Welcome to Molecular Monday! On the first Monday of the month, we take a closer look at the atoms and molecules that make up our physical universe. Today, we’re looking at:
AMMONIA AND ALIEN LIFE
Water, Water Everywhere…
You know how water has that Mickey Mouse shape? That shape is really important. That slight asymmetry allows electrical charges to accumulate on opposites sides of the water molecule.
The polarization of water molecules makes water a good solvent for other polar molecules, like amino acids. This is a big reason why water is essential to life (or at least, life on Earth). Without the ability to dissolve amino acids, we’d have an awfully hard time getting them to form peptides or proteins or DNA molecules.
But could life on some alien planet substitute another chemical for water?
Ammonia, Ammonia Everywhere…
This is an ammonia molecule (chemical formula NH3).
At first glance, you might think ammonia molecules are symmetrical, with three hydrogen atoms evenly spaced around the central nitrogen atom. Symmetrical molecules have all their electrical charges perfectly balanced, and therefore are non-polar and do not act as good solvents for amino acids.
But when you turn the ammonia sideways, things look rather more promising.
The three hydrogen atoms bend toward each other, just as the two hydrogens in water do. There’s a slight asymmetry, meaning electrical charges can form. Ammonia is a polar molecule after all!
And ammonia has a few other things in common with water:
- They’re both fairly common in the universe (though water is more common).
- They both can be liquid under fairly ordinary temperature/pressure ranges (though water’s liquid phase is wider than ammonia’s).
- They can both act as a base, meaning they can accept a proton from an acid (though ammonia is slightly more basic than water).
- They can both act as an acid, meaning they can both donate a proton to a base (though water is slightly more acidic than ammonia).
The most noteworthy difference seems to be that ammonia burns easily in the presence of oxygen. That could pose serious challenges to the evolution of complex, multi-cellular organisms that need the extra kick of energy oxygen provides.
Still, water and ammonia are similar enough to attract the attention of astrobiologists, and a lot has been written about the possibility of life emerging on some distant planet in an ammonia sea.
Links
Hypothetical Types of Biochemistry from Wikipedia.
Alternatives to Water from Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization.
Thalassogens from Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization.
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A special thank you to Kirov99 for suggesting this topic. My research tends to focus on the planets and moons of the Solar System, rather than hypothetical environments we might find elsewhere in the universe, so without the recommendation I would have probably missed this.
Planet Pailly 
Glad you liked the topic! I find it really interesting, but you should really be thanking Frontier Developments and their game Elite: Dangerous, as that’s how I was turned on to the subject.
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Oh, cool. I’ve been hearing a lot of good things about that game.
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Interesting on ammonia. Seems like it might increase the size of the habitable zone, since it’s a liquid at lower temperatures, although the oxygen part might be catastrophic if ammonia life’s photosynthesis leaves it as a byproduct as it did on our planet.
I’ve been doing some research lately on cell biology, and one thing I’ve been struck by repeatedly is the importance of the shape of molecules. When it comes to proteins, the shape becomes all important. (To the extent that a transmissible change in their shape can be a type of disease, prion disease, or “mad cow’s” disease.)
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Yeah, if ammonia-based microbes started producing oxygen as a waste product, they might literally set themselves and their whole planet on fire. Either that, or they’d slowly convert their whole ocean into molecular nitrogen and water.
I’m still trying to wrap my head around organic chemistry. I’ve only just barely touched on cell biology, but I’ve definitely gotten the sense that even a small change to a molecule–just one hydrogen atom out of place–can completely change the molecule’s function within a cell.
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I admire your determination on trying to understand organic chemistry. Myself, I’m resigned that I won’t understand its intricacies to any competent degree. I’m satisfied with just getting a broad idea of how it’s used in the construction and machinations of ribosomes, enzymes, proteins, DNA, RNA, etc.
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There will always be a certain level of detail that I don’t want to get into. I’m just a science fiction writer, after all. But I’ve come to realize that chemistry, especially organic chemistry, is fundamental to almost everything I write about, from alien life to rocket propellent to planetary science. So I want to know as much about it as my brain can handle.
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Yeah, the idea of life developing in liquid ammonia is intriguing. Oxygen would be a problem, but anywhere you’d find that much ammonia, you probably wouldn’t have that much oxygen in the first place.
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That makes sense to me. Unless there’s something I’ve overlooked or misunderstood, I take it that all the oxygen would be used up in reactions with the ammonia.
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