Let us begin by observing that the origin of ocean water is a problem that is separate from the origin of ocean basins and that it has two parts: the source of the water and the source of the many substances dissolved in it. The key role of the ocean in earth history has been discussed before; its origin and initial composition remain to be considered here.

For the atmosphere, the main issue is its evolution to its present composition. Compared to the rest of the solar system, our atmosphere, with its 78 percent nitrogen, 21 percent oxygen, and minor amounts of carbon dioxide, water vapor and rare gases, is an odd one. It is so odd that the atmospheres of Venus, Mars, Jupiter, Saturn, and of some of their moons offer little help in understanding our own.

WHENCE THE WATER IN THE SEA?

Whatever model one espouses for the accretion of the earth, the existence at an early time of a magma ocean and the composition of its parent material, the carbonaceous chondrites with their 15–20 percent water, render the early release of large amounts of water and gases inevitable. Total melting is unnecessary; a 50-km-deep magma ocean, itself a conservative guess, would yield every drop of water one needs. Consequently, around 4 by ago the earth had a true ocean, although perhaps not a full one, and an atmosphere, although one quite unlike the present one.