Water is, beyond doubt, one of the most remarkable substances in the universe and the one most inextricably linked with life of all kinds. A classic essay on the many peculiar properties of water that make it uniquely suited to the requirements of living things is included in The Fitness of the Environment by L. J. Henderson (1958). Among other things, Henderson discusses the remarkable heat-regulating properties of water (its extraordinarily high heat of vaporization and its high heat of fusion), its anomalous expansion when cooled below 4°C, the low density of ice, the incomparable powers of water as a solvent, its high dielectric constant, and its high surface tension. In a lighter vein, even James Joyce in Ulysses expounded on the marvelous properties of water.
Man and his entire ecology are utterly dependent on water. It can be said categorically that a habitable planet must have fairly large open bodies of liquid water, for without oceans there could be no extensive precipitation and hence no salt-free ground water to provide supplies of fresh water. Precisely what ratio of ocean surface area to total planetary surface area is necessary may be very difficult to determine. It is clear that a certain critical total quantity of water is necessary on the surface of a planet before bodies of water can appear. If there were less than this amount, then all of the water would be in the form of water vapor or watei absorbed on the surfaces or held interstitially between the solid particles of rock of which the crust is composed. On the other hand, a planet completely covered with water, a pelagic planet without permanent dry land, could hardly be considered habitable from man's point of view. On the Earth, a multiplication of our present water supply by a factor of four would be required for complete inundation of all our continents.
Humidity in a breathable atmosphere is also highly important. The uncomfortable effects of high levels of humidity at high temperatures are well known; but there may also be adverse physiological effects due to extremely low levels of water vapor pressure in the air, particularly at the higher temperatures. This latter condition causes very rapid drying of the mucous membranes of the nose, mouth, and throat; and continuous exposure to very low levels of water vapor pressure might well be intolerable.
Regarding water, then, a habitable planet must be able to retain it and must have open bodies of fluid water on its surface, but the proportion of surface area covered by water must be somewhat less than 90 per cent.
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