Life’s True Essences
by Peter Jekel
You cannot depend on your eyes, when your imagination is out of focus.
One of the most provocative questions in science is whether we are alone in the universe or whether the cosmos is teeming with life. So far, it would appear that the former is true. Though, our space probes have only touched a tiny fraction of the universe at large they have found nothing. We have seen nothing of the previously envisioned swamps of Venus or the advanced civilization of Mars. Even when the Viking spacecraft landed on Mars and performed their experiments of life, they came up with no evidence of life as we know it. Then, again, maybe we have discovered life on other worlds and not even known it.
When the Viking landers performed their experiments on Mars, perhaps we were being too critical of the results or perhaps not imaginative enough. Thus far, there is no universally accepted chemical explanation for the results of some of the tests performed by the Vikings. There are some in the scientific community that speculate that perhaps if we looked at Martian life as having hydrogen peroxide and water as its intercellular material, that may explain why no life as we know it was detected. The experiments would destroy any hydrogen peroxide/water lifeform that may exist there.
Water in terrestrial life is essential to provide a medium for the chemical reactions of life to occur. It provides a fluid in which molecules of nutrients are absorbed and the method by the molecules of waste are eliminated. It helps to regulate internal temperatures of organisms especially important in higher organisms.
To accomplish this enormous responsibility, it has many essential properties such as remaining a liquid over a wide temperature range, a high heat capacity useful for temperature regulation, a high heat of vaporization which translates into the fact that organisms lose little water via evaporation which is important in heat stabilization. Water is also often dubbed, “the universal solvent,” which means that most of the chemicals needed in life reactions dissolve in water. Water can range widely in its pH which means that it can act as an acid or an alkaline product, which is essential in many of the chemical reactions crucial to life. It has a high surface tension which would allow for the aggregation of molecules long before the first cell arose; it is, in fact, the medium in which life was able to evolve. Water is also polar in its structure which allows for the alignment of molecules so that many of the reactions of life can occur at an adequate speed. In a simple summary, water is the perfect solvent of life.
Why is hydrogen peroxide suggested as a plausible alternative or additive to water? After all, isn’t it an efficient disinfectant that would kill microbes? There is at least one earthbound lifeform that produces hydrogen peroxide, so that it certainly is not incompatible with life on earth. The beetle (Brachinux crepitans) produces a 25% mixture of hydrogen peroxide which it uses to warn off predators. We also must realize that organisms on a different world would have followed a very different path of evolution so that hydrogen peroxide may not be toxic to them.
Water in a cold and arid climate would have its limitations as biological solvent. It will expand with freezing thus potentially rupturing cell structure. Hydrogen peroxide, however, has a much lower freezing point than water at -56.5 degrees Celsius and it has the advantage of not crystallizing but supercools when the temperature is dropped; therefore, less likely to cause cell damage.
Hydrogen peroxide has another advantage in that it is hydroscopic; that is, it attracts water. An organism that utilizes a combination of water and hydrogen peroxide as its medium for life sustaining chemical reactions to occur would have an enormous advantage on the world of Mars today.
As we have seen, the use of hydrogen peroxide as an alternative to water or in addition to water as a solvent, makes life on nearby Mars all that more likely. We just have to know not only where, but how, to look.
What other solvents are there that could be used as the medium for life processes to occur? In 1954, J. B. Haldane, while speaking at the Symposium on the Origin of Life, suggested that ammonia could be substituted as a solvent for water. Ammonia has many of the properties of water. For one, it can dissolve most organic molecules. It even can dissolve many metals as well. In fact, in many cases, it often accomplishes this task better than water.
There are, however, many inherent problems with ammonia as a substitute for water. It has a low heat of vapourization making an ammonia-based creature unable to properly regulate its temperature, it has a weaker surface tension thus reducing its ability to concentrate polar molecules. Ammonia is also easily oxidized and combustible making it unlikely as a substitute for water in an earthlike atmosphere. Organisms would have to find other options to oxygen to respire, not impossible though. Many microorganisms on earth, themselves, find oxygen toxic and only survive in anaerobic (no oxygen) environments. Still other microbes can survive in both an anaerobic or aerobic (oxygenated) environment. Another weakness of ammonia is that it will freeze solid due to the fact that its solid state, unlike that of water, is denser than its liquid state. Such a feature can make multi-cellular ammonia-based organisms less likely, though it is not a weakness when speaking of simple one-celled organisms or even colonies of cells.
