The trouble with silicon…

As alternative biochemistries go, a fair amount of both serious science and science fiction has suggested the idea of silicon based life — life which uses silicon to form its main structures as opposed to the carbon that makes up life on Earth. After all, despite the fact that carbon-based life is the only kind of life we know of, it would be blinkered to assume that no other life is possible. In his book, The Cosmic Connection, Carl Sagan coined the phrase “carbon chauvenism” to describe such assumptions. It’s an interesting concept to think about, largely because of how utterly alien silicon-based life could be!

The basic assumption behind the idea of silicon-based life is simple. Silicon sits right underneath carbon in the periodic table, so it should have similar chemistry. The two certainly do share a few things in common — both can form four bonds, both are reasonably reactive (but not too reactive) and both have fairly extensive chemistries. For instance, both react with hydrogen to form methane (CH4) and silane (SiH4).

Sadly for silicon, there are a lot of ways in which it’s simply not as good as carbon. The biggest difference is that carbon can catenate. In other words, it can form strong chemical bonds with itself, meaning that carbon can form rings, chains and cages in all manner of shapes and sizes. The number of possible structures that carbon can make is theoretically infinite, given enough carbon atoms. Silicon, on the other hand, doesn’t much like to catenate. Silicon-silicon bonds are relatively weak, so silicon chains are unstable. So much so that, at least under Earth-like conditions, it’s extremely difficult to form any silicon chains at all. Let alone rings or anything else. Disilane will actually combust spontaneously in air. Water too, will break it apart (forming silicic acids). Also, as I found out previously, silicon isn’t even too fond of bonding with carbon.

Even considering the possiblity of siloxanes (chains made up of silicon and oxygen), there’s one other big fact standing in the way of silicon biochemistry. Earth’s crust is full of silicon. Oxygen too. Actually, Earth is rather odd in the Universe, in that it contains much more silicon than carbon. Given that life chose carbon anyway and mostly ignored silicon is surely an indicator that either silicon is unsuitable, or that carbon simply works better.

So it’s unlikely we’ll find anything like the Horta living on any of these new planets being discovered. Sorry Trek fans.

It’s worth mentioning though, that while Earth life isn’t actually based on silicon, some life does use it. Diatoms create their tiny but beautiful shells out of silica. Even you and I contain silica.

But carbon’s the bit that counts…

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
This entry was posted in astrobiology, chemistry, Imported from Livejournal. Bookmark the permalink.

24 Responses to The trouble with silicon…

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  4. invaderxan says:

    Very good point! I totally forgot to mention double bonds. Silicon seemingly can form π-bonds, and even take part in aromatic systems. But it really rather wouldn’t if it had any choice in the matter. :)
    And thank you. Glad you liked it!

  5. Anonymous says:

    Silicon also really does not like to form double or triple bonds, since it costs less energy to hybridize its 3s and 3p orbitals into 4 sp3 orbitals. No double bonds = no aromatic rings = most of the structural variety present in living molecules aren’t happening (hormones, many amino acids, etc etc etc).
    Cool post!
    Joel (infiniflux.blogspot.com)

  6. ryttu3k says:

    The other quids are cooler ;)
    I had a look on eBay (cannot use Amazon, the shipping is EVIL!), and it only had one copy in the States :( And I don’t have PayPal, so…

  7. invaderxan says:

    $80!? Holy crap! :6
    I got my copy second hand from Amazon. I think it cost me about a quid.
    (That’s a quid as in £1, not… you know…)
    :P

  8. ryttu3k says:

    Pretty much! XD
    I’ve still yet to find an actual copy of Cosmic Connection, though :( I’ve even asked the library about buying it off them, but they want eighty dollars for it!

  9. invaderxan says:

    Re: Astronomy Link List
    Cool. :)

  10. invaderxan says:

    So basically, you went on a Sagan quest! :D

  11. invaderxan says:

    This is true — so I gave you a bit more detail. :)
    (I hope, not too much!)

