It may surprise that there is no exact definition of what “life” is - it is often described as a system in thermodynamic disequilibrium with it´s own environment and therefore forced to actively seek, incorporate and transform matter and energy. Part of the acquired energy and matter is used by this system to create copies of itself and so to survive it´s own death.
Some of these properties are however shared also by inorganic entities, like the order, growth or twinning of crystals, and a virus can´t grow or replicate without infecting a living cell.
Mister Spock is well aware of this fact:
“Life as we know it, is universally based on some combination of carbon compounds.”
However when the crew of the Enterprise is contacted in the episode “The Devil in the Dark” by the miners on Janus VI, supposedly attacked by an unstoppable “monster”, soon this fact is questioned. The creature can´t be tracked by the sensors of the Enterprise, nor by the tricorder and seems immune to direct hits with the phasers.
“But what if life exists, based on other element. For instance silicon.”
McCoy disagrees with Spock´s deduction, noting that it is a physiological impossibility, however Kirk agrees as it may improbable, but not impossible. Already in 1891 the German astrophysicist Julius Schreiner argued that silicon (Si) could replace carbon in molecules supporting complex chemical reactions and therefore a hypothetical metabolism. Three years later science-fiction author H.G. Wells speculated in an article of popular science about such life forms:
“One is startled towards fantastic imaginings by such a suggestion: visions of silicon - aluminium organisms – why not silicon-aluminium 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.”
In theory, silicon could form polymers and complex compounds together with metals or elements like boron, nitrogen, phosphorus and sulfur (in fact almost all known minerals involve silicon). Such molecules could work also in other liquids than water, liquids like liquid nitrogen, methane, ammonium, various alcohols and even acid solutions.
The Horta, as the mysterious native life form of Janus VI refers to itself, defends itself and digs tunnels in solid rock using a strong acid. It´s curious to note that certain acids, like sulfuric and hydrocyanic acid could act as solutions for an extraterrestrial metabolism.
However silicon as replacement to carbon has also it´s disadvantages. Complex silicon compounds are unstable and ineffective under terrestrial conditions. McCoy notes this, claiming that silicon life would not survive in the artificial (?) human environment created on Janus VI. But as it seems the creature called Horta comes from within the planet, where may suitable conditions exist. Silicon life forms may could exist in oxygen-free or poor (as oxygen would oxides the silicon molecules), dry and very cold environments. Such for humans highly prohibitive conditions would also reduce the metabolism and reproduction rate of this hypothetical life form, may we would not even realize that it is alive. In the Star Trek episode the Horta is the last of it´s kind, only the eggs – appearing to us like inanimate silicon-concretions - survive, however needing thousands of years to develop and be finally ready to hatch.
One of the great strengths of Star Trek was to pose both ethic as intriguing scientific questions, may some will be answered by future generations, when they one day may really encounter life, but not as we know it ... I find that a fascinating vision for the future!
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