Xenobiology

© 1981 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., “Xenobiology,” Analog Science Fiction/Science Fact, Vol. 101, 30 March 1981, pp 30-41

URL: http://www.xenology.info/Papers/Xenobiology.htm

 


This paper contains material originally drawn from the book Xenology (1979) by Robert A. Freitas Jr.


 

Xenology is the study of all aspects of life, intelligence, and civilization indigenous to environments other than Earth. Over the last three decades xenology has advanced rapidly on many fronts. Biochemists have studied the origin of life on this planet, knowing that if they can duplicate the major early steps of “abiogenesis” in the laboratory then the evolution of alien life is a very likely – maybe inevitable – event. NASA biologists have spent much time developing sophisticated life detection instruments such as the miniature biochemical automated test laboratories carried to Mars by Viking in 1976. There is growing interest in SETI, the Search for Extraterrestrial Intelligence, in which radio scientists look for powerful transmissions or leakage radiation from advanced extraterrestrial supercivilizations. (Most recently the search has been broadened, by myself and a few others, to include the possible observation of alien interstellar probes and artifacts here in the Solar System.) Astronomers are also looking for direct evidence of planets circling nearby stars, a task which will be much easier once the Space Telescope is launched into Earth orbit as early as 1983.

Xenobiology – the study of alien lifeforms – is a major subdiscipline within the xenological sciences. Its subject matter is the set of all possible life systems in the universe, rather than just the biology of a single world. The common assertion that xenobiology is “a science in search of a subject” because no extraterrestrials have yet been found ignores the long evolutionary history of our planet. From the cosmic point of view, Earth is an alien world as exotic as any in the Galaxy.

 

Biological Chauvinisms

The term “chauvinism” derives from the name of Nicolas Chauvin, a highly jingoistic soldier born at Rochefort in the late 18th century. In 1815 Chauvin achieved notoriety by his stubborn, bellicose attachment to the lost cause of Napoleon’s crumbling empire. Since that time the word has come to be associated with any absurd, unreasoning, single-minded devotion to one's own race, nationality, sex, religious persuasion. or, more generally, to one's own peculiar point of view. Chauvinisms usually are associated with ignorance – in view of our lack of hard knowledge about lifeforms elsewhere in the universe, chauvinisms are predictably common in xenobiology.

For instance, there used to be the notion that oxygen (O2) is absolutely required for higher life. Many xenobiologists today categorically reject this proposition. Oxygen was largely absent during the first few billion years of evolution on Earth, and many organisms today still do not need this element to survive. Experiments have shown that plants grow better in air containing only about half the normal amount of oxygen, and the presence of O2 in the nuclear regions of contemporary living cells is usually fatal. Human scuba divers are poisoned by the gas at more than a few atmospheres pressure. Large creatures on any world may need some strong oxidant to power their bodies, but it may not have to be oxygen.

Another early biological chauvinism was the insistence that life is an especially fragile phenomena limited to a very narrow range of environments. During the 1960s scientists examined the extremes of terrestrial life and found that the flora and fauna of Earth (especially microorganisms and other simple lifeforms) resist death even when subjected to conditions that would quickly kill a human being.

For example, Thiobacillus microbes flourish in some of the strongest acids known to man whereas the blue-green algae Plectonema nostocorum thrives in the strongest bases. The rugged tardigrade can survive periods of total dehydration and may be frozen to near absolute zero or heated to more than 120 °C without dying. Biological growth and reproduction have been demonstrated in the laboratory from -243 °C up to 104 °C, and deep sea bacteria and other animal lifeforms survive exposure to pressures in excess of 8000 atm (Earth-normal at sea level is 1 atm). Micrococcus radiodurans and several algal species are found happily growing in the core water of nuclear power plants, enduring radiation that would kill a person almost instantly. When a TV camera was retrieved from the American lunar probe Surveyor 3 by Apollo astronauts, a colony of Streptococcus mitis bacteria was found growing inside the lens. These hardy microbes evidently survived three years of hard vacuum, no food or water, exposure to cosmic rays, and temperatures ranging from well above the boiling point of water in the daytime to -160 °C during the night.

