ABSTRACT
Earth is unique amongst the planets of the solar system in that its surface has abundant free water, in a constant state of movement termed the hydrological cycle (Figure 1.2). In 1998 water was found on the now-frozen surfaces of the Moon and Mars, leading to speculation about potential for life outside the Earth. On a global scale, the Earth's surface may be viewed as a giant distillation unit, powered by the Sun, where water is continuously evaporated from the oceans and the land, and returned as precipitation. The oceans form the largest reservoir, holding around 97% of the Earth's surface water.
