In the Right Place at the Right Time

The coming together of two hydrogen (H) atoms and one oxygen (O) atom produces something that is both marvelous and altogether greater than the sum of its parts: water (Figure 5.1). Symbolically written as H2O, water is the medium of life as we know it. Without water there is no life—end of story. A liter of pure water contains an incredible 3 x 1028 H2O molecules, and while this seems (and is) an astronomically large number, we are fortunate to live in a universe that is predominantly composed of hydrogen (to the level of about 75% by mass fraction), with oxygen coming in as the third most-abundant element, after helium (He), which accounts for about 24% by mass fraction of the universe.

Water is the most-abundant molecule on the Earth's surface, and although predominantly found in its liquid form, it is also

Figure 5.1. A typical view of the Earth's surface. Since some 70% of the Earth's surface is covered by oceans (recall Figure 4.12), the most typical vista that might be seen if placed at random on the Earth is that viewed from a boat. The tsunami-warning buoy, however, is not a typical foreground object. Image courtesy of NOAA.

Figure 5.1. A typical view of the Earth's surface. Since some 70% of the Earth's surface is covered by oceans (recall Figure 4.12), the most typical vista that might be seen if placed at random on the Earth is that viewed from a boat. The tsunami-warning buoy, however, is not a typical foreground object. Image courtesy of NOAA.

M. Beech, Terraforming, Astronomers' Universe 81

DOI 10.1007/978-0-387-09796-1_5, © Springer Science+Business Media, LLC 2009

present as a gas and a solid. This remarkable ability of water to exist on the Earth's surface in three distinctly different phases has immensely important consequences for our very existence and for the long-term stability of the biosphere.

Water is indeed a very special substance, and human survival is entirely dependent upon there being a ready supply of it to drink. The existence of and access to a supply of liquid water is often taken to be a minimum requirement for any terraforming program; so in the sections below, we look at some of the many remarkable properties of H2O and determine the physical properties required of an atmosphere such that liquid water can exist on the surface of a planet.

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