Chapter 8.  Exotic Biochemistries

"OXYGEN: An intensely habit-forming accumulative toxic substance. As little as one breath is known to produce a life-long addiction to the gas, which addiction invariably ends in death. In high concentration, it causes death quickly, but even in a 20% dilution few survive more than 0.8 century."
          -- Anonymous (1956)¹⁹⁹

"An exotic biochemistry based on silicon is basically a comic-book idea."
          -- Dr. Norman Horowitz (1966)730

"Suppose that in the early days of our own planet, when life was first forming in the primordial ocean, a thousand different schemes of life set sail. Let us further assume that one particular scheme won out over the rest, perhaps through the sheerest chance. The survival of that one scheme could now give us the false impression that it is the inevitable and only possible scheme."
          -- Dr. Isaac Asimov (1967)⁹⁶

"E.M. Hafner: Do you know of any program of experimentation in your laboratory or in any other laboratory that is aimed at discovering forms of life that are not carbon-based?
G. Levin: In my laboratory? No!"
          -- from Exobiology: The Search for Extraterrestrial Life (1969)⁶³⁰

"Indigenous alien life would not have to be like ours. In fact, it would be rather strange if it were. Our life is ideally adapted to terrestrial conditions, and it would show a surprising luck if other planets with widely different conditions had life based on the identical design."
          -- Dr. Peter M. Molton (1973)¹¹²⁹

"Fomor is a relatively small planet with almost no atmosphere and few interesting features. It does, however, possess several fungi which are biologically related to other fungi found in the Trans-Coalsack Sector, and their manner of transmission to Fomor has stimulated an endless controversy in the Journal of the Imperial Society of Xenobiologists...
          -- Larry Niven, Jerry Pournelle, from The Mote in God’s Eye (1974)⁶⁶⁸

In the previous chapter, we asked the question: What is the likelihood that life may evolve somewhere else in the universe? We answered by showing that, given a primitive environment similar to that of ancient Earth, some form of proteinous life is not unreasonable.

But how deterministic are the processes that occurred on this planet four eons ago? What are the chances that life must follow the identical biochemical pathways taken by organisms on Earth? It is the principle aim of xenobiology to ascertain where life exists in the universe, and what form it takes.

At first glance, the Hypothesis of Mediocrity might seem to rule out the possibility of alternative life biochemistries. There are no silicon beasts or chlorine-breathers present on this world, ergo natural selection does not favor them and they cannot exist.

This is, however, an incredibly chauvinistic argument. The only rigorous conclusion that can be drawn from the lack of exotic biochemistries on Earth is that contemporary conditions do not favor those other systems. Since a rich diversity of habitats is possible in the Galaxy, peculiar life chemistries cannot be categorically ruled out.

Life adapts itself to its environment. Change the environment, and the nature of life itself will change. It may be that no negentropic life-system can arise spontaneously under non-Earthlike conditions, but it is poor science to tie one’s hands with this assumption from the outset. Owing to the unique adaptivity of living things, the Hypothesis of Mediocrity must be applied cautiously when we venture out into new environs.

Experimental investigations have brought to light new facts which appear to indicate that significant variations on terran biochemistry are possible -- even probable -- on other planets.

[Note: See also the author's article "Xenobiology", published in 1981.]

8.1  The Argument for Diversity

The word chauvinism is derived from the cognomen of a highly jingoistic French soldier by the name of Nicolas Chauvin, born at Rochefort in the late 18th century. In 1815 Chauvin gained great notoriety by his obstinate, bellicose attachment to the lost cause of Napoleon’s crumbling imperium. The term has since become identified with the absurd, unreasoning, single-minded devotion to one’s own race, nationality, sex, or, most recently, to one’s own point of view.

Chauvinisms are predictably common in xenology. For example, we have argued against what might be called "G-star chauvinism," the idea that a home sun exactly like Sol is a prerequisite for life.¹⁵ Although our sun is class G, F stars and K stars undoubtedly are also hospitable to life. But stars may not be necessary at all. The possibility exists that interstellar space may contain a large number of starless planets, objects having jovian or superjovian mass. Neglecting such an alternative could be condemned as "solar chauvinism."¹⁴⁷⁰

There have been discussions of "planetary chauvinism," the belief that life can only exist on the surface of planets. Fred Hoyle exposes this parochialism in his science fiction novel The Black Cloud. After humans manage to open a communications link with the gaseous lifeform, the interstellar electromorph is quite astonished. "Your first transmission," says the Cloud, "came as a surprise, for it is most unusual to find animals with technical skills inhabiting planets -- which are in the nature of extreme outposts of life."⁶²

And atmospheric chauvinism? Carl Sagan has imagined organisms trapped on a planet whose air is slowly leaking away to space. Over time, such creatures might evolve mechanisms to cope with what is essentially an interstellar environment.¹⁵ Another possibility might be an advanced spacefaring civilization that had set up outposts in deep space or on airless worlds.

There are many biochauvinisms in xenology, various preconceptions relating to indigenous alien lifeforms. For instance, one biochauvinism this author finds exceedingly difficult to overcome is "phase separation chauvinism."^2371,2393 The requirement that all organisms must retain some sort of boundary layer between themselves and their surroundings seems to follow directly from the basic thermodynamic nature of life processes.

Other things seem not so fundamental. The extremes of life on Earth are well documented; microorganisms are especially hardy.

Acontium velatum and Thiobacillus thiooxidans flourish in some of the strongest acids known (pH = 0.0), while a blue-green algae known as Plectonema nostocorum thrives in the strongest bases (pH =13.0). (Normal water is neutral, with pH = 7.0).

