4
$\begingroup$

I’m mostly interested in chlorine since (as far as I understand) it’s the most viable, but I’m open to others as well (if it’s worth it).

I’ve been working with a sort of “worldbuilding challenge” on-and-off for a few years now and still don’t have the atmosphere for the main planet just right, yet. One rule that I have to work around is that the planet cannot have any significant trace of oxygen, so I’ve been toying around with how the local race could figure out fire, or if I even want it. I understand halogens such as chlorine and fluorine can act as oxidizing agents in combustion. Fluorine is a bit too… reactive, I feel, so I’m avoiding that. Chlorine (likely in its allotropic dichlorine form) I know less about. If possible, how would fire behave in an oxygenless atmosphere with a significant presence of chlorine gas? What would some other important implications be? How viable are other halogens?

If it helps, the atmosphere also has to mostly-to-significantly consist of an ambiguous gas, with which I’m forced to be less rigorous in terms of scientific justification. The mystery gas is the primary agent of respiration for my race (which was bioengineered to survive in this hellscape and derived from a basal form of Earth life). If it assists in the process of controllable combustion it can probably be given a relevant property.

If the halogens don’t work, I’ll either be lazy and give the mystery gas similar properties to oxygen, or I’ll just challenge myself even more and lay out an intelligent society without ease of flame. Thank you all in advance, and apologies for the drawn-out question!

$\endgroup$

5 Answers 5

9
$\begingroup$

Oxygen is an excellent oxidizer, however Chlorine is perfectly effective as well. Chlorine will support a fire almost as well as oxygen. As Stephen said, fluorine is also an excellent answer for your non-oxygen fire. Fluorine will, however, burn hotter and faster than oxygen. Bromine and iodine in your atmosphere will enhance the burning of fire as well, although they are not combustible on their own.

$\endgroup$
6
$\begingroup$

Chlorine is perfectly fine oxidiser for your flame. In fact, there is no reason why your race wouldn't breathe it! If they were bioengineered to thrive in this environment that would be the perfect element for respiration! If you are robust enough to survive in chlorine atmosphere you should be robust enough to breathe it. No need for the mystery gas, as it would have the exact same function: to be the oxidiser for metabolism. And while chlorine is a bit better oxidiser than oxygen the difference is not THAT huge. It would serve better than any other elemental gas.

$\endgroup$
2
  • $\begingroup$ Thank you so much! I’ll only add that I do need to keep the mystery gas for other lore reasons. That’s true though, with respiration as a redox reaction it would imply that the mystery gas is an oxidizing agent as well… how insane would it be to have a significant presence (>20%?) of two oxidizing gases in an atmosphere? Or does an oxidizing agent that can serve in respiration not necessarily one that serves much in combustion? $\endgroup$
    – Octopodes
    Feb 27 at 21:10
  • 3
    $\begingroup$ @Octopodes It doesn't make sense to have ONE oxidizing agent in an atmosphere at such a high presence. The only reason we have so much oxygen in ours is because it is a byproduct of photosynthesis and is constantly being made. So you need something to generate your gasses. Also, if you have chlorine in the atmosphere other parts should be relative inert (like our nitrogen, and carbon dioxide), as otherwise they would react with chlorine. $\endgroup$
    – Negdo
    Feb 28 at 8:35
4
$\begingroup$

Chlorine would make a perfectly acceptable oxidising agent for fire. It has a slightly greater reduction potential than oxygen so 'stuff' would burn slightly more energetically, but otherwise with no particular issues. Fluorine is indeed way more reactive, as evidenced by its propensity to 'burn' things that we would naively already consider fully oxidised, like glass. It would be a remarkable biomechanical process that could 'photosynthesise' free fluorine without it then immediately eating the enzyme, but chlorine seems very plausible.

Chlorine's flame colours are all in the ultraviolet spectrum (which is why metal chlorides are customarily used in school chemistry flame experiments), so it won't provide any special colour tint to your native fires; you'd see the same sort of flame colours you would from terrestrial materials, although all fires would burn slightly hotter than they would on Earth, all else being equal.

$\endgroup$
2
$\begingroup$

To make fire you need a combustible and an oxidiser, which doesn't have to be necessarily oxygen. Halogens work pretty well as oxidisers too.

Your fire would behave in the same way, just the flame color could differ.

$\endgroup$
1
$\begingroup$

You could just have less oxygen. This paper simulates burning of various objects in the Mesozoic era. suggests that forest fires may be harder to start if you went down to 15% oxygen, rather than the earlier accepted value of 12%, but there is still enough to have dinosaurs. If you want a margin for error, oxygen levels had been as low as 9%.

Chlorine is rarer than oxygen in the Earth's crust. There is a lot of it in seawater because it is reactive and makes soluble salts, but there is a lot more oxygen.

$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .