Page:The Solar System - Six Lectures - Lowell.djvu/70



is certain because of the changes which we can see going on in the surface markings; for without air no change could take place, and changes are indisputable. Water is relatively scarce.

That change goes on upon the planet's surface has been known for a long time. The polar caps were the first telltale. Sir William Herschel, at the end of the eighteenth century, observed that they waxed and waned periodically, and that their period was timed to that of the planet's year. They were therefore seasonal phenomena.

They behaved like ice and snow, and this they are generally supposed to be. Some astronomers find difficulty in conceiving of enough heat on Mars to permit them to be water, and carbonic acid has been suggested instead. But certain phenomena connected with the melting prove that carbonic acid cannot be the substance. The evidence is now very strong that they are what they look to be, and that the necessary heat will some- how be explained.

Up to the time of Schiaparelli, not much beyond this behavior of the polar caps and the general permanency of the dark and light markings was known about the planet. Its physical condition was likened to the Earth's, the white patches being polar snows, the dark markings oceans and seas, and the light markings land.