Must Life Drink Water?
Star Trek used to portray aliens made up of different stuff than the carbon and water chemistry which comprises Earth-based life. For years, most scientists who considered the possibility of life in space, including Carl Sagan and Stanley Miller, admitted, somewhat reluctantly, that the periodic table of the elements admits no practical alternatives to water as the solvent of life. This question has been reopened at a December conference of physicists, chemists, biochemists and microbiologists sponsored by the Royal Society, reports Philip Ball in the Jan. 1 issue of Nature.1 Life needs more than just a liquid, any liquid. Philip Ball reminds the casual observer that though life needs a liquid, liquidity is not enough:
But there is much more to water than that. It has long been recognized as a profoundly anomalous liquid, with properties that set it apart from all others. High heat capacity, expansion on freezing, maximum density at 4 �C, high dielectric constant � all of these so-called anomalies, and others, seem critical to its biological role. They are in fact relatively easy to rationalize on the grounds of water’s hydrogen-bonded structure, which joins the H2O molecules into a fluctuating, three-dimensional network (J. Finney, University College London). Unlike ‘simple’ liquids, water’s molecular structure is dominated not by the hard core repulsions between molecules but by the directional, attractive interactions of hydrogen bonds. (Emphasis added in all quotes.)
While not admitting to the viability of any other possibilities, he keeps the door open a tad:
There seems to be no simple molecule that can mimic all of the useful biological functions of water. One school of thought asserts that it is therefore futile to look for replacements for any one, or even simultaneously for several, of its ‘virtues’: the biological importance of water lies in their synchronous operation in a single molecular system. But what we really need is a way of asking which, if any, of those functions is generic to life. Is there, for example, a temperature limit that rules out other tetrahedral liquids such as silica, because of the complications introduced by molecular excited states at high temperatures? At low temperatures, would slower diffusion rates prevent effective exploitation of thermodynamic equilibria? In other words, is there a habitable zone not just in physical space but in chemical and thermodynamic space too?
1Philip Ball, “Astrobiology: Water, water, everywhere?” Nature 427, 19 – 20 (01 January 2004); doi:10.1038/427019a.
Asking a question is fine, but calls to mind Ahab’s proverb, “Let not the one who puts on his armor boast like the one who takes it off” (I Kings 20:11). The last warriors who fought this question gave up, singing, All day I faced the barren waste without replacing water, cool water. To astrobiologists looking in the cosmic deserts for a different elixir, we say: Keep a-movin’ Dan, don’t you listen to him Dan, he’s the devil not a man, and he spreads the burning sand with ammonia.