How many biological substrates of life?

[energyscholar]

How many biological substrates that support life do we know of? There's our current Chemistry in three dimensions substrate based on carbon. Are there any others? What are some likely prospects?

[gus_massa]

One (ours), nobody is sure if there are others.

There is some handwriting about others, but I'd ignore them. More linkbait than science.

[energyscholar]

Are you sure about that? Is that a considered and well-researched opinion or a hot take? For example, One, can you list some prospective substrates that ought to be considered? Or not thought about it enough ...

[gus_massa]

It's my well-researched opinion. Feel free to disagree. You mostly need some element that can form nice chains.

Nitrogen likes to transform explosively into N2. Keep them at a long distance.

Oxygen likes to react with everything. Oxygen on Earth is a byproduct of cyanobacteria and appeared very late in history. Chains also like to do weird thing and we use short chains is epoxy resines.

Boron, I never understood Boron chemistry, but IIUC they like to reorganize so it's difficult to get a nice tidy of Boron. There is a chance, but I guess you get only weird goo.

Next line:

Aluminium form covalent unions if you ask nicely, but most of the unions are ionic.

I never saw lines of Silicon, but you can interleave it with Oxygen and get silicone that is very useful, but the side chains with Carbon are very important. The main chain is probably too stable to be useful.

I don't remember long chains of Phosphorus, but if you interleave Oxygen there are small chains of phosphate in our cells. They store too much energy so they are difficult to produce, but are handy for task that involve short term energy storage.

I remember tiny chains of Sulfur, but I never heard about long ones.

The rest of the table is hopeless, so you have to stick to Carbon chains, that have already been found in planets without life. IIRC the natural ones have also Nitrogen, because it's easy to get non biological reactions with cyanide NCH. My guess is that we are a little Carbon heavy compared to non biological goo.

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Now, you can try weird setups. I don't remember any interesting one, so let's imagine a very silly one that I don't remember reading anywhere. Like the "chemistry" of the grains and dislocations inside the huge iron cores of planets. I'm not sure if they can do something interesting enough to call it alive. And I don't remember any interesting discussion of natural structures that can hold enough information an reactivity to guess they can eventually form something interesting enough to call it alive.

Perhaps it's possible, but and I/we have not enough imagination or knowledge. Anyway, most "groundbreaking" post in press articles about non-Carbon-life are just non-Carbon-crap.

[downboots]

Does turing machine count?

[energyscholar]

Well, downboots, a Turing machine computes. That's a step in the right direction. What are some requirements of life:

* Sustained energy throughput, aka thermodynamic disequilibrium - depends upon the substrate in which our Turing machines operates

* Sufficient degrees of freedom to perform computation - Turing machines compute, therefore any substrate that supports Turing machines has this. This includes autocatalytic Closure.

* Dissipative Non-linear reactions - not supported in conventional substrates that support turning machines. Are there perhaps some more exotic substrates that DO support this item?

So no, a Turing machines BY ITSELF does not qualify. That said, it's a step in the right direction. That's probably why Alan Turing pivoted away from digital computers and towards Morphogenesis towards the end of his life.