k = Ae(−Ea/RT)
where e is a mathematical constant, Ea is the activation energy, R is the universal gas constant, and T is the temperature of interest. The more unusual factor, A, is a constant for each chemical reaction; it defines the frequency of collisions having the correct orientation.
What does this exponential relationship between temperature and the reaction rate mean for life?
Consider a reaction with an activation energy of 50,000 joules, which is the energy needed to get the reaction going. Drop the temperature of the environment from 100°C to 0°C, and the reaction rate decreases by just over 350 times. However, drop the temperature by another hundred degrees, from 0°C to −100°C, and the reaction rate decreases by a staggering 350,000 times! At the temperature of liquid nitrogen (about −195°C), the rates of reaction are 1023 (100,000 billion billion) times less!
The optimistic might immediately hit back with the rejoinder that catalysts could accelerate reaction rates, but even the best enzymes and chemical catalysts increase reaction rates by only a few orders of magnitude. This exponential relationship may not be a problem: life can merely operate at these slower rates, maybe replicating many times less frequently than typical Earth life. However, in most planetary environments, life is subjected to constant damage that must be repaired. One source of this damage is background radiation.
Thus, life is confronted by a problem. It must be able to repair radiation damage to prevent the damage from accumulating to fatal levels. In the deep subsurface of Earth, where there is little energy available to grow and reproduce, microbes might divide very infrequently. And yet even here, they must get enough energy to repair damage from radiation. In the Earth’s rocks, natural background radiation would kill the most radiation-resistant microbes known after about forty million years if they remained dormant. On Mars, as the atmosphere is thinner than Earth’s, the surface has the additional problem of higher levels of cosmic radiation. Here, even a radiation-resistant dormant microbe, if such a thing ever existed there or is accidently dropped there by human or robotic explorers, would be killed within thousands of years, or much less.
If the chemical reaction rates in the cold-temperature life form are many thousands, millions, or trillions times less than those in the life we are familiar with, it is likely that this cold life form will accumulate a great deal of damage and be unable to repair itself sufficiently fast to remain alive.
There may, however, be some more optimistic news for the low-temperature life form. Some challenges it faces depend on temperature. The formation of reactive oxygen species, the decay of amino acids, and the thermally caused decay of DNA base pairs depend on temperature: the lower the temperature, the slower the damage occurs. Although the low-temperature life form may well incur damage, some of this degeneration will be correspondingly slow, partly making up for the slow rate at which it can repair itself. However, direct damage to molecules caused by radiation can be essentially independent of temperature. Live life too slowly, and you may not keep up with this inevitable damage.
Besides slow repair and growth rates, the environment may run away from our sluggard creature. Any environment alters over time. Indeed, for chemical reactions to yield energy for life, there must be turnover and dynamic alteration in the environment to provide these contrasts. At extremely low temperatures, cellular chemical reaction rates are such that by the time a cell has commanded its metabolic pathways to take advantage of some short-term change in its habitat, there is a good chance that the conditions will have shifted again. On the larger scale, there is the problem that if reaction rates are reduced by many trillionfold, then it is likely that conditions on a planetary scale will have transformed before metabolic pathways have had a chance to respond to the initial conditions to which they were exposed. Life would be engaged in a futile catch-up game, trying to capture energy sources or respond to physical and chemical conditions long since gone.
There is likely to be a temperature range optimal for living things. Life’s capacity to adapt and repair itself probably has a temporal correlation with the rate of radiation and other geochemical and geological perturbations in most environments in the universe. At extremely low temperatures, the processes of life may be generally out of sync with many processes that happen in and on planets.
It always helps, when thinking about the prospects for alien chemistries, to find real places in the Universe to test ideas. Many of the best-known examples of frigid environments in our own Solar System, such as the oceans in the icy moons of the gas giants or the glaciers of Mars, may not be much colder than places we know on the Earth. However, even in our own cosmic backyard, we do know of locations where there are liquids at temperatures significantly lower than anything found on Earth. Do we find any reason to be optimistic that they might harbor self-replicating evolving systems of matter?
There is a cold place in the universe where people have entertained the idea of life—Saturn’s moon Titan. The surface of this remarkable moon was presented to a stunned human audience by the Cassini spacecraft and its lander, Huygens, which in 2004 returned jaw-dropping images of this ethereal world as it descended through the atmosphere. Rivers of methane carve through a landscape, creating sinuous tributaries and lakes much like the features of our watery world. In this frigid landscape, at a chilly −180°C, water ice behaves physically like rocks on our planet.
On Titan, the solvent on offer to life is methane, whose behavior as an organic molecule differs greatly from water. Unlike water, methane has no polarity, and so it has difficulty dissolving many ions and charged molecules that are vital in terrestrial biochemistry. Most proteins we know would be ineffective.
