In the early days of planetary exploration – exploration carried out solely at the eyepiece of a telescope – astronomers searched in vain for oceans on other worlds. Clouds enshrouded Venus, although they might hide a carbonated sea. Mars was a desert world, Mercury was hotter than the boiling point of water, and the outer worlds were great spheres of gas. Earth, it seemed, was the water world, blessed with liquid, life-giving oceans. But with the advent of space exploration, we know better; our Solar System is awash in oceans of many kinds, and was in the past as well (Fig. 7.1).