Spacecraft sent to other planets in the last decade have returned a fascinating wealth of information about our nearest neighbors in space. Jupiter, long considered too cold for life, is now believed to have an atmosphere rich in organic compounds and cloud temperatures warm enough to permit liquid water to exist. The Jovian moon Europa may have an ocean of water as deep as Earth's seas trapped beneath its frozen surface (which could harbor life), and Io, another Jovian satellite, is thought to possess great underground pools of molten sulfur and tenuous sulfur dioxide air outgassed from the interior by active volcanoes. Titan, the largest moon of Saturn, has a thick atmosphere possibly containing hydrocarbons and other organic substances, and the presence of ammonia may produce a warming “greenhouse effect” which could raise surface temperatures up into the range of Earthly biology. The Viking mission to Mars found no unequivocal evidence for life, though some may have survived from an earlier, wetter epoch, yet escaped detection by hibernating in the Martian polar regions or deep underground. Finally, the Pioneer Venus spacecraft discovered water vapor in the Venusian atmosphere just under the main cloud deck in concentrations up to 0.5%. This is somewhat dry by terrestrial standards but still plenty wet for biology to retain a precarious foothold if it exists. The search for life in our Solar System is only beginning.

 

Carbon/Water Lifeforms

All living creatures we know about are made up of complex carbon compounds immersed in liquid water. It may be that all life in the universe must take this form. Earthly biology has two main components: DNA (chains of nucleic acids), the carrier of genetic information and the blueprints for inheritance, and protein (chains of amino acids), the raw materials and tools with which to build living beings. If we limit ourselves to lifeforms using DNA and protein in a carbon/water biochemistry, are there any workable alternatives within the basic terrestrial format?

For years it has been assumed that the genetic code is universal to all Earth life. This code, used to write the instructions (called “codons”) on chromosomal DNA for protein production by cells, was believed to be shared by plants, people and bacteria alike. In late 1979, scientists of the Medical Research Council in Cambridge, England and researchers at Columbia University in New York made a truly amazing discovery: The code is not universal! Apparently several of the codons used in mitochondria (the main sources of metabolic energy in cells) have a different meaning than the same codons would have in the surrounding, more “normal,” cellular cytoplasm.

This research raises a number of intriguing questions. How much variation is permissible in genetic coding schemes? Might primitive lifeforms from earlier stages of terrestrial evolution have had some different system altogether? Arc alternative inheritance codes possible, with genes written in a self-consistent language untranslatable by human cells? Could nucleic acids other than the five in our biochemistry appear in some alien genetic system?

Dr. Alexander Rich, a prominent MIT biochemist, believes that the functions of Earthly nucleic acids are not unique. He has suggested that other molecules could be used in organic chains called polymers which might be used as information carriers in extraterrestrial living systems. At the Conference on the Origin of Life held at Princeton in May 1967, Dr. Rich commented that “it would be amusing to make a chemical system of complementary polymers based on monomers that are not nucleic acid derivatives, simply to demonstrate that it can be done. In about ten years' time I think we will have a well-developed field of synthetic polymeric information carriers that will give us a great deal of insight into our own terrestrial system. That another system is possible might have relevance, if not to biology on this planet then perhaps to another.”

