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How to change the earth's atmosphere to chlorine based?

  1. Jun 2, 2015 #1
    Hi, I'm working on a commissioned story, the basic jist of it is aliens colonize earth, begin a lengthy process of terraforming it to fit their toxic homeworld atmosphere. I read somewhere chlorine is the most viable alternative to oxygen in terms of energy production and it's fairly common in the universe and toxic to humans so I went with that. The flipside would be that oxygen would be toxic to the aliens, so it would have to be removed.

    How would one go about practically doing that? Depleting earth's oxygen supply and creating free elemental chlorine?

    It would have to be fairly closely based on existing earth technology as the aliens would be a small but powerful invading force unable to bring much of their own technology, instead adapting ours.
     
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  3. Jun 2, 2015 #2
    Chloroforming a planet

    The abundance of chlorine in the Earth's crust is about 145 parts per million (ppm). Oxygen is 461 ppm. Chlorine is pretty rare in Earth's orbital zone (and the comet implactors) which should be pretty obvious to anyone with some science background.

    Wikipedia doesn't easily inform me about chlorine's abundance on Saturn or Jupiter. I'm trying to think of a plausible source of Chlorine. Maybe the gas giants would a good, fictional source of chlorine, since the low abundance of chlorine in interstellar gas and dust (which formed the solar system) is not well known.

    The Earth's early atmosphere was reducing--it had scant free oxygen, before carbon fixing organisms polluted the atmosphere with their effluents. Perhaps some taylored organisms would reverse the process, fixing the oxygen. ("Fixing" oxygen means binding it to something.)
     
    Last edited: Jun 2, 2015
  4. Jun 2, 2015 #3

    Vanadium 50

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    This is so far from the realm of "existing earth technology" that you will have to invent some kind of techno-magic.

    Over the last couple centuries, the full industrial capacity of the earth has changed the atmospheric content of the earth by about 100 ppm. You need to change it by 200,000 ppm.

    Worse, chlorine is more reactive than oxygen. Whatever you do, the reactions are going to want to go in the other direction: binding chlorine and releasing oxygen.
     
  5. Jun 2, 2015 #4
    Ok, I'm definitely open to altering this part. What would otherwise be a good substitute for chlorine in the story? It has to be 1) harmful for humans and 2) provide enough energy to be feasible for a technologically evolved race to have evolved breathing it.

    Again, time frame is not really an issue and there can be some suspension of disbelief as to the exact mechanics of it, but I'd like it to be reasonably believable chemically. If a global scale forest fire or the simultaneous burning of all fossil fuels would get us to depleting all of the world's oxygen supply within decades, I could work with that. Or if there was a way of funneling chlorine or whatever other substance from another planet like Stedwards was speculating, that could work too. I just need to build a plausible industry around it and have an idea of how long it's been going on to be nearly complete.
     
  6. Jun 2, 2015 #5
    Would "terraforming Earth conditions to awful one" good enough for you?

    Aliens need higher temperature, so you have to boost temperature by 30 Celsius. Effective way - ultra high emission of greenhouse gasses, especially sulfur hexafluoride. It's within realm of possibility.

    http://en.wikipedia.org/wiki/Sulfur_hexafluoride

    (if that good enough for you, I can give you link to studies concerning terraforming Mars in this way)
     
  7. Jun 3, 2015 #6
    Sounds interesting, but hard to sustain a group of human characters in a climate like that. Benefits of having the aliens chemically alter the composition of the atmosphere as opposed to attempting to boost temperature is its easier for human characters to survive with oxygen tanks and gasmasks.

    I'd love to read that link still though.
     
  8. Jun 3, 2015 #7

    SteamKing

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    Who cares how much chlorine is in the earth's crust?

    There's a much larger global repository of chlorine than that, and one that's easier to access, to boot. Any guesses?
     
  9. Jun 3, 2015 #8
    (I failed to find exactly the link that I wanted)
    Not bad one: http://www.pnas.org/content/98/5/2154.full.pdf

    I'd think that my chances (as unprepared) are higher in case of dealing with heat than with non-breathable atmosphere. There are some mountains / polls to escape.

