# What would the chemical make-up of a silicon based life-form be like?

I've recently become interested in the possibility of silicon based life. Assume these creatures developed on a planet with zero carbon (don't question, just accept it), so they have no carbon in their bodies. For this question, it would also be helpful to know what kinds of conditions would be conducive towards silicon based life. (I've excluded carbon because I know it is generally more effective for life-forms to evolve with it.)

Now, what would the chemical make-up of these creatures be like? Assume they are nearly like humans, except with silicon. Would they also be made up of lots of water, or would they have ammonia blood? Would they have higher internal body temperatures? Would anything else have to be different?

• It seems that the useful silicon molecules we use (like rubber and greese) contain carbon too, in particular conventional organic side-chains. – JDługosz Jun 5 '16 at 9:35

Based on silicon's properties, I would guess these characteristics of a silicon-based life form:

• silicon compounds resist temperature better than their carbon counterparts so the life form would live in a hot environment or have a high body temperature.

• silicon can combine with itself like carbon, but forms weaker links so the life form might not have good structural integrity. It may not grow to a large size or it would live on a small planet with low gravity or thick atmosphere or in a liquid.

• silicon oxides are stable compounds so the chemical reactions needed for metabolism might go slow. The creature could have a long life or move slow, like turtles.

• silicon oxidizes to the solid silicon dioxide - a life form would have difficulty disposing of a solid so it would not breathe in the same sense as we do.

• silicon-based reaction occurs rapidly, so the life form might produce large amounts of energy quickly, but this might also mean that it would have difficulty controlling its own survival mechanisms.

• I think hat fits really well with my setting. My silicon creatures live on a hot planet and a watery planet, and both have ways to rid themselves of excess energy. – Xandar The Zenon Jan 25 '16 at 1:07
• Point 4 makes me imagine a lifeform that breathes gas and excretes solids rather than exhaling... eww. – evilscary Jan 25 '16 at 16:33

For nearly any organic molecule you can make a Silicon analogue. The bond is a bit weaker and the properties cann be very different. Take $CO_2$ and $SiO2$ one is a colorless gas trees like to breath and the other is quartz. So we can't have a critter that breathes in $O_2$ and out Quartz. or rather you can and probably should.

Silicones are the silicon versions of hydrocarbons they don't store energy nearly as well but are used to mimic them in prosthetics. I think that your silica men might be softer and more rubbery than they are usually depicted in Science Fiction where they are usually rock people.

The silica Sugar analogue would probably be useless so phosphorous and nitrogen would probably be used to store energy. Maybe this is where the ammonia blood could come in but water would be fine.

• So silly putty is really horta poop?! I'm shocked! – JDługosz Jan 25 '16 at 17:37
• Doesnt rubber melt at high temperatures? If they live on a hot planet where silicon is more likely to be their biochemistry it seems unlikely they would be made of rubber. – Stephanie Mar 5 '16 at 18:30
• Silicone is pretty stable up to about 500f – King-Ink Mar 7 '16 at 16:58

Silicon is a lot less versatile than Carbon. There are no stable double bonds between Silicon atoms, nor ring structures with delocalised electrons. The single silicon to silicon bond is weak compared to the silicon-oxygen bond, hence silicones. There are no long silicon chain silanes.

Despite which, it has been suggested that the first sort of proto-life might have been based on aluminosilicate minerals. Clays. They form a huge variety of nanocrystals and have some ability to catalyse and organise carbon organic molecules.

Over some hundreds of millions of years the ungrateful carbon molecules learned how to get a life without needing the clays. Just possibly, somewhere else evolution took a different path and clay minerals are there an essential component of all living beings.

Note this theory is as speculative as any consideration of the origin of life must be. Only one known example and almost all evidence lost in very deep time.

• Yea, the big differencr is the number of hybrid bond possibilities in carbon. I think Si based life would have some things in common like long chains, but hace a much smaller catalog of things that fit together. – JDługosz Jan 25 '16 at 17:39

Silicon and Carbon have the same bonding order but there are a lot of organic compounds, especially high complexity compounds like amino-acids for which there are no stable Silicon analogues to the Carbon chains. Silicon is too big to form a lot of molecules because it distorts the normal (Carbon based) bond angles between the various components. Silicon-Hydrogen bonds are also weaker than Carbon-Hydrogen so long chain Hydrosilicons undergo more rapid racemization, and because Silicon=Silicon bonds are unstable there are a variety of organic chemical species that it is just plain impossible to form using Silicon instead of Carbon.

Silicon based life may exist but it's probably not going to look anything like the Carbon-Water chemistry we're used to seeing. I would expect to see something like either; a highly doped silicate rock that bore more resemblance to a computer chip in it's operation than an organic brain, or a semi-crystal that undergoes some sort of "electron-ripple" chemistry similar to that seen in artificial chlorophyll where a slight instability is propagated through interlocking chemical chains in the form of a single electron being "flicked" from atom to atom leaving the chain structure intact but transferring charge along it. Either structure would allow complex thought processes.

