Are silicon critters “…as much a staple of [science fiction] as barroom brawls and pistol duels are to westerns.”? (Peter Ward in Life As We Do Not Know It , Penguin Books, 2007)
Not quite, but no wonder silicon critters are so popular in science fiction—it’s an amazing element. In rummaging through my file cabinets this week, I found some intriguing articles published in the 1980’s by Science magazine.(Example: Science vol. 225, number 4667, p.1114, 14 Sept 1984 by Robert West, University of Wisconsin) After 70 years of trying, several stable disilenes with double Si=Si bonds were finally synthesized. They were seen to have structure and chemical reactions very unlike their carbon relatives. Some were heat resistant, and some reacted to make unusual types of molecules.
What we have learned since the ‘50’s suggests that there is no use looking for silicon monsters in this galaxy’s suns’ Goldilocks Zones. Temperatures warm enough to provide liquid water as a solvent would doom complex organic silicon molecules. In liquid nitrogen, or in the ethane and methane surface lakes of Titan, or somewhere on Triton, they might do fine, according to Peter Ward. They would be mighty slow, however.
How would we recognize such slow life? How would it change its environment? Could it evolve from a simple microbial form to something more complex?
Silicon microbes might do better than their carbon cousins at panspermia attempts, but only if they could escape the gravity of their home planets (which is not probable according to Bob Pappalardo.) Perhaps comets could be the delivery system for silane life between Triton and Titan.
Electrons could be used as sources of energy in silicon critters, unlike the proton shuffle carbon life uses. Ward suggests there might be enough light on these distant moons to produce electronic effects, possibly something parallel to photosynthesis.
Silicon rich clays are another possibility for giving silicon-based life a chance. Clays can have three planes—Si-O, Al-O, and OH. SiOH could be food. A.G. Caerus-Smith proposed that the first life on Earth might have been formed with a silicon-based clay.
The most likely use for silicon by extraterrestrials would be its use as a silicate structure (in a cold environment with no oxygen), enclosing or supporting a carbon-based metabolism, like Earth’s diatoms.
Here, carbon has outdone silicon in the Metabolism and Life Complexity Contest, even though silicon is 925 times more abundant on Earth than carbon. Carbon wins because of the nature of the bonds it forms. Natural selection has favored those elements that bond easier, are the most flexible, and are the most stable in liquid water. Water is the solvent of choice for life when it’s available. (Another topic for another day.)
To round out the pessimistic view, silicon is rare in the interstellar medium. Its abundance on Earth is found in silicates Si(OH)4, rocks and minerals that are liquid only at high temperatures and need high temperatures to react. If mica (crystal sheets of silica) should become liquid, it could form a scaffold for life, but how would such organisms metabolize and reproduce?
Enough for now. In the second article on silicon to follow shortly, we’ll consider low temperature possibilities for silicon life, define the various forms silicon can take, and explore its amazingly useful properties for us humans.
Here is some fiction that features silicon critters: Books: James White-Sector General, T. Pratchett-Discworld, Fred Hoyle-Black Cloud, A.D.Foster-Sentenced to Prison, A.C.Clarke-CrusadeTV: Star Trek-“The Devil in the Dark” and “The Disease,” Star wars Expanded Universe series, “Space Slug and Myrock,” X-files-“Firewalker” episode, Stargate Atlantic-“Remnant,” petrosapian species in “Ben 10”?Movies: “The Monolith Monsters (1957), Island of Terror (1966).