In December 1979 Dr. Rich and his colleagues at MIT announced the discovery of a new form of synthetic DNA polymer. Normal genetic material, called B-DNA, has the familiar double-helix structure with a right-handed direction of twist. The new DNA, synthesized for the MIT group by scientists in the Netherlands, has both a left-handed twist and a markedly different molecular surface structure than the B form. Rich calls it Z-DNA, after the zigzag pattern of the phosphate groups which serve as the molecular backbone for DNA (see figure below). Since the Z-DNA was synthesized from scratch, nobody knows yet if this unusual form of genetic material occurs naturally in living cells. Dr. Rich’s team has begun a series of experiments to find out. If researchers can identify proteins that bind to the synthetic structure, then it is likely that Z-DNA may be found somewhere in nature. This raises an even more interesting question for xenobiologists. Terrestrial chromosomes are constructed (so far as we know) entirely of B-DNA, but could extraterrestrial biochemistries exist which transmit genetic information mainly using Z-DNA? Dr. Rich thinks that there may be proteins which preferentially bind to left-handed rather than right-handed DNA. These proteins might even consist of amino acids that normally play no role in terrestrial life systems. Alien creatures based on a Z-DNA code doubtless would find our foods, drinks, drugs, cells and bodily fluids wholly incompatible. There is little danger that such visitors to our planet could transmit a lethal pathogen during first contact and start a plague on Earth.

 


Z-DNA vs B-DNA (courtesy of Alexander Rich)


 

How about alternative proteins in a carbon/water biochemistry? Chemists long have been aware of literally hundreds of amino acids in addition to the normal 20 which make up all protein molecules coded by DNA in human biology. No one knows exactly why these particular 20 were selected by evolution for the job. There seems nothing to prevent each extraterrestrial carbon/water DNA/protein life chemistry from having its own unique set of amino acid building blocks. A simple combinatorial calculation reveals the extent of this uniqueness: There are more than 1077 ways randomly to choose a set of 20 amino acids for use in alien protein from a total of, say, 200 possible amino acids, or about one million completely different protein systems theoretically available for every planet of every star in the universe.

Like Z-DNA, alternative protein structures may readily be imagined. To see how we must go back to the prebiotic synthesis experiments conducted by Dr. Stanley Miller and others in the 1950's. Dr. Miller (now at the University of California at San Diego), in order to reproduce the composition of Earth's atmosphere before life began, mixed together methane, hydrogen, ammonia and water in a closed vessel from which all oxygen had been removed. The gases were circulated past an electric spark discharge to simulate the effects of lightning. After a week Miller removed the contents for analysis and found a startling variety of organic compounds important to terrestrial life – including amino acids.

The basic shape of an amino acid is a short chain of carbon atoms with a small amino group (-NH2) stuck on somewhere. There arc two common forms of amino acids, called alpha and beta. Most all terrestrial proteins are of the alpha variety, in which the amino group appears near the tail end of the amino acid molecules. The beta forms, with the amino group displaced more to the front of the chain, are virtually absent.

The explanation for this selection may lie solely in the order in which water is introduced during the early stages of prebiotic evolution, according to Dr. Peter M. Molton, formerly of the Laboratory for Chemical Evolution at the University of Maryland. Miller-type experiments demonstrate that if water is present initially in the primordial “soup,” only alpha amino acids are produced and life evolves with alpha proteins. But if the early products of chemical evolution don't encounter water until much later, then the beta amino acids predominate. If this happens on another world, the resulting extraterrestrial lifeforms could have beta rather than alpha proteins and would probably not be edible by humans. They might even be poisonous, a fact of considerable importance to future interstellar colonists, tourists and soldiers.

 

Biology without Water

The reactions of terrestrial biochemistry take place in water, an amazing substance with a whole set of properties ideal for our kind of life. Over the years one of the most persistent and seemingly most reasonable biological chauvinisms has been the contention that water is the only good biochemical solvent But this view is slowly changing.

Today, xenobiologists regard ammonia (NH3) as the leading alternative to water for hypothetical alien life chemistries. Ammonia is known to exist in the atmospheres of all gas giants in the Solar System and is thought to have been plentiful on Earth during the first billion years of our planet's existence. The only solvent more abundant in the universe is water, and at pressures below about 10 atm water freezes out as solid ice over the entire temperature range for which ammonia is a liquid. We can easily imagine ammonia lakes or seas on other worlds, though these may not be necessary for the origin of ammono life. A thick mist concentrated in some stable layer in a quiescent Jovian atmosphere like that of Saturn might suffice.

