The equatorial and average temperatures are given under the assumption that Mars possesses an atmosphere as efficient as our own in equalizing the temperature of the whole planet. If, on the other hand, its atmosphere has no such regulating power, then under the zenith Sun the upper limit of the temperature of a portion of its surface reflecting one-eighth would be, as shown in the Table, 64°C. This would imply that the temperature on the dark side of the planet was very nearly at the absolute zero. “If we regard Mars as resembling our Moon, and take the Moon’s effective average temperature as 297° abs., the corresponding temperature for Mars is 240° abs., and the highest temperature is four-fifths of 337° = 270° abs. But the surface of Mars has probably a higher coefficient of absorption than the surface of the Moon—it certainly has for light—so that we may put his effective average temperature, on this supposition, some few degrees above 240° abs., and his equatorial temperature some degrees higher still. It appears as exceedingly probable, then, that whether we regard Mars as like the Earth or, going to the other extreme, as like the Moon, the temperature of his surface is everywhere below the freezing-point of water.”[14] As the atmospheric circulation on Mars must be languid, and the atmosphere itself is very rare, the general condition of the planet will approximate rather to the lunar type than to the terrestrial, and the extremes, both of heat and cold, will approach those which would prevail on a planet without a regulating atmosphere.
There is another way of considering the effect on the climate of Mars and its great distance from the Sun, which, though only rough and crude, may be helpful to some readers. If we take the Earth at noonday at the time of the equinox, then a square yard at the equator has the Sun in its zenith, and is fully presented to its light and heat. But, as we move away from the equator, we find that each higher latitude is less fully presented to the Sun, until, when we reach latitude 64½°—in other words just outside the Arctic Circle—7 square yards are presented to the Sun so as to receive only as much of the solar radiation as 3 square yards receive at the equator. We may take, then, latitude 64½° as representing Mars, while the equator represents the Earth. Or, we may take it that we should compare the climate of Archangel with the climate of Singapore.
Now the mean temperature of latitude 64½°, say the latitude of Archangel, is just about freezing-point (0°C.), while that of the equator is about 28°C. We should therefore expect from this a difference between the mean temperatures of the Earth and Mars of 28°; that is to say, as the Earth stands at 16°C, Mars would be at -12°C. But, on the Earth, the evaporation and precipitation is great, and the atmospheric circulation vigorous. Evaporation is always going on in equatorial regions, and the moisture-laden winds are continually moving polewards, carrying with them vast stores of heat to be liberated as the rain falls. The oceanic currents have the same effect, and how great the modification which they introduce may be seen by comparing the climates of Labrador and Scotland. There appear to be no great oceans on Mars. The difference of 28° which we find on the Earth between the equator and the edge of the Arctic Circle is a difference which remains after the convection currents of air and sea have done much to reduce the temperature of the equator and to raise that of high latitudes. If we suppose that their effect has been to reduce this difference to one half of what it would have been were each latitude isolated from the rest, we shall not be far wrong, and we should get a range of 56° as the true equivalent difference between the mean temperatures of Singapore and Archangel; i.e. of the Earth and Mars; and Mars would stand at -40°C. The closeness with which this figure agrees with that reached by Prof. Poynting suggests that it is a fair approximation to the correct figure.
The size of Mars taught us that we have in it a planet with an atmosphere of but one half the density of that prevailing on the top of our highest mountain; the distance of Mars from the Sun showed us that it must have a mean temperature close to that of freezing mercury. What chance would there be for life on a world the average condition of which would correspond to that of a terrestrial mountain top, ten miles high and in the heart of the polar regions? But Mars in the telescope does not look like a cold planet. As we look at it, and note its bright colour, the small extent of the white caps presumed to be snow, and the high latitudes in which the dark markings—presumed to be water or vegetation—are seen, it seems difficult to suppose that the mean temperature of the planet is lower than that of the Earth. Thus on the wonderful photographs taken by Prof. Barnard in 1909, the Nilosyrtis with the Protonilus is seen as a dark canal. Now the Protonilus is in North Lat. 42°, and on the date of observation—September 28, 1909—the winter solstice of the northern hemisphere of Mars was just past. There would be nothing unusual for the ground to be covered with snow and the water to be frozen in a corresponding latitude if in a continental situation on the Earth. Then, again, in the summer, the white polar caps of Mars diminish to a far greater extent than the snow and ice caps of the Earth; indeed, one of the Martian caps has been known to disappear completely.
Yet, as the accompanying diagram will show, something of this kind is precisely what we ought to expect to see. The diagram has been constructed in the following manner: A curve of mean temperatures has been laid down for every 10° of latitude on the Earth, derived as far as possible from accepted isothermals in continental countries in the northern hemisphere. From this curve ordinates have been drawn at each 10°, upward to show average deviation from the mean temperature for the hottest part of the day in summer, downward for the deviation for the coldest part of the night in winter. Obviously, on the average, the range from maximum to minimum will increase from the equator to the poles. The mean temperature of the Earth has been taken as 16°C, and as representing that prevailing in about 42° lat. The diagram shows that the maximum temperature of no place upon the Earth’s surface approaches the boiling-point of water, and that it is only within the polar circle that the mean temperature is below freezing-point. Water, therefore, on the Earth must be normally in the liquid state.
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Thermographs of the Earth and Mars
In constructing a similar diagram for Mars, three modifications have to be made. First of all, the mean temperature of the planet must be considerably lower than that of the Earth. Next, since the atmospheric circulation is languid and there are no great oceans, the temperatures of different latitudes cannot be equalized to the same extent as on the Earth. It follows, therefore, that the range in mean temperature from equator to pole must be considerably greater on Mars than on the Earth. Thirdly, the range in temperature in any latitude, from the hottest part of the day in summer to the coldest part of the night in winter, must be much greater than with us; partly on account of the very slight density of the atmosphere, and partly on account of the length of the Martian year.