so-called canals might be traced in drawings of earlier dates by Dawes, Secchi, and Holden. So much for 1877. In December, 1881, the planet was again in opposition, but farther off in distance, and therefore smaller in size than in 1877. It was, however, higher up in the Heavens as seen at Milan and the weather appears to have been more favourable. In these altered circumstances Schiaparelli again saw his canals, but this time they were in at least as many as twenty instances seen in duplicate; that is to say, a twin canal was seen to run parallel to the original one at a distance of from 200 to 400 miles, as the case might be. The existence of not only single canals but of twin canals seems an established fact, for Schiaparelli’s drawings and descriptions have been confirmed by competent testimony; but explanation is nowhere; especially in view of Schiaparelli’s own idea that the duplication of his canals is perhaps not a permanent feature but a periodical phenomenon depending on, or connected in some way with, Mars’s seasons.

Fig. 13.—Mars, August 27, 1892 (Guiot).

Fig. 13.—Mars, August 27, 1892 (Guiot).

Several points stand out clearly established by the observations of Mars during the opposition of 1894, especially the correctness of Schiaparelli’s discoveries and maps. Most of the canals originally seen by him were again seen, and thus their existence was confirmed, whilst new ones were also noticed. Many of these canals were double. The great extent of the S. Polar cap and its rapid disappearance as Mars’s summer approached was also a special feature of the observations of 1894. It dwindled until it became almost invisible, or at best showed as a tiny speck. It is thought by some observers that as the Polar cap melts, the water collects round the Pole, and thence flows over the planet’s surface, giving rise to the phenomenon of canals, and that this is the way the planet’s surface is irrigated. It may here be remarked that the word “canal,” which has been given to these dark streaks crossing and cutting up the large areas of land in Mars, is an unfortunate one, suggesting as it does artificial agency. But these Martial canals are probably, especially the largest, a great many miles in width and hundreds of miles in length, though some are smaller; and they are probably nature’s method of distributing over the continents and lands of Mars the water which collects round the Pole during the rapid melting of the Polar snows.

The idea of the presence of cloud or mist on Mars also received strong confirmation in 1894. Large portions of the planet’s disc were found to be hidden from view. “Herschel I. Continent” and the “Maraldi Sea” (both well-known markings on Mars, readily visible with small telescopes) were at times quite obscured by cloud. Indeed, the Maraldi Sea was occasionally quite blotted out: other well-known markings were also either blotted out or only faintly seen. These facts seem almost to prove conclusively the existence of cloud and vapour in Mars, especially as some of these markings subsequently again assumed their ordinary form and colour. Bright projections too were seen at times on the terminator of Mars, giving rise to the belief that there are high mountains on the planet, though some observers regarded these projections as high clouds powerfully reflecting the Sun’s light.

Mars rotates on its axis in 24h. 37m. 22s., a period so nearly coincident with the period of the Earth’s rotation as greatly to facilitate the mapping of Mars’s features by work continued from day to day by observers who have the necessary instrumental means and artistic skill in handling the pencil.

Mars has an atmosphere which may be said to be no more than moderately dense; that is to say much less dense than the Earth’s atmosphere. Of course the existence of snow, which has been taken for granted on a previous page, carries with it the existence of water and aqueous vapour—a fact capable of independent spectroscopic proof.

The inclination of Mars’s axis to the ecliptic has not been ascertained with all desirable certainty, but if Sir W. Herschel’s estimate that the obliquity on Mars is 28¾° (the Earth’s obliquity being 23½°) is correct, it is evident that there must be a very close similarity between the seasons of the Earth and the seasons of Mars, thereby furnishing another link of proof to support the statement made at the commencement of this chapter that, taken all in all, Mars is the planet which bears most resemblance to the Earth.

The apparent absence of satellites in the case of Mars was long a matter of regret to astronomers; they seemed to think that such a planet ought to have at least one companion. At last, in 1887, two were found by Hall at Washington, U. S., using a very fine refractor of 26 inches aperture. These satellites, which have been named Phobos and Deimos, are, however, very small, for Phobos at its best only resembles a star of mag. 11½, whilst Deimos is no brighter than a star of mag. 13½; from this it will be understood that only very large telescopes will show either of them. Phobos revolves round Mars in 7½ hours at a distance of about 6000 miles, whilst Deimos revolves in 30 hours at a distance of about 15,000 miles. It has been thought that neither of them can be more than about 6 or 7 miles in diameter, and therefore that they can not afford much light to their primary.

Mars revolves round the Sun in 686d. 23h. 30m., at a mean distance of 141 million of miles, which the eccentricity of its orbit may increase to 154 millions or diminish to 128 millions. The planet’s apparent diameter varies between 4 in conjunction and 30 in opposition. Owing to the great eccentricity of the orbit the planet’s apparent diameter as seen from the Earth varies very much at different oppositions. The real diameter is rather more than 4000 miles.

                                                                                                                                                                                                                                                                                                           

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