In an inquiry as to the probability of the existence of life, one of the most important points to be taken into account is the amount of heat available. Now, Mars is at such a distance from the sun that on the whole it would receive about two-fifths as much solar heat as we do. This does not, however, give the amount of heat that is actually received on the surface of the planet, a considerable proportion being absorbed by the atmosphere; and since our atmosphere is so much denser and thicker than that of Mars, it follows that we lose a much larger percentage of the solar heat. To calculate the exact amount of heat absorbed by a given thickness of air is a very difficult, if not impossible, problem; but it seems likely that, taking everything into account, the inhabitant of Mars will receive more heat from the sun than we do. This would have the effect of making the evaporation very large, and if so, the Martial atmosphere would be mostly composed of water-vapour.
According to Professor Langley, the true colour of the sun is blue; and its yellowness is due to the dirt always present in the air. To the inhabitants of Mars, it would most probably appear nearly white, unless, indeed, they also have volcanoes to fill the air with lava-dust.
Let us now sum up the facts we have stated, and determine as far as we can what sort of man the inhabitant of Mars must be.
In the first place, the force of gravitation at the surface is only just over one-third of its equivalent on the earth; a pound would therefore weigh about six ounces in Mars. If, therefore, we assume that the men are of such a size that their weight and activity are the same as ours, they would be about fourteen feet high on the average. This would make their strength very great; for not only would it be actually superior to ours, but, as every weight is so much smaller, it would be apparently proportionally increased. We should, therefore, expect to find that the Martialites have executed large engineering works; perhaps also their telescopes are much superior to ours, and we have been objects of interest for their observers. With regard to telescopes, it may be interesting to examine what is the effect of the highest magnifying power we can use. At his nearest approach, the distance from us to Mars is about thirty-seven million miles; and assuming that the highest power that can be used with advantage is twelve hundred, we approach with our telescopes to a distance of thirty thousand miles, so that houses, or towns, or indeed any artificial works, would be hopelessly invisible. With regard to the supply of heat and light, we have seen that the Martialite is not worse off than we are. To him the sun would appear as a white, or perhaps blue disc about two-thirds of the diameter that it appears to us. The Martial day differs but slightly from ours; his year, however, is much longer, being about six hundred and eighty-seven of our days, which is about six hundred and fifty Martial days. The inclination of his axis to the plane of the orbit is such that his seasons would be very similar to ours. It is difficult to reconcile the idea of an extensive vegetation with his peculiar red colour; it is just possible, however, that some of the green patches, generally supposed to be seas, may in reality be large forests.
The most valid objection to the habitability of Mars lies in the fact of the extremely low atmospheric pressure, which, as we have seen, would probably average about five inches of mercury. The lowest pressure that a man has ever lived in, even for a short time, is about seven inches, which was reached by Coxwell and Glaisher in their famous balloon ascent. The aëronauts, however, narrowly escaped perishing, not only on account of the low pressure, but also because of the extreme cold.
It seems impossible that a man constituted exactly as we are could live for any length of time breathing air only one-sixth of the density of ours. But it is rather going out of our way to assume that the Martialites would be exactly the same as we are in every way; the chances are a million to one against it; and on the other hand, a very slight modification of the lung arrangement would suffice to make life perfectly possible under such conditions.
The nights on Mars would be very dark, for he has no satellite like our moon. He has, it is true, two moons, but they are so small that their illuminating power is nil, being respectively only sixty and forty miles in diameter. The smallest of these presents the curious phenomenon that it revolves round Mars faster than the planet turns on his own axis, and therefore would appear to rise in the west and set in the east.
Our earth, as seen from Mars, when at his nearest, would appear about the same size as Jupiter does to us; that is to say, would subtend an angle of about forty seconds. At his furthest distance, this would be reduced to fourteen.
We thus see that there is ample reason for assuming that this, the most interesting of all the planets, is the abode of creatures not essentially different from ourselves. Being considerably older than we are, the Martialites are probably much further advanced in the arts and sciences; and perhaps there may be some truth in the story of the Italian astronomer who says he has lately detected lights on the planet moving about in such a way as seems to indicate a deliberate intention to open communication with the earth. What the language of the lights is, we have not been informed; let us hope it is something more practical than the proposal of the Russian savant to communicate with the moon by cutting a huge figure of the forty-seventh proposition of Euclid on the plains of Siberia, which, he said, any fool would understand.