Authors: Don Lincoln
We can see the merits of the various materials. Ammonia has good thermal properties, but a limited temperature range over which it is liquid. On the one hand, hydrogen fluoride has a broad temperature range in which it is liquid, and it takes considerable energy to warm up the liquid, with the downside that it can convert to the gas phase very easily. It also has an attractively high dielectric constant. On the other hand,
figures 6.7
to
6.10
show that fluorine is quite rare in the universe. Further, it reacts quickly with water to make hydrofluoric acid and with silicon-bearing rocks to make silicon fluoride. This is an inert material, which would tie up the fluorine and make it unavailable for respiration.
Note that methane is an interesting material; although not a polar solvent, it is a popular substance to consider when thinking about alternative biological chemistry. Methane can be found in its liquid form on the surface of Saturn’s moon Titan for instance.
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 survived.
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.