SIZE, ATMOSPHERE, AND TEMPERATURE
In size Mars is one of the smallest members of our solar family. Its mass is a little more than one-ninth that of the earth, and its entire surface is only about one-third as great as ours. It is the merest trifle more dense than Mercury, but only about sixty-six one-hundredths as dense as the earth. Its force of gravity is about thirty-six one-hundredths as powerful as that of the earth. A man weighing two hundred pounds here would be relieved of about one hundred and twenty-four pounds of his weight if transported to Mars, weighing there only seventy-six pounds, which would greatly increase his efficiency if he were in other respects the same.
It would necessarily follow that Mars, having such small force of gravity, could not long retain a heavy atmosphere, even if it had set out with such a one. No molecule of gas moving at a greater speed than three and thirteen-hundredths miles a second could be held by Mars in its atmosphere, and so much as it may have had of the rarer gases which move with great rapidity must have escaped long ago. But it did not begin life with an atmosphere heavy in proportion to that which the larger planets have. We have seen, in the case of Mercury, that being one of the small planets entails many restrictions in development. Such planets not only lose their atmosphere more quickly than the larger ones, but it is less dense to begin with. The atmosphere of Mars is probably no denser than we have at the tops of our highest mountains, more than likely not even so dense as that. There is some water vapor, and there are a few clouds most of the time; but in the main the atmosphere is so clear and thin that we can without any doubt see the actual surface of the planet. It is not certain that the clouds we see are formed from water vapor, as clouds of the ordinary kind are. It has been suggested that they may be simply dust-clouds. But this is as yet not much more than a suggestion, and nothing convincing has been offered to substantiate the idea. Even dust-clouds would need currents of air to create and carry them; so, whether dust or vapor, the presence of clouds implies an atmosphere.
The famous white polar caps, which furnish so many news items to the journals, are also of uncertain origin, and their true nature can be determined only by a fuller knowledge of the atmosphere of Mars. They appear in the winter season on the planet and disappear in its summer, so there seems to be no doubt that they are dependent in some way on the temperature in the polar regions of Mars. If they are hoar-frost or snow, they are condensations of water vapor; and, in that case, when they disappear there must be sufficient heat to melt them. It has been contended that the sun’s rays fall too obliquely on the poles of Mars to melt more than a few inches of snow, but that the caps may be light snow or frost, and thus capable of being dissolved by even such oblique rays of sunlight as they receive. Also it has been suggested that the deposit resembling snow may be carbon dioxide, which condenses into a white substance at a temperature more than a hundred degrees (-109° Fahr.) lower than is necessary to produce snow and melts at a correspondingly low temperature. What the nature of the phenomenon seen at the poles of Mars is depends largely upon what the temperature is; and the temperature in turn is dependent in some measure on the density and constitution of the atmosphere, as well as the planet’s distance from the sun.
The normal temperature of an unprotected body at the distance of Mars from the sun is about thirty-two degrees blow zero (Fahrenheit); and since we know Mars has no dense atmosphere to retain the heat it acquires, it is natural to suppose the existence there of a very low temperature, and one incompatible with our ideas of life and growth. The most favorable conclusions do not place the mean temperature higher than forty-eight degrees Fahrenheit. It is certain that the planet must be subjected to great extremes of temperature within its range, since its filmy robe of atmosphere cannot protect it to any extent from the direct rays of the sun during the day, nor prevent the heat from escaping with great rapidity at night; so that, whatever heat it may gain in the daytime, it probably loses much of it during the night. Until we know more of the constitution of the atmosphere of Mars we can know nothing certainly about its temperature beyond the fact that it is much colder than ours and more subject to variations. Anything much more definite than this is speculative at present. But with all the observation that is now given to Mars, and with the always increasing facilities for the work, many uncertainties regarding the planet are likely to be made clear before long. The spectroscope will probably be the final resort for facts concerning the atmosphere.