Some say that one advantage of methane is that it is less reactive than water. Those hydrolysis reactions that damage life’s molecules on Earth would be nonexistent. Although this may be the case, water’s tendency to react with some molecules is an essential part of its ability to maintain molecular flexibility and to choreograph dances and communication between molecules. The reactive nature of water, although sometimes unpropitious for life, is usually beneficial for any living thing.
A popular line of argument is that some chemists actually prefer to do some syntheses in non-water solvents, where the reactive nature of water can be avoided, proof that life would be better off without water entirely and that liquid methane and similar fluids may be a step up for life. However, chemists like doing reactions in organic solvents since their objective is to maximize the yield of compounds they are trying to make. They want to lose as little as possible in unwanted, reactive chemistry. But this is not life’s game. Life employs reactivity to drive active biochemical processes. Methane’s comparative lack of reactivity and its inability to dissolve polar molecules is unlikely to constitute some sort of advantage to life in the same way that these properties might entice industrial chemists.
With some imagination, however, we can think of how to build biochemical structures in the presence of this organic compound. Consider how to make a membrane like those used by life on Earth to enclose a cell. One way we might make such a membrane on a Titan-like world is to flip it around, to make an inside-out membrane. The charged heads would point inward toward each other to escape the water-hating methane, into which the long fatty acid tails would point. By turning the lipids around, we could make a vesicle appropriate for a methane world. To make this work, we could not incorporate the fatty acid tails that life uses on Earth, as they would be almost solid and immobile in the cold methane lakes of Titan. Instead, using chemical modeling, a team at Cornell University invented a membrane formed from acrylonitrile, a nitrogen-containing compound known to be present on Titan. Their azotosome, as they call it, would have polar heads rich in nitrogen; the heads would be attracted to one another to form the membrane, with tails of short-chained carbon compounds sticking out. Using this chemical compound, the whole structure would maintain fluidity on Titan similar to membranes on Earth.
Not
content with models and speculation, we might look at real data. Researchers have approached the possibility that Titan could host life by comparing measurements of gases on the moon with the ways that life might make energy. They proposed that by reacting hydrocarbons such as acetylene and ethane with hydrogen, present in Titan’s atmosphere, life could make energy, producing methane as the waste product. These ideas have even received some boost from the observed depletion of hydrogen in the atmosphere of the moon and an apparent depletion of acetylene near the surface, suggested as tantalizing circumstantial evidence for life. These data are highly provocative. In applying Occam’s razor, a principle that cautions us to accept scientific explanations with the minimum number of assumptions—a principle particularly important for thinking about alien life—we should remember that our still very limited knowledge of Titan and its methane cycle might well hide other geological and geochemical explanations for these observations. Nevertheless, the ideas are fascinating.
As we can see, with a little imagination, we can construct an internally self-consistent picture of life on Titan. However, the presence of possible energy sources for life, an abundance of organic molecules and other noncarbon atoms, and potentially even lipid-like compounds on Titan may not be sufficient for life if low temperatures in most of its lakes and landmasses prevent a viable living system.
As with all these discussions, we probably have some chemical bias because we focus our research efforts on the solvent we know so well—water. Our knowledge of ammonia, liquid nitrogen, hydrogen fluoride, liquid methane, and other solvents is less complete and as we have no example of a biochemistry that operates in them, we are left to a fair amount of speculation. If we ourselves used another solvent, could we predict how the strange solvent dihydrogen monoxide (H2O, water!) could interact with a reproducing, evolving, self-replicating organism? Even though we are a water-based intelligence, our knowledge of its role in biochemistry has only recently advanced rapidly and yet remains in its infancy.
Nevertheless, even with this caveat, the solvent seems an incredibly versatile substance. Water has an extraordinary capacity to play leading roles and bit parts in the theater of life. As yet, other solvents hosting organic chemistry or even other biochemical architectures of life have not been shown to possess this multitasking capacity. Equally important, water is liquid in a range in which chemical reaction rates are commensurate with the need to deal with biologically damaging agents, such as radiation and changing conditions at microscopic scales right up to momentous rearrangements at the planetary scale. Alongside its chemical promise as a solvent for life, water has a cosmic copiousness, suggesting not only that its physics is suitable for living things, but also that the physics of the wider universe makes it a common solvent on offer for any emergent planetary experiment in evolution.
Twelve billion light-years away, there is a quasar, an ancient object with the rather unmemorable name of APM 08279+5255. Astronomers have a penchant for names like this. I’m a biologist. Let’s call it Fred. Now Fred harbors a black hole about twenty billion times more massive than the Sun. Astronomers do not yet understand quasars. You will appreciate that since Fred is twelve billion light-years away, we are observing light from near the beginning of the universe. Quasars are very old objects. Nevertheless, within this curious distant fuzz, there is a vast quantity of water—a voluminous 140 trillion times all the Earth’s oceans combined!