    Maybe 30 would be a bit excessive but resistance active in mountains and during nights is something that would fit more or less the genre. ;)

    [EDIT] Anyway, regardless of your choice, I rather see one problem - aliens are so ultrapowerful that can quickly terraform planet, but not powerful enough to crush the resistance[/EDIT]

    Steamking:
    Wouldn't this chlorine would like to be dissolved in sea water back?
     
    Last edited: Jun 3, 2015
  10. Jun 3, 2015 #9

    Vanadium 50

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    It's in the oceans. But how do you propose getting it out? And more to the point, how do you get the O2 out of the atmosphere?
     
  11. Jun 3, 2015 #10

    SteamKing

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    Sea water already contains about 2% chloride ions dissolved in it. It's not clear how much gaseous chlorine could be dissolved on top of that.

    http://en.wikipedia.org/wiki/Seawater
     
  12. Jun 3, 2015 #11

    SteamKing

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    Any sufficiently advanced alien culture should have discovered electrolysis.
     
  13. Jun 3, 2015 #12
    And after electrolyzing all the oceans you evaporate them to remove the sodium hydroxide (to prevent it from reacting back with the chlorine), and you bury it in all the holes you made to dig up ALL the fossil fuels ever produced, which you then burn to get rid of the oxygen. After that it's only a small task to dig up enough siclicate rock and let it weather to get rid of the excess CO2.
     
  14. Jun 3, 2015 #13

    SteamKing

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    That's such a crude method.

    Presumably, these same aliens had also discovered chemistry by this point in their civilization. One need only separate out the salts from the rest of the seawater and electrolyze those to recover the chlorine and whatever else they need. Even humans are sophisticated enough to know that:

    http://www.mortonsalt.com/salt-facts/salt-production-and-processing

    I'm beginning to think that these aliens may not be as advanced as everyone seems to think. I think Earth can take them and avoid this whole mess. Teach these aliens not to mess with us!
     
  15. Jun 3, 2015 #14

    Vanadium 50

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    You still are facing the problem that chlorine is more reactive than oxygen. You make your chlorine, you stick it in the atmosphere, and it finds an oxide somewhere and turns it into a chloride. Now you have even more oxygen in the air.

    I think you also have a problem that you are making dilute sodium hydroxide in the oceans. That will react with the chlorine in the atmosphere and send it back into the ocean. Filling the oceans with lye and bleach will make people sad, but what it won't do is replace the atmosphere with chlorine.
     
  16. Jun 4, 2015 #15
    Though the replies here have been awesome (and highly entertaining), I'm afraid Vanadium may be right - the question becomes what other gasses would be preferable to use rather than oxygen or chlorine for breathing? What would be energy efficient enough to sustain the evolution of a highly advanced alien race?

    Biologically, the producer wanted them to be invertebrates or at least appear distinctly inhuman, slimy with multiple flexible tendrils/tentacles, if that's any clue as to their native living conditions. They're dependent almost entirely on technology (similarly to War of the Worlds). He showed me some concept art reminiscent of what can only be described as a telepathic cyborg-Cthulhu. Honestly that description alone is why I signed up.
     
  17. Jun 4, 2015 #16
  18. Jun 4, 2015 #17
    Chlorine in fact has a lower electronegativity than oxygen, though reactivity depends mainly on the enthalpies of the compounds that can be formed. The main problem is that it is rare, so chlorine atmospheres would basically never form naturally anywhere. It also reacts with water, forming hypochloric acid as well as hydrochloric acid, the latter, the equivalent of water in this context, is much more reactive than water and would react with (almost) every compound it would encounter in Earth's crust.
     
  19. Jun 5, 2015 #18
    How about something like the aliens require some trace element that is lethal to humans?
    For example perhaps they need to extract sulfur from air, so they seed the atmosphere with hydrogen Sulphide (H2S)
    The recommended exposure limit is around 10min @ 10ppm

    H2S might tie into the story well, as a species we have a good deal of experience with the gas from sour oil&gas production. So aliens dumping tonnes of it into the atmosphere would make it tough for us to survive but wouldn't be an immediate defeat. Allowing for survivors.
     
  20. Jun 5, 2015 #19

    Vanadium 50

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    Or NOx. That has the advantage of the precursors already being in the atmosphere.
     
  21. Jun 5, 2015 #20
    Or N2O... It can serve as very good oxidiser and would make any poisoning much more interesting...
    (plus it would ruin ozone layer and cause global warming)
     
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