ABSTRACT
Water is a substance of most intriguing characteristics. Its various states, mobility, and ability to transport solutes allows it the leading role in a host of geological processes. Deep in the earth, water lowers the melting temperature of rock to generate magma that builds the continents. As chloride-rich brine, deep crustal waters carry gold and other metals to form ore deposits. Water of the ancient oceans provided the crucible for early life to evolve, and freshwater is the basis of life in the terrestrial environment. As vapor, water provides the earth with some 30 degrees of greenhouse warming and makes the planet habitable. Yet, condensed as clouds it reects solar radiation and regulates our climate. Its high heat capacity (4200 joules per kilogram per degree), coupled with the enthalpy of vaporization (2.4 million joules per kilogram are released during condensation), fuels hurricanes and typhoons. Crystallized as ice, it has a remarkable open-tetrahedral framework that is less dense than liquid water and so, unlike most other compounds, its solid phase is buoyant. Fortunately so, otherwise aquatic habitats could freeze to the bottom, and there would be no surface ice to regulate seasonal albedo. Even the search for life on Mars following the traces of water. All who have plunged their face into a fountain of cold, clear water to slake their thirst on a hot day, whether from a mountain spring or a garden hose, appreciate its quality for sustaining life.
