Fig. 18. Photograph of Mars, enlarged and retouched; taken by Lampland
Fig. 19. Appearance of Mars April 8, 1907, as observed by Quénisset. Below one pole a dark line is plainly visible.
Fig. 20. The south pole spot on Mars observed by Jarry-Desloges July 10, 1909.
Fig. 21. The appearance of Mars during an observation by Antoniadi in 1909. The whole disk is somewhat hazy; below, the details are hidden by clouds of sand.
In the vicinity of the snow-white polar cap, whether there be continent or sea, bodies of water occur with solidly frozen surface covered by crystals of very hygroscopic salts, such as the chlorides of calcium, magnesium, and sodium. When the summer warmth returns and the polar cap is heated, the hoar-frost evaporates, and the now comparatively humid air spreads over the surrounding territory. We observe also frequent mist-formations in these places. The ground near the edge of the polar snow assumes then often a dark hue on account of the moisture ([Fig. 19]). Occasionally canals and lakes appear in the polar cap (see [Fig. 20]). This is evidently due to hot emanations along the cracks. The moist air sweeps over the salts, which then absorb water and dissolve into concentrated solutions. New quantities of water vapour are supplied from the pole as they distill over toward the other pole, where winter now exists, and push on toward the equator which they finally pass. In their course they dissolve the salts in the depressions along the fissures and particularly at the deep crossings where the centres of collapse or the so-called “oases” are located. Lowell has observed that the “canals” in this manner gradually “liquify” from 78° N. Lat. to the equator in fifty-two days.
The canal theory presents great difficulties to the explanation of this curious phenomenon. In order to make the water flow it must be assumed that the surface of Mars is entirely smooth or at least very nearly so and that the inhabitants convey the water melted at the poles through pumping stations. The canals vary in width; according to Lowell, their mean is 16 km. (10 miles), according to Flammarion between 300 and 60 km. (185 and 37 miles) which latter estimate probably is too high. The same canal differs widely in breadth in successive years and sometime disappears altogether. When the supply of water vapour is scant, only the most hygroscopic salts are dissolved, i. e., those deposited in the deepest furrow of the canal, but when the moisture sweeping over the canal is more abundant the broader portions absorb water, darken and thus become visible. The same holds true in regard to the inland lakes (“oases”). As the water vapour diffuses in the air, the canal becomes liquid along its entire length independent of the altitude of its various parts.
All agree upon the desert climate of the mainland on Mars. Like most deserts on the Earth, it is, therefore, probably a table-land, where one plateau mounts above the other, each one nearly level. By the action of the wind, the upper layers have been transformed into fine sand. On the dead planet no further sediments are deposited by the sea. The only accretions to the planet are meteorites and cosmic dust which slowly rains down. It contains among other substances iron, partly metallic and partly in the form of protoxides (which have a light green colour).[5] The oxygen in the atmosphere of Mars transforms these compounds to ferro-oxide which has different colours according to its coarseness, but generally is ochre. The surface of Mars is also described as possessing this colour. Dross has, therefore, assumed that the Martian soil is mingled with ferro-oxide. The finest dust, however, is yellow while larger crystals tend toward violet. We often observe on Mars that the details are covered by a yellow veil. This is of course finely powdered ferro-oxide probably mixed with less coloured sand which the desert wind whirls up over large portions of Mars. Vast sections of the planet bore such wrappings in the autumn of 1909, as observed and described by Antoniadi in Paris (see [Fig. 21]). Similar observations have previously been made by W. H. Pickering and others.