[extracrunchy]

hey guys, i have to write a paper about this and i needed some ideas... the question was: "why is life on earth based on carbon and not on silicon", and what kind of planet/environment could silicon-based lifeforms exist in ??

now, i know both carbon and silicon have a valence of four and that carbon is one of the most versatile atom.. it combines readily and easily with most other elements and the isomers have something to do with it being so vital for life ...

but i cant find any specific details ... why carbon and why not silicon

and what kind of aliens would silicon make

[agrobert]

It depends how you want to answer this question.

From a chemical standpoint you are right both Si and C are tetravalent and form many similar bonds.  Si however can not form long chains bonding to itself. Long hydrocarbons are essential to life and include many fatty acids and hydrophobic molecules.  Also Si forms bonds and complexes with oxygen readily.  Meaning that the energy or bond overlap between Si and O is favored.  Thus any Si polymers would interact with water and alcohols.  While this is a generalization of Si's attributes you may find a more extensive answer in further research.

As for Silicon aliens?  I think this is up to your imagination.  I don't know? maybe sandy? implants?

[Rabn]

You know I've considered this before.  One approach I thought about using was to try and find conditions: temp mainly; under which silicon would form bond energies that are similar to carbon. For instance compare Gibbs free energy of formation of CO₂ and SiO₂.  Then find a set of condition under which they are equal.  I never actually sat down and calculated it, but I thought it would be interesting. You would then need to do another comparison...glucose would be a pertinent choice...then using the conditions found previously, see how similar a molecule of Si₆H₁₂O₆ is to a molecule of glucose at standard conditions in terms of free energy of formation. Obviously free energy is only part of the overall schema but I thought it would be an interesting approach. You may find that you need an absurd temp...for instance millions of Kelvin or maybe only a few to allow the reactions to be equal. 

[Arkcon]

I used to read about silicon life when I was in grammar school.  Beyond the common complaint, the Si doesn't form double bonds as readily as C, there's also the issue of comparison with Earth's carbon cycle.  It's easy for plants to fix carbon from CO₂, and for animals to release CO₂ after they've extracted energy from more complex carbon skeletons, but SiO₂ is a solid, or at least, exists as a sol or gel, that adds particular physiological complications to the lifeforms, that you'll have to work in.  Which would be cool to read about -- but this is really science fiction, not science.

[DrCMS]

Si however can not form long chains bonding to itself.

That is rubbish!  Si_nH_2n+2 are only known up to 9 and are spontaneously flammable in air but Si_nR_2n+2

where R = Me chain to ~3000 and cyclic rings of up to 40 Si's are known 
and if you change to R = n-hexyl chains to ~37000 are known.

[Arkcon]

That is rubbish! 

One can argue that the entire thread is rubbish, because it's science fiction.  In fact, that observation usually shows up in any sci-fi thread on these boards, sooner rather than later.  However, as I heard it, silicon is more likely to form a network, than a chain.  It's better at making rocks, than polymers.

[P]

That is rubbish!  Si_nH_2n+2 are only known up to 9 and are spontaneously flammable in air but Si_nR_2n+2

where R = Me chain to ~3000 and cyclic rings of up to 40 Si's are known 
and if you change to R = n-hexyl chains to ~37000 are known.

Yes - I think you'd need something like a Wurtz synthesis to do it (Hot toluene and molten sodium catalyst!).  But they do not occur 'naturally' on earth.   

[Rabn]

Since this is fantasy...who says that you need to get silicon to act like carbon. Maybe the fact silicon forms networks instead of polymers means that silicon life could already exist.  Who the heck are we to say that large pieces of rock don't act like neural networks?  I wonder what a large mountain thinks, let's speculate: "hmmm, I wonder what I'm going to do today...think I'll just sit around, no sense in moving.  Oh damn, it's raining on the leeward side again...I hate the rain.  It gives me leprasy. Really, seriously, you humans have it made.  Imagine if half your skin would constantly be over moisturized and the other dry as bone. You should talk to my brother near the sahara desert.  One side of him is tropical and mildewy and the other is hot and dry....no wonder he can't get married.  Oh and the Appalachians, who wouldn't want to be one of them, temperate all around."

[Borek]

hmmm, I wonder what I'm going to do today...

