Life thus far has been couched with the qualifier “as we know it.” That’s carbon-based life, operating in a water-based medium, with higher forms metabolizing oxygen. All forms of life on Earth—from slimy bacteria to sentient humans—share this same basic biochemistry. And on the basis of what we now know about the various chemical elements, carbon would seem to be the atom best suited to form the long-chain molecules needed for life. But are we being chauvinistic? How do we know that other biochemistries aren’t possible?

Silicon-based Life Conceivably, other kinds of biology might be so different from life on Earth that we know neither how to study them nor how to test for them. For example, the abundant element silicon (Si) has chemical properties similar to those of carbon and thus might be an alternative to carbon as a basis for living organisms. Such weird biochemistries might have real advantages, implying that silicon-based life might be selected for survival in odd nooks and crannies on our planet, or especially in alien environments on extraterrestrial bodies.

Why, then, are there no silicon-based life forms on Earth, especially given that silicon is ~135 times more abundant than carbon on our planet? The answer is that, although silicon would seem to have an advantage in intense heat, carbon prevails within typical environments at or near Earth’s surface. That is, at so-called room temperature (~300 K), carbon bonds to other atoms more strongly, and especially so to other carbon atoms. Specifically, carbon, with its 4 unpaired outer electrons, can form tight chemical bonds by sharing those electrons with other elements. In this way, as shown in Figure 8.22, ¹²carbon atoms can achieve maximum stability by attracting other atoms to each of its 4 sides.

FIGURE 8.22 — The orbital arrangements of electrons for (a) 12carbon and (b) 28silicon.

Likewise, ²⁸silicon is a possible alternative to carbon, yet to see why it’s not as robust as carbon, consider the following. Listed in the same column as carbon in the periodic table of the elements (consult Figure 3.45), silicon also has 4 unpaired electrons in its outermost orbital. Alas, as noted above silicon cannot bond to other atoms as well as carbon can. As shown in Figure 8.22(a), a carbon atom's 4 unpaired electrons normally reside in its second orbital. Since 8 is the maximum number of electrons allowed in the second orbital of any atom, this orbital becomes full and complete when carbon binds with other atoms on 4 sides. For this reason, a carbon chemical bond is among the strongest of all. By contrast, silicon's 4 unpaired electrons normally reside in its third orbital, as shown in Figure 8.22(b). The maximum number of electrons permitted in the third orbital of any atom is 18. Although silicon might normally have atoms bonded to 4 of its sides, just like carbon, the silicon bond isn’t as strong as the carbon bond. That’s because the outer orbital of silicon is unlikely to have a full complement of electrons, even when it's bonded to other atoms. Generally, carbon bonds are twice as strong as silicon bonds.

Of even greater importance, carbon links most strongly to other carbon atoms. This is especially true for diamond, which is made of carbon atoms bonded to one another, as sketched in Figure 8.23. In fact, diamond is the hardest substance known; hardness results from great bond strength. The carbon bond is also unaffected by water, giving it another advantage. Silicon, on the other hand, doesn’t bond as well to other silicon atoms, and not well at all in the presence of many liquids. Chains of silicon are especially unstable in water; they break apart.

FIGURE 8.23 — Diamond (a) is the hardest substance known. It's made of pure carbon atoms, each atom strongly bonded to 4 others (b).

The fact that the carbon-carbon bond is stronger than the silicon-silicon bond, especially when immersed in liquid, is an important reason favoring carbon-based life. Another reason is reluctance on the part of silicon to form double and triple bonds, which normally add great strength to a group of two or more atoms. A third argument favoring carbon-based life is the high cosmic abundance of oxygen. When C chemically reacts with O, the result is CO₂. This is a gas and thus can easily combine with other compounds; in our case, humans exhale CO₂ after inhaled O has reacted with the C in our bodies. When silicon (Si) reacts with O, however, the result is quartz (SiO₂), which is a solid unlikely to interact easily with other compounds. Besides, can you imagine a living creature exhaling quartz bricks each time they take a breath? Thus we shouldn’t be surprised that silicon plays no biochemical role on Earth, despite its large abundance on our planet.

Given the proper conditions on any planet, both carbon-based and silicon-based life might initially form. Other types of life might also emerge—perhaps based on the rare element germanium, which also has 4 electrons in its outer (fourth) orbital. However, carbon-based life would doubtless eventually eradicate all other types of life. Carbon clearly has greater bonding flexibility and strength and can adapt better to changing dry-wet conditions. On chemical grounds, then, carbon is best suited to act as the backbone of the long-chain molecules required for life.

