Shared from xenology book

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.

 

Table 8.2 Acid-Base Reactions for Ammonia-based Life
INORGANIC 	
HCl
	+ 	
NaOH
	 —> 	
NaCl
	+ 	
H2
	Aqueous-life chemistry
  	
HOH
	+ 	NaNH2 	
—>
	
NaOH
	+ 	
NH3
	Ammonia-life chemistry
								
ORGANIC
	
CH3COOH
	+ 	
NaOH
	
—>
	
CH3COONa
	+ 	
H2
	Aqueous-life chemistry
  	
CH3CONH2
	+ 	NaNH2 	
—>
	
CH3CONHNa
	
+ 
	
NH3
	Ammonia-life chemistry
  	
Acid
	+ 	
base
	
—>
	
Salt
	
+ 
	
Solvent
	 

 

 

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
Solvent
	
Anions
	Cations
H2
	
OH -
	
O=
	  	
H +
	
H3O+
NH3
	
NH2-
	
NH =
	
N º
	
H +
	
NH4+

    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.

 

Table 8.4 Physical Constants for Xenobiochemical Solvents352,879,1578,2082
Possible Life Solvent
	
Chemical
Formula
	
Molecular
Weight
	
Liquidity
Range
	
Melting
Point
	
Boiling
Point
	Heat of
Fusion 	
Heat of
Vaporization 
	
Typical
Dielectric 
Constant
	
Typical
Viscosity
		(gm/mole) 	
(K)
	
(K)
	
(K)
	(kcal/mole) 	(kcal/mole) 		(centipoises)
Sulfur 	S2 	
64.1
	
331.8
	
386.0
	
717.6
	
---
	
23.2
	
3.48
	
1
Sulfuric acid 	H2SO4 	
98.1
	
327.6
	
283.5
	
611.1
	
2.56
	
12.0
	
100.
	
48.4
Glycerol  	H2H8O3 	
92.1
	
271.4
	
291.7
	
(563.1)
	
4.42
	
18.19
	
42.5
	
102 - 106
Phosphorous sesquisulfide 	P4S3 	
220.1 
	
234.0
	
447.1
	
681.1 
	
2.002
	
16.06
	
---
	
0.10
Acetic anhydride 	(CH3CO)2O 	
102.1
	
213.1
	
200.0
	
413.1
	
---
	
6.76
	
6.3
	
0.851
Ethanol 	C2H5OH 	
46.1
	
192.8
	
158.6
	
351.4
	
1.20
	
10.9
	
24.3
	
1.078
Formamide 	HCONH2 	
45.0
	
190,4
	
275.7
	
466.1
	
---
	
15.0
	
111.
	
3.31
Methanol 	CH3OH 	
32.0
	
162.5
	
175.3
	
337.8
	
0.759
	
8.42
	
32.6
	
0.544
Carbon disulfide 	CS2 	
76.1
	
157.1
	
162.4
	
319.5
	
1.05
	
6.7
	
3.0
	
0.436
Arsenic trichloride 	AsCl3 	
181.3
	
143.0
	
260.1
	
403.1
	
---
	
12.6
	
12.6
	
1.23
Phosgene 	COCl2 	
98.9
	
136.2
	
145.1
	
281.3
	
1.37
	
6.22
	
4.34
	
---
Hydrazine 	N2H4 	
32.0
	
111.7
	
274.9
	
386.6
	
---
	
10.2
	
53.
	
