Page:Is Mars habitable - Wallace 1907.djvu/95

74 in Siberia, in Lat. 67 1/2°N. a cold of −88°F. has been attained; while over a large portion of N. Asia and America above 60° Lat. the mean January temperature is from −30°F. to −60°F., and the whole subsoil is permanently frozen from a depth of 6 or 7 feet to several hundreds. But the winter temperatures, over the same latitudes in Mars, must be very much lower; and it must require a proportionally larger amount of its feeble sun-heat to raise the surface even to the freezing-point, and an additional very large amount to melt any considerable depth of snow.

But this identical area, from a little below 60° to the pole, is that occupied by the snow-caps of Mars, and over the whole of it the winter temperature must be far lower than the earth-minimum of −88°F. Then, as the Martian summer comes on, there is less than half the sun-heat available to raise this low temperature after a winter nearly double the length of ours. And when the summer does come with its scanty sun-heat, that heat is not accumulated as it is by our dense and moisture-laden atmosphere, the marvellous effects of which we have already shown. Yet with all these adverse conditions, each assisting the other to produce a climate approximating to that which the earth would have if it had no atmosphere (but retaining our superiority over Mars in receiving double the amount of sun-heat), we are asked to accept a mean temperature for the more distant planet almost exactly the same as that of mild and