The Trouvelot astronomical drawings manual

by E. L. Trouvelot

On account of the inclination of the axis of rotation of this planet to the ecliptic, it is rare that both of these spots are visible on the disk at the same time; and when this occurs, they are seen considerably foreshortened, as they are then both on the limb of the planet. Usually only one spot is visible, and it appears to its best advantage when the region to which it belongs attains its maximum of inclination towards the Earth.

The polar spots change considerably in size, as they do also in form. Sometimes they occupy nearly one-third of the disk, as is proved by many of my observations; while at other times they are so much reduced as to be totally invisible. It is to be remarked that the reduction of these spots generally corresponds with the summer seasons, and their enlargement with the winter seasons of the hemispheres to which they respectively belong. From these well-observed facts it would appear that a relation exists between the temperature of the two hemispheres of Mars and the variations of the white spots observed at its poles. A similar relation is known to exist on our globe between the progress of the seasons and the melting away and the accumulation of snow in the polar regions. Astronomers have been led, accordingly, to attribute the polar spots of Mars, with all their variations, to the alternate accumulation and melting of snows. On this account, the polar spots of Mars are sometimes also called "snow-spots."

Errors have certainly been made by astronomers in some of their observations of the so-called polar snow-spots, other objects occupying their place having been mistaken for them. A regular series of observations on this planet, which I have now continued for seven years, has revealed the fact that during the winter seasons of the southern hemisphere of Mars, the polar spots are most of the time invisible, being covered over by white, opaque, cloud-like forms, strongly reflecting light. In 1877, during more than a month, I, myself, mistook for the polar spots such a canopy of clouds, which covered at least one-fifth of the surface of the whole disk. I only became aware of my error when the opaque cloud, beginning to dissolve at the approach of the Martial summer, allowed the real polar spot to be seen through its vapors, as through a mist at first, and afterwards with great distinctness. In this particular case, the snow-spot was considerably smaller than the cloudy cap which covered it, and it is to be remarked that it was not situated at the centre of this cloudy cap, but was east of that centre; a fact which may account for the so-called polar spots not being always observed on exactly opposite sides of the disk. From my observations of 1877, 1878 and 1880, it appears that at the approach of the autumnal equinox of the southern hemisphere of Mars, large, opaque masses, like cumulus clouds in form, began to gather in the polar regions of that hemisphere, and continued through autumn and winter, dissolving only at the approach of spring. These clouds, which varied in form and extent, were very unsteady at first, but as the winter drew nearer they enlarged and became more permanent, covering large surfaces for months at a time.

That the large white spots under consideration are real clouds in the atmosphere of Mars, and are not due to a fall of snow, is proved by the fact that these spots covered both seas and continents with equal facility, even in the equatorial regions of the planet. Snow, of course, could not cover the seas of Mars, unless these were all frozen over, even in the equatorial zones; therefore, if the dark spots of Mars are assumed to be due to water, these large white spots cannot well be ascribed to snow.

The real polar spots of Mars seem to be in relief on the surface of the planet, since the southern spot often appeared slightly shaded on the side opposite to the Sun during my observations in 1877. In certain cases, when they are on or very near the limb, they have been observed, both by others and by myself, to project from the disk slightly.

The polar spots of Mars are doubtless composed of a material which, like our snow or ice, melts under the rays of the Sun; although it seems difficult to admit that the Martial snow is identical with our terrestrial snow, and that it melts at a like temperature. The south polar spot of Mars entirely disappeared from sight in its summer season in 1877, although the planet receives less than one-half as much heat as we receive from the Sun; yet on our globe the arctic or antarctic ices and snow are perpetual—never melting entirely. An important fact disclosed by the melting away of the southern polar spot is, that in melting it is always surrounded with a very dark surface, which takes the place of the melted portion of the spot, as observed by myself in 1877-78. When the polar spot had entirely disappeared, its place was occupied by a very dark spot. Now, if the polar spot is really ice, and the dark spots are actual seas, this polar spot must be situated in mid-ocean, since, on melting away, it is replaced by a dark spot. If the polar spots are composed of a white substance melting under the rays of the Sun, as seems altogether probable, its melting point must be above that of terrestrial snow.

Many of the dark spots of Mars, and especially those whose northern border forms an irregular belt upon the equatorial regions of this planet, are bordered on that side by a white luminous belt, following all their sinuosities. These white borders are variable. Sometimes they are very prominent and intensely bright, especially at some points, which occasionally almost equal the polar spots in brilliancy; while at other times they are so faint, that they can hardly be distinguished, or are even invisible; although the atmosphere is clear and the dark spots appear perfectly well defined. While these white borders were invisible, I have sometimes watched for several hours at a time to see if I could detect any traces of them in places where they usually appear the most prominent, but generally without success. On a few occasions, however, I had the good fortune to see some of these spots forming gradually in the course of one or two hours, at places where nothing of them could be seen before.

These whitish fringes forming and vanishing along the coasts of the Martial seas have been very little studied by astronomers. From my observations made during the last seven years, it appears very probable that this belt and its white spots are mainly due to the condensation of vapors around, and over high peaks, and extensive mountain chains, forming the Martial sea-coasts, as the Andes and Rocky Mountains form the sea-coasts of the Pacific Ocean. These high mountains on Mars, condensing the vapors into fogs or clouds above them, or at their sides, as often happens in our mountainous districts, would certainly suffice to produce the phenomena observed. Some of the highest peaks among these mountain chains may even have their summits covered with perpetual snow, or some substance partaking of the nature of snow. The temporary visibility and invisibility of the white spots seen on Mars, as well as the rapid transformations they sometimes undergo, may be explained as caused by clouds having a high reflective power and a liability to form and disappear quickly.

The assumption that these irregular whitish bands and spots are formed by the condensation of vapors on mountain chains, and elevated table lands, is supported by my observations made in 1877 and 1879. When such white spots were traversing the terminator at sunrise, they very often projected far into the night side, thus indicating that they were at a higher level than that of the general surface. Indentations in the terminator, corresponding to large dark spots crossing its line, also clearly indicated the depression of the dark spots below the general surface. The highest mountainous districts thus observed on Mars, are situated between 60° and 70° of south latitude, towards the western extremity of Gill Land. The mountain chain, which almost completely forms the surface of this land, is so elevated at some points, that they not only change the form of the terminator when they are seen upon it, but also the limb of the planet, as seen by myself. They then appear so brilliant, that the principal summit among them has been mistaken by several observers for the polar spot itself, as proved by the wrong position assigned to it on their drawings. It seems probable that this high peak, which appears always white, is constantly covered with snow, or the similar material replacing it on Mars. This high region is situated between longitudes 18o° and 190°.

The highest mountainous parts belonging to the hemisphere represented on Plate VIII., which are nearly always more or less visible as whitish spots and bands, form a coast line along the northern (lower) border of De La Rue Ocean. This great spot, which is not so simple as it has been represented by observers, is in fact divided by two narrow isthmuses, one in the north, the other in the east, both joining, in the interior of the great ocean, a peninsula heretofore known as Hall Island. Upon the south-eastern extremity of this peninsula, a white spot, called Dawes Ice Island, was observed in 1865, but it soon disappeared, and was after that seen only now and then. It is very probable that this so-called Ice Island was due to clouds forming around the summit of some high peak of this peninsula.