Since the late 19th century, scientists have speculated about what life might look like beyond the Earth. Since carbon-based life-forms here on Earth represent life as we know it, what about critters based on different chemistries? For example, if living things evolved on other worlds based on silicon, ammonia, boron, or nitrogen-alien chemistries suggested by respected space researchers-would we even recognize them as living organisms?
In the 1890s, German astrophysicist Julius Scheiner was the first person to imagine silicon as the basis for extraterrestrial organisms. Such creatures, he speculated, might appear as animated crystals to human eyes. They might also appear as loosely spun threads, like living fiberglass, woven together within a delicate architecture.
In 1893, chemist James Emerson Reynolds gave an unusual lecture at a meeting of the British Association for the Advancement of Science. The lecture was a review of evidence he found that showed that silicon compounds-being resistant to high temperatures-could serve as the basis for life on broiling planets that would quickly destroy Earth-based life.
Science fiction writer H. G. Wells dabbled in the silicon-life idea, too. Also joining Scheiner and Reynolds in the 1890s-a decade fertile in both scientific and sci-fi speculations about exobiology-Wells wrote his classic alien invasion tale "The War of the Worlds".
The year before Wells penned the classic that would inspire Orson Welles' Halloween 1938 radio broadcast and several movies, he wrote a short, science-fact article for the Saturday Review about what he termed "another basis for life".
According to Wells, "One is startled towards fantastic imaginings by such a suggestion: visions of silicon-aluminum organisms. Why not silicon-aluminum 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..."
Beautiful speculation, but is silicon biochemistry really possible?
According to David Darling, whose popular "Internet Encyclopedia of Science" website speculates on this and other outer-space topics, "Silicon does look like a promising organic alternative to carbon... Silicates are analogs of carbonates, silicon chloroform of chloroform, and so on. Both elements form long chains, or polymers, in which they alternate with oxygen. In the simplest case, carbon-oxygen chains yield polyacetal, a plastic used in synthetic fibers, while from a backbone of alternating atoms of silicon and oxygen come polymeric silicones."
However, Darling's reading of mounds of technical literature has uncovered an Achilles Heel.
"Conceivably, some strange life-forms might be built from silicone-like substances were it not for an apparently fatal flaw in silicon's biological credentials," he notes. "This is its powerful affinity for oxygen."
In Earth life, carbon is oxidized as part of the respiratory mechanism-it is converted to a waste product, and greenhouse gas, known as carbon dioxide.
"The oxidation of silicon," Darling adds, "yields a solid because, immediately upon formation, silicon dioxide organizes itself into a lattice in which each silicon atom is surrounded by four oxygens. Disposing of such a substance would pose a major respiratory challenge!"
Does astronomy reveal anything in the universe that might support or refute Julius Scheiner's original hypothesis of silicon-based life? Well, it appears that no complex compounds such as silanes or silicones-the likely basis of silicon life-have ever been detected in space.
But now the whole discussion turns back to Earth: Did silicon play a role in the evolution of life here?
"A curious fact is that terrestrial life-forms utilize exclusively right-handed carbohydrates and left-handed amino acids," according to Darling. "One theory to account for this is that the first prebiotic carbon compounds formed in a pool of primordial soup on a silica surface having a certain handedness."
Ironically, while extraterrestrial life based on silicon chemistry seems unlikely-due to the lack of silane compounds and silicone existing in any appreciable quantities outside of laboratories-the basis of artificial intelligence, now making its first baby steps, is founded on a very high silicon content; it's popularly known as the computer chip.
What's in the Sky: This week, two bright planets are visible around 8 p.m. Venus can be seen setting in the west as ringed Saturn can be seen rising in the east.
Lou Varricchio, M.Sc., is a former NASA science writer. he is a member of the NASA/JPL Solar System Ambassador program in Vermont.