Ammonia, therefore, if is acts as a solvent substitute for water, may exist on a very cold world, those that exist beyond any water based habitability zone such as we would find on the moon Titan which orbits Saturn. It has also been speculated that ammonia-based lifeforms may exist in Jupiter’s atmosphere. It was, in fact Carl Sagan, who along with co-author Edwin Saltpeter, in a 1976 paper published in the Astrophysical Journal Supplement, who suggested an entire ecosystem not unlike that of earth’s oceans that exists in the atmosphere of Jupiter. Arthur C. Clarke, in his short story, "A Meeting with Medusa" and in Ben Bova’s novel, Jupiter, Sagan’s and Saltpeter’s ecosystem is described in great detail.
Ammonia in a liquid form could also exist in atmospheres of high pressure and therefore, need not be restricted to a cold world. Theoretically, ammonia-based life could exist under the extreme pressures found on Venus. At sixty atmospheres (earth at sea level is at one atmosphere) ammonia would remain a liquid at normal temperatures with a melting point of -77 degrees Celsius and a boiling point at 98 degrees Celsius.
With its markedly lower melting point than water, an ammonia/water solvent has even been suggested for any theoretical organisms that may live beneath the frozen surfaces of Europa, Callisto and Enceladus, with the ammonia acting as a kind of antifreeze. One science fiction author, Robert Forward imagines just such an ammonia/water based biochemistry in his Rocheworld series of novels.
Hydrogen fluoride is another chemical that could act as a substitute to water. However, it is rarity of fluorine as an element in the universe that makes it unlikely that we will ever encounter planets with oceans of hydrogen fluoride as opposed to water. The likelihood of life based on hydrogen fluoride is not likely but given the enormity of the universe, certainly not impossible.
Are there any other solvents that could be substituted for water especially on a hotter world? One idea that has been tossed around is hydrogen sulfide. It has many of the properties of water including polarity, a large range of temperature between melting and boiling. Its problem is that it is highly corrosive though some organic chemicals can survive its touch including many alkalines. Silicone has been found to survive the corrosiveness of sulfuric acid. In today’s computer age, it does not require great leaps of imagination to speculate on a silicon-based lifeform.
What about methane or ethane lakes that have been found on Titan by the Cassini spacecraft? Both ethane and methane have many of the features of water but lacks its polarity. If we substitute proteins as the building blocks of life, as suggested by the late Isaac Asimov, science fiction author and biochemist, with polylipids, the polarity of water loses its advantage over other solvents.
Perhaps the most detailed description in fiction of methane-based lifeforms appear in Hal Clement’s novels, Mission of Gravity and its sequel Starlight. The novels follow human interaction with the methane-based lifeforms on a highly unusual planet in orbit around 61 Cygni. He also wrote Heavy Planet, which is a collection of short stories based on the planet Mesklin. An interesting sidebar is that Hal Clement, a pseudonym for Hal Clement Stubbs, was a chemist by trade which would explain the exquisite detail that he provides on the lifeforms of Mesklin.
In Hal Clement’s Half-Life, Stephen Baxter's Titan and Ben Bova’s Titan, lifeforms on the Saturnian moon are speculated but certainly not in as much detail as Clement’s earlier novels.
A derivative of methane, methanol is perhaps a better substitute as a solvent of life. It is an excellent solvent dissolving many organic chemicals, a good temperature regulator with a wide range of liquidity. It is also very common in the universe, but in the end, it is not known to exist anywhere in our solar system even on the hydrocarbon rich world of Titan.
Hydrazine (a chemical very similar to ammonia) has been speculated as a possibility in that it has many of the features of water but, like ammonia, it is very reactive and decomposes rapidly in the presence of oxygen, so the oxygen breathing hydrazine based lifeform is impossible. Again, a different atmosphere than we have on earth may make such a lifeform possible.
What about carbon dioxide as a solvent for life? It is abundant of Earth and becoming more so with climate change. On Venus it makes up a major component of its thick blanketing atmosphere. It is found in the thin atmosphere of Mars and even in the dry ice caps of Mars and in comets. Carbon dioxide is essential in many biological processes including photosynthesis.
Carbon dioxide, though a gas for all intents and purposes, at least on earth, can act as a liquid under large pressures such as those found on Venus. It also has the advantage of being heavy like water but at the same time very easy for molecular motion to occur since the carbon dioxide molecules do not stick together as tightly as other liquids. Many enzymes that are placed in liquid carbon dioxide also works as well as they do in many other solvents.