Despite these strong statements, we shouldn't close our minds entirely to weird biochemistries. Some planets may have odd physical conditions that actually favor strange types of life. For example, heat comes to mind as one such property that would perhaps favor silicon chemistry over carbon chemistry. Silicon-oxygen bonds can withstand heat to ~600 K, and silicon-aluminum bonds to nearly 900 K. By contrast, carbon bonding of any type breaks down at such high temperatures, making carbon-based life impossible. This heat-resistant property of silicon is the main reason that silicone compounds are often used as industrial lubricants; even hot machinery runs smoothly with silicon-based grease.

Should silicon-based life arise on a hot planet somewhere in the Galaxy, its flexibility and adaptability would still be severely limited. Simple, primitive types of silicon-based life might well reside on such alien worlds. But based on everything known about chemistry, it’s hard to imagine anything as complex as intelligent life based on silicon.

Non-Water Liquids What about the medium in which life operates? Must it be liquid? And if so, then what’s the best type of liquid to enhance clusters of complex molecules? Must it be water? Answers require us to speculate about the best way for complex organic molecules to move around and interact with one another.

A solid is a poor interaction medium, unless perhaps the solid is pulverized as powder; atoms and molecules within hard solids have little mobility lest they be on the verge of liquefying. Gases are also poor substitutes for liquids; a gas doesn't easily stay put unless restrained by gravity or in a container of some sort. This type of loose reasoning leaves the liquid phase as the most reasonable interaction medium. But do liquids seem best only because we ourselves are partly made of liquids? We again wonder if our conclusion is chauvinistic, and if some other liquid could substitute for water on another planet.

Several arguments do favor water as the most likely liquid medium for life, the best one being that the water molecule is made of two of the most abundant atoms—hydrogen and oxygen. According to our knowledge of cosmic abundances, these atoms are expected to be plentiful everywhere. Another reason favoring water as a preferred medium for life is its widely separated freezing and boiling points; for the conditions typical of Earth’s surface, that range is 100 K, allowing vital biochemical reactions to proceed anywhere between 273 and 373 K (i.e., 0-100^oC). (This range, however, depends on the pressure and is true as stated only for the conditions on Earth's surface.) Yet another unique property of water is its reversal in density just before freezing—that is, while cooling from 277 to 273 K; ice is less dense than liquid water, a statement that is untrue for any other known substance—hence the reason ice floats. If ice were denser than liquid water, it would sink and water would solidify from the bottom up, as for all other chemicals. Collecting at the bottom of a lake or ocean each winter, ice would hardly have a chance to melt each summer. It wouldn’t be long before entire bodies of water, including whole oceans, became solid blocks of ice. Fortunately, water’s peculiar density property prohibits this, ensuring lots of terrestrial liquid in which molecules can freely interact.

Ammonia (NH₃, and therefore made of the common elements hydrogen and nitrogen) is sometimes proposed as a possible liquid medium in which life might develop, at least on a planet cold enough for ammonia to exist in the liquid state. Like water, ammonia is also made of abundant atoms and, in many other respects, resembles water. However, it's not an entirely appropriate replacement for water.

To remain fluid, pure NH₃ must be several tens of degrees colder than H₂O. Ammonia’s liquid range (on Earth) spans about 200 to 240 K, although, as for water, this range is pressure dependent; room temperature, remember, is nearly 300 K. The presence of ammonia might then enable life to prevail on a cold planet where water is normally frozen. But such low temperatures would inevitably cause metabolisms to slow as less energy is available to drive biological reactions. Although admittedly billions of years are available, the molecular interactions in liquid ammonia needed to produce more complex life forms would be a good deal more sluggish than on watery Earth. Furthermore, NH₃ doesn’t have the peculiar density reversal near its freezing point, as for H₂O, so large bodies of NH₃ would likely freeze solid.

Together or separately, these and other alternatives to life as we know it would surely give rise to organisms with radically different biochemistries from that encountered on Earth. Still, scientists have little empirical data about non-carbon, non-water biochemistries, for the very good reason that we have no examples of them to study. Nor is there much incentive to theorize about non-carbon-based, non-water-based life when today’s biochemists themselves are clearly made of ~80% water laced liberally with carbon. For health and medical reasons alone, we tend to study ourselves. Speculation will continue to run amuck about alien life beyond Earth, but currently it remains a subject for which there are no data.