Hurrah for the Horta! (Notes on the Possibility of Silicon Based Life)

The Horta: 'The Devil in the Dark'
Star Trek: The Original Series (S1/E25, 1967)

Image: startrek.com

I. C (6)

Carbon - as everybody knows - is the key component of terrestrial life and it's commonly assumed that, if there is life elsewhere in the universe, then it too will be carbon-based.

The reason for this, explains astronomer and popular science writer David Darling, "is not only carbon's ability to form a vast range of large, complicated molecules with itself and other elements, especially hydrogen, oxygen, and nitrogen, but also its unique facility for maintaining the right balance of stability and flexibility in molecular transformations that underlie the dynamic complexity of life".

Nevertheless, this is an assumption and Darling concedes that we may - as carbon-based life-forms ourselves - suffer from what Carl Sagan termed carbon chauvinism; i.e., a form of prejudice that prevents us from seriously considering viable alternatives. And so, whilst it's true that scientists have yet to find anything in the chemistry of other elements that suggests they might be able to give rise to organic compounds, we shouldn't dismiss the idea out of hand.

Indeed, it seems to me perfectly legitimate to consider silicon, for example, as a possible basis of alien life. For not only is silicon a similar element to carbon, but it's also an important constituent of many living cells. In fact, silicon is the great white hope of many astrobiologists and science fiction writers who dream of strange and beautiful possibilities of being ...

II. Si (14)

People began speculating on the suitability of silicon as a basis for life at the end of the 19th century and they have continued to do so to the present day. In 1894, and drawing closely on the ideas of his time, H. G. Wells wrote:

"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."

Over sixty years later, American screen-writer Gene L. Coon conceived of a silicon-based entity called the Horta in an episode of Star Trek.

Basically a living rock, the Horta was both sentient and sensitive - a bit too touchy-feely for me, as a matter of fact - and moved through rock like a hot knife through butter, shitting bricks as it went, thereby solving one of the main problems that would face siliceous life (one of the flaws in silicon's biological credentials is that the oxidation of silicon yields solid waste material that would be difficult - to say the least - for a creature to excrete). 

Sadly, even if silicon has had a part to play in the origin of life, the astronomical evidence suggests it's unlikely we're going to be encountering any silicon-aluminium organisms, or mind-melding with Horta, in the near future. For as Darling notes:

"Wherever astronomers have looked - in meteorites, in comets, in the atmospheres of the giant planets, in the interstellar medium, and in the outer layers of cool stars - they have found molecules of oxidized silicon (silicon dioxide and silicates) but no substances such as silanes or silicones which might be the precursors of a silicon biochemistry."

See: 

David Darling, entry on carbon in his online encyclopedia of science: click here. 

David Darling, entry on silicone-based life in his online encyclopedia of science: click here. 

H. G. Wells, 'Another Basis for Life', Saturday Review, (December 22, 1894), p. 676.