Tuesday, November 06, 2007

Is there more silicon-based life in the universe than in Katie Price's bra?

It's been a staple of science fiction for years; the idea that life could be based upon silicon rather than carbon, as all life on Earth is. Think of Terry Pratchett's rock-eating trolls, the planet-wide life form in Arthur C Clarke's Crusade, the alien in the Alien Trilogy, and the crystalline entity and the Horta of Star Trek's Silicon Avatar and The Devil in the Dark episodes. Silicon-based aliens are everywhere. But why do sci-fi writers pick on silicon as opposed to other elements like helium or mercury or calcium? It's basically because silicon is the closest thing to carbon in terms of structure.In this post, I'll try to explain the science behind the concept of a silicon-based organism. It'll be as plain English as I can make it (I'm a writer and artist - not a scientist).

Star Trek's 'Crystalline entity'
It's interesting to speculate what a silicon-based life form would be like. It's more interesting to ask why there are none on Earth (as far as we know) as silicon is far more abundant than carbon here. At first glance, silicon looks like a good basis for an alternative biology. Carbon and silicon both have something that chemists and physicists call a valence of four. That means that individual silicon atoms can make four bonds with other elements to form chemical compounds; like a jigsaw puzzle piece that has four sides and can therefore connect with four other pieces. Carbon can form over 10 million different compounds with other elements, among them many that are essential for life as we know it, such as sugars, celluloses, chitins, alcohols, fats, antibiotics, amino acids and proteins. Silicon can form almost as many, although different, compounds.
Star Trek's silicon-based Horta

Secondly, both silicon and carbon can bond with oxygen. Not all elements can. And, as far as we know at this time, nearly all living organisms need to process oxygen.

Thirdly, by using oxygen as a kind of 'glue', both silicon and carbon atoms can form long chains called polymers. Two examples of this are the carbon-based poly-acetal, a kind of plastic, and silicon-based polymeric silicones, which we use for waterproofing and lubrication. It is carbon's ability to form long complex chains that led to the formation of DNA ... and it is entirely conceivable that silicon could evolve something similar. However, while there are marked similarites between the two elements (which is why they appear so close together on the Periodic Table), there are some major differences too.

A silicon-based Lithovore by Terence Dickinson and Adolf Schaller

You'll have seen that an atom is often represented in books and articles as looking something like a mini planetary system with a central nucleus surrounded by circling electrons (although in newer reference works the orbiting electrons are sometimes represented as a cloud around the central nucleus). Well, with the carbon atom there are six electrons whizzing around a nucleus made of six protons and six neutrons (hence carbon's Atomic Number of 6). Silicon has 14 electrons circling a nucleus made of 14 protons and 14 neutrons (Atomic Number 14). What this means is that the 'cloud' of electrons around the silicon nucleus is bigger and, therefore, the forces that bind it all together are weaker. Consequently, the silicon atom does not form as strong bonds with other atoms as carbon does (which is why even the hardest silicon-based rock is not as hard as carbon-based diamond). This alone makes it difficult to imagine a silicon-based molecule the length of DNA forming and remaining stable.


Secondly, the chemistry of life is difficult (although not impossible) to visualise for a silicon-based life form. When carbon unites with oxygen (oxidises) during breathing, it forms carbon dioxide, a gas. We breathe it out and plants breathe it in. However, when silicon oxidises, it forms a solid called silicon dioxide. It's hard to imagine a creature that breathes in oxygen and breathes out something that is essentially sand. There would also be some 'disposal' issues for a silicon-based life form as it would excrete similar silica-based substances (it adds a whole new dimension to the expression 'Shi**ing a brick'!).

Thirdly, where would a silicon-based life form draw its energy from? All living things need a way to collect, store and utilise energy. Once absorbed or ingested, the energy must be released exactly where and when it is needed within the body. Otherwise, all of the energy might liberate its heat at once, incinerating the life-form. In a carbon-based life-form, storage takes the form of carbohydrates (the curse of us fatties). A carbon-based life-form 'burns' this fuel in controlled steps using speed regulators called enzymes. Carbohydrates (the clue is in the name) are carbon-based compounds that oxidise to form water and carbon dioxide, which are then exchanged with the air. Silicon doesn't form many compounds that will duplicate the function of enzymes so it's hard to imagine how a silicon-based living organism could function.
Of course, as we've so often said on this blog, it would be narrow-minded to assume that life can only exist in the so-called 'Goldilocks Zone' of liquid water, oxygen atmosphere and moderate temperature that we do.

The Web, a silicon-based artificially created life form used to trap the spaceship Liberator in Blake's 7

Raymond Dessy, professor of Chemistry at Virginia Polytechnic Institute and State University has this to say:

'It is possible to think of micro- and nano-structures of silicon; solar-powered silicon forms for energy and sight; a silicone fluid that could carry oxidants to contracting muscle-like elements made of other silicones; skeletal materials of silicates; silicone membranes; and even cavities in silicate zeolites that have handedness. Some of these structures even look alive. But the chemistries needed to create a life-form are simply not there. The complex dance of life requires interlocking chains of reactions. And these reactions can only take place within a narrow range of temperatures and pH levels. Given such constraints, carbon can and silicon can't.'

And UK astronomer and science writer David Darling points out that:

'The absence of silicon-based biology, or even silicon-based prebiotic chemicals, is also suggested by astronomical evidence. 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.'

A vacuum 'breathing' silicon life form (artist unknown)

And I'll leave the final words to ATS and their speculative study of the possibility of silicon-based life:

'A silicon-based organism might live on a planet without oxygen. There is no good reason why an organism could not use another gas as a phosphorylation catalyst, like hydrogen, nirogen or other reactive gases. It is also possible that methane might replace phosphor in the metabolic pathway. This would allow totally different matabolisms which might not have the problem of producing a lot of solid waste. A major component of glass is silicon. It might be possible that an organism based on glass exists. An organism like that may get its energy from solar-cell like cells. Zeolites, microporous materials based on silicon, aluminium and oxygen, might play a role in these organisms.'

Then, of course, there's life based on nitrogen, phosphorous, arsenic, ammonia ...


Links:

David Darling
Silicon-Based Life (SCIAM)
Zeolites
Silicon-Based Life (University of Winnipeg)
Wikipedia
Silicon-Based Life (UCLA)

2 comments:

Anonymous said...

Absolutely amazing! I am wondering though, aren't we limiting ourselves by considering only Silicone as a base of life, besides Carbon?

Yes, Silicon is the closes in structure to Carbon, but just because we cannot imagine life based on other elements, it doesn't mean that it doesn't exist.

cheers,
Bruce, Vancouver, BC

Daniel M Mafra said...

People lacks imagination,
that allowd Einstein to discover the space-time delights and Hawking to explore the heart of a black hole.

Silicon out of glass and of silanes, silicone and othe compounds. What paper would heat and eletricity have in such organisms? What is the part that could behave like and eletro-eletronic living machine or some other odd probability, easily denied by science theology and common-sense, difficult to acess through our limited imagination?