Before we talk about other properties of silicon that stimulate both imagination and technology, we had better review the chemical jargon.
1) Silanes: Silicon can react with four hydrogen atoms to form silane, SiH4. At Earth temperatures, it is a gas and combusts in air. Silanes can form chains, but Si-H and Si-Si bonds need very low temperatures to be stable. Silanes also react with water, their long chains decomposing.
The carbon equivalent to silane is methane. Carbon reacts with oxygen, nitrogen, sulfur, phosphorus, iron, magnesium and zinc. It can form polycyclic aromatic hydrocarbons that form complex organics and eventually generate amino acids and nucleotides (thus DNA, genes and us). Silicon is not so reactive. It forms only a few monotonous molecules, since its bonds are large and form double bonds with difficulty.
2) Silicones are chains of silicon and oxygen. They are more stable than hydrocarbons in a sulfuric acid environment. Swell. Si-O reacts with very few other chemicals.
3) Silicates (SO2, sand) are insoluble solids where water is liquid. At high temperatures they become liquid. Silicates are old friends to us on Earth—our rocks and minerals. Si(OH)4 forms the outer crust of rocky planets (90% of Earth’s), and the mantle over the iron core of planetary moons. Silicon takes up oxygen quicker than carbon to form this inert mineral that is so stable. As such, it needs very high temperatures to react.
Life forms might be possible if silicon critters could breathe oxygen and exhale sand. At least, they could form backbones at low temperatures. The bad news–no complex polymers of silicon have been found in meteorites.
However, the element silicon is a treasure to us humans because of its unusual features: 1)Polysilanes form long chains with two side groups on each silicon atom with smeared electrons giving them unusual nonlinear optical properties. 2) Others are so sensitive to light that they are interesting to semiconductor manufacturers. 3) Polysilanes were thought to produce strong silicon carbide fibers.
4) Organo-silicon compounds can mediate organo synthesis, forming C-C bonds under controlled ionic conditions and making specific isomers.
5) Silicon-hydrogen bonds are sensitive to acid and bases, and hydrosilanes can oxidize explosively in air.
6) Chains of silicon and –CH3 are quite stable.
7) Single crystals of silicon have a boxy structure that is harder than most metals and strong when repeatedly tensed and compressed, i.e. it can’t wear apart like metal.
8) Silicon can also act as a heat sink and pressure sensor. In 1953 silicon was first used to make very small electronic components, using micromachinery and chemical etching procedures. As a
semiconductor, it is unusual in that it can be oxidized to silicon dioxide when exposed to steam. Such surface layers protect silicon during batch fabrication.
These features have served us well over the years, but they also tease our imagination, wondering how a live silicon being might find advantage with them.
To recap silicon’s life possibilities: Triton is seen by some as the most possible environment for silicon life. If made of polysilanes, life would require intense low temperatures, no liquid water, high pressure, no reactive oxygen, little carbon, and methane or methanol as solvent. These conditions could be present on Triton. There, oxygen is frozen as CO or O2 so would not be dangerously reactive. Ultraviolet radiation of icy silicate grains could exist but would not be mobile. In liquid nitrogen, polysilanes could form a backbone for life in icy inclusions or fissures. Life would likely be biofilms of chemoheterotrophs that might support consumers. Methane would be a problem as a solvent, since its carbon would compete with silicon in making working organics. Methane or methanol solvents would work for life only under high pressure.
On Titan, silicates would not work, but subsurface pools of ethane and methane could house life with membranes of silanes under reducing conditions at low temperatures. The surface ice would house granular organics, if any.
Silicon life on Venus would be possible if it evolved separately from carbon-based life. At high temperatures Si-O is stable, as are silicates, though life would have to tolerate liquid magma. No life
forms have been found in Earth lava yet. In spite of the bad news, I keep wondering if the hydrosilane defense organs (HSDOs) I designed for my critters in The Crystal Diadem (the basis for two musicals, so far) could be made to work. In any case, silicon chemistry will continue to be great fodder for the imagination.