The planets, which have always been objects of wonder and curiosity to those with the opportunity or need to lift their eyes to the heavens, now in our times shine with new and strange lights revealed to us by the far seeing instruments carried upon space craft. The Moon, Mars, Venus and Mercury all bear on their surfaces the crater scars of innumerable meteorites that have fallen upon them from the beginning of the solar system. The Earth alone has an active surface that has obliterated those scars. The fluid surface of Jupiter is in constant and vigorous motion, driven by heat flowing out from the interior or, it may be, brought to it by the ultra-violet radiation from the Sun or by the solar wind. The Medicean satellites of Jupiter now' present to us strange and individual faces: would Galileo who first saw the mountains on the Moon or the spots on the Sun have been surprised by the eruption of sodium and sulphur from Io and the cloud of gas within which it moves, or by the strange stress patterns upon other of the satellites? Seeing these strange and varied faces of the planets, each apparently different from any other, who can forbear to ask, what bodies are these, how are they made up, that their appearances are so distinctive? Why are some active, and others apparently dead, some dry, and others thickly covered with atmosphere or ocean?