© 1975-1979, 2008 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm

8.2.2  Alternatives to Water

Can living processes be based on a liquid other than water (Figure 8.1)? To answer this question we must address a more fundamental problem: What are the properties of a good solvent for life?

First of all is availability. If the substance is exceedingly rare, there will not be enough of it around to sustain an ecology. Next, it should be a good solvent for both inorganic and organic compounds, and in this regard an acid-base chemistry is highly desirable. Further, the fluid ought to have a reasonably large liquidity range, so that organisms will enjoy a wide span of temperatures in which they remain biochemically operational.

A high dielectric constant is preferable -- the liquid medium should provide adequate electrical insulation from the surroundings. Also, a large specific heat would be nice, because this would give the organism thermal stability in the face of sudden or extreme temperature variations in the environment. Finally, the solvent ought to have a low viscosity -- it should not be too thick and resistant to flow (not an essential characteristic but certainly convenient).

Figure 8.1 "Ammonia! Ammonia!" (from Bracewell⁸⁰)

J.B.S. Haldane, speaking at the Symposium on the Origin of Life in 1954, speculated on the possible nature of life based on a solvent of liquid ammonia.²³²⁸ The British astronomer V. Axel Firsoff picked up on this a few years later, and extended the analysis considerably.^352,1217 Today, ammonia is considered one of the leading alternatives to water. Let’s see why.

Ammonia is known to exist in the atmospheres of all the gas giant planets in our solar system, and was plentiful on Earth during the first eon of its existence. Ammonia may be a reasonable thalassogen, so it should be available in sufficient quantities for use as a life-fluid on other worlds.

Chemically, liquid ammonia is an unusually close analogue of water. There is a whole system of organic and inorganic chemistry that takes place in ammono, instead of aqueous, solution.^1579,1584

Ammonia has the further advantage of dissolving most organics as well as or better than water,²³⁴⁵ and it has the unprecedented ability to dissolve many elemental metallic substances directly into solution--such as sodium, magnesium, aluminum, and several others. Iodine, sulfur, selenium and phosphorus are also somewhat soluble with minimal reaction. Each of these elements is important to life chemistry and the pathways of prebiotic synthesis.

The objection is often heard that the liquidity range of liquid NH₃ -- 44° C at 1 atm pressure -- is a trifle low for comfortable existence. But as with water, raising the planetary surface pressure broadens the liquidity range. At only 60 atm, far less than Jupiter or Venus in our solar system, ammonia boils at 98 °C instead of -33 °C. ("Ammonia life" is not necessarily "low temperature life.") So at 60 atm the liquidity range has climbed to 175 °C, which should be ample for life.

Ammonia has a dielectric constant about ¼ that of water, so it is a much poorer insulator than H₂O. But ammonia’s heat of fusion is higher, so it is relatively harder to freeze at the melting point.* The specific heat of NH₃ is slightly greater than that of water, and it is far less viscous (it is freer-flowing) too.

The acid-base chemistry of liquid ammonia has been studied extensively throughout this century, and it has proven to be almost as rich in detail as that of the water system (Figure 8.2). The differences between the two are more of degree than of kind. As a solvent for life, ammonia cannot be considered inferior to water.

Compelling analogues to the macromolecules of Earthly life may be designed in the ammonia system. But Firsoff has urged restraint: 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.¹¹⁷²

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 (Table 8.3). In the NH₃ solvent system, acids and bases are different than in the water system-acidity and basicity, of course, are defined relative to the medium in which they are dissolved.

Table 8.3 Dissociation of the Vital Solvent¹²¹⁷

In the ammonia system, water, which rests with liquid NH₃ to yield NH₄⁺ ion, would seem as a strong acid, quite hostile to life. Ammono-life astronomers, eyeing our planet from their chilly observatories, would doubtless view the beautiful, rolling blue oceans of Earth as little more than "vats of hot acid."

After all, water and ammonia are not chemically identical. They are simply analogous. There will necessarily be many differences in the biochemical particulars. Molton has suggested, for example, that ammonia-based lifeforms may use cesium and rubidium chlorides to regulate the electrical potential of cell membranes. These salts are more soluble in liquid NH₃ than the potassium or sodium salts used by Earth life.¹¹³²

Dr. Molton concludes: Life based on ammonia instead of water is certainly possible (Figure 8.2), theoretically, at the superficial level. If we delve further into the complex biochemistry of the cell, we could find some insuperable barrier to ammonia-based life -- but it is hard to conceive of any obstacle so insuperable that it would rule it out altogether.