What about oxygen difluoride as a solvent? One science fiction author, Robert Forward, has imagined just that. In his book, Camelot 30K, he envisions an ecosystem on a Kuiper system planetoid where life is based on oxygen difluoride. In terms of shape, it is similar to water in that the two fluoride atoms adhere to the oxygen core in much the same way that hydrogen atoms adhere to the oxygen core. However, it does have different properties than water in that it is a strong oxidizer, unlike water. It is probably only through Forward’s imagination that this solvent solution is a possibility.
Hydrocyanuric acid has been speculated as another possibility but it has many limitations. It has the temperature regulating facility that is closer to that of ammonia than water and it has limited ability to dissolve the molecules of life into solution.
Therefore, we see that it is possible to have organisms based on a solvent other than water. In fact, it may explain the results that we have seen with the Viking lander experiments.
What about the structural part of an organism? On earth, life is based on carbon. Carbon is an ideal chemical for creating the complex molecules required for life processes over a wide range of temperatures. In fact, carbon forms the most stable macromolecules in the universe of any non-metal. It also fits well with water as being a solvent.
There are other considerations though that can be considered and one in particular makes its way into science fiction literature periodically. That chemical is silicon. It was as early as 1891 that a German astrophysicist, Julius Scheiner, mused about silicon-based life. In 1893, intrigued by the idea, English chemist James Emerson Reynolds pointed out that silicon compounds would allow life to exist at extremely high temperatures because of their stability. It was H. G. Wells, who was the first science fiction author to speculate about such an alien lifeform was actually excited by the idea. In one of his musings he wrote:
One is startled towards fantastic imaginings by the suggestion: visions of silicon-aluminum organisms-why not silicon-aluminum men at once?
Silicon has a number of properties that make it a suitable alternative to carbon. It is abundant in the universe as is carbon. However, there is a shortcoming of silicon that makes it unlikely to replace carbon as the base of life. When organisms respire on earth, they produce carbon dioxide which is easy for the organism to remove from its body as it is a gas at normal temperatures. However, the analog of silicon is silicon dioxide which is essentially sand. Imagining the disposal of sand during the respiratory process is a difficult one to imagine, but by no means impossible. Stanley Weisbaum may have come up with a novel solution to this problem in A Martian Odyssey. The story has a creature over half a million years old that takes over ten minutes to move and deposits a brick, thus overcoming the limitation of the solid respiratory byproduct of a silicon-based lifeform. Again a solution would be to look for silicon-based life on worlds that are not earthlike.
Another limiting feature of silicon is its inability to display handedness or chirality of its molecules. In carbon chemistry chemicals can take on a right or left form and it is this handedness that allows enzymes to be created which regulate the processes of life.
The environments where silicon-based life may be found, if it exists, are on high pressure worlds such as Venus or cold environments such as those found in Jupiter’s atmosphere or the surface of Titan where ammonia would be found in its liquid form. Ammonia can dissolve silicon making it a better solvent for any silicon-based lifeform.
Instead of silicon oxides or silicone, silicon hydrogen molecules called silanes may provide an alternate solution to carbon. Silanes have been found to be common in the universe but they have the disadvantage of spontaneously decomposing. With this disadvantage, it is more likely that a silane-based lifeform would be a single cell at least or create a cellular colony at best. A silane-based lifeform would have the same problem of disposing of the silicon dioxide waste byproduct from respiration, but in a single cell or cellular colony it is easier to accomplish than in a complex multicellular organism. Silane-based lifeforms would thrive in an environment similar to that of silicon- based life. Not quite silane, Greg Egan, in Diaspora writes of organisms that are polysaccharide carpets, a form of cellular colony.
Alan Dean Foster wrote Sentenced to Prism a story in which the hero is trapped on a world based entirely on crystalline silicon-based life. Joseph Green’s Conscience Interplanetary is about a planet with intelligent silicon-based plants. H. Beam Piper’s story, Uller Uprising speculates about silicon-based life as well. Arthur C. Clarke, in his short story, Crusade, speculated on a planet-wide lifeform that was based on silicon and superfluid helium. Not limited to science fiction imaginings alone, one scientist at least, A. G. Cairns-Smith, proposed the idea that life on earth actually began with clay minerals based on silicon in his 1985 book Seven Clues to Origins of Life. The silicon formed the templates for future carbon-based molecules, the molecules of life.
There is also the possibility of a carbon/silicon blend of lifeform. In fact, on earth, we have such organisms. Diatoms have a silicon based cell wall and sponges incorporate silica into their cell membranes. Humans and other more complex organisms even have silicon present in many connective tissues.
