EDIT: The planet/moon has a high mass, and has a similar atmosphere to Jupiter or Saturn. These organisms will be silicon-ammonia based, and breathe hydrogen. There will be very little oxygen in the atmosphere. Ocean depth and land ratio are similar to Earth.

Let's face it: our search for extraterrestrial life has been entirely speculative so far. We are constantly searching for "water" and nothing else, and while I have researched the reason for this, I'm not convinced that life can't exist without water.

Here's a scenario: A planet or moon with an ideal temperature for liquid ammonia - which I have researched as being one of the top candidates to replace water.

So, let's assume that life was formed somehow on this planet or moon. I'm not concerned with how life formed. For this question, just know that it happened.

I've been thinking about what an ammonia-based life-form would look like... I do know that it most certainly would not resemble anything that we have seen here on Earth... But what would it be like? Would evolution even work the same way? Would it remain as single-celled organisms? Would there even be "cells" in terms of the biochemistry of this alien life?

This is not like other questions that have been asked:
What would a world whose atmosphere is made up of primarily ammonia be like?
And this one:
An ammonia - not water - based alien race that breaths hydrogen. Is it believable/possible? Although it contains good information, it does not address the evolution of such a life-form.

How would life on an ammonia-based planet or moon evolve? Assume that it is like an ammonia version of Earth: vast oceans, lakes, and rivers of liquid ammonia; large, rocky landmasses; The temperature could vary between -77 °C (ammonia melting point) and -33 °C (ammonia boiling point), allowing occasional solid ammonia glaciers.

I am unaware what conditions would require the evolution of ammonia-based life, so I am unable to answer what the main composition and atmosphere of the planet or moon is. But I'm not asking about the chemical processes a life-form undergoes.

I am not looking for opinions or purely speculative answers. I want an answer that demonstrates research and/or expertise on evolution and how it could apply to an ammonia-based organism.
A good answer will say "Based on my knowledge and research on evolution and biochemistry, an ammonia-based life-form might evolve in the following ways..."

I am not an expert on any of this. I have already done research into this and couldn't find anything that helped me. One of the planets in my fictional solar system has ammonia-based life forms and part of my story will involve a series of horror chapters where the life-forms kill the explorers and scientists in terrible and unimaginable ways. Not being like anything they've ever seen, they won't know how to handle it.

I want to know what kind of creatures I need to design, but I want it to be plausible and based on hard science. (ie. I don't want to make an ammonia-based tree if that kind of structure wouldn't be possible using an ammonia-based solvent)

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Sep 25 '19 at 2:57

In short, evolution is perfectly possible provided you crack the incredibly hard problem of non-water DNA. However, it's going to be veerrrrryyy slow.

Darwin's theory of Natural Selection is all about nonrandom genetic changes that mean a branch of a species is better adapted to living in a certain environment. Early AI took a similar approach, so this is definitely not limited to water-based organisms.

So considering that evolution as a process is invariant to biochemistry, let's consider the rate at which genetic mutations would occur within an ammonia-based life form compared to a water-based one. Here's the first problem:

enter image description here

Scientists have just shown water is the key to binding DNA strands. The reason is to do with the polarity of water, which is something that's theoretically possible with ammonia but I wouldn't even know how to begin approaching that problem at scale. I'm aware you said the 'how' of life doesn't matter. If you can form ammonia-based DNA, evolution is (relatively speaking) a cinch with genetic mutations.

With this in mind, the (far) hydrogen bonds between ammonia molecules reduce its power to concentrate non-polar molecules in a hydrophobic way. This means ammonia DNA is going to have to be far more stable, stronger or just much less error-prone in some way to counteract for the fact that it's more likely to fall apart than water-based DNA. The likely effect of this is the evolutionary matrix (described here) can't afford to take as many chances as water-based DNA. Thus, the chance of mutations is going to be significantly diminished to promote chemical stability.

Phew. In short, the weaker hydrogen bonds in ammonia are going to cause you a list of problems, but if you get ammonia-based DNA worked out it will probably produce natural selection-esque evolution at a rate far slower than that on Earth.

  • $\begingroup$ Would it be better if they breathed hydrogen? $\endgroup$
    – overlord
    Sep 24 '19 at 19:19
  • 1
    $\begingroup$ You would make my evening if they breathed hydrogen. I'm sure you could explain it with some worldbuilding in whatever you're working on, but if you simplify it down I can (try) to give you some math to work with. It's just an added touch- no biggie. $\endgroup$
    – mcRobusta
    Sep 24 '19 at 19:22
  • 1
    $\begingroup$ I just edited the question, they breathe hydrogen now. When I put chlorine, I was thinking that maybe the organisms would have a way to stabilize or even have a natural defense mechanism involving the explosive nature of chlorine-ammonia reactions $\endgroup$
    – overlord
    Sep 24 '19 at 19:28
  • 1
    $\begingroup$ Maybe your atmosphere could be some hydrogen-chlorine blend: I only say hydrogen because it's super simple (relatively) in respiration calculations. As a sidenote, I happen to know the reaction of hydrogen and chlorine under UV light is something you do NOT want to be anywhere near. $\endgroup$
    – mcRobusta
    Sep 24 '19 at 19:34
  • 1
    $\begingroup$ also, ammonia beings live in planets that are much colder then Earth. Because of this, reactions will be much slower and that will make evolution even slower. $\endgroup$
    – Geronimo
    Sep 24 '19 at 19:47

I'm not a Biochemist or a Xenobiologist, but my Google-fu is strong, and you may find this useful:

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. Another option is hydrogen, which can be used to break down larger compounds into methane, but that reaction releases much less energy, so life which uses it may have to be small, slow, or work in bursts of activity.

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, ammoniacal 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 98 degrees.

Other Properties

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, ammoniacal 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.

  • $\begingroup$ "The restrictions on water likely also extend to oxygen - it too is probably going to be rare on the planet." - So, there won't be an earthlike lithosphere? Most materials we call rocks are oxygen-rich compounds. This leaves us with the very unpleasant carbon and iron planets, both of which will mess a lot with the ammonia oceans. $\endgroup$ Sep 24 '19 at 19:13
  • $\begingroup$ @TheDyingOfLight or gas giants. Who says you need a lithosphere at all? $\endgroup$ Sep 24 '19 at 19:20
  • $\begingroup$ Maybe. Explaining the lack of oxygen will still be hard though, as it is the third most common element in the universe. $\endgroup$ Sep 24 '19 at 19:22
  • 1
    $\begingroup$ @TheDyingOfLight In the universe, yes. In Gas Giant atmospheres, not so much: en.wikipedia.org/wiki/Jupiter#Atmosphere $\endgroup$ Sep 24 '19 at 19:24
  • $\begingroup$ "There may also be a thin layer of water clouds underlying the ammonia layer. Supporting the idea of water clouds are the flashes of lightning detected in the atmosphere of Jupiter." - from your source. The oxygen compounds would be in the core of the giant. This still might actually work, but then there is the issue that gas giant atmosphere biospheres are somewhat dubious propositions. Even if the locals use ammonia instead of water, beeing microscpoc sounds like a superior strategy in an atmosphere. Then again, who knows... $\endgroup$ Sep 24 '19 at 19:31

Not the answer you're looking for? Browse other questions tagged or ask your own question.