Silicon vs. Carbon

Life on Earth is carbon based. This simply means that the chemistry for life on Earth uses carbon to form complex molecules that are used for various life functions, such as information storage. We find carbon in everything from cell membranes, to hormones, to DNA. For years, scientists and science fiction writers have dreamt about the possibility of life based on something other than carbon. To replace carbon with another element, we would need to carefully choose a competitor. Carbon’s contender should be an element that is abundant since it will be a major constituent of so many vital molecules. In addition, we would need to consider elements that have the ability to bond with themselves as well as with a variety of other elements to create complex, and more importantly stable, molecules for life.

It is well known that different elements can possess similar chemical characteristics. These similarities stem from the fact that all atoms are essentially put together in the same way. The periodic table is an organized list of all the elements and is presented in such a way as to reflect patterns in the arrangement of the nuclear particles within atoms. For example, as you read the periodic table from left to right, the number of protons and electrons per atom increases. All of the elements in one column have the same number of electrons in their outer electron shells. Typically it is only the outer shell of electrons that plays a role in chemical reactions. This means that elements in the same column tend to participate in chemical reactions similarly. If we look at the column that begins with carbon, we can read down the column and see that this column includes various other elements such as silicon (Si), germanium (Ge), tin (Sn), and lead (Pb). In most of the fantasies about alien life, silicon is the candidate proposed to replace carbon since its location in the periodic table is directly beneath that of carbon. For the remainder of this discussion, we will compare silicon to carbon as the fundamental element of life.

Silicon has the same number of electrons in its outer shell, meaning that it can form four bonds just like carbon. It is also very abundant, comprising much of the rock that is beneath your feet. Silicon can bind readily to itself to make Si-Si bonds just like carbon can make C-C bonds. With just this information, one might think that we are on to something with this silicon atom. After all, C-C bonds are the basis for complex molecules on Earth. However, we are neglecting some rather important details. Although Si-Si bonds, as well as silicon-hydrogen and silicon-oxygen bonds, are easily made we have not yet considered the relative strengths of these bonds. Si-Si bonds are much weaker than C-C bonds – they are only half as strong! Si-H bonds and Si-O bonds are stronger than Si-Si bonds, whereas the carbon analogs for all three of these types of bonds are nearly equal in strength. This means that while it is very easy to create long chains and rings of carbon atoms, it is unusual to have long chains or rings of silicon atoms linked together. In fact, it is extremely rare to find any molecules that have strung together more than three silicon atoms.

Some of the more common carbon molecules that we are familiar with on Earth, such as carbon dioxide (CO2) and methane (CH4) do have silicon derivatives. Silicon is very attracted to oxygen and therefore combines readily with oxygen even at lower temperatures, forming silicon dioxide, SiO2. If silicon were to combine with the most abundant element in the universe, hydrogen, it would form silane, SiH4. However, silicon doesn’t react as easily with hydrogen as it does with oxygen. Even in the most reducing conditions and with plenty of excess hydrogen, silane won’t form below temperatures of 1000 K. And when you compare silane to methane, we notice that silane is much less stable than methane, igniting when exposed to air.

We have plenty of evidence of SiO2 formation on Earth, as it is a primary constituent of rocks. The most common form of SiO2 is quartz. Although commonly identified on Earth, SiO2 has vastly different properties than the also abundant CO2. Here on Earth, CO2 is gaseous at most temperatures, is very soluble in water (and is therefore available in aqueous solution for life), and can be broken down into carbon and oxygen. In stark contrast, SiO2 does not exist as a gas except at extremely high temperatures, well over 2000 degrees Celsius. As can probably be anticipated by the fact that it comprises many rocks on Earth, SiO2 is almost completely insoluble in everything. Finally, because silicon has a high affinity for oxygen, it is very difficult to break SiO2 into it constituent atoms. Consequently, carbon dioxide wins the competition against silicon dioxide for being most useful to life. With respect to living organisms, SiO2 can be considered a very inert molecule and therefore useless for life processes.

So far we have compared silicon to carbon primarily within the context of what we know here on Earth. However, what might the conditions be like on another planet? How might life elsewhere evolve to use silicon instead of carbon? In 1894 the famous writer H.G. Wells wrote,

“ One is startled towards fantastic imaginings by such a suggestion: visions of silicon-aluminium organisms - why not silicon-aluminium men at once? - wandering through an atmosphere of gaseous sulphur, let us say, by the shores of a sea of liquid iron some thousand degrees or so above the temperature of a blast furnace. “

We do know that silicon-oxygen compounds form easily and are therefore quite common. Might life somehow take advantage of this? On Earth we know that some fairly large molecules can be made from Si-O bonds. Silicones are an example of such molecules; they are comprised of Si-O bonds and contain carbon. Silicones are very stable, so stable that they don’t react with other molecules much. Although silicones could be used by life to store and transmit large amounts of information, their inability to easily engage in chemical reactions makes them an unlikely choice for any type of life. This leads us back to the same problem that we noted with SiO2, silicones wouldn’t be very useful for chemical reactions.

Maybe we are being too narrow-minded with how we are considering basic chemistry. Do the “rules of chemistry” work in the same way throughout the universe? Would we observe silicon behaving differently on another planet? Based on observations made by astronomers, the answer is probably no. Astronomers have examined the cosmic environment: the interstellar medium, interstellar clouds, meteorites, comets, and stars. In all of these places, carbon molecules run rampant and not just simple carbon molecules, but also some of the more complex organic molecules as well. Oxidized silicon, like silicon dioxide, is quite common in the cosmic environment. However, silicon molecules such as silane and silicones that we would consider silicon-based life molecules are seldom identified. Carbon chemistry appears to be ubiquitous in the cosmos.

So far, the evidence suggests that it is unlikely for life to be based on silicon chemistry. However, that doesn’t rule silicon out as far as playing a role in the origins of life. Many carbon molecules used for life exhibit something known as “handedness” or chirality. They can exist as either right- or left-handed molecules. A right-handed sugar molecule is the mirror image of the complimentary left-handed sugar molecule, just as your left hand is a mirror image of your right. When you shake hands, the two hands involved are either both right hands or both left hands. A handshake just doesn’t work well when one left and one right hand is involved. Similarly, life has developed to use only molecules with a particular chirality. Silicon molecules seldom exhibit this trait; they are usually achiral – exhibiting only one “handedness”. One proposition for the origin of life on Earth is that the first organic molecules may have formed on the surfaces of silicates. This would have determined the handedness of the organics used by life today.

Despite the pessimism surrounding the prospects of silicon based like, science fiction writers haven’t given up hope of an alien life form that departs significantly from that which we are most familiar – carbon based life. The chances for silicon-based life are very slim, but that shouldn’t restrict our minds from exploring the unimaginable.

Wells, H. G. "Another Basis for Life," Saturday Review, p. 676 (December 22, 1894).

Copyright © 2000-2008 Authors/Editors Chris Impey & Erika Offerdahl
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