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The Limits of Organic Life in Planetary Systems
TABLE 6.2 Critical Temperature and Pressure for Selected Substances
Critical Temperature (K)
Critical Pressure (atm)
It is useful to divide the solvents into three groups: polar solvents that are not water, nonpolar solvents, and cryosolvents.
6.2.1 Polar Solvents That Are Not Water
Ammonia is analogous to water in many of its properties. Ammonia, like water, dissolves many organic compounds, including many polyelectrolytes. Preparative organic reactions are often done in ammonia in the laboratory. Ammonia, like water, is liquid over a wide range of temperatures (195 to 240 K at 1 atm). The liquid range is even broader at higher pressure. For example, at 60 atm ammonia is liquid from 196 to 371 K. Further, liquid ammonia may be abundant in the solar system. A large amount of the inventory of liquid ammonia in the solar system exists, for example, in clouds in the jovian atmosphere. However, as noted earlier by the committee, some view clouds as unlikely places to harbor life. However, if clouds are not transient and broken (as on Earth) but are rather more continuous (as on Venus), this view may need modification.
As compared with water, ammonia’s increased ability to dissolve hydrophobic organic molecules suggests an increased difficulty in using the hydrophobic effect to generate compartmentalization in ammonia, relative to water. This in turn implies that the liposome, a compartment that works in water, generally will not work in liquid ammonia. Hydrophobic phase separation is possible in ammonia, however, albeit at lower temperatures. For example, Brunner reported that liquid ammonia and hydrocarbons form two phases, where the hydrocarbon chain contains from 1 to 36 CH2 units.5 Different hydrocarbons become miscible with ammonia at different temperatures and pressures. Thus, formation of ammonia-phobic and ammonia-philic phases, analogous to the hydrophobic and hydrophilic phases in water, useful for isolation would be conceivable in liquid ammonia at temperatures well below its boiling point at standard pressures.
The greater basicity of liquid ammonia must also be considered. The species that serve as acid and base in pure water are H3O+ and HO−. In ammonia, NH4+ and NH2− are the acid and base, respectively. H3O+, with a pKa of −1.7, is about 11 orders of magnitude stronger (in water) as an acid than NH4+, with a pKa of 9.2 (in water). Likewise, NH2− is about 15 orders of magnitude stronger as a base than HO−.
The increased strength of the dominant base in ammonia,as well as the corresponding enhanced aggressivity of ammonia as a nucleophile, implies that ammonia would not support the metabolic chemistry found in terran life. Terran life exploits compounds containing the C=O carbonyl unit. In ammonia, carbonyl compounds are (at the very least) converted to compounds containing the corresponding C=N unit. Nevertheless, hypothetical reactions that exploit a C=N unit in ammonia can be proposed in analogy to the metabolic biochemistry that exploits the C=O unit in terran metabolism in water (Figure 6.1).6 Given this adjustment, metabolism in liquid ammonia is easily conceivable.
Most interestingly, ammonia is a potent antifreeze for water. Recently recovered data from Titan suggest that that moon is periodically being resurfaced by a liquid having a viscosity comparable to that of a water-ammonia eutectic, which is liquid even in an environment that experiences methane rain. Water-ammonia eutectics, which