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BY JAMES BREEN, F.R.A.S.
F all the planets of the system, Mars, which for the last is that whose topographical details are best known to us. By glancing over the accompanying pictures of its telescopic aspect, we immediately perceive how much is revealed of its surface,of its islands, continents, seas, and snows, by means of powerful optical aid. It is the planet which most strongly resembles the Earth in the duration of its days and seasons; the existence of an atmosphere is everywhere apparent: being proved by the dimness of the dark streaks and spots at the circumference, as compared with their distinctness at the centre of the planet (for at the former the solar light has to penetrate through a dense stratum of air, and is again refracted through the same thick medium);by occasional clouds passing over its surface ;—by the snowzones piled up and stretching over vast spaces at its poles in the winter, which melt away gradually as the Sun ascends above the horizon in the summer, and dissipates the frost and darkness which for months previously had reigned in those arctic and antarctic regions. In the planet Jupiter a small telescope may more readily show the dark belts and spots; but those are ever changing and drifting about with variable velocity; the terra firma is scarcely perceived on this immense body, which appears to have an economy of its own, hidden from us by great masses of cloud, through an occasional break of which we perhaps sometimes catch a glimpse of the dark body of the planet. On the surface of Mars, on the contrary, the dark spots preserve the same position and relative dimensions, and we appear to be looking at a miniature globe pencilled over with dim seas and continents. From year to year the sea does not appear to encroach upon the land, nor the land upon the ocean; all the changes which are perceived are purely meteorological—the presence of clouds and murky weather, and snow during the winter,--of a clear atmosphere and sunny clime throughout the summer.
The first circumstance we detect in looking at the planet Mars, is its exceedingly red light, which is quite different from that of the other bodies that circulate about the Sun. This does not appear so prominently, however, when looked at with a telescope, as when seen by unaided vision. Still, however, even with the former, the orange light is very decided, and if compared with the Moon or a neighbouring white star, the contrast is sufficiently striking. Viewed when the whole disc of the planet is illuminated, its form appears quite circular, and no suspicion is aroused of a flattening at the poles, or bulging forth of the equatorial regions. But when the micrometer is applied, and careful measurements are made of its polar and equatorial diameters, several observers have agreed that there is a slight variation from the circular form, although the results which they have obtained are very discordant. Herschel was the first who suspected the elliptical form of the planet, and who patiently set about to determine the amount of this variation. To arrive at a knowledge of the figure of the Earth requires long and arduous labour ; but in the case of the planets the method is more simple, and the diameters at different parts of the disc may be said to be measured with the same facility as if it were a palpable object. Herschel found that the proportions of the equatorial diameters of Mars were as 1355 to 1272, or near as 16 to 15. Schroeter could not perceive any such ellipticity, and was of opinion that the two diameters were in the proportion of 81 to 80. Arago found them to vary in the proportion of 31 to 30. The Greenwich observations of late years give this variation as 52 to 51, and as 62 to 61. Other observers, among whom is Bessel, have not been able to detect the slightest difference between the diameters. Herschel, however, states, that on one occasion he showed the planet to some scientific friends, one of whom considered that it was as considerably bulged out at the equator as the globe of Jupiter. At certain times, as the whole surface of Mars is not illuminated, it will appear of the same figure as the Moon when three or four days before or after full; when this was the case, the flattening at the poles was still readily perceived by Herschel. There are a few circumstances which militate against the correctness of those measures ; sometimes the white cap of snow seems to project over the edge of the planet, at others the equatorial margins are exceedingly bright and radiating. According to theory, the proportion of the polar to the equatorial zone should be as 192 to 193; but different degrees of density at various parts of the globe would, of course, alter this. By observations made in September and October of 1862, Mr. Main concludes its polar diameter to be 4,221 miles, and its equatorial 4,332 miles,-a great difference for a small body like Mars, which has almost the same density as the Earth. In the latter body, the difference between the
polar and equatorial diameters amounts only to twenty-six miles; these quantities being respectively 7,899 and 7,905 miles.
Be the planet circular or elliptical in figure, however, it is easy to see that it rotates on its axis, and that too in about the same time as Mercury, Venus, and the Earth. In other respects, also, the similarity is striking, these planets being nearly of the same size, globular figure and density, and differing greatly from the huge exterior planets in those respects. The rotation of Mars is readily perceived by watching it from hour to hour throughout the evening, when it will be seen that the dark spots pass over the disc from west to east in the same sense in which the Earth is rotating. In about twenty-four hours and thirty-seven minutes, or on the following evening, the spot which was first observed will be found to have returned exactly to the same position, and the others will follow in the same succession as on the previous day, the polar snow-spot retaining nearly the same place. By further watching, the patient observer will find that the equator of Mars is only inclined a few degrees more to the plane of its orbit than that of the Earth, and that, consequently, their seasons are about the
From the longer year of Mars, however, the interval during which the polar regions are hidden from and exposed to the
rays of the Sun, is very different. At the latitude of the British islands, the shortest day on Mars is only about six hours, whilst on the Earth it is between seven and eight. The longest day on Mars, at the same latitude, would be nineteen hours, whilst on the Earth it is only seventeen hours. At seventy degrees of north latitude the Sun remains above the horizon for sixty-nine days, whilst on Mars at the same latitude it is above the horizon for 169 days. The pole of Mars is exposed to the Sun for 338 days, and hidden from it for the same time; whilst on the Earth the polar day or night is only 180 days.
