Life as we know it requires water. Water acts as a biological solvent, dissolving various chemicals into solution, so that they may be transported more readily via the body's cells. Water also helps regulate temperature within the body. Water is in every cell of life here on earth (see also cytosol). However, it's possible that alien biology might subvert that assumption. Just as alien life might be based on silicon instead of carbon, it's possible that alien biochemistry might find another solvent. One of the most likely alternatives is liquid ammonia. Though largely toxic to animal life on our planet, ammonia actually has a number of properties similar to water that may make it attractive as an option for life elsewhere. It dissolves numerous chemicals, forms compounds without being either too stable nor too reactive, and it is relatively common in the universe.
Pick A Proper Planet
In order for life to evolve incorporating ammonia, a number of conditions need to be in effect. For starters, ammonia itself needs to be very common on the planet. Many gas giants, such as Jupiter, have ammonia-rich atmospheres, so that may be a likely place for ammonia-loving life to evolve.
Water should be relatively rare on the planet in question. Partly this is because ammonia is less-suited than water to do water's job. More importantly, however, is the fact that ammonia is a base. Biochemistry based on ammonia will likewise be more base in nature, and thus water would function like an acid to it. This requirement of water to be rare might be waived if life on the planet were based on silicon, as the acidic environment may not be as big a danger to silicon-ammonia as it is to carbon-ammonia life.
The restrictions on water likely also extend to oxygen - it too is probably going to be rare on the planet. This is because of the chemical composition of ammonia - it's made from nitrogen and hydrogen. An abundance of oxygen would lead to much of it joining with the hydrogen to form water. In the process, it would oxidize and break down the ammonia. So, ammonia-utilizing life will need something else to breathe. One chemical that could serve as an alternative for respiration is chlorine, however, most ammonia-chlorine compounds are explosive. Probably the next most common oxygen-alternative proposed is methane, which is also explosive. It's likely the atmosphere of such a planet will be quite volatile.
Ammonia has a far lower boiling point and freezing point than water. This means it's a suitable solvent on planets too cold to support liquid water. So, ammonia life may develop on planets outside the goldilocks zone, and instead happen on planets where what little water there is comes in the form of solid ice. Such life will likely be comfortable at the rather chilly temperatures between -70o and -40oC. However, boiling points are flexible, and subject to pressure. An extremely large planet, such as a gas giant or super-earth with a thick atmosphere would have enough pressure for ammonia to remain liquid at room temperature. Therefore, ammoniated life is not merely restricted to super cold planets, it may also exist on warmer planets, provided they have enough gravity and/or a strong enough magnetic field to form a dense atmosphere. As you increase pressure, the boiling point of ammonia rises, but the freezing point stays roughly the same. In pressure similar to Jupiter or Venus, ammonia will remain liquid up to 98o.
Incompatibility with Humanity
From the factors enumerated in the previous section, we can conclude that any planet likely to be sprout life using ammonia as a solvent is going to be inhospitable to human life. It's atmosphere is full of chemicals toxic to humanity, and lacks the oxygen we breathe. If that doesn't kill us, either the temperature or the pressure will. Either we'd freeze, or be crushed. Chances are, these life-forms will have minimal interactions with humanity. The worlds that are useful to us, are not so to them, and vice-versa. Our oceans would be like vats of burning acid to them, because their pH is several points above that of water. We might well be toxic to each other, even lethal to the touch or breath.
Ammonia life will be adapted for the cold, or for immense pressure. It will probably be highly reactive to acid, and intolerant of humidity and high-temperatures. Ammonia is very combustible, so life utilizing it may be especially vulnerable to fire. The very atmosphere it breathes is probably explosive or at least highly flammable.
Ammonia is not nearly as good an insulator as water. One might conclude from this that ammonia-based life is not as good at regulating it's temperature as our watery life is. Like cold-blooded or hibernating creatures, ammoniated life may be active in cycles depending on the temperature or weather. However, it would be capable of surviving down to extremely cold temperatures, and is resistant to freezing.
Ammonia is less viscous and freer-flowing than water, and surface tension is also less. Speculating wildly, I'd suggest this may lead to chemicals traveling through the body more rapidly than in our carbon-and-water lifeforms. Chemical and hormonal effects might be faster, food might be digested quicker, etc.
The hydrogen bonds in ammonia are weaker than in water, reducing it's ability to concentrate other materials together in solution. This may imply the opposite of the previous paragraph, resulting in less efficient biochemistry. Or, it may simply mean that it takes longer for life to evolve in an ammonia atmosphere, as more time and random factors are needed before self-replicating patterns are formed.
An ammonia-rich atmosphere may be home to life based on nitrogen and phosphorus, or silicon, either of which is particularly compatible with using ammonia as your biological solvent. In such a model, ammonia remains the solvent, but the cell walls, proteins, and amino-acid equivalents are made from silicon or a nitrogen/phosphorus blend.
Another option is that life might evolve using both water and ammonia as solvents, or using one while tolerating or expelling the other. Ammonia and water aren't terribly stable together, but there's plenty of chemical reactions that can result in both existing in quantity within an environment.
It's noteworthy that a solution of ammonia and water has a much lower freezing point than water on it's own. Therefore, it's possible for life more like that which is familiar to us on earth to evolve in a much colder environment, provided that ammonia keeps the water from freezing.
Whether this life processes, produces, utilizes, or just eliminates the ammonia is probably up to the GM. There's not much material about it on the internet.
Such life would, of course, evolve to resist ammonia's toxicity as well as water's (relative) acidity, and be harmed by neither.
Game and Story Use
- Jupiter, Uranus and Titan are three places in our own solar system that could potentially harbor ammonia life.
- Let's say your science fiction setting is populated by races that evolved on three different worlds. You've got creatures that use water as a solvent, creatures that use ammonia, and creatures that evolved in a planet with an ammonia-water solution. As indicated above, the first two species might not be able to interact in any meaningful way. However, there's a possibility that the third species might be able to interact with the others, needing a light environment suit on either planet (including refrigeration when visiting Earth). They'll still be somewhat acidic relative to the lifeforms that are pH-balanced to ammonia, but it might be only minor discomfort instead of total toxicity.
- The "middle species" might be allied to one or other species, and act as natural diplomats between them.
- Or, the middle species might be competing against the other two. They have no desire for each other's worlds, but any planet usable by the middle race is likely at least somewhat viable to one of the two others. This resource competition would align the two species that can't inhabit the same world. The Enemy Of My Enemy is my friend, after all…
- See Alien, Alien Biology, Hypothetical Types of Biochemistry, Silicon-Based Life, Nitrogen- and Phosphorus- Based Life, Chlorine-Breathing Life and Methane-Breathing Life for more ideas.