Alien Universe

by Don Lincoln

  Hydrocarbons like methane have some advantages over water. Certainly empirical evidence suggests that the reactivity of organic molecules is comparably versatile in hydrocarbon solvents. However, since hydrocarbons are not polar, they are less reactive to some unstable organic molecules.

  The surface of Titan is an excellent test case for many of these considerations. Titan is not in thermodynamic equilibrium, it has ample carbon-containing molecules, and it is covered with a liquid solvent. The temperature is low, which allows for a broad range of covalent and polar bonds. Indeed, it has many of the essential features that seem to be important to life. This leads us to speculate that if life is an inevitable outcome of chemistry, then Titan should have at least primitive life. If it turns out to not have life, then we must begin to suspect that there is something unique about the environment of Earth, perhaps including the use of water as a solvent. It is therefore not surprising that a probe to the methane oceans of Titan is a high-priority goal in NASA’s exobiology plans.

  Evolution Matters

  The last property that seems to be necessary for alien life and definitely Aliens is some sort of Darwinian evolution. However life comes into being, it won’t spring forth, fully formed, as an intelligent Alien, any more than it did here on Earth. Simple life-forms will be the beginning. They will encounter unstable environments, competition from members of the same species and others, predation, and so on. There must be a mechanism whereby organisms can change and adapt. If not, they will die out. It’s just that simple.

  However, precisely how this works is up for grabs. For instance, on Earth, the blueprint for life is stored in our DNA. Four nucleic acids—adenine, guanine, cytosine, and thymine—are the building blocks of the familiar spiral ladder of life. These nucleic acids make up the “rungs” of the ladder, while the sides of the ladder are called the backbone and consist of the sugar phosphoribose, which separates the rungs of the ladder.

  Evolution occurs through a series of small changes that culminate in larger changes in the organism. The organism then competes in the ecosystem and may experience enhanced reproductive success. This is all pretty standard stuff.

  What is a little more subtle is the realization that changes means just that … changes. It is imperative that the molecular structure that holds the genetic code be stable against small changes. The chemical properties of the DNA backbone must dominate the structure. Swapping a nucleic acid in or out must not cause the whole ladder to fall apart. This is critical. If the change causes the whole structure (and therefore organism) to be nonviable, then this is a disaster.

  We can generalize these ideas beyond the specifics of DNA. The genetic molecules of any Alien must be able (1) to change without destruction of the molecule and (2) to replicate accurately with the new change. Self-replicating systems are well known in chemistry, but ones that can generate inexact copies, with that inexact copy also being faithfully replicable, are not. This might suggest that the genetic code of Aliens might need something analogous to the backbone of DNA, where the code can be “snapped in” like LEGOs. Surely the details of the molecules will be different, but the functionality is probably necessary.

  Extremophiles

  Extremophiles are organisms that live under conditions injurious to many forms of life. Now, from my observation, this should include people who enjoy being outside in Houston in August or a colleague of mine who summers in Antarctica, but extreme is actually quite a bit more extreme than that. Mankind has used extreme environments for a long time to preserve food. We now know that this is because these techniques kill or suppress the bacteria that would otherwise cause spoilage. A few techniques are to heat (i.e., cook) the food, refrigerate it, salt it, or even irradiate it.

  And we all know this works. We have refrigerators and freezers. We have been admonished to cook rare roast beef to an internal temperature of about 140°F or as much as 180°F for well done beef or all poultry. The reason is to both cook the meat—to convert it from something raw to something yummy—and to kill the bacteria living in the raw meat.

  There are other methods for preserving food that you have encountered in your local grocery store. There are dried vegetables, fruits, and meats, which have been starved of water, inhibiting bacterial growth. Nuts and other foods come vacuum packed to reduce the oxygen available in the package. Processing food by using high pressure can kill microbes. This is used for many products, including guacamole and orange juice.