Fred is not unusual. Water can be found everywhere: it is a common volatile. In our own Solar System, we have the ocean under the ice cover of Jupiter’s moon Europa and the jets of water—geysers—erupting from the south pole of one of Saturn’s moons, Enceladus, an unpretentious moon less than five hundred kilometers in diameter. Then there are the ice caps of Mars and those frozen comets, about one to ten billion of these objects with a diameter greater than one kilometer in the Kuiper Belt alone.
How the water found in Fred was produced is a matter of some conjecture, but regardless, it is thrilling that astrochemists have schemes for how it might be formed in these very alien environments. Look at the reaction scheme below:
H2 + cosmic irradiation → H2+ + e−
H2+ + H2 → H3+ + H
H3+ + O → OH+ + H2
OHn++ H2 → OHn+1+ + H
OH3+ + e− → H2O + H; OH + 2H
The chemical details need not concern us, but the simplicity is beautiful and worth remarking on. Molecular hydrogen is bombarded with some radiation, perhaps from a dying star. Some hydrogen ions are produced and can react with oxygen atoms, the oxygen itself having been produced and strewn across interstellar space in supernova explosions. Then the ions containing hydrogen and oxygen react with more hydrogen ions to produce an OH3+ ion that can mop up an electron and become water. I have underlined the water above.
So we take hydrogen from the big bang and some oxygen from exploding stars, mix in some radiation and electrons, and we produce water, everywhere across the universe.
These reactions may not be the only ones that give Fred its water, but they show that the pathway to water is simple and requires no special conditions. The water that Earth obtained in its early history, once thought to be from comets, probably mainly came from water-rich asteroids, the water within them having been originally formed in reactions like the ones described above. Fred tells us that these processes have been going on for billions of years. In one location in space, over seven billion years before Earth was formed, before life on our planet would emerge, trillions of oceans of water were produced around just one object.
The other solvents that have attracted attention as plausible candidates for life tend to be rarer. The ocean of water thought to exist under the surface of Titan may contain 30 percent ammonia, a water alternative likely to have been one of the components in early Earth’s atmosphere. Today, ammonia is one ingredient of the atmosphere of Jupiter. The compound is out there, but probably not as abundant as water. Sulfuric acid, the more eccentric suggestion of a liquid for life, is even rarer. As for hydrogen fluoride oceans, they seem unlikely. Fluorine is about a hundred thousand times less cosmically abundant than oxygen. Whatever their chemical versatility, these alternative solvents and others fail to match the sheer quantity of water in the universe. Other fantastical life-giving liquids in the universe, oceans of sulfuric acid or ammonia in which fishy life forms swim, are likely to be much rarer than our comely water oceans. The physical properties of water make it both abundant and versatile as a solvent in which to assemble life.
CHAPTER 10
THE ATOMS OF LIFE
TO BEGIN A CHAPTER of a book on life with an excursion into Star Trek may not seem like a very auspicious development, but this television and film series that began in 1966 from a concept by Gene Roddenberry is an example of that pervasive view that biology is limitless. Across the galaxy, the crew of the starship Enterprise gallivant around, encountering strange life forms and trying to figure out ways of defusing their often irascible tempers or aggressive tendencies. The theme of this series, that the universe contains a never-ending supply of unpredictable biological potential, is a common idea in science fiction. Star Trek constructed decades of television and film from this simple notion.
I absolutely deny being a Trekkie, but I agree with William Shatner, Captain Kirk’s real-life incarnation, that the best episode made was titled “The Devil in the Dark,” which aired in 1967. Fifty miners lie dead on a planet called Janus VI, apparently killed by an annoyed creature that sprays things it encounters with a corrosive substance. The creature is tracked down and turns out to be a silicon-based life form made of the same silicate substances from which rocks are assembled. The silicon nodules that the miners have been collecting (one nodule sits on the desk of the mine’s director) are not mere boulders, but the eggs of these creatures, the Horta. After some reconciliation with the crew of the Enterprise, some Horta etch NO KILL! into the rock, and with this sentient coming-together of cultures, the Horta help the miners locate precious
metals in exchange for being left alone to tend to their eggs. Everyone couldn’t be happier.
The Horta and their offspring reflect another fundamental question in biology: can the elements from which life is constructed, the atomic building blocks of life, differ from those we know on Earth? With this most basic of questions, we continue down into the hierarchy of life, now moving into the atomic scale to peruse how physical processes might shape and channel its construction at this more fundamental level of matter.
Life on Earth uses a vast range of elements as the atomic chassis of its basic molecules, but the predominant element that forms the backbone, if you will, of the vast pantheon of molecules that come together to building living organisms is carbon. This element occupies group 14 of the periodic table. Silicon, the element below it, belongs to the same group and shares similar chemical characteristics. So then, imaginative people ask, why couldn’t silicon replace carbon in life as the next best alternative? With the universe full of silicon, there is no shortage of the substance for building living things. As Kirk might well have pondered, what’s not to like about the Horta?
The Equations of Life Page 21