Chapter XII
on the abandonment of a planet by its water. Deserts are simply another sign of the same process. The very aging which began by depriving a body of its seas takes from it later its forest and its grass. A growing scarcity of water is bound to depauperate the one, as it depletes the other. We have positive proof of the action in our own deserts. For these bear testimony, in places at least, to not having always been so, but to have gradually become so within relatively recent times. But we have more general proof of the action from the position occupied on the earth’s surface by its deserts.
The significant fact about the desert-making so stealthily going on is that only certain zones of the earth’s surface are affected. Those belting the two tropics of Cancer and Capricorn, for several degrees on either side of them, most exhibit the phenomenon. Such positioning of the deserts is not due to chance. Directly, of course, desertism is due to dearth of rain. This in turn depends on the character and condition of the winds. If a wind laden with moisture travel into a colder region of the globe, its moisture is precipitated in rain and we have a fertile country; if it voyage into a warmer clime it takes up what little moisture may be there already and a desert is the result.
Now our system of winds is such as to produce a fall of rain for the different latitudes, as tabulated by Supan, thus:—
Zone I 40°N-27°N Little rain in summer but much in winter. II 27 N-19 N Little rain at all seasons. III 19 N- 7 N Little rain in winter but much in summer. IV 7 N- 1 N Abundant rain at all seasons. V 1 N-17 S Little rain in winter but much in summer. VI 17 S-30 S Little rain at all seasons. VII 30 S-35 S Little rain in summer but much in winter.
Zones II and VI, the zones of minimum rainfall, are also those in which the deserts occur. The northern one traverses southern California, Arizona, New Mexico, the Sahara, Arabia, and the Desert of Gobi; the southern, Peru, the South African veldt, and central Australia. The belts are wavy bands which by their form betray both a general underlying trend to drought at these parallels and also the effect of local topography in the matter.
From being distributed thus in belts, it is evident that the deserts are general globe phenomena, and from their being found only in the zones of least rainfall, that the earth has itself entered, though not far as yet, upon the desert stage of its history. Once begun, the desert areas must perforce spread as water becomes scarce, invading and occupying territory in proportion as the rainfall there grows small.
Now the axial tilt of Mars is almost exactly the same as that of our Earth, the latest determinations from the ensemble of measures giving 24° for it. Here, then, we have initial conditions reproducing those of the earth. But from the smaller size of the planet that body would age the earlier, since it would lose its internal heat the more rapidly, just as a small stone cools sooner than a larger one. On general principles, therefore, it should now be more advanced in its planetary career. In consequence, desertism should have overtaken more of its surface than has yet happened on earth, and instead of narrow belts of sterility we should expect to find there Saharas of relatively vast extent.
Now, such a state of things is precisely what the telescope reveals. The ochre tracts occupy nine tenths of the northern hemisphere and a third of the southern. Three fifths, therefore, of the whole surface of the planet is a desert.
[Illustration: Desert areas.]
Of cosmic as well as of particular import is the correlation thus made evident between the physical principles that effect the aging of a planet and the aspect Mars presents. Experimental corroboration of those laws is thus afforded, while, reversely, confidence in their applicability is increased. With continued observation the planet appears more desiccate as improved conditions bring it nearer. Dry land as it was thought to be proves even drier, something which lacks water for the ordinary necessities of a living world.
[Illustration: Desert areas.]
The picture the planet offers to us is thus arid beyond present analogue on Earth. Pitiless as our deserts are, they are but faint forecasts of the state of things existent on Mars at the present time. Only those who as travelers have had experience of our own Saharas can adequately picture what Mars is like and what so waterless a condition means. Only such can understand what is implied in having the local and avoidable thus extended into the unescapable and the world-wide; and what a terrible significance for everything Martian lies in that single word: _desert_.
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