Chapter 3

I. The Polar Cap

To the solution of this inquiry, also, the planet's polar cap turns out to hold the key. For just as the fact of change in the cap proves the presence of air, so the manner of that change implies the presence of water. It not only does this; it turns out to do a deal more. For to the whole water question it appears to play the part not only of occasion but of cause. In more senses than one, it is in that great glistening white patch that our water problem takes its rise.

On the 3d of June, 1894, the south polar cap stretched, almost one unbroken waste of white, over about 55 degrees of latitude. A degree on Mars measures 37 miles; consequently the cap was 2,035 miles across. Inasmuch as the inclination of the Martian equator to the plane of the Martian orbit is, according to Schiaparelli, 24 degrees 52', it must have then covered more than the whole south frigid zone of the planet.

Now, to take in the full meaning of the condition of the cap at this time and of the changes that ensued, we must begin by determining the Martian time of year. This is done by fixing the dates at which the Martian pole reached its maximum tilt toward or from the Sun, and the dates at which it was not tilted either to or from, but sideways to, the Sun; the former gives us the Martian solstices, and the latter the Martian equinoxes. It thus appears that on April 7, 1894, occurred the vernal equinox of the Martian southern hemisphere, on August 31, its summer solstice, and on February 7, 1895, its autumnal equinox. From these dates it is easy to transform the one calendar into the other. On the 3d of June, 1894, therefore, it was about May 1 on the southern hemisphere of Mars.

On May 1, then, Martian time, the cap was already in rapid process of melting; and the speed with which it proceeded to dwindle showed that hundreds of square miles of it were disappearing daily. As it melted, a dark band appeared surrounding it on all sides. Except, as I have since learned, at Arequipa, this band has never, I believe, been distinctively noted or commented on before, which is singular, considering how conspicuous it was at Flagstaff. It is specially remarkable that it should never have been remarked upon elsewhere, in that a similar one girdling the north polar cap was seen by Beer and Madler as far back as 1830. For it is, as we shall shortly see, a most significant phenomenon. In the first place, it was the darkest marking upon the disk, and was of a blue color. It was of different widths at different longitudes, and was especially pronounced in tint where it was widest, notably in two spots where it expanded into great bays, one in longitude 270 degrees and one in longitude 330 degrees. The former of these was very striking for its color, a deep blue, like some other-world grotto of Capri. The band was bounded on the north, that is, on the side toward the equator, by the bluish-green areas of the disk. It was contrasted with those both in tone and tint. It was both darker and more blue.

The band not only varied in width at different longitudes, but its width corresponded to the amount of the blue-green areas of the disk visible at these longitudes below it. It was widest where these were greatest in extent, and narrowest where they were least. If we consult the map of Mars we shall see that below the bay in longitude 330 degrees lies the great dark area, the Syrtis Major, and, below the one in longitude 270 degrees, the Syrtis Minor. This correlation was highly suggestive in itself. As if, however, to remove all question as to possible coincidence having a hand in the matter, the agreement in position was emphasized by visible connection. Two long dark streaks appeared joining respectively each bay to its corresponding Syrtis.

But the most significant fact about the band was that it kept pace with the polar cap's retreat toward the pole. As the white cap shrank it followed pari passu so as always to border the edge of the snow. It thus showed itself not to be a permanent marking of the planet's surface, since it changed its place, but a temporary one, dependent directly upon the waning of the cap itself. In short, it was an associated