Whence came our water?

In Chapter 2 we thought briefly about the origin of the Earth’s abundant water. The evidence is fairly clear that water was acquired from elsewhere in the universe some considerable time after the Earth’s origin. Just how the water came is less certain, the favoured idea being that it was carried on the debris of comets, but water generation by sulphide-dependent chemoautotrophic bacteria as a by-product of photosynthesis is also likely. The existence of some water in the stellar dust grains that coalesced to form the original Earth has also not been completely excluded.

What is clear is the close association between water and life. The Earth’s life forms depend for their composition on water and carbon, as well as some other elements, and indeed the present interest in finding other planets in our galaxy, the Milky Way, which may harbour life, is chiefly pursued by looking for planets with water and a median temperature between 0°C and 100°C. Since it now seems that the Milky Way, with its 200–400 billion stars, is also characterised by the presence of one or more planets in association with most of these stars, the hunt is on for another planet other than Earth with some life-favouring characteristics. The moon Europa, which orbits around Jupiter, has abundant ice and water and is a current candidate. It seems that although water and carbon are the hot favourites for substances essential for the evolution of living systems, there are also some extreme situations where ammonia could substitute for water and silicon for carbon. But for the present, the point which emerges strongly is the strong correlation between water and life. Without the former we would not have the latter.