Microbes can tolerate poisons of many kinds in their environment, such as corrosive sublimate (mercuric chloride), sulfuric acid, and arsenic. The tardigrades can stand prolonged periods of virtually complete desiccation,* and organisms have survived pressures ranging from the vacuum of space to more than 8000 atm (the barotolerant deep sea bacteria).

Growth and reproduction have been demonstrated from -24 °C (psychrophilic bacteria) up to 104 °C, and a few organisms (tardigrades, spores) have been frozen to near absolute zero, or heated to more than 120 °C, and survived the ordeal.

Radiation resistance is low in mammals and other higher lifeforms -- whole body lethal dose for man is a few hundred roentgens. But Deinococcus radiodurans and certain algae have endured as much as ten million roentgens of neutron bombardment, owing in part to special protective chemicals contained within their cells. Ultraviolet chauvinists claim that life is impossible on Mars because of the intense, unshielded solar radiation (UV) there, but many protective adaptations readily can be imagined.^15,26,1238

Perhaps one of the most persistent biochauvinisms is "oxygen chauvinism." A few decades ago, before the matter was given the serious thought it deserves, it was alleged that any planet lacking this "vital" gas was ipso facto uninhabitable. However, O₂ is not a requirement for survival for many organisms alive on Earth today (e.g., yeasts, tetanus bacillus, etc.) and was not present in appreciable quantities on the primitive Earth when the origin of life occurred.

It has been shown that the present level of O₂ is not optimal for plant growth. Greenery evidently grows more luxuriantly in an atmosphere containing only about half the normal amount of oxygen.⁵³ Human scuba divers are poisoned by the pure gas at more than a couple atmospheres of pressure. The presence of O₂ in the nuclear regions of contemporary living cells is usually fatal.

Oxygen is basically a reactive, toxic gas which chemically combines with and degrades virtually all useful biomaterials. The disastrous and widespread contamination of Earth’s atmosphere with O₂ a few eons ago (the first real "smog crisis") might have spelled the end of life on this world had nature not been able to quickly readjust to the new situation. It is as Arthur Clarke says: "Oxygen needs life," rather than the other way around.⁶⁰⁹

The mere absence of oxygen on a planet cannot, by itself, argue against the presence of life there.

Perhaps a more general biochemical question is whether or not the chemistry of life must occur in the liquid state. Most biologists would probably insist on a liquid solvent.

But life in the gaseous state cannot categorically be ruled out. One can imagine a "soap-bubble beast," laced with innumerable compartments and sub-compartments throughout. Probably a creature of the air, its metabolism might consist of chemical redox reactions taking place within its many "cells" in a controlled manner with the reaction products slowly diffusing outward. Because of the lower concentration of chemicals in such a gaseous medium, the organism’s structure, complexity, size and behavior would be sharply limited.

Solid life, too, is not out of the question. Although it has been alleged that reaction rates would be too slow for such lifeforms to exist, we know that timescales are relative and highly subjective. Trees often take hundreds of years to grow to full maturity, and many are thousands of years old. There is nothing a priori absurd in positing a form of life which has extremely slow negentropic processes.

Of course, it must be admitted that the liquid phase seems rather more convenient than the solid, gaseous or plasmic phases. Ions form easily, transport is greatly simplified, the breakup and recombination of chemical bonds is facilitated, and crude environmental stability is assured. The liquid phase is probably the preferred mode of existence for extraterrestrial lifeforms.

* The kangaroo rat, a common resident of American deserts, never needs to drink water. Its metabolism breaks down chemical compounds in sufficient quantities to enable it to live on the water manufactured from the food it eats. Other animals, such as the flour beetle, are known to have similar abilities, and camels can sustain themselves for weeks in this fashion. In the plant world, the Spanish moss can grow without contact with any groundwater -- when humidity is high, it can extract the needed moisture directly from the air.

8.1.1  Temperature Chauvinism

Any life chemistry will inevitably be subject to a narrow, or at least specific, temperature range. This is because a successful biochemistry is based on large assemblages of complex, delicately balanced molecules. These molecules must walk the thin line between overstability and overreactivity. Too cold, and the system grows sluggish and grinds to a halt; too hot, and reactions become uncontrollably rapid and the metabolism destabilizes.

The dedicated temperature chauvinist wants to restrict the viable range of all lifeforms to less than 100 °C, hardly enough to cover the gamut of terrestrial organisms alone. More sophisticated arguments suggest that even unfamiliar carbon-based systems probably could not exist much above 500 °C, because large carbon macromolecules shake themselves to pieces long before things get even that hot.

At the cold end of the scale, carbon-based biochemistries may be much less successful below about -100 °C. Reaction rates become extremely low, and there are fewer and fewer solvents in which the life-chemistry may proceed.

But are these valid limits for all conceivable living systems?

Perhaps not.* Table 8.1 gives the energy of various chemical bonds that might possibly occur in biologically significant molecules. If a structure is given more than this energy, the bonds may snap and the molecule falls apart. The higher the bond energy, the more stable the molecular structure. And stability is essential for any chemistry that aspires to live.

