To Campbell’s critique Very answered by the suggestion that the meteorological conditions during the Mount Whitney observations should have been exceptionally unfavourable. The entire south-west of the United States and the north of Mexico were visited at that time by cloudy weather and heavy downpours. Very contends that this humidity should partly have extended to the high strata above Mount Whitney and therefore rendered the calculation of the moisture content of the air entirely unreliable.
Simultaneously (August, 1910) new measurements were published of Slipher’s photo-plates from February, 1908, which Very had examined. The result was now that the rain-band “A” was 2.5 times more pronounced in the spectrum of Mars than in that of the Moon. Furthermore, the oxygen absorption-band “B” was 1.5 times stronger for Mars than for the Moon. Great quantities of water vapour and oxygen should, therefore, undoubtedly exist in the atmosphere of Mars.
In the meantime Campbell had not been idle. The difficulty with the older measurements consisted in the fact that the absorption line of water vapour in the atmosphere of Mars occupies the identical place of the line due to vapour in the Earth’s atmosphere. There exists, however, a method, as already pointed out by Campbell in 1896, of separating the two, which method is available when Mars either approaches or departs from the Earth with sufficient velocity. The latter could be determined both from the known motions of the two planets and from the displacement of certain spectral lines of the Sun. These two determinations were in almost perfect agreement; for instance January 26–27, 1910, astronomical calculations gave a relative velocity of 19.1 km. (11.86 miles) per second and spectroscopical measurements 19.2 km. (11.93 miles) per second, while on February 3–4 the relative velocity was 18.1 km. (11.24 miles) a difference of 1 km. (.62 miles) per second. This trial shows the accuracy of the method. Among the absorption lines of water vapour and of oxygen there was, however, none due to the atmosphere of Mars. Campbell assumes that such lines would certainly have been visible if they had been only one-fifth as strong as the so-called tellurian lines. The advantage of this method is evidently that the “martian” and the “tellurian” lines lie close beside each other on the same plate so that differences in sensitiveness, exposure, and atmospheric conditions are entirely eliminated.
From these and the following data we may calculate water content and temperature of the atmosphere on Mars anew: water vapour at the point of observation was 1.9 grammes (29.3 grains) per cubic meter (1.3 cu. yds.), zenith-distance of Mars 55° and incident as well as reflected sun-rays formed an angle of 70° with the surface of Mars; hence, the amount of moisture at the surface was only 0.12 gramme (1.85 grains) per cubic meter (1.3 cu. yds.), corresponding to -38° C. (-36.4° F.) for saturated air and to -27° C. (-16.6° F.) for air of 31 per cent. saturation. Oxygen content per cubic meter (1.3 cu. yds.) at the surface of Mars would be only a sixteenth part of corresponding numerical value on the Earth. This determination is more accurate than any of the previous ones and reduces the temperature another 10° C. (18° F.) below the lowest value derived earlier in this chapter. We should remember, however, that, during the trial of September, 1909, the sun stood practically in zenith on Mars, while in January and February, 1910, we are concerned with a point where sunrise had occurred about four and a half hours previously. The latter observation should give a value close to, but slightly above, the mean diurnal temperature on Mars.
No determination comparable in precision with this one by Campbell appears to have been made. We must therefore recognize it as conclusive.
We may easily calculate the surface temperature of a planet from the intensity of the solar radiation received, or insolation, provided the surrounding vapour shell contains no heat retarding gas. The most important gases of this kind are water vapour, which, as we just have seen, is very sparse in the atmosphere of Mars, and carbon dioxide, of which there probably also, for reasons stated below, is only a scant supply in the martian gas shell. Such calculations were first performed by Christiansen of Copenhagen, who assumed 2.5 calories as the solar constant on Earth, i. e., the amount of energy received through insolation per minute by each square centimeter (.15 sq. in.) of the Earth’s surface when at right angle to the radiation and on mean distance from the Sun. On Mars, the radiating energy received under similar conditions is only about 1.1 calories. The surface of the planet is heated until it radiates as much energy into space as it receives from the Sun. In this way we obtain an average temperature of -37° C. (-34.6° F.) for the entire surface of Mars. The regions, exposed to the Sun in zenith at noon, might, if heat were not conveyed therefrom, possibly reach a daily mean temperature of +8° C. (46.4° F.) and perhaps slightly more at noon. Probably not even the freezing point is reached, as the heat is rapidly carried away by the freely circulating air. The above-mentioned mean temperature of -37° C. (-34.6° F.) seems on the whole to agree well with the observations by Campbell on Mount Whitney.