To be useful in biology a solvent must remain in the liquid state. Some scientists have claimed that the liquidity range of ammonia is too small for it to replace water in any alternative biochemical system. The liquidity range of water at normal pressure is exactly 100 °C (0 °C to 100 °C as compared to only 44.3 °C (-77.7 °C to -33.4 °C) for ammonia. But is this difference really relevant? Virtually all lifeforms on Earth occupy environments between 0-44 °C. and prebiotic chemical evolution as presently understood docs not require a medium much beyond this temperature span. Further, the liquidity range of ammonia may be broadened by increasing the pressure. At just 60 atm, far less than the pressures encountered near the surface of Venus or in the dense clouds of Jupiter and Saturn, ammonia boils at 98 °C rather than -33.4 °C. (Ammono life is not necessarily low-temperature life.) Ammonia thus compares favorably to water at elevated pressures.

Much is often made of water's virtually unique property of expanding when it freezes. Ice is less dense than the liquid, so it floats on top and protects creatures swimming beneath the surface. Ammonia seas would freeze from the bottom up because the solid is more dense and sinks, causing the entire environment to solidify during periods of extreme cold and smothering to death any organisms living there. Ergo, ammonia is an inferior solvent.

But this neglects the fact that water freezing within the cells of living tissue opens the door to a new hazard also unique to water – mechanical damage by expansion. Ammonia shrinks when it freezes, so cellular damage is far less likely. The very property which might create massive oceanic freeze-ups should also allow ammonia-based lifeforms to be much more successful hibernators in a frozen milieu.

The acid-base chemistry of liquid ammonia has been studied extensively throughout this century and has proven as rich in detail and complexity as the water system. Ammonia dissolves most organic compounds as well as or better than water and has the unprecedented ability to take many metals directly into solution, including sodium, magnesium, aluminum and several others. Iodine, sulfur, phosphorus and selenium also are somewhat soluble with minimum reaction, each important in terrestrial life chemistry or prebiotic synthesis. As a chemical solvent for life, ammonia cannot be considered inferior to water.

Several series of ammonia-based analogues to the macromolecules of Earthly life have been designed by enterprising xenobiologists. including substitutes for alcohols, amino acids, fatty acids, proteins and carbohydrates. But we don't know if ammono life will have the same chemical forms found in terrestrial biochemistry. Undoubtedly there are as many different possibilities in carbon/ammonia as in carbon/water biologies. One suggestion by Peter Molton is that ammono life 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 and sodium salts required by watery life on Earth.

Numerous other biological solvent systems have been proposed from time to time, as for instance sulfur dioxide, hydrogen fluoride, methane, hydrazine, chlorine and sulfur. Though its solution chemistry is virtually unknown, liquefied carbon dioxide (CO2) is yet another plausible alternative. Under normal pressure CO2 exists only in the gaseous or solid (“dry ice”) states, but above 5.2 atm pressure the liquid state can also be reached. If the planet Venus (with 96 pure CO2 air at 90 atm pressure) suddenly were moved to an orbit somewhere near the Asteroid Belt, most of the atmosphere could precipitate out and fall as rain, creating planetary oceans of liquid carbon dioxide about a kilometer deep.

 

Biology without Carbon

Most biochemists find it difficult to imagine a biology based on any element other than carbon. Carbon atoms easily bond together to make long polymeric chains and can combine with a large number of other elements to form a bewildering variety of different molecules with many useful biochemical properties. But is it unique in this respect? Since at least the beginning of this century a few researchers have speculated on the possibility of replacing carbon with silicon in living systems. This suggestion arises mainly because silicon lies directly below carbon in the Periodic Table of the Elements (a systematic grouping of elements according to their properties) and as such is carbon's nearest neighbor chemically.