 

Table 8.1 Chemical Bond Energies for Some Combinations of Xenobiological Interest

Bond Type	Energy	Bond Type	Energy	Bond Type	Energy
	(eV)*		(eV)*		(eV)*
N = N	9.8	H - Br	3.8	S – Cl	2.6
C = N	9.4	Si – O	3.8	Cl – Cl	2.5
C = C	8.4	Si – Cl	3.8	S – S	2.2
C = O	7.4	C – O	3.7	O – Cl	2.2
C = C	6.4	C – C	3.6	N – Cl	2.1
H – F	5.9	S – H	3.5	Br – Br	2.0
Si – F	5.6	C – Cl	3.4	Si – Si	1.8
O – H	4.8	P – Cl	3.4	N – N	1.7
C – F	4.6	H – P	3.3	F – F	1.6
H – H	4.5	H – I	3.1	I – I	1.6
H – Cl	4.5	Si – H	3.1	O – O	1.4
N = N	4.4	Si – C	3.0	O2N – NO2	0.57
C – H	4.3	C – N	3.0	Hydrogen bonds	0.08-0.45
N – H	4.1	H – Se	2.9	van der Waals	0.04
* 1 eV = 1.60 × 10-19 Joules			Room temperature (25 °C) ~ 0.026 eV

 

Carl Sagan suggests that for life to exist, the fraction of bonds disrupted due to random thermal motions must be no larger than 0.0001%. If this is true, then lifeforms whose biochemistry is based solely on van der Waals forces (a weak attraction between atomic electrons and the nucleus of an adjacent atom) alone could survive at temperatures as high as 40 K. Biochemistries relying on hydrogen bonds alone could exist up to 400 K. Bonds of strength 2.0 eV or higher would suffer less than 0.0001% random breakage up to 2000 K, and for 5 eV bonds the molecules survive up to 5000 K.²³⁵⁸

This spans the range of temperatures from the coldest worlds to the surfaces of stars. Concludes Dr. Sagan: "There seem to exist chemical bonds of appropriate structural stability for life, and it would appear premature to exclude the possibility of life on any planet on grounds of temperature."

 

* Hal Clement’s two science fiction novels, Mission of Gravity (low temperature life)²⁰⁶⁹ and Iceworld (high temperature life),²⁹² are highly entertaining.

 

 

8.2  Alternative Biochemistries

Professor G.C. Pimentel, chairman of a NASA Study Group on Exobiology in 1966, remarked that perhaps the most interesting and important discovery that could ever be made in the entire field of xenobiology would be the detection of extraterrestrial lifeforms based on a chemistry radically different from our own. Space probe experiments designed solely to search for Earthlike organisms cannot firmly rule out the possible presence of life solely on the basis of a negative result. Cautions Pimentel, we must beware of the hazards of "down-to-earth thinking."²³⁵³

Yet all living creatures with whom we are acquainted are comprised of complex carbon compounds immersed in liquid water. Two classes of molecules always seem to be present: Nucleic acids, the blueprints of inherited instructions, and proteins, the materials and tools with which the architecture of life is constructed.

Must life always be based on carbon chemistry in aqueous solution? If we can agree that a biochemistry is the proper format for living systems, and that a liquid phase is probably essential, does it follow that carbon and water are our only choices in the matter?

8.2.1  The Limits of Carbon Aqueous

Carbon chemistry in terrestrial organisms proceeds by chemical reactions in the medium of water -- an amazing substance with a whole set of properties which make it ideal for our kind of life. Some have even contended that "water is the only possible candidate material."

In 1913, Harvard University biochemist Lawrence J. Henderson published a little book entitled The Fitness of the Environment in which he assembled for the first time the many points in favor of water as a life-fluid.⁸⁷⁹ Henderson’s analysis extends to the other molecules of life as well, and his main contribution is to show that the very chemical properties of the elements gives each of them a certain unique status and irreplaceability.

Among the many advantages of water, Henderson notes that it is an excellent solvent for countless substances, making it quite useful as a mediator of chemical activity in the liquid phase. Water, too, is an ionizing solvent, which means that an acid-base chemistry is permitted and an ever wider range of reactions can take place. (Acid-base chemistry is fundamental to Earth life but is not necessarily a requirement for all life.¹⁰⁷⁴)

Hydrogen bonding between water molecules gives the liquid a high heat capacity -- the ability to store lots of heat without changing temperature very much. Organisms which use water are thus at a distinct advantage in an environment in which sudden swings between hot and cold are common. This same bonding force also holds biomolecules together so that reaction rates are enhanced⁶⁴ (although it has been pointed out that H-bonding may not be absolutely essential for life²³⁵³).

Furthermore, water has a comfortably wide liquidity range -- a full 100 K under normal terrestrial conditions. However, the extent to which this temperature span may be broadened is not generally appreciated. Saturated salt water may freeze as low as 250 K; under 100-200 atm of pressure, the boiling point may be elevated to as much as 640 K.

In the proper environment, water could remain a liquid over a range of 400 degrees. It is not unreasonable to conclude that H₂O may well be the solvent of choice from 250-500 K, particularly in view of its extremely high cosmic abundance.

Serious laboratory work aimed at defining and measuring the limits of carbon-based, aqueous biochemistries has just gotten under way in earnest in the 1970s. Consequently, direct evidence is only beginning to emerge from the scanty data.

In spite of this handicap, there are early signs that many alternatives are possible even within the confines of a carbon-water system.

Dr. Peter M. Molton at the University of Maryland has suggested that simple changes in the early prebiotic environment may drastically affect the chemical species which later turn up as the dominant actors on the biochemical stage of evolution.¹⁰⁹⁴ His example is drawn from Miller-type experiments involving the prebiotic synthesis of amino acids, the building blocks of proteins.

In the lab, chemists have learned that there are two common structural forms taken by amino acids. They are called alpha and beta.