  Meat is cured by salting, as in the familiar bacon and ham. The high salinity kills germs. Smoking meats is also a way to store them. Sugar, even though it is rich in calories, is a good way to preserve fruits. Jellies and glacéed fruits can sit a long time without going bad.

  Alcohol, aside from its mood-altering side effects, is also used to preserve some fruits. This is usually done in conjunction with using sugar as a preservative.

  Changing the acidity or alkalinity of the food is another way to lengthen its lifetime. While salting plays a role in making pickles (and pickling in general), the use of vinegar (with its attendant acidity) can extend the shelf life of food. And, if you are of Scandinavian descent, you might enjoy lutefisk, which is fish prepared with lye, which is highly alkaline.

  Atmosphere modification is also a useful technique. Food, such as grains, can be put in a container and the air replaced with high-purity nitrogen or carbon dioxide. This removes the oxygen and destroys insects, microbes, and other unwanted intruders.

  The real point is that mankind has known about various ways to preserve food for millennia. Spoilage of food originates from undesirable creatures (typically microbes of some sort) “eating” the food and releasing waste products. Through some combination of the techniques mentioned above, we have learned to kill the undesirable bacteria that would otherwise ruin our food.

  Our experience has led us to some understanding of the range of conditions under which Earth-like life can exist. However relatively recent scholarship has revealed that life is actually hardier than we thought.

  Biologists have given the name “extremophile” (meaning “lover of extreme conditions”) to organisms that thrive in environments that would kill familiar forms of life. While the study of extremophiles is still a fairly young science, we can discuss some of the range of conditions under which exotic life has been found.

  At the bottom of the oceans, sometimes at extraordinary depths, there are spots where magma has worked its way from the interior of the Earth to the ocean floor. At these points, called hydrothermal vents, superheated water streams away from the magma. This water can be heated to well above the familiar boiling temperature of 212°F, but the huge pressure at the bottom of the ocean causes the water to stay in its liquid form. Water inside these hydro-thermal vents can be nearly 700°F, certainly high enough to kill any form of ordinary life.

  Only a few feet away from these vents, the temperature of ocean water can be very close to freezing, about 35°F. In this temperature gradient grows an unusual ecosystem. At the top of the food chain are relatively common types of clams and crabs who consume food in standard ways. However at the base of the food chain are thermophilic (heat-loving) bacteria that can live at temperatures above the usual 212°F boiling point of water. These bacteria do not use the same biochemical pathways of ordinary life. Rather than using oxygen as an electron receptor, they use sulfur or occasionally iron. These materials are spewed copiously into the sea, dissolved by the water from the magma source.

  In fact, current thinking is that these prokaryotes are perhaps closest in nature to the last universal common ancestor (LUCA) of life on Earth. How could this be? Well, we should remember that LUCA was itself a sophisticated life-form and certainly not the only one around at the time it existed. While the following is purely speculation, we could imagine that this life-form might have survived a late strike on Earth by a comet or something similar. The impact would have vaporized the oceans and only the deepest-dwelling, most heat-resistant life might have s
urvived.

  Heat-resistant, sulfur-breathing life is not the only type that exists in extreme environments. On the other end of the spectrum are the cold-loving cryophiles. While pure water freezes at 32°F, salty water can remain liquid at temperatures much colder than that. Life-forms at the cold end of the spectrum have quite different problems compared with their thermophilic cousins. If water freezes, it expands and can rupture cell membranes. Plus the reduced temperature can significantly lower the rate of chemical reactions experienced by the life-form. In essence, cold life “lives slower.” Further, just like cold butter is hard to cut, while warm butter is nearly a liquid, cold can stiffen the cellular membranes of cold life. Chemical adaptations are needed to mitigate the problems of the cold.

  As of our current understanding, we know of no eukaryotic life that can exist at temperatures outside the range of 5 to 140°F. While the lower number is below the freezing point of ordinary water, water with high salinity can remain liquid at these temperatures. Microbial life has been observed over a temperature range of –22 to 250°F. An example of a cryophilic organism is Chlamydomonas nivalis, a form of algae that is responsible for the phenomenon of “watermelon snow,” in which snow has the color and even the slight scent of watermelon.