Traditionally the idea of silicon life has received a chilly reception from the scientific community at large. One common complaint is that carbon atoms are about ten times more plentiful in the universe than silicon atoms: All else equal, carbon should be favored every time. But most biochemical evolution probably takes place on planetary surfaces. We know that silicon comprises roughly 25-30% of the total surface composition of all terrestrial worlds whose soil has actually been tested. In Earth's crust silicon atoms are a hundred times more numerous than carbon atoms. Carbon is even rarer on the Moon, and in the Martian crust there is no trace of it down to the parts-per-billion level.

One often hears that carbon chemistry is far more diverse than silicon chemistry, hence more suitable for the intricacies of living systems. More than two million carbon compounds are known today as compared to a paltry 20,000 silicon-based substances. But vastly more research effort has been expended on behalf of carbon due to its obvious biological and medical importance. Serious interest in silicon chemistry awakened only a few decades ago and has since moved at a relatively slow pace. The apparent poverty of silicon compounds may in large part be due to a lack of commitment among chemists.

The customary coup-de-grace of the carbon chauvinists is to assert the inability of silicon atoms readily to hook together into long polymeric chains. No chains, it is alleged, means no biochemical complexity, therefore no life. Is this a valid conclusion?

Many think not. It is quite true that silicon atoms have trouble linking up to form stable noncrystalline structures, but this really says little about the possibility of Si-life. Earthly proteins, carbohydrates and nucleic acids – the three most important polymeric substances in terrestrial biochemistry – rarely include more than a few consecutive carbon atoms. Organic side chains may contain up to eight carbons, and certain fats and vitamins many more, but the basic molecular backbone of life makes do with only a few. For example, most proteins consist of a repeating sequence of just two carbon atoms and a nitrogen of the form -C-C-N-C-C-N-. Life needs stable macromolecules, not merely long chains of identical backbone atoms.

Silicon life may actually be possible. In combination with oxygen, nitrogen, and several other elements Si makes a variety of ring-shaped and straight-chain polymers stable in high ultraviolet fluxes and at very low temperatures. Silane (SiH4), the silicon analogue of methane (CH4), might serve as solvent for a cold silicon biochemistry under anhydrous reducing conditions. Si-Si bonds tend to break up in the presence of water or ammonia. Fortunately the liquidity range of silane (-185 °C to -112 °C) lies well below the melting point for either of these more abundant solvents, so both water and ammonia should be frozen out in “mineral” form. Pools of liquid silane may seem incredible, even preposterous, to many. But we should remember that natural lakes of molten sulfur were regarded as utterly ridiculous too – until scientists discovered them on Earth beneath the volcanic crater of Volcan Poas in Costa Rica in 1977 and on the Jovian moon Io during the Voyager flybys in 1979.

In recent years chemists have discovered a wealth of inorganic polymer classes based on various unusual elements. Silicones, for instance, are polymers with alternating silicon and oxygen atoms stable to very high temperatures. Long carbon chains easily link to the silicone backbone, offering the interesting possibility of a carbon/silicon hybrid biochemistry. The element boron also is now known to form large stable molecular structures in some cases superior even to carbon. Boron is an amazingly versatile element – literally thousands of compounds have recently been synthesized (many with extreme high temperature tolerance) and more than 40 elements, nearly half the Periodic Table, have been successfully incorporated into boron's unique molecular “cage” architecture. Boron-nitrogen compounds, often referred to as “pseudocarbons” by specialists in the field, represent a whole series of remarkable direct analogues to carbon-based organic substances.

 

General Xenobiology

It is useful at this point to ask exactly how biochemical biology is relevant to our study of alien beings. A xenologist who speaks of “lifeforms” most generally refers to an entity or material system that metabolizes both matter-energy and information. (Metabolism is any mechanism which accepts a set of inputs and then processes them to produce a specific set of outputs.) A few might also demand that each metabolism be survival-oriented or geared for reproduction, but it is clear that the lifeform concept is vastly broader than traditional notions based on Earthly life. Xenobiologists would argue that while all biochemical biological entities must be lifeforms, not all lifeforms need be biochemical in constitution.

