When I was in junior high school, I seldom missed Star Trek (the original series) on TV. In one of my favorite episodes the crew of the Enterprise encountered a silicon-based life-form, called a Horta, on the mining planet Janus VI. Even as a teenager I found this idea intriguing. Could life be based on an element like silicon (or perhaps arsenic, as we'll see later) instead of carbon?
Life as we know it on Earth consists of carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur (CHONPS). But could other elements constitute life as we don't know it?
Not merely a discussion topic for science-fiction buffs, this question bears on origin-of-life discussions and on the search for extraterrestrial life. Carbon-based life requires a strict set of conditions. But perhaps life based on an element like silicon can exist under more extreme conditions. Few places in our solar system, and presumably beyond, can conceivably support carbon-based life. But for life built upon silicon, habitable sites may well abound throughout the universe.
However, of the 112 known chemical elements, only carbon possesses sufficiently complex chemical behavior to sustain living systems.1 Carbon readily assembles into stable molecules comprised of individual and fused rings and linear and branched chains. It forms single, double, and triple bonds. Carbon also strongly bonds with itself as well as with oxygen, nitrogen, sulfur, and hydrogen.
Silicon belongs to the same chemical group as carbon and should display similar chemical properties, prompting some astrobiologists to propose that life could be based on this element. But while silicon does form rings and chains, these structures lack the stability and the range of complexity found in carbon-based compounds. Silicon-silicon bonds are much weaker than the corresponding carbon-carbon bonds, and unlike carbon-carbon bonds, they are susceptible to oxidation.2
What about arsenic? Physicist and astrobiologist Paul Davies suggests there may be unusual life-forms that use arsenic instead of phosphorus.3 Phosphorus plays a key role in forming important biomolecules like DNA. Arsenic appears below phosphorus in the same column of the periodic table and displays similar chemical behavior, thus again prompting speculation that life could make use of arsenic.
However, though arsenic and phosphorus share some chemical properties, the two elements display significantly different chemistries as well. Phosphorus is a nonmetal. Arsenic is a metalloid.
Phosphorus reacts with oxygen to form chemical compounds called phosphates. These groups take part in the linkages that constitute the backbone of the DNA molecule by reacting with the sugar, deoxyribose. The sugar-phosphate linkages are described as phosphate esters. Arsenic will also form a compound called arsenate by reacting with oxygen. Arsenates and phosphates display some chemical similarities, but esters formed with arsenates are unstable. They could never be used to construct the backbone of DNA––considered indispensable for life––or another similar compound.
It's laudable for science to boldly go where no man's gone before, but it looks like life based on elements other than CHONPS is truly science fiction.
1 T. W. Graham Solomons, Organic Chemistry, 2nd ed. (New York: Wiley, 1980), 48–49.
2 F. Albert Cotton and Geoffrey Wilkinson, Advanced Inorganic Chemistry, 5th ed. (New York: Wiley, 1988), 234–304.
3 Paul Davies, "Are Aliens Among Us?" Scientific American (December 2007), 63–69.