Here’s a question I haven’t really got an answer for; imagine a planet with an atmosphere rich in ammonia rather than oxygen, and maybe some other non-reactive gases like nitrogen, argon and possibly carbon dioxide. Probably such a planet would be considerably colder than Earth, but ammonia is a very reactive compound, so could it support combustion on its own?
I know ammonia is the basis for several rather nasty explosives, and has been involved in several explosions and IEDs over the years. In an atmosphere where ammonia was the predominant reactive component, could certain compounds (maybe organic compounds of some sort) produce a visible flame, or even a conflagration? Any suggestions would be welcome.
Don’t worry too much about the chances of an ammonia-rich atmosphere - this might be the result of biological activity, like the oxygen in our own atmosphere.
Our atmosphere is only about 21% oxygen, the rest is almost all nitrogen. If it was all oxygen everything on earth that could burn or oxidize in some way would have already. What is the complete makeup of your planet’s atmosphere, percentage of ammonia, other gases, pressure?
My chemistry is rusty, but a quick look through internet sources indicates that ammonia is not an oxidising agent (no matter how much some people want it t be), so it can’t take oxygen’s role in combustion.
Chlorine, by contrast, is such an oxidizer, and has been used a lot in science fiction as an alterntive atmosphere. You can see demonstrations of burning in chlorine atmosphere in several YouTube videos.
Science fiction author Hal Clement was working on a story set in a world with an ammonia-rich atmosphere. He called it “Bleachworld”, and spoke about it at conventions. He even did artwork related to the stories he was thinking about (but to my knowledge, never wrote). I can’t find anything about Bleachworld on the internet, though.
Ammonia or ammonium nitrate? Many high energy compounds decompose to nitrogen and other gasses. But the decomposition of ammonia to hydrogen and nitrogen is endothermic. You can get an explosion with an oxidizer. E.g., there are some ammonia/air mixtures that are explosive, but AFAIK most fatal ammonia accidents are due to ammonia not being very good for humans in high concentration.
Not pertinent to the ammonia question, but Titan has a hydrological-type cycle similar to Earth’s, and is the only known body in the solar system other than Earth that has such a cycle, resulting in rain and rivers, lakes, and seas. Only instead of water, the cycle consists of liquid methane (with some ethane), basically liquid natural gas.
The gas giant planets have atmospheres that are mostly methane and ammonia. They also have vigorous weather systems that produce lots of lightning. If ammonia supported combustion, they would have gone up like the Hindenburg eons ago.
Just from a basic logical exercise, it should be obviously true that any flammable atmosphere would be inherently unstable and wouldn’t last any significant length of time. At some point, whether days or weeks or years or decades, there would be some event on the planet that would cause a spark (i.e. lightning, meteor impact, volcanic activity, etc.), and thus the atmosphere would ignite and be gone (or replaced by a different atmosphere).
So it’s probably impossible that there would be any significant celestial body with a flammable atmosphere.
I’m not sure the OP is talking about a flammable atmosphere as such. IMO they’re talking about an atmosphere that can support combustion of some fuels, but which isn’t using oxygen as the oxidizer.
Very different proposition.
Now the Earth’s atmosphere contains both hydrogen and oyxgen. An obviously flammable pair. But what it doesn’t have, per @iiandyiiii’s logic, is large amounts of both. If for whatever reason a large concentration of hydrogen appears, pretty quickly it gets oxidized and hydrogen is once again locally rare.
The converse question might be reasonable. Ammonia is a reducing agent, not an oxidiser. It isn’t generally inert. So we might posit a planet where life drives ammonia into the air, and there are oxidising compounds around, that might be the result of life’s activity. You could then have a perfectly happy fire. There is nothing that says that the gas needs to be the oxidiser.
Depends what you mean by “flammable”. Above I just described Titan’s hydrological-like cycle which cycles methane instead of water. It has oceans, lakes, and rivers that are basically liquified natural gas, and the cycle evaporates that into the atmosphere where it eventually condenses and falls as methane rain. In the atmosphere, methane exists as a gas until it condenses, comprising about 5% of the atmosphere. So, a flammable atmosphere comparable to a natural gas leak in a building on Earth.
But … there’s no oxygen. The other 95% of the atmosphere is nitrogen. So you have natural gas in both liquid and gaseous form, but it can’t burn.
Nearly every substance has something it’ll react with. So for any given atmosphere other than a pure noble gas, there will be some substance that will be “flammable” in that atmosphere.
I think that it’s actually the partial pressure of oxygen that matters, not the proportions. An atmosphere 21% as thick as ours of pure oxygen (i.e., like ours, except with the nitrogen removed) would be no more nor less dangerous than ours, and an atmosphere five times as thick as ours with the same proportions would be just as dangerous as pure oxygen at 1 atm.
Oxygen supports combustion, yet it persists in our atmosphere, because it is constantly replenished by photosynthesis. If the oxygen level on Earth was too high, most of the biomass would burn and photosynthesis would cease, or would be inhibited until oxygen levels reduced to a safe level. If photosynthesis ceased altogether, almost all free oxygen would disappear into the crust by oxidation (eventually).
My hypothetical world has no free oxygen, or perhaps just trace amounts.
This is the sort of situation I am looking for. Organic, oxygen-rich compounds that could burn in ammonia, perhaps some sort of carbohydrate, might fit the bill.
Of course, it doesn’t have to be organic. The reaction of nitric acid and ammonia is exothermic; has anyone tried burning nitric acid in a pure ammonia atmosphere?
I’m finding it difficult to imagine a reaction that would cause a visible flame in an ammonia atmosphere. Probably there would be lots of exothermic reactions that could create copious amounts of heat, so a smouldering and sputtering reaction could occur; but a gaseous ‘flame’ like that of a candle or burning torch would require the release of a flammable gas (a gas that is ‘flammable’ in an ammonia atmosphere), and that could be more difficult.
I recall some other thread with people arguing about what constitutes fire/burning. Without giving that much thought, I think we’re talking about reactions where solid or liquid fuel volatilizes and undergoes a redox reaction with an atmospheric component in a relatively stable, self-sustaining manner that releases visible light and heat.
I don’t see a simple acid/base reaction (nitric + ammonia) as fitting. Bromine + ammonia might qualify.
In answer to my question, I found this: “No, you cannot have fire without oxygen; oxygen is a crucial element for fire to exist, as it acts as the oxidizer in the combustion process, meaning that without oxygen, a fire cannot start or continue burning.”
As one poster pointed out, if conditions existed for mass atmospheric burning, it would have happened already. Jupiter, for example doesn’t seem to have any fires, so there must not be enough oxygen to support random combustion even if there is a huge amount of methane or ammonia or some other combustible gas in the atmosphere.