I'm trying to work out some really basic biochemical details for cold-world aliens whose bodily fluids are based on liquid methane rather than water. I figure they have to be hydrogen breathers, because you won't get free oxygen on a world with methane oceans! But that means that their metabolism would produce water and ammonia as waste products- and both of those are very solid at liquid methane temperatures.

Additionally, although the solubility of water in liquid methane is apparently unexpectedly high according to Nature, it's still extremely low on an absolute scale. It seems this would be a bit of a problem for anything larger than a microbe that needs to expel metabolic wastes- or for "plants" that need to consume water and ammonia to get oxygen and nitrogen supplies for synthesis.

So, how might a methane-based alien go about transporting waste water and ammonia in its blood? I imagine special transport molecules would be required, but is there something relatively small and simple that could act like a reverse-soap to dissolve these polar solids in methane, or would it necessary require something large and highly specialized, like the hemoglobin we use to transport oxygen?

  • $\begingroup$ Peristalsis as plants too. But u are far away from real problems at the moment, let say from what they are consists? So handwave - your best choose in this case. $\endgroup$
    – MolbOrg
    Jul 13, 2016 at 1:16
  • $\begingroup$ If I may ask, on earth, our atmosphere is a mixture, they 78/21/1 N/O/x mix. If an alien life looked at earth, the would see a world with liquid water and a Nitrogen atmosphere... but we breathe O2... does your atmosphere contain a methane /x mixture? $\endgroup$
    – Joe
    Jul 13, 2016 at 2:58
  • $\begingroup$ @Joe I'm thinking mostly nitrogen, with a large percentage of biogenic hydrogen analogous to Earth's biogenic oxygen, and a small percentage of methane vapor analogous to Earth's water vapor, plus a haze of suspended complex hydrocarbons due to photochemical reactions and thin ammonia ice clouds. $\endgroup$ Jul 13, 2016 at 19:50
  • $\begingroup$ After some lengthy though and recalling long-ago biochem, I think two ideas bear some examination; Terrestrial life isn't composed of the most common elements, but the most bio-versatile. Earth is loaded with silicon, yet virtually no life uses it. The second point is that much of terrestrial life relies on enzymes to break down/build up or convert molecules, although your ambient temperatures mean ionization energies are off-scale, other analogs of enzymes could use N2 and free H to create hydrocarbons, and N+O variations that might be fluid at -250F... hope this helps ! good luck! $\endgroup$
    – Joe
    Jul 14, 2016 at 21:10
  • $\begingroup$ @joe but we are made of the most abundant elements, in the same order (after striking out helium)! $\endgroup$
    – JDługosz
    Jul 19, 2016 at 4:09

1 Answer 1


Ok basically you want a bio version of a chemical membrane that's resistant to methane. So, let's start with the materials what we know on this planet used to resist or keep methane contained.

Low-density polyethylene (LDPE) is a thermoplastic made from the monomer ethylene. Aluminum is also used as part of the membranes used to keep methane products out of the soil. Basically, plastic and aluminum. Here's link to a waste disposal site which uses this in methane situations.

We make plastic industrially, but who is to say that an alien body couldn't make it, in conjunction with aluminum to hold water and other waste products. Or it could just be aluminum, an important part of the aliens' diet. I would say it would have to specialized and "large" like hemoglobin, and that it would dump the waste in a "holding tank" made out of much the same materials, that would then be expelled out of a specialized tube, like the anus.

  • $\begingroup$ What good would a methane-resistant membrane do? I don't see how something else that is intended to not dissolve in methane is going to help with the problem of transporting molecules that don't dissolve in methane. $\endgroup$ Jul 22, 2016 at 14:44
  • $\begingroup$ You just need a system that carries waste out. Why should it have to dissolve? It just needs to be transported out. Having a resistant membrane prevents solubility. It would still be a solid. And why shouldn't it be? Just have the molecules attach to a specialized system made of plastic or aluminum. In smaller creatures, that just means pushing the solids towards a simple out hole via a biological mechanism quickly rather than anything more complex. For larger creatures waste management gets more complex. $\endgroup$ Jul 22, 2016 at 19:25
  • $\begingroup$ Because it's the "larger creatures" I'm concerned about. They'll be producing insoluble solid wastes in every cell. It doe seem particularly plausible to me to have a network of microscopic tubes transporting solid waste away from every tissue. But even if they did go that route, I don't see how attaching the insoluble solid waste to some other insoluble solid molecule is going to help at all. $\endgroup$ Jul 23, 2016 at 20:36
  • $\begingroup$ You want it to be soluble? Isn't the idea to move it out rather than have it dissolve and be part of the system? The only thing I can think as far as water is concerned is a mechanism that separates the hydrogen from the oxygen almost immediately and keeps them apart. At the temps for liquid methane, those two are still liquid. $\endgroup$ Jul 23, 2016 at 22:52
  • $\begingroup$ Ammonia is far trickier. If that's broken down, nitrogen freezes at that temperature. The hydrogen should be fine, but the nitrogen will be solid. $\endgroup$ Jul 23, 2016 at 22:55

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