The basic layout of an amino acid molecule is a chain of carbon atoms with a small -NH₂ ("amino group") stuck on somewhere. In the alpha form, the amino group appears near the tail end of the molecule. In the beta form, the amino group is displaced more towards the front of the chain.

All amino acids used in terrestrial biochemistry, with one minor exception, are of the alpha variety. The beta forms are absent. Why?

Molton shows that this peculiarity may be due to nothing more complicated than the order in which water is introduced during the early stages of chemical evolution. If H₂O enters into the prebiotic reactions when the first simple compounds are being synthesized, then life will evolve with proteins consisting exclusively of alpha amino acids.

This was probably the situation on the primitive Earth, eons ago.

But what if the initial products of chemical evolution never come into contact with water at all in the early stages? According to Molton, when water is thus absent the beta amino acids will predominate. The proteins comprising the resulting extraterrestrial lifeforms would then be of the beta, rather than the alpha, variety.*

The next step, says Molton, is to try to synthesize plausible alternative nucleotides in the laboratory, simply by altering the prebiotic conditions under which they arise. Scientists are just beginning to see the myriad possibilities that may be open to carbon-water biochemistry on other worlds.

 

* Proteins made from the beta forms would probably not be edible by humans. Indeed, they might even be poisonous -- a fact of considerable importance for future interstellar astronauts and colonists.

 

 

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

 

 

8.2.3  Alternatives to Carbon

Why do lifeforms prefer carbon?

Few elements can compete with its ability to combine with many different kinds of other atoms. As for its ability to form long, polymeric chains, carbon knows no equal. There are many who believe that the element is "uniquely qualified" for the job of life. They may well be correct.

The idea of living systems founded on a radically different chemical basis from ours has been around for a long time. It was already old hat in 1908 when Dr. J.E. Reynolds, a British biochemist, delivered a paper on the subject at a meeting of the Royal Institution in London. The reviewer for Chemical Abstracts wearily reported:

... It contains no new matter. The author advances a speculative theory as to the probability of a "high temperature protoplasm" containing silicon in place of carbon and phosphorus in place of nitrogen, and points out that silicon found in certain animal and plant cells may actually be a constituent of the protoplasm of such cells.¹⁶⁰⁸

Among xenologists, the possibility of silicon (Si) -based extraterrestrial lifeforms was raised by the British astronomer Sir Harold Spencer Jones as early as 1940.⁴⁴ In more recent times, silicon-based structures have become perhaps the best-known and most commonly advanced proposal as an exotic biochemistry for aliens (Figure 8.3).

 

Figure 8.3 Depiction of a silicon-based lifeform in science fiction

The Horta, a silicon-based lifeform depicted in an episode of Star Trek, crouches in fear of the approaching humans. The small mineral nodules littering the subterranean lair are the creature’s eggs.

 

This is because Si lies directly below C in the Carbon Family of the Periodic Table of the Elements (Table 8.5). Members of the same family are expected, more or less, to have similar chemical properties and to form analogous compounds.

There have been numerous objections to silicon life from all quarters of the scientific community.

A common protest, for example, is based on the relative cosmic scarcity of Si as compared to C. From Table 8.6, we note that carbon is roughly an order of magnitude more abundant than silicon in the universe.

 

Table 8.6 Cosmic Abundance of the Elements1413
Atomic Number	Element	Symbol	Abundances of Atoms (normalized to Si = 103)		Atomic Number	Element	Symbol	Abundances of Atoms (normalized to Si = 103)
1	Hydrogen	H	31800000.		44	Ruthenium	Ru	0.0019
2	Helium	He	2210000.		45	Rhodium	Rh	0.0004
3	Lithium	Li	0.0495		46	Palladium	Pd	0.0013
4	Beryllium	Be	0.00081		47	Silver	Ag	0.00045
5	Boron	B	0.350		48	Cadmium	Cd	0.00148
6	Carbon	C	11800.		49	Indium	In	0.000189
7	Nitrogen	N	3740.		50	Tin	Sn	0.0036
8	Oxygen	0	21500.		51	Antimony	Sb	0.000316
9	Fluorine	F	2.45		52	Tellurium	Te	0.00642
10	Neon	Ne	3440.		53	Iodine	I	0.00109
11	Sodium	Na	60.		54	Xenon	Xe	0.00538
12	Magnesium	Mg	1061.		55	Cesium	Cs	0.000387
13	Aluminum	Al	85.		56	Barium	Ba	0.0048
14	Silicon	Si	1000.		57	Lanthanum	La	0000445
15	Phosphorus	P	9.6		58	Cerium	Ce	0.00118
16	Sulfur	S	500.		59	Praseodymium	Pr	0.000149
17	Chlorine	Cl	5.7		60	Neodymium	Nd	0.00078
18	Argon	Ar	117.2		62	Samarium	Sm	0.000226
19	Potassium	K	4.2		63	Europium	Eu	0.000085
20	Calcium	Ca	72.1		64	Gadolinium	Gd	0.000297
21	Scandium	Sc	0.035		65	Terbium	Tb	0.000055
22	Titanium	Ti	2.775		66	Dysprosium	Dy	0.00036
23	Vanadium	V	0.262		67	Holmium	Ho	0.000079
24	Chromium	Cr	12.7		68	Erbium	Er	0.000225
25	Manganese	Mn	9.3		69	Thulium	Tm	0.000034
26	Iron	Fe	830.		70	Ytterbium	Yb	0.000216
27	Cobalt	Co	2.21		71	Lutetium	Lu	0.000036
28	Nickel	Ni	48.		72	Hafnium	Hf	0.00021
29	Copper	Cu	0.54		73	Tantalus	Ta	0.000021
30	Zinc	Zn	1.244		74	Tungsten	W	0.00006
31	Gallium	Ga	0.048		75	Rhenium	Re	0.000053
32	Germanium	Ge	0.115		76	Osmium	Os	0.00075
33	Arsenic	As	0.0066		77	Iridium	Ir	0.000717
34	Selenium	Se	0.0672		78	Platinum	Pt	0.0014
35	Bromine	Br	0.0135		79	Gold	Au	0.000202
36	Krypton	Kr	0.0468		80	Mercury	Hg	0.0004
37	Rubidium	Rb	0.00588		81	Thallium	11	0.000192
38	Strontium	Sr	0.0269		82	Lead	Pb	0.004
39	Yttrium	Y	0.0048		83	Bismuth	Bi	0.000143
40	Zirconium	Zr	0.028		90	Thorium	Th	0.000058
41	Niobium	Nb	0.0014		92	Uranium	U	0.0000262
42	Molybdenum	Mo	0.004