Today? They don't think that fast, more like "What I am going to do this eon... Perhaps I will strech a little bit... (earthquakes rolling) Hmm, Atlantic really grows fast, I remember him being just a small crack. I wonder where all these dinosaurs have gone, they were all around moments ago."

According to Prattchet they can think fast only when cooled close to absolute zero

[Arkcon]

Oh and the Appalachians, who wouldn't want to be one of them, temperate all around."

Yeah, it's nice to be them.  But only because they've been beaten down by erosion since they uplifted.

Yes - I think you'd need something like a Wurtz synthesis to do it (Hot toluene and molten sodium catalyst!).  But they do not occur 'naturally' on earth.   

Yeah, the star for a silicon life world would have to be much more powerful than our little yellow sun.  But then, an astrophysicist will tell you, more energy output means a shorter lifespan, and life needs time to develop the trial and error process of organization.

[Valdorod]

Anyone seen the movie Evolution with David Duchovny and Julianne Moore.  If I recall correctly it is about silicon based life forms, or maybe nitrogen based, anyways, they figured that since arsenic is posinous to humans and selenium is right next to selenium then selenium must be poisonous to the aliens.  They end up killing the alliens with head and shoulders. 

Valdo

[Arkcon]

Anyone seen the movie Evolution with David Duchovny and Julianne Moore. 

Naw, didn't rush out and see that one.  Probably wouldn't have liked it, if that's the science in the film.  I'm pretty generous with suspension of disbelief with most films, and I do like my sci-fi, but I need ... somethin'.

[shelanachium]

There is a hypothesis due to Cairns-Smith that life actually began as self-replicating clay crystals, which are silicates. These then associated with organic molecules and the organics finally took over ('Seven Clues to the Origin of Life').

Because it is likely that the origin of life involved processes of low probability, we need material that is exceedingly abundant to give it a chance of starting. If you need a 'miracle' give it as much space, time and matter as possible to happen in! The elements most vital to life, H,C,N,O are also the most abundant reactive elements in the Universe, and there are vast quantities of compounds of C,N and O with H in Space. These compounds readily form more complex molecules which can be precursors to living matter.

Si, whilst abundant, has so high an affinity for O that it is all converted to silicates; such substances as SiH4 which might be precursors to more complex molecules do not form. So it is unlikely that life-forms based on polysilanes rather than hydrocarbons could form either.

Boron, which has an exceedingly rich chemistry, has also been proposed as a basis for life, but it is an exceedingly rare element in the Universe, and like Si also has a very high affinity for oxygen.

If life based on boron ever turned up, it would be proof of extraterrestrial intelligence, since it could never form naturally. There are no vast quantities of boranes out there in which it could start. There are, however, vast quantities of hydrocarbons - and silicates, relevant if Cairns-Smith is right!

[tasmodevil44]

The way I understand it, carbon has weaker bonds with other atoms in an organic molecule than the stronger bonds of inorganic silicon compounds.This is necessary for life to exist, because the bonds of organic compounds must be easily broken and reunited again very readily: .. if the chemistry of life can easily occur.Because inorganic silicon bonds are a lot stronger and tougher to break, more energy definitely must be involved in silicon chemistry.This implies higher temperatures.The planet in which such life form exists would definitely have to be in an orbit much closer to the parent star. If farther away from star, not feasible at all.That is, even if speculation about silicon-based life forms being even possible in the first place according to the strictly defined limits the rules of physics such chemistry would have to abide by.

[tasmodevil44]

And then there's the liquid medium substitution and compatibility problem with silicon-based life forms. We are talking about good ol' fashioned H2O here. It is perfect for all sorts of organic chemistry like what occurs in living cells. And it exists in the liquid state at relatively moderate temperatures that are neither too cold (freezing) or too hot (boiling). Planet Earth is a unique planet covered with vast oceans of it.
     
      But like I stated before in my previous post on this subject,silicon forms stronger bonds than carbon that are harder to break. This implies higher working temperatures for silicon-based chemistry and life forms.

      However,water can't exist in the liquid state if the temperature is too high. This implies some other high temperature liquid medium with a much higher boiling point than water being substituted as the liquid medium for silicon chemistry. And it would have to be compatible with silicon chemistry in a somewhat similar way water is to organic chemistry.

      But what? Seas of molten sodium? Hot liquid lead? Such far-out, sci-fi concepts for silicon-based life forms do not even seem very probable.