What about other chemical bases of life? There is the possibility of looking at highly toxic substances (to terrestrial life) as the bases of life. Arsenic, for example, may have actually originally formed the backbone or base of the DNA molecule (deoxyribonucleic acid-molecule of heredity), now made of phosphorus. Arsenic reactions have been observed in some microbes. Some marine algae use arsenic in some of their molecular structure such as arsenosugars and arsenobetaines. Fungi and bacteria produce volatile methylated arsenic compounds. The idea of having arsenic-based life did not escape the attention of one science fiction author, Joan Slonczewski, in her book Brain Plague about intelligent arsenic-based microbes that live symbiotically with their human hosts.
Sulfur has been suggested as a carbon alternative. It can form long chain molecules; in fact, it rivals carbon in this capacity but is limited by its high reactivity. Sulfur , though, is often substituted in some bacteria, as an alternative to oxygen for respiration. It has been suggested that it may be the basis of lifeforms in the clouds of Venus. Ben Bova mused about such Venus lifeforms in his book, Venus.
Boron may be another potential alternative to carbon. Together with nitrogen, it can form molecules that mimic carbon/carbon bonds when present in the same molecule. The difficulty of suggesting boron as a basis of life is its rarity in the universe. Its bonds with nitrogen are weaker than that of carbon/carbon bonds and therefore, life reactions would occur at a quicker pace, making life processes potentially difficult.
Nitrogen on its own could also form the basis of life in that, in cold environments, it can form complex chains of molecules, though they have a tendency to break apart easily.
Phosphorus can also form complex molecules and forms the backbone of ATP (adenosine triphosphate-energy source for cells) and nucleic acids in earth based life, but they are relatively unstable and cannot survive over a range of temperatures.
Although metals are not often looked upon as favourable structural elements on which to build a lifeform, Stephen Baxter, in Manifold: Space imagines just that with robotic lifeforms made of iron. Perhaps the best series of novels that explore the full scope of biochemical alternatives to life is James White’s novel and short story series, Sector General. The series is about a hospital whose patients come from all possibilities of lifeforms.
Though carbon and water appear to be the best components of life in the universe, we should always be aware of the carbon bias that pervades the thinking of many scientists. Plausible alternatives are out there. Given the immensity of the universe before us, it is not only plausible but likely that other alternatives are indeed out there. In fact, we may find, given the conditions of the worlds in our immediate cosmic neighbourhood, that life is only a relative stone’s throw away. We only have to be able to understand where the stone landed.
Further Reading :
-Alison, A. (1968) "Possible form of life." Journal of the British Interplanetary Society 21:48.
-Ball, R. S. W. (1894) "The possibility of life on other worlds." Fortnightly Review 62:718.
-Cairns-Smith, A. Graham (1985) Seven Clues to the Origin of Life. Cambridge University Press.
-Darling, David (2001) Life Everywhere: The Maverick Science of Astrobiology. Basic Books.
-Feinberg, Gerald and Robert Shapiro (1980) Life Beyond Earth. Morrow.
-Firsoff, V. A. (1962) An ammonia-based life. Discovery 23:36-42.
-Firsoff, V. A. (1965) "Possible alternative chemistries of life." Spaceflight 7:132-136.
-Goldsmith, D. and T. Owen (1992) The Search for Life in the Universe. Addison-Wesley.
-Haldane, J. B. S. (1954) "The Origins of Life." New Biology. 16:12-27.
-Jakowsky, Bruce (1998) The Search for Life on Other Planets. Cambridge University Press.
-Jones, Harry (2001) "Extraterrestrial Ecology." Proceedings of the 31st International Conference on Environmental Systems.
-Jones, Harry (2002) "Exochemistry and the search for alien life." Proceedings of the 32nd Conference on Environmental Systems.
-McKay, C. P. (1992) Mars: A Reassessment of its Interest in Biology. Exobiology in Solar System Exploration, ed. G. Carle, D. Schwartz and J. Huntington. NASA.
-Molten, P. M. (1973) "Terrestrial biochemistry in perspective: Some other possibilities." Spaceflight 15:134-144.
-Reilly, Michael (2008) "Early life could have relied on arsenic." New Scientist. 198 (2653).
-Sagan, C. and E. Saltpeter (1976) "Particles, environments and possible ecologies in the Jovian atmosphere." Astrophysical Journal Supplement 32:737.
-Scharf, C. (2009) Extrasolar Planets and Astrobiology. University Science Books.
-Shapiro, Robert (1999) Planetary Dreams: The Quest to Discover Life Beyond Earth. Wiley.
-Wells, H. G. (1894) "Another basis for life." Saturday Review 676.