I've always favoured the version of silicon-based life that is hot but more-or-less human, they have a type II semi-crystalline brain (just because it's more easily reconfigured thus better for having evolution) that works faster the hotter they get. The rest of the nervous system and the functions thereof would be performed by the same sort of system. They have a body temperature that bottoms out in the mid-1500s Celsius but can be much much higher, they may "breath" when they need to cool off but they don't respire in the sense of taking on chemical inputs from the atmosphere. Their muscles are an electro-responsive Silicon polymer, stable at high temperatures but prone to brittleness below their lowest operating temperature. They build up a layer of "dead" material that acts as skin and thickens as they age giving them an appearance similar to coarse volcanic rock. They eat only to expand or to replace decayed transuranic elements that have given up their heat to fuel the silicate thermocouple metabolism, they're not really "alive" as we define it (they fall short of MRS GREN on a couple of counts) but they'll do.

To the best of my knowledge, there two broadly plausible possibilities: one operating at very low temperature, the other at very high temperature.

The somewhat less plausible route is to use silane (and silene/silyne) compounds, in as-direct-as-possible analog to carbon compounds. Silanes are not as stable as carbon chain molecules, and so would need a colder environment. This means you need a cryogenic solvent, as well- water won't do. Silica, the direct analog for carbon dioxide, dissolves well in various organic solvents (which are used to manufacture, e.g., silica aerogels) with low freezing temperatures, but if there are organic solvents around, you'd be much more likely to end up with cryogenic carbon-nitrogen life instead, or maybe a hybrid that occasionally incorporates silane molecules into a primarily carbon-based chemistry. So, yeah, you need to pretty much eliminate carbon altogether, and that means, most likely, using an ammonia-water mixture as the primary biosolvent. (A mixture of water and ammonia has a lower melting point than either liquid alone.) Like organic solvent, ammonia will react with oxygen, which means you can't have much free oxygen in the atmosphere, so these organisms would need to be hydrogen breathers, or use some other alternative metabolic cycle- perhaps one that doesn't require breathing at all, and relies entirely on simple decomposition of energy-rich molecules.

The more plausible option is something based on silicone (-Si-O-) backbones, possibly with organic side-chains. Silicones can make conveniently soft and flexible molecules at human-comfortable temperatures (as evidenced by silicone baking ware), but they withstand high temperatures quite well (again, as evidences by silicone baking ware...) and become more versatile at higher temperatures. In this case, there is a reasonable geophysical scenario that would predispose the development of this sort of life over purely carbon-based: a world with large quantities of sulfuric acid, which decomposes hydrocarbons. So, you have a world somewhere between Io and Venus, which gets much hotter than our world, resulting in a loss of water and hydrogen, and concentration of sulfuric acid. Not as small and cold as Io, and not quite as hot as Venus, because you ideally do want to retain some surface liquid, not just clouds. Conveniently, sulfuric acid also reacts with chloride and fluoride salts, producing hydrochloric and hydrofluoric acids, and hydrfluoric acid will dissolve silica, which would serve to make it bioavailable and provide a means of disposing of metabolic wastes.

So, you have a mixture of highly concentrated sulfuric acid with a little water and small quantities of hydrochloric and hydrofluoric acid as the biosolvent, operating somewhere between 50 and 300 degrees Celsius. You don't need to eliminate carbon from the environment--the solvent choice makes the evolution of purely carbon-based life impossible anyway--and you don't want to, because carbon will be useful to incorporate into side-chains and functional groups, just like we incorporate nitrogen into lots of our own "carbon-based" biomolecules. The solvent won't react with oxygen, so there can be plenty of free oxygen (and possibly some chlorine and fluorine) in the air as well, although there may be little motivation for purely oxygenic biosynthesis (whether photosynthesis, chemosynthesis, or what have you); you could instead end up with a mixture of free oxygen and a bunch of sulfur trioxide in the air, either of which could be used as oxidizing agents for animal metabolism. (In our environment, sulfur trioxide is a solid, but it's a gas above 45 degrees Celsius.)

1. I think A silicon based life form would...
• live in a hot enviroment since silicon compounds are too stable in a not that hot enviroment. (around 1100 degrees, approximately the melting point of silicon sulfite, wich it disposes like we dispose co2)
• live inside planets since only there is enough silicon and heat.
• consume sulfur instead of oxygen, since there no oxygen available inside planets (oxygen most likely only as oxide).
• dispose liquid sulfur dioxide.
• have "bones" out of some silicon based crystal, if any.
• swim in magma, since burrowing through stone would be a huge energy effort.
• get petrified when taken out of magma since most silicon compounds are minerals.
• never be noticed by humans even if it lived in earth since they can't isolate magma and keep it hot. (they would noever notice that some rocks are petrified lifeforms)

I too am interested in the chemistry of Life As We Do Not Know it. Silicon is one of my favorites since I read Hal Clement's Iceworld and later when I saw the "Devil in The Dark" episode of Star Trek with the Horta. There is a lot of skepticism about silicon as a basis for life but here is a relatively recent sci-fi story I published in the net that tries to deal with the objections and even gives a sketch of a hypothetical 'biochemistry' of a higher temperature silicon life form. Learning Curve

• Hey Dan. Can you excerpt the relevant bits from your story or post a synopsis, in case the link goes dead? Then your answer does not depend on the link. – Willk Jan 22 at 17:55