  Chemical considerations can give us insights into the ultimate constraints on the temperature of carbon-based life. Due to the bond strength involving carbon atoms, it’s hard to imagine life at standard pressure much higher than 620°F; about as hot as the hottest your oven can bake. Of course, pressure can affect the rate at which molecules break apart and the decomposition of molecules can be slower at high pressure. It’s probably safe to say that carbon-based life is not possible above about 1000°F at any pressure.

  Water is critical to life, however it may be that there are extremophiles that don’t need much of it. Looking for life in locations with little water is a way to better understand the realm of the possible. And Earth does have some extremely dry places. The Atacama Desert is commonly called the driest place on Earth. Some places in the desert get about a fraction of an inch of rain per year and some weather stations have never recorded any rain at all. There are tall mountains (over 22,000 feet tall), which one would expect to be glacier-covered, that are completely dry. In fact, there are empty river beds that have been estimated to have been dry for as many as 120,000 years. There are a few places in the Atacama Desert that are thought to be the naturally occurring place on Earth with conditions comparable to Mars. In fact, NASA has done some work there to help design Martian probes. They have gone so far as to experiment on searching for life in the sands of the Atacama Desert, using techniques that are hoped to definitively answer the question of life on Mars.

  There are also forms of life that are halophiles (salt loving). In the Dead Sea region of the Middle East, most life couldn’t survive. However, there are lichens and cellular life that have adapted their chemistry to maintain their inner environment in such a way as to thrive. Some of these forms of life actually need the high salt environment to live at all. It’s hard to believe that an environment that can cure a ham is actually a comfortable place for life to live and yet it’s true.

  As with the other food-preserving extremes, life has been found in highly acidic and basic environments and even in the presence of radioactivity a thousand times higher than would kill the hardiest normal forms of life. These observations have certainly broadened scientists’ expectations of the range of environments that life can successfully inhabit.

  With the discovery of these extremophiles, scientists have intensified their search for the niches that life can occupy on Earth. We have pulled life out of well cores taken from a couple of miles under the surface of the Earth. Life has been found floating in the rarified air of the stratosphere. Microbes have been found as high as 10 miles above the ground. This environment is extremely harsh. The temperature and pressure is very low, the flux of ultraviolet light is very high, and there is nearly no water. Survival in this hostile environment inevitably raises questions of “panspermia,” which is the premise that life might have arrived on Earth from some other body … perhaps Mars. While this seems improbable, it is not ruled out. But life had to start somewhere, so the questions we have discussed here are still relevant, even if life started elsewhere. Of interest to us here is the understanding that some primitive forms of life can exist in an environment that would kill creatures that live closer to the Earth’s surface. However, this primitive form of life wouldn’t be an Alien. But it does give us some additional information on precisely how resilient Earth-based life, with our carbon and water-based biochemistry, can be.

  Silicon-Based Life?

  In science fiction, there is soft SciFi and hard SciFi. In hard SciFi, the writer tries to advance the plot line constrained by the best-known science of the time, while in soft SciFi, more liberties are taken with the science. In the case of stories about alien life, a common alternative to our familiar type of life is one based on the silicon atom. The arguments presented earlier about the advantages of carbon (specifically the four bonds available and the rich chemical complexity that comes with it) are rather compelling, suggesting the four available bonds are a necessary condition of complex life. In fact, chemists have cataloged more molecules involving carbon than all the known molecules that exclude carbon. Think about that. If you took all elements except carbon and made every known compound, you’d have fewer compounds than the ones that have been found and contain carbon.

  Given the benefits of four bonds, it is therefore natural that a hard SciFi writer who wants to break away from carbon-based life would then invoke silicon as the next candidate base element around which to build a fictional ecosystem. There’s only one problem: it isn’t as simple as that.

