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and does not bear much aqueous vapour. Now snow can fall only through the condensation of aqueous vapour in the atmosphere. It does not therefore seem likely that much snow can fall on the polar regions of Mars.

Another consideration is that the power of the sun's rays to melt snow is necessarily limited by the amount of heat that they convey. In the polar regions of Mars the rays fall with a great obliquity, and even if all the heat conveyed by them were absorbed, only a few feet of snow could be melted in the course of the summer. But far the larger proportion of this heat must be reflected from the white snow, which is also kept cool by the intense radiation into perfectly cold space. We therefore conclude that the amount of snow that can fall and melt around the polar regions of Mars must be very small, being probably measured by inches at the outside.

As the thinnest fall of snow would suffice to produce a white surface, this does not prove that the caps are not snow. But it seems more likely that the appearance is produced by the simple condensation of aqueous vapour upon the intensely cold surface, producing an appearance similar to that of hoarfrost, which is only frozen dew. This seems to me the most plausible explanation of the polar caps. It has also been suggested that the caps may be due to the condensation of carbonic acid. We can only say of this, that the theory, while not impossible, seems to lack probability.

The reader will excuse me from saying anything in this chapter about the possible inhabitants of Mars. He