How would the Horta actually work?
June 21, 2010 1:28 PM   Subscribe

"Silicon based life" is a perennial sci-fi trope - I guess because silicon and carbon share similar-ish valency properties. Has anyone actually tried to flesh out what a silicon chemistry complex enough to support protein analogues would look like? Or more broadly, has anyone proposed schemata for complex, self-replicating chemistries that differ substantially from those we know?
posted by nicolas léonard sadi carnot to Science & Nature (16 answers total) 15 users marked this as a favorite
Response by poster: Having JUST posted, I stumbled across this amazing document, which is absolutely on point. Am still keen for further input, though!
posted by nicolas léonard sadi carnot at 1:31 PM on June 21, 2010 [2 favorites]

Not silicon, but as mentioned in this previous post, some Nasa scientists are speculating about Methane based life on Titan after recent Cassini findings.
posted by chrisulonic at 2:01 PM on June 21, 2010 [1 favorite]

That's a great find, M. Carnot. Would make a good FPP.

Also see here -
While carbon and silicon can theoretically form very similar kinds of structures, complicated carbon-based molecules tend to the stable, while complicated silicon-based molecules tend to fall apart (especially in water)....
if a silicon-based biochemistry exists, it probably doesn't use silicon the way we use carbon, and we might even have a difficult time recognizing it as life.
posted by Electric Dragon at 2:22 PM on June 21, 2010

One of the things that makes carbon-based life possible is the fact that carbon binds to itself to form chains, and these chains are both strong and versatile. The properties that make carbon so good at forming chains with itself are due in large part to its place on the periodic table. Notice that silicon is right there underneath it. That means that silicon has the same valence electron configuration as carbon, which means it shares many of the same binding characteristics. The problem is that silicon is bigger, heavier, and forms weaker bonds, both with itself and with some of the other important elements for life (e.g., oxygen and nitrogen). It can also form more than the standard four bonds, which presumably complicates matters. As I understand it, the current thought is that these factors make it unlikely that life, at least as we understand it, could arise from a silicon source.
posted by dephlogisticated at 2:32 PM on June 21, 2010 [1 favorite]

Also worth noting that methane-based life does not mean life made out of methane. Methane is just CH4—that is, a carbon and four hydrogens, bonded to nothing else. I think what they're talking about is life either using methane as fuel or producing it as a biproduct.
posted by dephlogisticated at 2:37 PM on June 21, 2010

Response by poster: Silicon-oxygen links are pretty stable, I think - I mean, they're the basis for most silicate minerals - so they could be a potential backbone, or structural mesh, at least. I definitely agree that any self-replicating structures constructed on such a basis would be exceptionally foreign, though.
posted by nicolas léonard sadi carnot at 2:41 PM on June 21, 2010

Response by poster: The NASA thing was about methane as an alternate solvent to water, which seems kind of viable - at least you're not moving away from the standard carbon model of life.
posted by nicolas léonard sadi carnot at 2:43 PM on June 21, 2010

Yeah, I think most people who have proposed this have considered silicones--alternating silicon-oxygen backbones--rather that pure silicon chains. There are plenty of stable silicone polymers out there.
posted by mr_roboto at 2:51 PM on June 21, 2010

Best answer: Hi! Graduate chemistry student who works with silicon here. This question excites me to no end, so I hope I don't geek up my answer too much.

Silicon-based life would have a very hard time of it, for these reasons:

1. Silicon is less electronegative than hydrogen, whereas C is more electronegative (Si is 1.9 on the Pauling scale, C 2.5 and H 2.1). This means nucleophilic attack (a common step in many chemical reactions relevant to biochemical processes) will happen at the Si site rather than the H.

(Also, while this difference means that Si can form very strong bonds thermodynamically with electronegative elements such as O (exemplified by the thermal stability of SiO2 in sand a glass) or F, these compounds can be fairly kinetically reactive, since the large difference in electronegativity results in a highly polarized bond, thus inviting further attack by nucleophilic species.)

2. Silicon has a very hard time forming chiral complexes (an important concept akin to the left or right "handedness" of a molecule, which determines the shape of molecules such as helices in proteins). The nitty gritty chemistry of it is that Si is larger atomically, and thus is less fussy about expanding its coordination sphere (which is a probable step that a molecule might go through in switching chiral modes) than C.

3. Silicon is much less abundant in the universe than C, about 5:1. (However, in the Earth's crust it's the second most abundant element, after oxygen. But if we're talking best chance at forming a viable life form when the current known odds of forming C-based life are at 1/10^veryverylargenumber, it might likely be a factor).

4. Silicon doesn't like very much to form double or triple bonds (a common motif in many biochemicals, such as many amino acids containing aromatic rings).

5. Silicon oxides form the solid SiO2 (an extended solid with high thermal stability-- think sand and glass) whereas C oxidizes to give CO2, a gas. The possibility of respirating a solid makes the concept of breathing as we know it a little difficult.

Anyway, there very well may be some possible method by which a Si (or other non-C)-based lifeform could live and reproduce. I hope if they do exist, they're friendly!
posted by beepbeepboopboop at 2:51 PM on June 21, 2010 [6 favorites]

Just to nitpick but aromatic bonds are not true double bonds, but your point still stands.
posted by Canageek at 5:19 PM on June 21, 2010

Very true- it's more that Si always prefers to be sp3 hybridized and avoid any other form of hybridization unless forced to do so. The energy difference between Si's 3s and 3p orbitals is much much less than C's 2s and 2p orbitals; therefore there is less benefit to hybridizing sp or sp2 instead of the whole hog sp3.

There are some neat examples out there where a "silyene" double bond is forced to form due to sterics, but that is a little esoteric when we're talking about primordial soups in my opinion.
posted by beepbeepboopboop at 5:52 PM on June 21, 2010

Is it possible to form a silicon equivalent of benzene? IANAC but it strikes me that whereas benzene is merely flammable, if a silicon equivalent existed it would be high explosive.
posted by Chocolate Pickle at 5:54 PM on June 21, 2010

The biggest problem with the Horta concept (as presented in classic ST) was that it lived by eating rocks. That isn't likely; most of what is in rocks is oxides.
posted by Chocolate Pickle at 5:58 PM on June 21, 2010

What if the Horta had some form of a reducing agent in it's stomach? I mean we have HCl or some such inside us right? Couldn't they have LiAlH4 or somesuch?
posted by Canageek at 1:42 PM on June 26, 2010

Canageek: think of it in terms of energy flow. When organic compounds are oxidized, energy is released. Conversely, it takes energy to reduce organic compounds. Plants use solar energy to reduce atmospheric carbon dioxide, thereby creating high-energy carbohydrates. The plant (or any organisms that eat that plant) can then oxidize those carbohydrates to release that energy and drive cellular processes.

In order to reduce mineral oxides, a rock-eating organism would require an outside source of energy (such as the sun) to drive the reaction. In other words, mineral oxides are at best a means of storing energy. They aren't themselves an energy source. A powerful reducing agent such as LiAlH4 would require huge amounts chemical energy to produce, and that energy has to come from somewhere.
posted by dephlogisticated at 8:05 AM on June 28, 2010

Well, not all reductions are endothermic, and not all oxidations are exothermic. It depends on the strength of the bonds broken and formed (and the entropic considersations). That being said, dephlogisticated is definitely right: there might be some definitely weird manner by which life in a very different form from our own might exist in a reducing environment, but they ain't getting out of the laws of thermodynamics.
posted by beepbeepboopboop at 12:21 PM on June 28, 2010

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