  We’ve already noted the simple objection that while we breathe out carbon dioxide as a gaseous waste product, silicon dioxide is solid and we are more familiar with it as sand. This particular fact was noted early on in the 1934 short story A Martian Odyssey, by Stanley G. Weinbaum, in which he described a Martian silicon-based creature that excretes bricks every ten minutes. These bricks were the waste products of respiration.

  However, the problems with silicon are much deeper and fundamental than this. Far more damaging are silicon’s issues with its stability in its interactions with other atoms and the rate at which silicon chemically interacts.

  A very important feature of how carbon bonds with other elements is that the bond strength between two carbon atoms (C–C) is quite similar to that of a carbon-hydrogen bond (C–H), as well as carbon-oxygen (C–O) and carbon-nitrogen (C–N). Because of this, it is energetically fairly easy for a reaction to swap out one atom and connect another. From an energy point of view, which of these elements participate in the bond doesn’t matter much and so these swaps occur pretty freely.

  In contrast, silicon doesn’t have this property. It turns out that silicon-oxygen (Si–O) bonding is much stronger than with hydrogen (Si–H), nitrogen (Si–N), or even other silicon atoms (Si–Si). Consequently, silicon binds easily to oxygen (making silicon dioxide), and it is very hard to break apart that bond and slip in another atom.

  What we’ve mentioned here is just a characteristic of single interatomic bonds. When we turn our attention to multiple bonds, carbon is again quite superior. It turns out that a double carbon bond takes about twice as much energy as a single bond, while a triple bond uses about three times as much energy. It didn’t have to be that way. The details of multiple bonds are different from single bonds, and carbon just got lucky.

  Silicon, in comparison, has a much harder time making double and triple bonds. This has to do with the size and shapes of the atoms. The pictures of figure 6.5 give an overly simplified impression of the shape of atoms. Silicon and carbon really look like spheres with bumps protruding out of them, with the bumps participating in the bonds. Because the silicon sphere is bigger than the carbon one, and the silicon bumps aren’t much bigger than the carbon on
es, the bumps are farther away between two adjacent silicon atoms. This makes it harder to get the bumps closer to other atoms to share electrons, which makes a second bond much weaker than the first one. Consequently, the strength of double bonds between adjacent silicon atoms aren’t much different from single silicon bonds. This makes complex chemistry using silicon that much harder. This point is illustrated in figure 6.12.

  FIGURE 6.12. Because of their size and shape, silicon atoms have a hard time making stable double and triple bonds. The strength of the second silicon bond is much weaker than the first silicon bond. This is in contrast to carbon, in which the second bond is comparable to the strength of the first bond. The black areas represent electrons available for bonding. In silicon, the electrons participating in the second, third, and fourth bonds are separated by a greater distance and consequently bond more weakly.

  Finally, the ease at which reactions can occur is much greater with silicon atoms. Consider a gas stove, inadvertently left on, so carbon-containing natural gas fills the house. The gas can fill the house, but it won’t explode without a spark to set events in motion. However, a similar “silicon natural gas” would spontaneously react without the spark. This speed of reaction reduces the time necessary to form complex molecules.

  So does this mean that silicon-based life is impossible? Could the rock people of planet X be having a discussion about the benefits of silicon-based life? Well, sure. It’s not like the factors mentioned in this chapter are definitive, nor should you think that we’ve exhaustively explored all options. But these factors are certainly strong reasons to not think of silicon-based life as equally likely as other worlds covered with carbon-based life. Even Carl Sagan was reported to have stated that while he was only a weak water chauvinist, he was a huge carbon chauvinist.

  So scientists must consider the possibility of alien life based on atoms other than carbon, but it isn’t considered to be highly likely. However, when we talk in this way about silicon life, we need to remember that we’ve been talking about life that evolved directly from non-living substances. There is another form of silicon life that we should keep in mind.