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Originally Posted by The Brummell
Didn't somebody say that Saturn's moons (some of them) might be covered in simple hydrocarbons? I forget where I saw the story, I think it was from one of the Cassini flybys or possibly the Huygens mission. Apologies to people who know about those missions, I'm probably mangling the mission details and misspelling the names.
If that were the case, we might expect life to evolve on a cold, hydrocarbon-rich world that does use the chemical energy stored in hydrocarbon molecules. If you can break down carbohydrates and get energy without ever involving free oxygen, can you do the same with butane? I think I saw a discussion in NatureNews about xenobiologists (love that word) looking into Formamide as a solvent rather than water. Formamide disolves nicely in water, but I think it's also good as disolving some nonpolar stuff like small hydrocarbons. Any physical chemists reading this?
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Or, indeed, an organic chemist or a biochemist.
It was known that Titan had an atmosphere rich in hydrocarbons (such as methane and ethane) before Cassini and Huygens were sent there. I think one of the reasons the mission was so exciting is that the atmosphere of Titan is rather similar to that which existed on Earth prior to the accumulation of molecular oxygen in the atmosphere.
It all depends on what you mean by "free" oxygen.
In our metabolism, carbohydrates are oxidised in reactions that are coupled to the reduction of a cofactor called NAD (nicotinamide adenine dinucleotide). In this context, oxidation is loss of electrons and reduction is gain of electrons. The reduced NAD (usually called NADH) is itself oxidised in a sequence of reactions that are linked to the production of ATP. This is called oxidative phosphorylation (ox-phos for short), and it is the part of our energy metabolism that requires molecular oxygen.
In the many reactions leading from glucose to CO2, one or two of them are directly coupled to the production of ATP without ox-phos. This anaerobic metabolism is thus able to release useable energy without molecular oxygen. But, it is less efficient than ox-phos (ox-phos generates a lot more ATP per glucose molecule than is otherwise the case) and it gives rise to toxic by-products. In mammals, this by-product is lactic acid. In yeast it is ethanol. Generally, a large, multi-cellular organism cannot use anaerobic metabolism for a prolonged period of time.
The case for hydrocarbon metabolism could conceivably be similar if it were not for one important fact: C-C bonds and C-H bonds are very stable. You get a lot of energy from breaking them, but it takes a significant effort to make that happen (the activation energy of the reaction is quite high). I therefore think it would be unlikely for a living system to develop using anaerobic metabolism of hydrocarbons.
Formamide is a useful solvent: I believe it will dissolve several short-chain hydrocarbons (probably methane, ethane, propane, butane). However, my experience of formamide is limited. I have never used it at -180°C, which is, I believe, a typical temperature on the surface of Titan. I think it might go a bit lumpy.