 

But the real business of biochemical evolution takes place on planetary surfaces. The Earth, Moon, and Mars are remarkably similar in their silicon content -- roughly 25-30% of the total topsoil. But on this planet, Si atoms outnumber those of C by more than two orders of magnitude (Table 8.7). Organics are present in lunar soil only to the extent of a few parts per million, and on Mars there is no trace of carbon in the crust even at the parts-per-billion level.

Carbon is actually rare!*

 

Table 8.7 Comparative Elemental Abundances6,96,1413,1470
(weight %)
Element	Universe	Earth’s Crust	Seawater	Human Body
Hydrogen	91. %	0.22 %	66. %	63. %
Helium	9.1 %	0.0000003 %	--	--
Oxygen	0.063 %	47. %	33. %	25.5 %
Carbon	0.035 %	0.19 %	0.0014 %	9.4 %
Nitrogen	0.011 %	0.015 %	0.000745 %	1.4 %
Neon	0.010 %	--	--	--
Magnesium	0.0031 %	2.2 %	0.033 %	0.013 %
Silicon	0.0029 %	28. %	0.00011 %	--
Iron	0.0024 %	4.5 %	0.0000005 %	0.0038 %
Sulfur	0.0015 %	0.026 %	0.017 %	0.049 %
Aluminum	0.00025 %	7.9 %	0.000014 %	--
Calcium	0.00021 %	3.5 %	0.006 %	0.3 %
Sodium	0.00018 %	2.5 %	0.28 %	0.041 %
Phosphorus	0.000028 %	0.026 %	0.0000016 %	0.21 %
Chlorine	0.000017 %	0.032 %	0.33 %	0.026 %
Potassium	0.000012 %	2.5 %	0.006 %	0.057 %
Titanium	0.000008 %	0.46 %	0.00000014 %	--

 

A few have suggested that since carbon-based Earth life exhales carbon dioxide, a gas, silicon-based lifeforms must surely "breathe out silicon dioxide, SiO₂, which is quartz: a painful process . . ."⁴⁹ It is difficult to find any merit to this biochauvinistic objection. Silicon organisms probably are able to survive only in a reducing, oxygen-free environment -- so SiO₂ should not be produced at all. Even if it is, it’s not clear why an extraterrestrial lithomorph should find the excretion of sand at all painful.

A seemingly more valid challenge is the contention that any available prebiotic silicon atoms will be irreversibly locked into large, heavy SiO₂ polymers, making it impossible for them to participate in any life chemistry. But silicon dioxide is far from absolutely stable. In fact, it is the original material in the synthesis of many silicon-organic molecules under the action of various chemical reagents.²⁶

Another common complaint is that the number of carbon compounds catalogued -- perhaps two million or so -- greatly exceeds the total number of silicon-based substances known to chemists today -- about 20,000, two orders of magnitude less.

But the only reason a class of compounds is found may be because someone went looking for them. As few as twenty-seven organosilicon molecules were known at the turn of the century, and real interest in silicon chemistry began to accelerate just a few decades ago. Furthermore, the pitiful number of scientists currently engaged in silicon research is dwarfed by the armada of pharmaceutical houses and petrochemists flying the flag of carbon.

As Carl Sagan notes with some amusement: "Much more attention has been paid to carbon organic chemistry than to silicon organic chemistry, largely because most biochemists we know are of the carbon, rather than the silicon, variety."¹⁵

The inability of lone Si atoms to readily hook together to form very long chain polymers is often cited as the fatal flaw in all silicon biochemistry schemes. But exactly how crucial is this ability to concatenate?

In Earthly proteins, carbohydrates, and nucleic acids -- the three most important and common polymer types -- the C-C linkages rarely include more than a few consecutive atoms. Organic side chains may contain up to eight, and fats and various vitamin complexes use even more successive carbons, but the basic molecular backbone of life is served by only a few. For instance, most proteins consist of a repeating -C-C-N- unit, a mere two carbons in a row.

Biochemistries need stable polymers, not long chains of similar backbone atoms (Figure 8.4).