Life requires metabolism, a systematic manipulation of matter-energy and information. But manipulation can only be accomplished by the application of force. Physicists tell us there arc just four fundamental forces in nature. Most powerful is the nuclear or strong “chromodynamic” force, responsible both for binding protons and neutrons together in atomic nuclei and for holding subnuclear “quarks” together within individual protons, neutrons and other particles. Less strong is the electromagnetic force, which provides the attraction or repulsion between charged objects and predominates in the chemical reactions of terrestrial biochemistry. The weak force mediates many processes of radioactive decay. Finally there is gravity, by far the weakest force, which manifests itself in the universal attraction of all matter-energy. We can imagine four broad classes of metabolic entities – chromodynamic or nuclear lifeforms, electromagnetic lifeforms (e.g., all Earth life, including humans), weak lifeforms. and gravitational lifeforms. Each is most likely to evolve in those environments where the forces upon which they most depend predominate over all others.

For example, gravitational lifeforms, should they exist, survive by making use of the most abundant form of energy in the universe. Gravity is also the most efficient – this is why a hydroelectric power station which converts the energy of falling water into electricity (essentially a controlled gravitational contraction of the Earth) can have an efficiency close to 100%. In theory, gravity beings could be the most efficient creatures in the universe. Their energy might be derived by arranging encounters of collisions between black holes, galaxies or other celestial objects, or by carefully regulating the contraction of various objects such as stars or planets. These beings need not be astronomical in size. Rotational and orbital motions of planetary bodies could serve as sources of gravitational power. Comparatively small lifeforms might survive by harnessing the energy of waterfalls, wind patterns, tides and ocean currents, or even seismic disturbances.

Chromodynamic creatures may evolve in an environment where nuclear forces are predominant. While the chromodynamic force is the strongest in nature, it is effective only over ranges of about 10-15 meter, so very special conditions might be required for such life to exist. These conditions possibly could be found inside a neutron star.

Neutron stars are heavy, rapidly spinning objects 10-20 kilometers in diameter with approximately the mass of a star. They have densities like nuclear matter, tremendous magnetic fields. surface gravities in excess of 100 billion Earth-gees, and are thought to be the energy source for pulsars. Neutron stars have atmospheres half a centimeter deep and mountains at most one centimeter high. Under the three-kilometer crust of crystalline iron nuclei a sea of neutrons circulates at a temperature of hundreds of millions of degrees. In this sea float a variety of nuclear particles including protons and atomic nuclei. Scientists believe that there may be neutron-rich “supernuclei” or “macronuclei” dissolved in the neutron sea. These macronuclei might contain thousands of nucleons (as compared to only a couple of hundred in normal matter) which could combine to form still larger supernuclei analogous to the macromolecules which make up earthly life. The neutron sea may be the equivalent of water in the primordial oceans of Earth, with macronuclei serving as the equivalents of amino acids, carbohydrates, and nucleotides in the prebiotic origin of life. It is possible to conceive of life evolving in neutron stars much as it did on our own planet nearly five billion years ago, but substituting atomic nuclei, supernuclei and neutrons for atoms, molecules and water.

Weak force lifeforms would be creatures unlike anything we can readily imagine. Weak forces are believed to operate only at subnuclear ranges, less than 10-17 meter. They are so weak that unlike other forces, they don't seem to play a role in actually holding anything together. They appear in certain kinds of nuclear collisions or decay processes which, for whatever reason, cannot be mediated by the strong, electromagnetic or gravitational interactions. These processes, such as radioactive beta decay and the decay of the free neutron, all involve neutrinos.

A weak lifeform might be a living alchemist. By carefully controlling weak interactions within its environment, such a creature could cause its surroundings to change from a state of relatively high “weak potential” to a condition of low “weak potential” and absorb the difference into itself. A state of high “weak potential” might be characterized by extreme instability against beta decay – perhaps these beings are comprised of atoms laden with an excess of neutrons and become radioactive only when they die.