Recent accurate determinations of the intensity of the solar radiation by Abbot, K. Ångström and others, indicate that it has been estimated about 20 per cent. too high. If we take the solar constant to an even 2.0 calories, which is a trifle high, we reach the conclusion that the mean temperature on Mars would fall about 50 degrees below freezing. Equatorial regions might then reach an average of -8° C. (+17.6° F.) and at noon the temperature might possibly rise slightly above zero (32° F.). A higher temperature yet might be attained at the pole where the Sun during the summer remains for months above the horizon or a high mark of +8° C. (46.4° F.) provided no heat were carried away by air currents. Such losses naturally must occur, and the temperature probably hovers around freezing. At the martian poles we might possibly imagine the existence of some low forms of vegetation (snow-algæ, etc.) during the height of the short summer.
When we hitherto on the authority of Lowell, Very, and others, have assumed an average temperature of +10° C. (50° F.) on Mars, we have done so on the supposition that the atmosphere of the planet contained great quantities of heat-conserving gases. This assumption appears to be no more tenable than the belief in the high temperature on Mars. After all, the temperature is probably about 10° C. (18° F.) higher than our last calculation would indicate—or about -40° C. (-40° F.)—because the air on Mars is very clear and admits, therefore, all sun-rays, retaining also a fraction by virtue of what little water vapour, carbon dioxide, and other heat-conserving gases there may be present in the atmosphere. The mean summer temperature at the martian equator (-27° C. or -16.6° F. acc. to Campbell’s data) would then lie about 13° C. (23.4° F.) above the mean for the planet. This agrees closely with conditions on the Earth where the highest mean in July at the equator is 27° C. (80.6° F.) and the mean for the earth 16° C. (60.8° F.).
We are consequently obliged to revise in their entirety our ideas about Mars. The belief that organic life (green vegetation) causes the colour of the so-called seas on Mars, as assumed by Lowell, or that the red tints belong to the gorgeous dress in which autumn arrays the plants before their leaves are shed under the attacks of frost, as intimated by Flammarion, must nowadays take its place in the shadowy realm of dreams.
Those who do not believe that the so-called canals are real waterways, devoted to freight carrying and irrigation, or illusions, which conception the photographs contradict (for example [Fig. 18]), generally consider that they signify cracks or fissures. As in the crust of the Earth, they generally run in nearly straight lines or in regularly bent curves ([Fig. 17] and [17a]). Flammarion mentions that the renowned physicist Fizeau looked upon the “canals” as cracks in the ice-coverings of the oceans on Mars. Penard, in 1888, expressed the more likely opinion that they correspond to the fissures in the crust of the Earth. Flammarion contends that such fissures do not have the rectilinear configuration of the “canals.” This is completely in error, as shown on the map here reproduced ([Fig. 16]). It is also stated that they are so inexplicably long, for instance the canal Phison is 2250 English miles (Lowell) or 3620 km. in length. The longest known earthquake crack along the entire length of which a dislocation has taken place at one time is 600 km. (373 miles) in extension; the violent earthquake in California, 1906, originated from this crack. There is no doubt, moreover, that a great fissure in the Earth follows the coast of Chile from Arica to the Strait of Magellan in a nearly north and south direction for a distance of about 32 parallels or 3560 km. (2210 miles). This fissure is almost as long as Phison on Mars. Such cracks exist along the entire coast of the Pacific Ocean. As yet, we do not know their position in much detail, because long stretches run below the sea or through territories not yet occupied by civilized people. As an example of a small fissure, a picture taken by Sederholm from Segelskär, east of Hangö in the Baltic, may serve (see [Fig. 15]). As the studies of earthquakes are prosecuted with increasing interest in later years, fissures of all dimensions will undoubtedly soon be discovered. The solid crust on Mars is, furthermore, somewhat thicker than that of the Earth as the cooling of that planet has progressed further. The sections broken off at the bursting of the Martian crust ought therefore to be much larger both in breadth and in length. No doubt, the facts that the intensity of gravity on Mars is only three-eights of its intensity on the Earth and that the curvature of the Martian surface is twice as sharp as the Earth’s contribute to this result. Imagine two vaults, one built with higher and broader wedge-shaped stones than the other and with half the radius and furthermore loaded only one-third as heavily as the other, and it will become evident that we can permit a much larger span in the former than in the latter case without fear of collapse. In other words, it requires a much more extensive caving or shrinkage of the molten mass beneath the crust of Mars to cause a rupture than under the terrestrial crust.