Silicon And Life

As someone interested in science fiction as well as regular science, I tend to watch the news for articles about life on other planets, especially outside our solar system.  The most recent news from the rovers exploring Mars, especially Curiosity, is the definite evidence that water existed on Mars in the distant past because it left behind stream beds that could only have been caused by running water.  But outside our solar system, we’ve only just begun to examine other planets, and there’s certainly no evidence for life on any of those yet.  I’m waiting for astronomers to get an image of an extra-solar planet good enough to be able to identify chlorophyll in its reflected light by spectroscopic examination.  That would be astounding, to say the least.

But today I’m more interested in discussing a somewhat more prosaic substance that will almost certainly be found on far distant worlds, namely, silicon.  Silicon is the second most common element on earth, behind the ubiquitous oxygen.  Sand is mostly silicon dioxide, and glass is made from sand, and pure silicon–as most of us are aware–is used in large quantities in the semiconductor industry.  Man-made silicone is used in insulators and adhesives.  But in spite of this superabundance, silicon has rarely been found in living organisms.  Some have suggested that life forms on other planets may have used silicon as the chief structural molecule of life, instead of carbon which is really what makes life possible here.  I suppose that could be true, but considering the almost total lack of the use of silicon by living organisms here on Earth, I wonder if living forms on other planets could really use that element in any substantial way.  As far as I’m aware, the only organisms that use silicon on Earth are diatoms, and they use it as silicon dioxide to manufacture a shell which encloses the soft body of the living microorganism.  The shell consists of two halves, one slightly larger than the other, which fit together like the two halves of a petri dish.  This forms a solid “shell” to protect the unicellular organism.  If you’ve heard of “diatomaceous earth,” that’s a powder composed largely of the empty shells of diatoms, so it contains a heavy proportion of silicon dioxide.

So, if silicon is so much a part of our earth, why don’t living organisms utilize it more?  Not being a chemist I can’t answer that directly, but it may have to do with the tremendous insolubility of silicon dioxide in water.  Think of it, if silicon dioxide were soluble in water to almost any extent at all, by now, billions of years after they were formed, all the beaches in the world would have dissolved into the seas and oceans and lakes and rivers.  Yet the sand remains.  Diatoms must have an extremely difficult time extracting it from the water.

Still, silicon and its dioxide are so far outside the sphere of living things, it seems unlikely that it could ever be used by extra-terrestrial life forms.  Let us assume a hypothetical planet composed of large quantities of sand (the “Dune” planet comes to mind).  On such a planet, siliconaceous beings could possibly arise, but they might be so brittle and delicate they could be destroyed with the mere whack of a rock.  Or even a hammer.  Still, stranger things have happened, so it remains within the sphere of possibility, though I feel it’s unlikely.  It will be a long time before we land on another planet outside our solar system, so all we can do now is hypothesize.

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