 

Figure 8.4 Carbon-Family Analogues for Life: Polymers of

Silicon (Si),^{1603,1649,2348} Germanium (Ge),¹⁵⁷² Tin (Sn),¹⁵⁹⁶ and Lead (Pb)¹⁶⁹⁶

CH3  CH3   CH3 |    |    | —Si—O—Si—O—Si— |    |    |  CH3  CH3  CH3	Polydimethylsiloxane -- A typical silicone polymer. Stable up to approximately 250 °C.
C6H5  C6H5 C6H5 |    |    | —Si—O—Si—O—Si— |    |    | O    O    O |    |    | —Si—O—Si—O—Si—  |    |    |  C6H5  C6H5 C6H5	Phenylsilicone "ladder polymer"-Remains stable up to about 300 °C.
H H CH3 H H CH3 | | |  | | | —C—C—Ge—C—C—Ge—  | | |  | | |    H H CH3 H H CH3	Dimethylated polygermane organopolymer.
H CH3 H CH3 H CH3  | |  | |  |  |  —C—Sn—C—Sn—C—Sn—  | |  | |  |  |   H CH3 H CH3 H CH3	Diorganotins -- Forms liquids or resinous, glassy, or rubbery polymeric solids, depending in the nature of the organic side-group.
	Butylpolystannoxane polymer, n ~10-13. Colorless, soluble in chloroform and CCl4. Reasonably stable to hydrolysis(water) and to heating up to about 250 °C.
C6H5 C6H5  C6H5 |    |    | —Pb—O—Pb—O—Pb— |    |    |   C6H5 C6H5  C6H5	Polymeric diphenyllead oxide.

 

Silicon, in combination with nitrogen and oxygen, forms a variety of ring-shaped and chain-polymer macromolecules stable in high ultraviolet radiation fluxes (such as might be found near a class F star or on the surface of an unshielded planet like Mars) and at low temperatures as well.¹⁵⁹⁷ Silane (SiH₄), the silicon analogue of methane with a repulsive odor, remains a liquid between 88.l K and 161.4 K. It might serve as a solvent for a cold silicon biochemistry under anhydrous reducing conditions. The Si halides might also work, though at somewhat higher temperatures.

Unfortunately, Si-Si bonds tend to break up in the presence of ammonia, oxygen, or water, all of which are more likely to appear on a colder world. This difficulty disappears in a hot environment in which the role of oxygen has been usurped by its chemical cousin, sulfur. The problem then becomes one of preventing the low-energy Si-Si bonds from tearing themselves to pieces in the blistering heat.¹¹⁷²

At present, the biggest obstacle is in devising plausible pathways of prebiotic evolution (Figure 8.5). Carbon seems more competitive under most conditions we can readily imagine.** Yet as Dr. Molton says, "this may be due to our own ignorance of silicon chemistry as much as to any inherent theoretical difficulty."¹¹³²

 

Figure 8.5 Possible Prebiotic Biochemicals Usable by Si or Si—C Life352,1132,1597,1649
H | H—Si—H | H	Silane (silicon analogue of methane)	H  H |  | H—Si—Si—H |  | H  H	Disilane (silicon analogue of ethane)
H  H |  | H—C—Si—H |  | H  H	Methylsilicane	Produced using silicon in H2, CH4 atmosphere in Miller-type experiment with electrical discharge and BCl3 catalyst.
H |   H—Si—NH2 | H	Silylamine	Analogue to methylamine in carbon chemistry. Many other Si-amines exist. C-amines of great importance in terrestrial biochemistry.
HO2SiSiO2H	Oxyprosiloxane	Silicon analogue of oxalic acid.
Cl Cl  [  Cl  ]  Cl Cl    |  |   |  |   |  |  | Cl—Si—Si— | -Si— | —Si—Si—Cl    |  |   |  |   |  |  |    Cl Cl  [  Cl  ]n Cl Cl		Perchloropolysilane -- Prepared by heating SiCl4 in H2 atmosphere at 1000 °C, using quartz catalyst. Demonstrates Si-Si bond stability up to n=21.
CH3  CH3 [ CH3 ] CH3 CH3     |    |   | |  |  |  | CH3—Si — Si— |—Si—| —Si—Si—CH3     |    |   | |  |  |  |     CH3  CH3 [ CH3 ]n CH3 CH3		Permethylated linear polysilanes -- Long chain organosilicon polymers up to n=8. Stable to oxygen and water.
	Cyclohexyltrichlorosilane	Produced in Miller-type experiment by sparking a mixture of SiCl4 and C6H12 in the absence of water.

 

In the last few decades a broad, new class of silicon polymers has been discovered which might serve as a basis for life. These substances, known as siloxanes to the chemist and as "silicones" in popular parlance, are extremely stable in the presence of oxygen and water. In fact, many silicones are formed by the action of water on the Si-Si bond.

This novel class of compounds is now under intensive investigation, as they have been found to exhibit a wide range of fascinating properties. There are rubbery silicones, analogous to soft living tissue, which remain flexible and "elastomeric" across a span of temperature that few organic polymers can match. There are hard silicone resins with impact and tensile strengths comparable to those of bone, and which retain their stoutness in hot environments.^1607,1610

Silicone liquids are useful as hydraulic fluids, and some of them have very handy peculiarities. For example, polydimethylsiloxane is an oil with variable mechanical properties strikingly similar to those of mammalian synovial fluid (a kind of bone joint lubricant).²³⁰

Some silicone rubbers are selectively permeable to specific gases. One rubber which passes oxygen has been tested in artificial gill devices designed to extract the dissolved gas from seawater for the benefit of human divers.²³⁴⁸ These compounds are generally less active chemically, stronger, more heat-resistant and more durable than their carbon counterparts.

The molecular architecture of the silicones is relatively simple. Silicones have a backbone, not of Si atoms alone but rather of alternating silicon and oxygen atoms. The side chains can be organic, and are as complicated as any in terrestrial organic chemistry. Silicones appear to possess an information-carrying capacity and a complexity of structure as required for a successful biochemistry.

