This entry is based, in part, on material from the section "Alternatives to Water" in the book Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization by Robert A. Freitas, Jr.

 

In 1954, J. B. S. Haldane, speaking at the Symposium on the Origin of Life, suggested that an alternative biochemistry could be conceived in which water was replaced as a solvent by liquid ammonia.¹ Part of his reasoning was based on the observation that water has a number of ammonia analogues. For example, the ammonia analogue of methanol, CH₃OH, is methylamine, CH₃NH₂. Haldane theorized that it might be possible to build up the ammonia-based counterparts of complex substances, such as proteins and nucleic acids, and then make use of the fact that an entire class of organic compounds, the peptides, could exist without change in the ammonia system. The amide molecules, which substitute for the normal amino acids, could then undergo condensation to form polypeptides which would be almost identical in form to those found in terrestrial life-forms. This hypothesis, which was developed further by the British astronomer V. Axel Firsoff,^{2, 3} is of particular interest when considering the possibility of biological evolution on ammonia-rich worlds such as gas giants and their moons (see life on Jupiter).

 

On the plus side, liquid ammonia does have some striking chemical similarities with water. There is a whole system of organic and inorganic chemistry that takes place in ammono, instead of aqueous, solution.^{4, 5} Ammonia has the further advantage of dissolving most organics as well as or better than water,⁶ and it has the unprecedented ability to dissolve many elemental metals, including sodium, magnesium, and aluminum, directly into solution; moreover, several other elements, such as iodine, sulfur, selenium, and phosphorus are also somewhat soluble in ammonia with minimal reaction. Each of these elements is important to life chemistry and the pathways of prebiotic synthesis. The objection is often raised that the liquidity range of liquid ammonia – 44°C at 1 atmosphere pressure – is rather low for biology. But, as with water, raising the planetary surface pressure broadens the liquidity range. At 60 atm, for example, which is below the pressures available on Jupiter or Venus, ammonia boils at 98°C instead of –33°C, giving a liquidity range of 175°C. Ammonia-based life need not necessarily be low-temperature life!

 

Ammonia has a dielectric constant about ¼ that of water, making it a much poorer insulator. On the other hand, ammonia's heat of fusion is higher, so it is relatively harder to freeze at the melting point. The specific heat of ammonia is slightly greater than that of water, and it is far less viscous (it is freer-flowing). The acid-base chemistry of liquid ammonia has been studied extensively, and it has proven to be almost as rich in detail as that of the water system. In many ways, as a solvent for life, ammonia is hardly inferior to water. Compelling analogues to the macromolecules of Earthly life may be designed in the ammonia system. However, an ammonia-based biochemistry might well develop along wholly different lines. There are probably as many different possibilities in carbon-ammonia as in carbon-water systems.⁷ The vital solvent of a living organism should be capable of dissociating into anions (negative ions) and cations (positive ions), which permits acid-base reactions to occur. In the ammonia solvent system, acids and bases are different than in the water system (acidity and basicity are defined relative to the medium in which they are dissolved). In the ammonia system, water, which reacts with liquid ammonia to yield the NH⁺ ion, would appear to be a strong acid – quite hostile to life. Ammono-life astronomers, eyeing our planet, would doubtless view Earth's oceans as little more than vats of hot acid. Water and ammonia are not chemically identical: they are simply analogous. There will necessarily be many differences in the biochemical particulars. Molton suggested, for example, that ammonia-based life forms may use cesium and rubidium chlorides to regulate the electrical potential of cell membranes. These salts are more soluble in liquid ammonia than the potassium or sodium salts used by terrestrial life.⁸

 

On the down side, there are problems with the notion of ammonia as a basis for life. These center principally upon the fact that the heat of vaporization of ammonia is only half that of water and its surface tension only one third as much. Consequently, the hydrogen bonds that exist between ammonia molecule are much weaker than those in water so that ammonia would be less able to concentrate non-polar molecules through a hydrophobic effect. Lacking this ability, questions hang over how well ammonia could hold prebiotic molecules together sufficiently well to allow the formation of a self-reproducing system.⁹

 

References

1. Haldane, J. B. S. "The Origins of Life," New Biology, 16, 12-27 (1954).
2. Firsoff, V. A. Life Beyond the Earth: A Study in Exobiology. New York: Basic Books (1963).
3. V. Axel Firsoff, "An Ammonia-Based Life," Discovery 23, 36-42 (January, 1962).
4. Gerhart Jander, Hans Spandau, C. C. Addison; eds. Chemistry in Nonaqueous Ionizing Solvents. New York: John Wiley, Interscience (1966).
5. Smith, Herschel. Organic Reactions in Liquid Ammonia. New York: John Wiley, Interscience (1950).
6. Franklin, E. C., "The Ammonia System of Acids, Bases, and Salts," American Chemical Journal, 47, 285 (1912).
7. Firsoff, V. A., "Possible Alternative Chemistries of Life," Spaceflight, 7, 132-136, (July, 1965).
8. Molton, P. M., "Terrestrial Biochemistry in Perspective: Some Other Possibilities," Spaceflight, 15, 134-144 (April 1073).
9. Feinberg, Gerald, and Shapiro, Robert. Life Beyond Earth: The Intelligent Earthling's Guide to Life in the Universe. New York: William Morrow (1980).