1.12
Phosphorus oxychloride 	POCl3 	
153.4
	
107.0
	
274.1
	
381.1
	
---
	
---
	
13.9
	
1.15
Hydrogen fluoride 	HF 	
20.0
	
102.7
	
190.0
	
292.7
	
1.094
	
7.23
	
83.6
	
0.256
Acetic acid 	
CH3COOH
	
60.1
	
101.5
	
289.7
	
391.2
	
2.76
	
5.81
	
9.7
	
1.16
WATER 	H2O 	
18.0
	
100.0
	
273.1
	
373.1
	
1.455
	
9.719
	
81.1
	
0.959
Formic acid 	HCOOH 	
46.0
	
92.3
	
281.5
	
373.8
	
3.04
	
4.77
	
58.5
	
1.804
Methylamine 	CH3NH2 	
31.1
	
86.0
	
180.6
	
266.6
	
3.47
	
6.47
	
11.4
	
0.236
Mercury dibromide 	HgBr2 	
360.4
	
82.3,
	
511.1
	
593.4
	
---
	
---
	
9.84
	
3.70
Fluorine oxide 	F2O 	
54.0
	
79.0
	
49.3
	
128.3
	
---
	
2.65
	
---
	
---
Formaldehyde 	HCHO 	
30.0
	
71.0
	
181.1
	
252.1
	
---
	
5.92
	
---
	
---
Chlorine 	Cl2 	
70.9
	
66.9
	
172.2
	
239.1
	
1.531
	
4.78
	
2.0
	
4.9
Sulfur dioxide 	SO2 	
64.1
	
62.7
	
200.5
	
263.2
	
1.969
	
5.96
	
13.8
	
0.429
Nitrosyl chloride 	NOCl 	
65.4
	
55.5
	
211.6
	
267.1
	
---
	
5.4
	
22.5
	
0.586
Ammonia 	NH3 	
17.0
	
44.4
	
195.4
	
239.8
	
1.84
	
5.64
	
22.
	
0.265
Hydrogen cyanide 	HCN 	
27.0
	
39.0
	
259.8
	
298.8
	
2.01
	
6.03
	
123.
	
0.201
Oxygen 	O2 	
32.0
	
35.4
	
54.8
	
90.2
	
---
	
1.86
	
1.51
	
---
Nitrogen tetroxide 	N24 	
92.0
	
33.6
	
260.8
	
294.4
	
3.5
	
9.11
	
2.42
	
---
Hydrogen chloride 	HCl 	
36.5
	
29.9
	
158.3
	
188.2
	
0.476
	
3.86
	
12.
	
0.51
Hydroxylamine  	NH2H 	
33.0
	
25.0
	
306.1
	
331.1
	
---
	
---
	
---
	
---
Hydrogen sulfide 	H2 	
34.1
	
24.8
	
187.7
	
212.5
	
0.568
	
4.463
	
10.2
	
0.432
Methane 	CH4 	
16.0
	
21.0
	
90.7
	
111.7
	
0.22
	
2.13
	
1.7
	
---
Chloroform 	CHCl3 	
119.4
	
124.7
	
209.6
	
334.3
	
2.10
	
7.5
	
5.61
	
0.70
Carbon Tetrachloride 	CCl4 	
153.8
	
99.7
	
250.1
	
349.8
	
0.783
	
8.27
	
2.23
	
1.33
Methyl chloride 	CH3Cl 	
50.5
	
73.2
	
176.1
	
249.3
	
---
	
5.38
	
12.6
	
0.183

 

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.

 

Table 8.5 The Periodic Table of the Elements
H
HYDROGEN
1
	  	  	  	  	  	  	
He
HELIUM
2
	First
Period
Li
LITHIUM
3
	
Be
BERYLLIUM
4
	
B
BORON
5
	
C
CARBON
6
	
N
NITROGEN
7
	
O
OXYGEN
8
	
F
FLUORINE
9
	
Ne
NEON
10
	Second
Period
Na
SODIUM
12
	
Mg
MAGNESIUM
12
	
Al
ALUMINUM
13
	
Si
SILICON
24
	
P
PHOSPHORUS
15
	
S
SULFUR
16
	
Cl
CHLORINE
17
	
Ar
ARGON
18
	Third
Period
K
POTASSIUM
19
	
Ca
CALCIUM 
20
	
Ga
GALLIUM
31
	
Ge
GERMANIUM
32
	
Ar
ARSENIC
33
	
Se
SELENIUM
34
	
Br
BROMINE
35
	
Kr
KRYPTON
36
	Fourth
Period
Rb
RUBIDIUM
37
	
Sr
STRONTIUM
38
	
In
INDIUM
49
	
Sn
TIN
50
	
Sb
ANTIMONY
51
	
Te
TELLURIUM
52
	
I
IODINE
53
	
Xe
XENON
54
	Fifth
Period
Os
CESIUM
55
	
Ba
BARIUM
56
	
Ti
THALLIUM 
81
	
Pb
LEAD
82
	
Bi
BISMUTH
83
	
Po
POLONIUM
84
	
At
ASTATINE
85
	
Rn
RADON
86
	Sixth
Period
Sodium
Family

Group I
	
Calcium
Family

Group II
	
Boron
Family 

Group III
	
Carbon
Family 

Group IV
	
Pnictide 
Family

Group V
	
Chalcogen
Family

Group VI
	
Halogen 
Family

Group VII
	
Noble Gas
Family 

Group VIII 
	 

 

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