There remain two problems with such silicon-oxygen lifeforms, which must be dealt with before the plausibility of their existence can be acknowledged.

First, many silicones tend to disassemble into ring molecules at temperatures of roughly 300-350 °C. (Similar behavior is observed in most complex carbon compounds, but at somewhat lower temperatures.) It would be difficult for silicones to remain stable in much hotter climes, and it is unclear whether this slight thermal advantage is enough to enable Si to out-compete C in a high temperature regime.

There do exist a few silicon polymers that can really get out of carbon’s league. Certain Si-C combinations are good to at least 500 °C, and various aluminum-silicon structures can reach 600 °C without destruction.

The second problem that must be faced is a familiar one: How do we arrange for a plausible prebiotic evolutionary sequence? Natural planetary conditions, by and large, are not conducive to the prebiotic synthesis of silicones.

Worse, recall that most of the complexity of the silicones is derived from the carbon side chains they possess. In spite of their greater thermal stability, these Si polymers may find themselves in an indirect competition with carbon-based macromolecules.

On any world in which the carbon chemistry had evolved sufficiently far to allow C side chains (as on the Si backbone) of the requisite complexity, it is far more likely that these carbon chains would form polymers among themselves rather than splicing onto an "alien" silicone backbone molecule.

Of course, silicon is not the only game in town. Other members of the Carbon Family might stand in for C, although this is much less likely.

Germanium has been suggested as an analogue to carbon in some biochemical systems. N.W. Pirie has cited some rather dubious evidence for germanium-based protobionts in Earth’s past: The excessive concentration of Ge in the Hartley coal seam in Northumberland, England.²³⁴⁷

But we are not restricted to the Carbon Family in our quest for analogues to C. One alternative not widely known outside specialist circles involves a tricky arrangement with the element boron (B).^{1172,2089,2446}

Looking at the Periodic Table, we see that boron lies just to the left, and nitrogen just to the right, of carbon. One might well suspect that a kind of averaging effect could take place if the two elements were combined, resulting in some sort of "pseudocarbon" system.

Indeed, this does occur. There are compounds made of alternating boron and nitrogen atoms which closely parallel their organic counterparts in many ways. They have the same types of bonds, similar molecular weights, similar physical and chemical properties, and so forth. A few possibilities are illustrated on the following page by comparing a series of common carbon compounds with their boron-nitrogen analogues (Figure 8.6).

 

Figure 8.6 Boron-Nitrogen Analogues
	Benzene (A common solvent in organic chemistry Colorless liquid Molecular weight = 78.11 Melting point = 279°K Boiling point = 353°K Density = 0.88 gm/cm3 insoluble in water		Borazine  ("Inorganic benzene") Colorless liquid Molecular weight = 80.50 Melting point = 215°K Boiling point = 328°K Density = 0.86 gm/cm3 Decomposes in water
	Methyl benzene (Toluene. another common organic solvent) Colorless liquid Molecular weight = 92.13 Melting point = 178°K Boiling point = 384°K insoluble in water		Methyl borazine ("Inorganic toluene") Colorless liquid Molecular weight = 94.53 Melting point = 214°K Boiling point = 360°K Decomposes in water
	Graphite (High-temperature lubricant) Slippery black powder Planar polymer in overlapping sheets Sublimes 3925-3970°K Insoluble in water Diamond (Compressed graphite) Colorless, cubic crystal Hard enough to scratch any substance  Burns in air at 1170°K		White Graphite ("Inorganic graphite") Slippery white powder Planar polymer in overlapping sheets Sublimes ~3300°K Insoluble in water Boron Diamond (Borazon)  ("Inorganic diamond") Colorless, cubic crystal Hard enough to scratch diamond Burns in air at 2170°K
	Dimethyl butene Colorless liquid Molecular weight = 84.16 Melting point = 199°K Boiling point = 346°K Soluble in organic solvents (ethanol, ether, etc.)		Dimethyl borine Colorless liquid Molecular weight = 84.99 Melting point = 181°K Boiling point = 338°K Soluble in organic solvents (ethanol, other, etc.)

 

While some B-N polymers are known to be stable to high temperatures, many such substances turn out to be less stable with heat. Borazine, the boron-nitrogen analogue to benzene, is more susceptible to chemical attack because of its greater reactivity. The presence of water tends to degrade most B-N polymeric compounds.

Part of these difficulties can be eliminated by switching to other combinations which also give a "pseudocarbon" effect. There are the boron-phosphorus (borophane) and the boron-arsenic (boroarsane) systems, which are known to be extraordinarily stable and inert to thermal decomposition. These substances might serve on high temperature worlds if the abundance problem could be licked.

A completely different kind of exotic biochemistry is the possibility of halogen life. Members of the Halogen Family, of which fluorine and chlorine are the most abundant, could conceivably replace hydrogen atoms in whole or in part. This would apply to biological macromolecules constructed on the basis of carbon, silicon, or any other viable backbone system.

An oxygen-poor star might give rise to planets with abnormally high concentrations of free halogen. This is not as unreasonable as it might sound at first. The element phosphorus, a common atom in Earthly biochemistry, has a cosmic abundance approximately equal to that of fluorine and chlorine. Thus, the availability and use of halogens by alien lifeforms cannot be categorically ruled out.