Such lifeforms seem impossible in our present universe, but all may not be lost. Modem cosmologists believe that at the beginning of time the weak and electromagnetic forces were fundamentally the same – both obeyed the same sort of inverse square law and both were about the same strength. During the Big Bang as the universe cooled to below 3 x 1015 K, a kind of “phase transition” is theorized to have occurred. Much like the sudden freezing of water, the weak interaction abruptly parted company with electromagnetism and became what it is today – weak and extremely short-range. Recently the Nobel laureate physicist Steven Weinberg suggested that there may exist regions in the universe where the weak force is still comparatively strong. Says Weinberg in The First Three Minutes: “When water freezes it does not usually form a perfect crystal of ice, but something much more complicated; a great mess of crystal domains, separated by various types of crystal irregularities. Did the universe also freeze into domains? Do we live in one such domain, in which the symmetry between the weak and electromagnetic interactions has been broken in a particular way, and will we eventually discover other domains?”

Electromagnetic lifeforms also may assume many different shapes. Any creature that makes use of electromagnetic atomic bonding, electron flows, or electric and magnetic fields is a member of this class. All biochemical life on Earth or any other planet meets this test, but there may be many other kinds of alien living systems which also qualify. For instance, the advancing intelligence and versatility of electronic computers suggests that some sort of solid state “machine life” may be plausible. Such entities would survive by manipulating electron flows and fields in order to process matter-energy and patterns of information.

Another outré possibility is the proposal by Jean Schneider of the Groupe d'Astrophysique at the Meudon Observatory in France that a crystalline nonchemical form of life is theoretically feasible using arrangements of crystal dislocations. Schneider describes a primitive memory process that provides a rich, stable information storage system, using what he calls “dislocation loops” which can react and interlock and are capable of being diffused into the surrounding medium in coherent form. Such crystalline physiologies might be found in any of four different places: (1) the rocks on Earth and other planets; (2) interplanetary or interstellar dust grains; (3) in the dense matter of white dwarf stars; and (4) in the crust or core of neutron stars.

Venturing still further afield, someday we may meet electromagnetic creatures such as those described in astronomer Fred Hoyle's The Black Cloud. In this science fiction classic, a great cloud of ionized gas approaches our Solar System and engulfs the Sun, shutting out its light and warmth. Scientists eventually discover that the Cloud is a giant living creature operating on the principles of plasma physics rather than the usual molecular biochemistry. Memory and intelligence are stored on an electrically conductive substrate of various solid materials. Streams of ionized gases carry “nutrients” to wherever they are needed within the Cloud, controlled purely by means of electromagnetic forces.

It is very likely that ours is just one possible life chemistry of many, and that all biochemical life is only one of many modes of xenobiological existence. But regardless of what shape they take, all lifeforms are worthy of our curiosity and respect as manifestations of the same fundamental unity and cosmic order that gave rise to life on Earth eons ago.

 

For Further Reading

Andrew H.J. Wang et al, “Molecular Structure of a Left-Handed Double Helical DNA Fragment at Atomic Resolution,” Nature 282(13 December 1979):680-686 (Report on Z DNA)

V.A. Firsoff, Life Beyond the Earth, Basic Books, New York, 1963. (Alternative low- and high-temperature biochemistries.)

Peter Molton, “Non-Aqueous Biosystems. The Case for Liquid Ammonia as a Solvent.” Journal of the British Interplanetary Society 27(April 1974):243-262. (A good but technical survey paper.)

Robert L. Forward, Dragon’s Egg, Del Rey Books, 1980. (Fictional account with technical appendix about life on neutron stars.)

Jean Schneider, “A Model for a Non-Chemical Form of Life: Crystalline Physiology,” Origins of Life 8(1977);33-38. (Crystal dislocation lifeforms.)

 


Last updated on 30 December 2008