Water is one of the essentials for life. Most of the H2O in the universe is either in the form of ice or vapor. Free-flowing water, so common on our planet that we do not even think about it, is apparently uncommon elsewhere and requires a rather narrow range of temperature to continue to exist in that state. On earth, water moves about in a continuous cycle, being drawn up in the form of water vapor, and then falling back to earth as rain or snow, flowing down into rivers eventually to return to the sea, where it begins the...