  12. invaderxan says:

    Well, lower temperatures effectively increase kinetic stability — less kinetic energy means reactions slow or even stop. If there’s a certain activation energy to react before a reaction can occur, then a low temperature means there’s less chance of it actually happening (assuming a normal distribution of energies in a population of molecules).
    In a total absence of oxygen, I’m not sure (without looking it up) if there are other compounds which could break up silicon chains. I suspect carbon or nitrogen might be able to though. Silicon’s quite electropositive, which is why its bonds are weak.
    The biggest problem in all this is that the best place for active chemistry or biochemistry to happen is in a solvent (water, in the case of Earth life). Most solvents would break silicon chains pretty easily…
    Hope that all makes sense!

  13. davidnm says:

    …And I’ve just noticed that got there first with a very similar comment. Drat – teaches me to look before posting, I suppose!

  14. davidnm says:

    So out of curiosity, what would the silicon-chain situation be like under cooler temperatures? Or in the absence of oxygen? (Although I suppose those create a host of problems all of their own.)

  15. Anonymous says:

    Astronomy Link List
    This article has been added to the Astronomy Link List.

  16. ryttu3k says:

    :p
    Some of them were pretty easy to get. Broca’s Brain and Billions & Billions I got with a voucher, Dragons of Eden was four bucks on eBay, Shadows of Forgotten Ancestors was $12 in a second-hand shop… Demon-Haunted World I did have to have ordered in from the local bookshop XD Contact and Varieties of Scientific Experience I got in the US (San Francisco Borders <3 ), I can't actually remember where I got Pale Blue Dot from, and Cosmos… heh. I had looked for it everywhere, yeah? Guess where I found it? The bookshelf upstairs -_- Mum forgot we had it!

  17. invaderxan says:

    Wow… That’s a seriously impressive collection!

  18. invaderxan says:

    On Titan and Triton? That’s interesting… Perhaps it’s something to do with a lower temperature and a lack of free oxygen. Personally though, I’d have thought all the water ice on both those planets would be a major hindrance to silicon based life.

  19. ryttu3k says:

    I own the Cosmos DVD boxset and… *counts* eight books. Cosmos, Pale Blue Dot, Demon-Haunted World, Billions & Billions, Broca’s Brain, Dragons of Eden, Shadows of Forgotten Ancestors, and The Varieties of Scientific Experience. Oh! And Contact! Loooove Contact. The book more than the film.

  20. invaderxan says:

    Everyone should own at least one thing (book/video/whatever) by Sagan. Everyone. :)

  21. I do sometimes wonder about arguments like that. Would a hypothetical silicon-based lifeform be able to deduce the existence of proteins and lipids?
    At any rate, I’ve read arguments that silicon-based lifeforms could work under non-Earth-like conditions. The book I just read, Life As We Do Not Know It, for example, suggested that silicon-based life (at least in microbial form) might be plausible on Titan and conceivably Triton. I can’t remember the arguments used, though.
    Personally, I’m rather fond of shade-of-blue-based lifeforms myself … ;-)

  22. ryttu3k says:

    Aww! It’s episode five – Blues For The Red Planet. But yeah, Cosmic Connection came a good seven years earlier, oops XD (That’s the only Sagan book that I’ve read that I don’t own ;_; )

  23. invaderxan says:

    Oh, I’m open to lifeforms not based on carbon… I’m just not sure about silicon. Don’t forget too — carbon is the fourth most common element in the whole universe. Gives it something of an advantage…
    Y’know, despite reading Cosmic Connection thoroughly, I’ve never actually seen that episode of Cosmos!
    (I know, I know…)

  24. ryttu3k says:

    Plus, carbon chemistry is just more fun XD
    I would be open to lifeforms NOT based on carbon, but I think the majority will be carbon-based. Carbon is just so GOOD at forming complex molecules. (And dude, I watched that episode of Cosmos YESTERDAY XD)

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