Our empire has been at the mercy of violent marauders who are not working with the Will of Dozin and instead trying to break away from our cosmic union. In our archives, we have found a xenoforming device we aspire to use as a form of leverage against these marauders.

It apparently has the capability to self replicate, create a complex of massive organisms that cover a world within the span of years and is able to eventually spread to other worlds until all garden worlds in the whole galaxy if not beyond are xenoformed into being chlorine worlds.

That is, it converts worlds of oxygen-water chemistry to chlorine worlds. However the mechanisms of this process we do not understand and want to know how it does it. Our own knowledge of chlorination is that it should only acidify the oceans- how does it create a chlorine rich atmosphere?

(In short, how do you chlorinate the atmosphere of an earthlike planet?)

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    $\begingroup$ Chlorine is highly reactive. It will not exist freely as pure Chlorine for more a brief period. It will react with practically any available substance to form a more stable compound. Note that most of the Chlorine we encounter in real life is in salt for precisely this reason. Look up Chlorine on Wikipedia to get more details. $\endgroup$ Aug 19, 2019 at 9:11
  • $\begingroup$ @StephenG oxygen is also known to be quite reactive, y'know. $\endgroup$ Aug 19, 2019 at 9:15
  • $\begingroup$ I understand that chlorine gas is highly reactive and won't exist as free chlorine for long, I already addressed this by nature of the WMD being seeded organisms that produce chlorine gas like how plants on Earth produce oxygen. $\endgroup$ Aug 19, 2019 at 9:21
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    $\begingroup$ @StarfishPrime I'd suggest an organism producing molecular Chlorine is less viable than one producing molecular Oxygen. Chlorine compounds are going to be very hard to build a viable biochemistry chemistry around to generate Chlorine. I'd suggest producing salts (alkali compounds) or acids containing Chlorine is a more viable route for an organism and also more likely to produce problems for normal organisms. Maybe something as "simple" as an organism that produces common salt - pretty tough environment for most organisms and hygroscopic as well. Still no idea how you'd do that, mind you. $\endgroup$ Aug 19, 2019 at 9:26
  • $\begingroup$ [ I'd suggest producing salts (alkali compounds) or acids containing Chlorine is a more viable route for an organism and also more likely to produce problems for normal organisms.] - I'll keep that in mind. $\endgroup$ Aug 19, 2019 at 9:29

1 Answer 1


TL;DR: there's not enough chlorine, not enough of that is it a convenient form, and there's far too much water to turn an earthlike world into an acidic chlorine world any time soon. Unless your attack timescales are very, very long, you'd best find an alternative attack mechanism. The timescales involved also give enemies lots of time to find counters to the xenoforming gloop.

Problem one is getting enough chlorine. In the solar system in general and earth in particular, it is good 4 orders of magnitude less common than oxygen, for example. This strongly limits how chlorinated your final world will be.

Now, the easiest place to find a good supply of chlorine on earth is in the sea, so your chlorine-producing organisms will need to be photosynthetic algae. Better hope your terraforming organisms are super efficient, because they'll be competing with existing phytoplankton that are very well suited to their environment (which chlorine organisms will probably not be) and are present in enormous numbers and will compete for nutrients.

Take the next bit with a pinch of, aha, salt, because I Am Not An Alchemist. The H-O bond in water (which is broken during photosynthesis) has a bond energy of a mere 467kJ/mol, vs the Na-Cl bond in salt of 769kJ/mol. So on top of all the other problems you have, you'll need a photosynthesis process which is significantly more efficient than the terrestrial kind (or whatever other process is on the "garden worlds" you're trying to destroy).

Next, when grabbing the hydrogen from water or HCl the other bit, the waste, is gas phase and so is easily lost. In this case though you end up with sodium and potassium ions, which will presumably form sodium and potassium hydroxide in pretty short order, which is going to really put a dent in your hopes to turn the sea into acid as both those chemicals are strong bases.

The free chlorine your algae produce will react with water to form hydrochloric and hypochlorous acids, and given that earth has quite a lot of water you'll have a real problem keeping your liberated chlorine (which is heavier than air, and so will naturally pool at low points... eg. over the sea) from simply dissolving back into the ocean, and then react with whatever else is available (all that sodium and potassium from the sea salt has to go somewhere, right?). Surface saline water may turn into weak acid, which will be obviously quite bad for marine ecosystems, but there's a lot of sea to dilute that acid and a lot of highly basic metalbolic waste from your algae, such as sodium and potassium hydroxides which will neutralise that acid and turn straight back into salt.

The process will also liberate oxygen from sea water, I'm not sure if this is enough to offset the loss of oxygen production from phytoplankton, but you never know.

In chlorine poor areas (like the land) and places which are comfortably above the level of any free chlorine liberation (like most land) it isn't obvious what sort of effects you'll actually see. If you're hoping for rains of acid and choking green fog then I suspect you'll be disappointed. It'll be an ecological catastrophe and one that might ultimately render the planet uninhabitable, but it'll take a looooong time.

Also relevant: What conditions are necessary to support a vitriolic chlorine planet? (and a few other questions on chlorine planets) suggest that you need a pretty hostile acidic atmosphere in order to end up with free chlorine. You'll need to handwave some other process to trigger that... some kind of massive runaway greenhouse effect to get a venusian world that then might be further xenoterraformed? In any case, it won't be the chlorine that renders the world uninhabitable.

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    $\begingroup$ So what I gather is that the organisms produced by this bioweapon will matter a lot. That being said, it wouldn't only be algae. More advanced eldritch alien biotech stuff. This bioweapon wouldn't be a case of taking chlorine world organisms and chucking them at targeted worlds. Even considering that, it does seem that a lot more of the terror will come from the organisms themselves than the xenoforming aspects. $\endgroup$ Aug 19, 2019 at 10:13
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    $\begingroup$ @MrKusnetsov the problem is, if you want to huff out chlorine, you need to go where the chlorine is. And for the most part, you'll find it in the sea or underground (where you can't photosynthesise). The open ocean is a hostile environment, and phytoplankton have already demonstrated that you can thrive there and perform significant atmospheric modification whilst you do. Any other bioweapon components of your weaponised xenoformer will just be run-of-the-mill terrifying xenomorphs. They won't be chlorine generators, cos they won't be able to get enough of the stuff. $\endgroup$ Aug 19, 2019 at 12:35
  • $\begingroup$ While it seems I can't get global chlorine world, I still don't see how this disqualifies regional chlorine infested biomes or the use of chlorine as something part of their tool box so to speak. However, it does seem that while Chlorine may be part of their tool box, it shouldn't be the primary aspect of how it devastates biospheres. $\endgroup$ Aug 20, 2019 at 6:25
  • $\begingroup$ @MrKusnetsov I think it is all but impractical on anything other than insigificant scales. I'd stick to more boring and conventional atmospheric mangling... massive increases in $\mathsf{CO_2}$ and/or $HS$, for example. There's still scope for boiling oceans and rains of acid! $\endgroup$ Aug 20, 2019 at 11:39
  • $\begingroup$ Hmm. Don't know what's boring about that, being able to pump CO2 more than the volcanoes could to rapidly change a global climate is pretty crazy as is. However, it did recently occur to me that I could have them use some fictional chemistry to get the effects I want. If this is a xenoforming WMD from some civilization that experimented way more than the 2 centuries we have with biochemistries and is capable of advanced synthetic biology, making some custom death gas may make more sense than using one that exists. However that would still have a constraint from local resources. $\endgroup$ Aug 24, 2019 at 9:29

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