The Horta: Silicon Life

Could silicon life exist? Not an A.I. in a computer, but a physical life-form capable of performing the activities of living?

 

For fans of the old Star Trek episodes, there’s the Horta. A rundown of the story (Season 1, Episode 26, “The Devil in the Dark”) can be found on this website.

 

The Horta is one strange-looking creature. One commentator described it as Hamburger Helper. I’ve never eaten Hamburger Helper so I have no further comment on that matter. However, as someone interested in chemistry and the origins of life, it’s interesting to consider if silicon life is possible – chemically speaking.

 

When Kirk encounters the creature (picture above), you can also see a clutch of metallic looking spheres in the background. In the TV show, they are supposedly made up of almost-pure silicon and might be Horta eggs, with the contents predominantly being Horta food (is my guess). Interestingly, here on Earth we make pure silicon spheres to define the kilogram – which also helps us to calculate Avogadro’s constant. And if you see non-faked pictures of natural metallic spherical objects in Earth’s oceans, those are manganese nodules, not silicon spheres.

 

But let’s get back to the chemistry of silicon and life.

 

Silicon sits just below carbon on the periodic table. An introductory chemistry course often declares that elements in the same row have similar chemistry. So if carbon can form the backbone of life’s molecules on Earth, couldn’t it be the backbone of life’s molecules on some alien planet?

 

Why is carbon the backbone element of Earth life? Carbon is tetravalent, meaning it can form up to four directional bonds with other atoms. Non-metals in other columns of the periodic table form three (boron, nitrogen), two (oxygen), or one (fluorine) directional bonds. (Metals form non-directional bonds for the most part.) This allows for a greater diversity of structures that can be formed. Silicon is also tetravalent. So far so good.

 

Adding to the diversity repertoire, carbon also forms double bonds and triple bonds. Silicon does so poorly, i.e., the second or third bonds (usually referred to as pi-bonds) are so weak, they break easily to form other single bonds. Also, the single bonds between silicon atoms are much weaker than the single bonds between carbon atoms – the larger silicon atoms can’t get as close to each other when “overlapping” (is my simplified explanation for today’s blog post; chemical bonding is complicated!).

 

The elements of life are often abbreviated as CHNOPS, which sounds almost like the name of some pharaoh. I’ve discussed why (the elements, not the pharaoh) in a previous post that included going into the details of the bond strengths alluded to above. But delving into the origins of metabolism, we find that the key metabolites have mostly just CHO (although S is implicated as being important in co-factor molecules).

 

Let’s break this down. We’ve talked about C. Why also H? You wouldn’t have very much diversity with pure carbon: there’s diamond, graphite, buckyballs, buckytubes, and the presently popular graphene (single graphite monolayers). That’s about it. Adding hydrogen allows you to “cap” a carbon atom’s valences giving rise to a huge diversity of hydrocarbon molecules. There are hundreds, thousands, millions, of combinations you could have. But besides adding diversity, carbon-carbon and carbon-hydrogen bonds are both strong and relatively inert, i.e., you get a diversity of stable compounds that aren’t overly reactive, especially in water (interesting chemistry takes place in liquids).

 

I used the word “backbone” deliberately. In life you’re trying to find that sweet spot between stability and reactivity. If you’re rock-solid-stable, nothing happens. If you’re crazy-unstable-reactive, you won’t hang around long enough to do anything interesting or be able to transduce free energy effectively. And mind you, energy transduction is the name of the game of life. That’s where oxygen comes in. The O atoms in CHO-containing molecules provide, not just additional diversity, but sites of reactivity while maintaining stable backbones. The entry of O₂ as an oxidant changed the game remarkably (I recommend Nick Lane’s book Oxygen). Need energy? Got fuel (hydrocarbons)? Just burn, baby, burn!

 

It’s difficult for Horta to survive in a terrestrial-like environment with water as the main liquid solvent driving chemistry. Silicon is simply not stable enough nor can it form a sufficient diversity of molecules to support the structure, flexibility, and adaptability of living organisms. At least in life as we know it. There might be a very slow rock-like-life that we’d say looks dead. And while Hamburger Helper Horta has rocky looking characteristics, he can scuttle off or react remarkably quickly. But on a different planet, with a very different atmosphere, hydrosphere, lithosphere – you could have a very different biosphere, and Horta might exist after all.

 

P.S. Extant life does incorporate silicon, mainly in plants, and almost exclusively in just two forms: molecular silicic acid, Si(OH)₄, or amorphous silica, SiO₂.