Saturn is in the same interesting condition. The surface in the photograph (Fig. 13) is steam, and Saturn is so far away from the sun that the vaporisation of its oceans must necessarily be due to its own internal heat. It is too hot for water to settle on its surface. Like Jupiter, the great globe turns on its axis once in ten hours--a prodigious speed--and must be a swirling, seething mass of metallic vapours and gases. It is instructive to compare Jupiter and Saturn in this respect with the sun. They are smaller globes and have cooled down more than the central fire. Saturn is a beautiful object in the telescope because it has ten moons (to include one which is disputed) and a wonderful system of "rings" round it. The so-called rings are a mighty swarm of meteorites--pieces of iron and stone of all sorts and sizes, which reflect the light of the sun to us. This ocean of matter is some miles deep, and stretches from a few thousand miles from the surface of the planet to 172,000 miles out in space. Some astronomers think that this is volcanic material which has been shot out of the planet. Others regard it as stuff which would have combined to form an eleventh moon but was prevented by the nearness of Saturn itself. There is no evidence of life on Saturn. THE MOON Mars and Venus are therefore the only planets, besides the earth, on which we may look for life; and in the case of Venus, the possibility is very faint. But what about the moons which attend the planets? They range in size from the little ten-miles-wide moons of Mars, to Titan, a moon of Saturn, and Ganymede, a satellite of Jupiter, which are about 3,000 miles in diameter. May there not be life on some of the larger of these moons? We will take our own moon as a type of the class. A Dead World The moon is so very much nearer to us than any other heavenly body that we have a remarkable knowledge of it. In Fig. 14 you have a photograph, taken in one of our largest telescopes, of part of its surface. In a sense such a telescope brings the moon to within about fifty miles of us. We should see a city like London as a dark, sprawling blotch on the globe. We could just detect a Zeppelin or a Diplodocus as a moving speck against the surface. But we find none of these things. It is true that a few astronomers believe that they see signs of some sort of feeble life or movement on the moon. Professor Pickering thinks that he can trace some volcanic activity. He believes that there are areas of vegetation, probably of a low order, and that the soil of the moon may retain a certain amount of water in it. He speaks of a very thin atmosphere, and of occasional light falls of snow. He has succeeded in persuading some careful observers that there probably are slight changes of some kind taking place on the moon. [Illustration: FIG. 17.--A MAP OF THE CHIEF PLAINS AND CRATERS OF THE MOON The plains were originally supposed to be seas: hence the name "Mare."]