Mars.
| NORTH POLAR CAP. | SOUTH POLAR CAP. | ||
| At maximum | full extent of white | At maximum | white |
| At minimum | inner circle | At minimum | nothing |
Now it so happens that these astronomic conditions affecting the Earth several thousand years ago, are in process of action on one of our nearest planetary neighbors at the present time. The orbit of Mars is such that its present eccentricity is greater than what the Earth ever can have had, and the winter solstice of the planet’s southern hemisphere falls within 23° of its aphelion point. We have then the conditions for glaciation if these are the astronomic ones supposed, and we should expect a southern polar cap, larger at its maximum and still more so, relatively, at its minimum, than in the opposite hemisphere. Let us now look at the facts, for we have now a knowledge of the Martian polar caps exceeding in some respects what we know of our own. The accompanying diagrams exhibit the state of things at a glance, the maximum and minimum of each cap being represented in a single picture and the two being placed side by side. It will be observed that the southern cap outdoes its antipodal counterpart at its maximum, showing that the longer, colder winter has its effect in snow or hoar-frost deposition. But, on the other hand, instead of excelling it at its minimum, which it should do to produce permanent glaciation, it so far falls short of its fellow that during the last opposition at which it could be well observed, it disappeared entirely. The short, hot summer, then, far exceeded in melting capacity that of the longer but colder one.
Let us now suppose the precipitation to be increased, the winters and summers remaining both in length and temperature what they were before. The amount of snow which a summer of given length and warmth can dispose of is, roughly speaking, a definite quantity. For it depends to a great extent only on its amount of heat. The summer precipitation may be taken as offsetting itself in the two hemispheres alike. If, then, the snowfall in the winter be for any reason increased daily in both, a time will come when the deposition due the longer winter of the one will exceed what its summer can melt relatively to the other, and a permanent glaciation result in the hemisphere so circumstanced. Increased precipitation, then, not eccentricity of orbit, is the real cause of an Ice Age. And this astronomic deduction we owe not to theoretic conclusions, for which we lack the necessary quantitative data, but wholly to study of our neighbor in space. Had any one informed our geologic colleagues that they must look to the sky for definite information about the cause of an Ice Age, they would probably have been surprised.
With this Martian information, received some years ago, it is pleasing now to see that Earthly knowledge is gradually catching up. For that increased precipitation could account for it, the evidence of pluvial eras in the equatorial regions, contemporaneous with glacial periods, indicates. But another and probably the chief factor involved was not a generally increased precipitation, potent as that would be, but an increased snow deposit due to temporary elevation of the ground.
Glacial map of Eurasia—after James Geikie.