Water: The Vital Fluid
by J. D. Hays (April 1996)
Introduction
Where did it come from and what's it like?
Water emerged from Earth's interior, through volcanic eruptions, probably within a few hundred million years of Earth's formation. It is now retained, mostly in the liquid state, at Earth's surface. This is in striking contrast with other planets in our solar system. For example, our nearest neighbors, Venus and Mars, have lost most of their water. Although Mars retains some, it is frozen in the planet's subsurface or polar ice and dry ice caps. Why Earth retained so much liquid water while its neighbors didn't is a question worth pondering, for it is the key to many of Earth's unique characteristics.
Liquid water is essential for life. Many organisms live in it and all organisms maintain an inner aqueous environment within which life's chemical reactions occur and by which life's materials (food, gas and waste) are transported.
Water has a number of unusual properties (Table I) which are a consequence of its hydrogen bonding. The hydrogen bond is a weak bond formed by the dipolar charge of the water molecule resulting from the asymmetrical arrangement of hydrogen and oxygen atoms (Fig. 1). This bond causes expansion on freezing, thus the solid phase floats on the liquid phase (Fig. 2b). Water freezing in rock crevasses is an important mechanism for mechanically breaking rocks. Breaking rocks into smaller fragments (mechanical weathering) increases their surface area and thereby promotes the breakdown of the rock by surface chemical reactions (chemical weathering). Water's dipolar charge also enhances its power as a solvent. Water molecules effectively surround cations and anions reducing the possibility of new compounds forming (Fig. 2a). This characteristic, as we shall see, is essential to water's chemistry and its role as the fluid of life.
The energy required to overcome water's hydrogen bond produces a higher heat capacity, heat of vaporization ( 540 cal/cm3) and heat of fusion (80cal/cm 3) than would be expected for its molecular weight. In addition, water's hydrogen bond holds together the double strands of the DNA molecule. Breaking these hydrogen bonds "unzips" the molecule, while forming new bonds between base pairs duplicates it.
Water resides in a number of reservoirs ( Table II), the ocean being by far the largest. It moves from the earth's surface to the atmosphere by evaporation and returns as rain. Solar energy powers this process and atmospheric circulation determines the planetary distribution of the rain (Fig. 3, Fig. 4, Fig. 5). The residence or replacement time for water in the ocean is about 40,000 years. In other words, if all water were removed from the ocean and the worlds rivers continued to flow at their current rate, it would take about 40,000 years to refill the ocean.