Figure 8.2 Living in Liquid Ammonia

Biochemical Type	Terrestrial Water-Life Form	Possible Ammonia-Life Analogue
Typical Alcohol	H H | |  H—C—C-OH | | H H	H H | |   H—C—C—NH2 | | H H
Typical Fatty Acid	H O |  ||  H—C—C—OH |  H	H O |  ||   H—C—C—NH2 |  H
Typical Amino Acid	H H O | |  ||  H—C—C—C—OH | |  H NH2	H H O  | |  ||   H—C—C—C—NH2 | |  H NH2
Typical Protein Polymer (could be identical molecule in both systems)
Typical Carbohydrate (Ribose)

There are many other life-solvents (Table 8.4) which have been studied to varying degrees, though none so extensively as ammonia. Hydrogen fluoride (HF), for instance, has often been proposed. HF is an excellent solvent in theory both for inorganics and organics vital to carbon-based life.

Hydrogen fluoride has a larger liquidity range than water and has hydrogen bonding as well as an acid-base chemistry (in which nitric and sulfuric acids act as bases!).¹⁵⁸³ It also has a large dielectric constant and a sizable specific heat. The major difficulty with HF is its extreme cosmic scarcity. However, this need not be a fatal objection in view of the widespread use of the equally rare element phosphorus in terrestrial biochemistry.

Liquid hydrogen cyanide (HCN) is another possibility. Unlike HF, hydrogen cyanide has a reasonably high cosmic abundance -- although it still may be too low to be of xenobiochemical significance. HCN is a good inorganic and organic solvent, has an adequate liquidity range, has hydrogen bonding, a large dielectric constant and specific heat, and a viscosity five times lower than that of water. Its chemistry, however, may be complicated by its tendency to polymerize.

Hydrogen sulfide (H₂S) is the sulfur analogue of water, in which S atoms replace those of oxygen. (The two elements are of the same family in the Periodic Table (Table 8.5), and have similar chemical properties.) We might expect that H₂S would have similar solvating abilities to water, but such is not the case. Hydrogen sulfide has only weak hydrogen bonding, a low dielectric constant, and is a very poor inorganic solvent.¹⁵⁷⁸ Its narrow liquidity range (25 °C) means that it should be suitable, if at all, only for planets with heavy atmospheres and small daily temperature variations.

Sulfur dioxide, another possible thalassogen, is an ionizing substance which is a good organic and a fair inorganic solvent. It has an adequate liquidity range, but a very low dielectric constant.

Carbon disulfide, a wide liquidity range fluid, solvates sulfur and a number of organic compounds. But it is relatively unstable with heat and is expected to be rare on most planetary surfaces.

Little is known about chemistry in liquid chlorine (Cl₂). While it has a good liquidity range, it is five times more viscous than water. One peculiar halogen hybrid, fluorine oxide (F₂O), is a direct analogue of water. This intensely yellow fluid is a good ionizing solvent, unstable at high temperatures but ideal for biochemistry below 100 K. At such temperatures, F₂O might serve as solvent for the coordination chemistry of the noble gases.¹¹⁷²

There are many, many other less likely solvents that have been discussed in the literature.**

* The point is sometimes made that water has the virtually unique property of expanding upon freezing, which means that ice will float atop a cooling mass of water and protect the lifeforms beneath. However, water freezing within the cells of living tissue exposes the organism to a new hazard -- mechanical damage by expansion. Since ammonia shrinks when it freezes, the very property responsible for massive oceanic freeze-ups should also allow ammono lifeforms to be much more successful hibernators in a frozen clime.

** Dr. Allen M. Schoffstall at the University of Colorado at Colorado Springs has performed some preliminary experiments with possible prebiotic syntheses in exotic solvents, such as formic acid, acetic acid, liquid formamide and other nonaqueous solvents. His experiments have demonstrated the feasibility of prebiotically converting nucleosides to nucleotides or nucleoside diphosphates in anhydrous liquid formamide -- an alternative solvent to water.²³⁸⁴ Similar research is just now getting started at several other laboratories.⁴⁰⁸⁶

Last updated on 6 December 2008