On planet Earth the substance water, in its three physical states, solid ( ice and snow), liquid (water and as droplets in clouds) and gaseous ( water vapor, steam), is probably the most critical component in the planet's geochemistry, although the surficial water inventory only makes up 0.02% of the planet's mass (Plates 9, 10). Inasmuch as its liquid phase is one of the essentials for self-reproducing life, it is a substance of supreme interest to humankind, for survival, for the environment and for the economy (Kuenen, 1955).
Water in one form or another appears to be present in most of the planets and their satellites except for Mercury and the Moon, but on none does it play such a significant role as it does on Earth. Its unique place in terrestrial biogeochemistry is directly related to the Earth's distance of approximately 1.5 × 108 km from the Sun. A little closer, and a Venusian hothouse would result (no liquid phase), and a little farther away, a Martian cold world (ice