There might exist water oceans and an atmosphere rich in chlorine or fluorine. Peter Molton has proposed a respiration-photosynthesis cycle for such a world, involving carbon tetrachloride as the halogen analogue of methane.¹¹³²

Going still further out on a limb, Isaac Asimov has set forth the possibility of fluorocarbon (Teflon) or chlorocarbon polymers floating in seas of molten sulfur. "No one," the Doctor gently chides, "has yet dealt with the problem of fluoroproteins or has even thought of dealing with it."²³⁴⁴ No one, that is, except science fiction writers.¹³⁵⁹

Actually, polymers of any kind should be of interest to xenobiologists (Figure 8.7). Since the basis of all life appears to be the polymeric organization of small molecules into larger ones, polymer chemistry seems a reasonable avenue to explore for alternative biochemistries.

 

Figure 8.7 Other Polymers of Possible Xenobiochemical Interest

Cyclosilazanes1599		Cyclosilthians1600
Polysilazanes1599		Polymeric diphenyltin1603
CH3      CH3               CH3                   |        |                 |  - O - Al - O - Si - O - Si - O - Al - O - Si - O - Al -       |        |        |        |        |        |          OC6H5     CH3      CH3     OC6H5     CH3     OC6H5
"Random" silicon-aluminum copolymer1603
Cl      Cl      Cl      |       |       |  = N - P = N - P = N - P =N-     |       |       |       Cl      Cl      Cl
Polymeric phosphonitrilic chloride1603		Polyphosphazine chloride trimer2348
CH3HN   CH3HN   CH3HN      |       |       |  = N - P = N - P = N - P =N-     |       |       |       CH3HN   CH3HN   CH3HN	SUBSTITUTED PHOSPHAZENES	CF3CH2O  CH3CH2O  CF3CH2O     |       |       |  = N - P = N - P = N - P =N-     |       |       |  CF3CH2O  CH3CH2O  CF3CH2O
(a water-soluble polymer)2348		(film-forming flexible crystalline thermoplastic)2348
	BORON POLYMERS
Dimethyl polyborophanes1574		Polymeric silyl orthoborates1573

 

In view of various deficiencies in normal carbonaceous organic chains, many other classes have been examined in recent times.²³⁴⁸ According to H. R. Allcock, a chemist at Pennsylvania State University, "a new revolution based on organic polymers is about to begin."

Silicon-nitrogen rubbers and oils have been known for many years. These compounds, called silazanes, are unstable in the presence of water or in an oxygen atmosphere.¹⁵⁹⁸ Inorganic polymers with alternating silicon and boron atoms have turned up recently, and a boron-oxygen-silicon linkage is used in the well-known "silly putty." Various carbon-boron ("carborane") polymers which are quite stable have been discussed in the literature,¹⁵⁷⁵ along with short-chain nitrogen, sulfur, and silicon-sulfur arrangements.

Phosphorus, nitrogen, and chlorine combine to form a kind of rubber in a water-free environment. These "polyphosphazines," as the chemists love to call them, are normally highly unstable in the presence of H2. However, it has recently been learned that short segments can be polymerized and made water-stable.

Soon after this discovery, the elated researchers wrote: "... it now seems likely that almost any set of required properties can be designed into the polymer by a judicious choice of side groups." The proposal that polyphosphazine polymers be used in biomedical applications to transport fixed metal ions²³⁵¹ suggests a wide range of xenobiochemical applications, perhaps analogous to the metal-containing complexes in chlorophyll and hemoglobin.

 

* While more than thirty carbonaceous molecules have been detected in the interstellar void by radioastronomers, only two silicon compounds -- the monoxide (SiO) and the sulfide (SiS) -- had been found as of 1976.¹⁰⁰² This may, however, reflect more the zeal and interests of the searchers than the true ubiquity of molecular species containing silicon.

** It should be noted that partial substitution of Si for C occurs even in terrestrial skeletal components (e.g., diatoms, some grasses, etc.) and in protoplasm.^1551,1649 Dr. Alan G. MacDiarmid, Professor of Chemistry at the University of Pennsylvania, has succeeded in forcing bacteria to take up silicon analogues of various carbon compounds in their nutrients. He has conducted similar experiments using analogues based on germanium (Ge),¹¹⁷² the element directly below silicon in the Periodic Table and whose compounds have long been known to possess certain medical properties.¹⁵⁷⁶

 

 

8.3  Exotic Lifeforms

What is the bottom line in xenobiochemistry?

It must be admitted that the mere ability of atoms to assemble themselves into polymers, while significant, is yet a far cry from the complex biochemistry needed to sustain a living system. There remains a vast gulf between the simple silicone and polyphosphazine polymers and the orchestrated symphony of life.

While chemists have been vaguely aware of the possibility of exotic life schemes for more than a century, no coherent, well-integrated alternative system has been proposed and none is on the horizon. Without actual specimens of alien organisms to examine, the task suddenly takes on staggering dimensions. Imagine trying to speculatively reconstruct our entire terrestrial biochemical basis, having no prior knowledge of its nature or even of its existence!

The evidence admittedly is against the existence of silicon-based life-forms: The evolutionary mechanisms and planetary conditions appear much too unwieldy. Ammonia life seems far more feasible, if for no other reason than it is so closely analogous to terran biochemistry.

It would also appear that carbon is the backbone element of choice in Earthlike environments, although this should impose no real restriction on diversity. One must agree with Shklovskii and Sagan when they assert: "It is quite premature to conclude that ours is the only, or even the best of all possible, biochemistries."²⁰

In spite of the difficulties, there probably exist many different kinds of life in our Galaxy, including some very exotic forms based on different physical interactions than ours. But we cannot be certain of this until we travel into space and seek them out.

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