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I'm playing with the idea of an alien race that uses ammonia, not water, as a solvent and breathes hydrogen. This is mainly because I want it to combust when exposed to our atmosphere. However, I want to know if this is a believable concept, i.e. not a dumb idea.

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    $\begingroup$ Do you have a metabolic cycle in mind for that hydrogen breather? The last article I saw on such a concept involved a world where you stored oxidizers for fuel, and breathed in reducers (like hydrogen) instead of our current process of storing reducers for fuel (like fats and sugars), and breathing in oxidizers. If so, you might not get the combustion you expect to see. You might want to ask about that in a separate question, see if others can weigh in. $\endgroup$ – Cort Ammon - Reinstate Monica Nov 10 '15 at 6:01
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    $\begingroup$ Your job as the author is not to prove that it is possible, but to convince your audience to suspend their belief that it is impossible, for the sake of the story. You don't need to get bogged down in the details as most likely very few of your readers will have the necessary scientific credentials to have a problem with it. It's good to think about the details but don't let them get in the way of a good story! $\endgroup$ – CJ Dennis Nov 10 '15 at 6:54
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    $\begingroup$ Here's an interesting article on ammonia-based alien life. treknews.net/2011/09/18/… $\endgroup$ – user22606 Jul 7 '16 at 0:54
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Using ammonia as a solvent would work, though if you want it to be pure ammonia, you'll have to keep your aliens quite cold, as it boils at -33°C. Aqueous ammonia solutions would work, but that does not seem to be what you're after. Ammonia isn't as broadly useful a solvent as water, but it does allow for lots of interesting chemistry to occur, including things like relatively stable solutions of solvated electrons, which are quite unstable in aqueous systems. Lots of biological reactions will produce water, so an anhydrous alien will need a way to excrete the water just as we have a way to excrete ammonia. Maybe your alien could excrete crystals of glycerol or urinate an alcohol or something like that. In any case, ammonia is a much better choice than a hydrocarbon.

Breathing hydrogen is a bit more problematic. The purpose of breathing oxygen is that we can break C-H bonds and combine oxygen with both carbon and hydrogen atoms to release energy in the metabolism of organic molecules. Oxygen is highly reactive stuff; we just don't realize it because we've evolved to walk around at the bottom of an ocean of it. If we breathed hydrogen instead, it would be far less reactive. Hydrogen will readily react with lots of things, but it typically doesn't release huge amounts of energy in the process.

One idea might be to have your aliens run a biological version of the Sabatier process. The reaction is spontaneous at low temperatures, though it would never actually occur without catalysis at liquid ammonia temperatures. This would give them a means of producing some basic metabolic building blocks from materials that are common in the universe as well as provide a reason for them to breathe hydrogen in the first place.

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    $\begingroup$ Note, though, that in order to have a useable hydrogen atmosphere it's necessary that there be no other gases, such as nitrogen. If this happens, the hydrogen will gravitationally separate, and the lower reaches of the atmosphere will be made of anything but hydrogen. $\endgroup$ – WhatRoughBeast Nov 10 '15 at 4:17
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    $\begingroup$ Also I suppose they would "exhale" methane. $\endgroup$ – Night_Fox Nov 10 '15 at 4:40
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    $\begingroup$ Instead of low temperature, you may use high pressure. Data give boiling point 98.3°C at 45600 mmHg (60 atm). $\endgroup$ – BartekChom Nov 10 '15 at 5:28
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    $\begingroup$ Water would be a waste product and not necessarily a toxin (although it might be a toxin). Sort of like how $CO_2$ is for humans. If this was a low temperature environment, the critters would excrete solid water though - imagine pooping ice cubes! I was also thinking that it might make more sense for the critters to breath methane than hydrogen. $\endgroup$ – Jim2B Nov 10 '15 at 17:55
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    $\begingroup$ @Night_Fox The Sabatier reaction actually releases energy at any sort of normal biological temperatures. The only reason it requires a hot catalyst is that it has a very activation energy. Basically, it's extremely difficult to get started, but does result in a net release of energy. The products would be water and methane; I figured that the methane could then be used for biological purposes, as a basic building block for organic molecules. The water would likely be waste, toxic or not. There are reactions that can occur in liquid ammonia that would be ruined by water (water = poison.) $\endgroup$ – Jason Patterson Nov 10 '15 at 23:16
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Ammonia can work as a solvent in place of water or in addition to water. Hydrogen is also very reactive so it could be a useful energy source. Also if a planet somehow had high concentrations of Boron then Boron Nitride could be a substitute for Carbon as together they form the same bonds as Carbon and life based on Boron and Nitrogen might use ammonia as a solvent as at the temperatures of liquid ammonia that reactions between Boron and Nitrogen could be more controlled. In this case the planet might have an atmosphere of Diborane, Hydrogen, Nitrogen, and Ammonia vapor. In this case the equivalent of plant life might get energy by reacting diborane and ammonia to produce sugar analogues and hydrogen while the equivalent of animals might react hydrogen and sugar analogues to produce diborane and ammonia. In this case the gasses that the organisms would breath would explode when in contact with air. Also liquid water in high concentrations would be too hot and too acidic for this type of life and if this life touched life on Earth both would mutually poison each other.

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