Some observers have complained of the striking monotony and uniformity of the surface of Mars. If we cast a glance at the accompanying pictures of the planet, we shall hardly be disposed to indorse this opinion. On the contrary, the variety of scenery is deeply interesting. The seas, or dark portions, are remarkably sinuous in their course; the indentation of the coast, caused by bays and creeks, is very picturesque. It is true that mountains and vales, such as those on the arid surface of the Moon, cannot be perceived; but by attentive watching, it will be seen that the bright portion of the planet is curiously dotted over with a mottled ground, and that the dark seas vary greatly in the intensity of their tint, even when the spot is at the centre of the planet, and is viewed most favourably. It
would appear, from a cursory view, as though the surface of Mars were pretty equally divided between land and water, the former, indeed, from Maedler's map, having somewhat the predominance, whilst on the Earth the ocean covers three times as much space as the land.
Whilst observing the spots on Mars, we are somewhat astonished to see that, as they pass from the centre towards the margins of the planet, they become rapidly fainter, and that at the edges they are entirely obliterated. Even the darkest spots on Mars (those which are seen between twenty and forty degrees of south latitude) undergo this change as they approach the margins of the planet. Occasionally the whole circumference of the planet is surrounded, as it were, with a luminous ring, as in Plate XII., fig. 1, which represents the planet as seen by the writer in the 20-foot refractor at the Cambridge Observatory, in the opposition of 1856. This is generally held to be due to its atmosphere, which is considerably thicker at the edges than at the centre, just as during a fog we can perceive the blue sky and stars over-head, which are quite invisible at the horizon, where the atmosphere is considerably deeper and thicker. We might thus conclude that Mars has an atmosphere of considerable density; and this is confirmed by other circumstances. The older observers attempted to prove the existence of a dense atmosphere in Mars, by the disappearance of stars even at considerable distances from the planet, which they imagined had been eclipsed by its extensive gaseous envelope. This has been repeatedly tested in modern times, and with the best instruments, but without success.
The elder Herschel followed very faint stars close up to the margin of the planet, and Sir J. South has repeated the experiment more recently without perceiving any change in the brilliancy or colour of the very small stars selected. Whilst Cassini could not perceive a star of the fifth magnitude within six minutes of the margin of the planet (on Oct. 1, 1672), Herschel could perceive stars of the twelfth and thirteenth magnitudes within three minutes of its disc. Herschel, however, fully believed in the existence of a dense atmosphere surrounding the planet,
having often seen, besides the permanent spots on its surface, occasional changes of partial bright belts. The late Dr. Pearson
saw a similar phenomenon in 1828, having noticed a dark cloudy spot to have changed its position, in an interval of four days, from the side to the top of the planet (see diagram). One very persevering observer, however (Maedler), thinks that many very natural mistakes may occur on this point, and considers that if the spots are not narrowly scrutinized each night, one of them may readily be mistaken for a cloud. This observer has, however, noticed that the spots on Mars are of varying colours at different times; some of them are black, others yellowish-red, and others again of a greenish tint. On the Earth, however, the same change of tint would be apparent; the great forests would be dark in comparison with the bright sandy deserts; the Black, White, and Red seas might appear of different tints; yellow fogs and the " brickdusters” of Australia would cause a proportionate change in the apparent colour of the countries which they enveloped, and over which they hung. This is perhaps a more natural explanation of the colour of Mars than that which accounts for its peculiar appearance by the tints assumed by the vegetation on the planet. Without, however, having recourse to accidental fogs and clouds, it has been supposed that the dense atmosphere of the planet absorbs all the violet rays of the solar light (which has to pass twice through its atmosphere before reaching the Earth), and that only the red rays -those of the rising and setting Sun when passing through dense mist or cloud-make their way to our planet, the others being either reflected or absorbed.
Let us now turn our attention to the snowy zones about the north and south poles of the planet. By direct measurement with an instrument for estimating light, it has been found that those white spots have twice the intensity of the dun-coloured portions of the planet. In the earliest years of telescopic discovery they were duly noticed, and their appearance and disappearance, and the various changes which they underwent, were curiously scrutinized for a considerable length of time, before, however, any one surmised that the changes were caused by atmospheric variations. By his observations between 1779 and 1784, the elder Herschel placed this matter beyond all doubt. In the latter year he detected all the changes which have since been observed and confirmed in this planet; and although Maraldi had seen the changes, and prophesied the extinction of the north snow-spot as far back as 1719, yet to Herschel is due the credit of their complete examination. He was the first to show that the snow was not exactly placed at the poles of the planet. This is very apparent from our 3rd, 4th, and 5th figures, which have been very carefully drawn by Secchi; the first in 1856, and the two latter in 1858, with fine optical and atmospheric circumstances in his favour, and which,