This question is in the same context (the same world) proposed by Steven L. Gillett in his book: "World-building". And actually there have been some other questions about this same chlorinated world. For example this:

Are plastic plants plausible?

Long history short: The world is Earth-like (same biochemistry, carbon based also and oxygen breathers) but the atmosphere contains 1% chlorine.

Steven L. Gillett explains about the atmosphere:

"Chlorine is colored; as all the chemistry texts say, it’s a “greenish yellow” gas. It absorbs blue light (and shorter wavelengths) strongly."

and explains about the water:

"the acidity of surface water will be about like that of undiluted vinegar."

I am trying to draw such a planet as accurate as possible, but I have a problem with the color of the polar ice caps and the snow. Although the planet has oceans and rivers of water, there will be a small amount of (an equivalent of) acid and bleach solution in the water. So, my question is: What color will the snow be in that planet?

EDITON: Justification of the chlorine in the planet (from the book also)

Over geologic time, geologic processes have gathered most of Earth’s chlorine together in the ocean, in which it’s present as the very stable chloride ion, Cl-. In fact, it takes quite a bit of energy to strip that electron away. But suppose some plant in the oceans evolved the capability to make chlorine gas from chloride by stripping that electron away again, using the energy derived from food. (And, of course, a special enzyme system.) Why would it bother? Well, say, as a defense mechanism. It could then incorporate the chlorine into its biomolecules to make itself a poor meal for predators—making its own natural chlorinated pesticides, if you will. But predators will eventually evolve a defense to that, too. Then, the continued escalation of the biological “arms race” may eventually result in plants releasing free chlorine—still as a defense mechanism, a natural gas attack.

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    $\begingroup$ Are you aware of how incredibly toxic chlorine is ? $\endgroup$ Commented Feb 15, 2019 at 2:40
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    $\begingroup$ @StephenG, are you aware of how incredibly toxic oxygen is? If the atmosphere is 1% chlorine, then anything evolved there will be able to handle it. $\endgroup$
    – Mark
    Commented Feb 15, 2019 at 3:07
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    $\begingroup$ Would chlorine become trapped in the snow just as it does when you make ice cubes from chlorinated water? The latter smell like swimming pool when they melt. $\endgroup$
    – NofP
    Commented Feb 15, 2019 at 12:31
  • $\begingroup$ @Mark I'm very skeptical that at 1% levels the extreme reactivity of chlorine would make life viable.The longest I've heard organisms survive in chlorine treated swimming pools is ten days, but that's a much lower concentration level. $\endgroup$ Commented Feb 15, 2019 at 14:24
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    $\begingroup$ My first thought was.... blue! Mostly because that would be pretty. Seems like chlorine would become chlorides pretty quick. And there's some pretty metal chlorides like copper chloride as it gains moisture and cold temps. Thinking of the pretty auroras in atmosphere at the poles including greens and blues makes me wonder if there could somehow be enough metal ions in atmosphere for something cool like blue or green- blue snow to exist on that world. I apologize if attempt to recall biochem is way off. $\endgroup$
    – N2ition
    Commented Feb 15, 2019 at 18:22

2 Answers 2


Since chlorine doesn't become a liquid until roughly −34 °C (−29 °F) your snow will be mostly white. As a liquid it is amber in color and this will give your snow a slight yellow discoloration.

However, it will likely react with the water and create a weak acid (HCl or HOCl). Its usually clear, but can have a yellow discoloration.

If anything, your premise appears to be a little incorrect here. Chlorine is highly reactive so if you had 1% it would quickly react with other elements and disappear from its pure form. Water in the form of moisture, snow, rain would quickly remove all the chlorine in your atmosphere.

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    $\begingroup$ "your snow will be mostly white" : Sorry, but I gotta ask, are you sure? $\endgroup$
    – Pelinore
    Commented Feb 15, 2019 at 1:06
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    $\begingroup$ @Pelinore Yes... 1% chlorine in the atmosphere is different from solid chlorine $\endgroup$
    – Shadowzee
    Commented Feb 15, 2019 at 1:09
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    $\begingroup$ Ah yes, barely enough to even tint the result really [+] $\endgroup$
    – Pelinore
    Commented Feb 15, 2019 at 1:09
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    $\begingroup$ Oxygen is highly reactive, yet it persists in the atmosphere. Or rather, is continuously replenished by photosynthesis. So this planet would have to have a parallel photosynthetic pathway that releases chlorine rather than oxygen. $\endgroup$
    – jamesqf
    Commented Feb 15, 2019 at 5:48
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    $\begingroup$ @jamesqf Chlorine is more reactive than oxygen (unless you are talking about a single oxygen molecule which can basically bind with anything but isn't common). Look at Zeodyns answer which looks into that aspect more than mine. $\endgroup$
    – Shadowzee
    Commented Feb 15, 2019 at 6:02

Chlorine reacts with water in an equilibrium: Cl2 + H2O <-> HClO + HCl

As long as the pH doesn't rise this means there will always be some elemental Chlorine left. So water alone isn't a problem for your chlorinated atmosphere. The problem is, that any reducing agent on the ground will over time react with the atmosphere. Wood for example or any metals. Oxygen only does that with enough starting energy or over long periods, chlorine will happily react at room temperature with any biomass. You can see that this happened on our earth in the past, which is why the oceans are full of NaCl. So you might be able to sustain such an atmosphere if the surface is inert to the reaction with Chlorine.

As to the color of snow: Chlorine won't be trapped or solved in the ice just as CO2 isn't. The crystalization purifies the water and only traces of Cl2 should be found in it. So your snow should be white.

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    $\begingroup$ But you could say almost exactly the same for oxygen. If you look at the effects of the first oxygen-releasing photosynthetic organisms, there was a period of ~200 million years or so where the oxygen they released reacted almost immediately with other things like dissolved iron. It's only when those were all used up that oxygen concentrations increased. (See e.g. en.wikipedia.org/wiki/Great_Oxygenation_Event ) So you could plausibly have a similar sequence of events with a chlorine-releasing photosynthesis. Of course the biochemistry would be different than ours. $\endgroup$
    – jamesqf
    Commented Feb 15, 2019 at 19:39

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