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I need a planet that is a combination of chlorine world and vitriolic world. The planet is extremely hot and under high pressure, covered in oceans of pure sulfuric acid and a chlorine/fluorine atmosphere replenished by silicon-based autotrophs.

What are the optimal conditions for such a planet to form and remain stable? Surface temperature, atmospheric pressure, and so forth.

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  • $\begingroup$ Science cannot answer this. Non-carbon based life wasn't proven possible, and many proposed structures are proved impossible. But of course it's easier to scientifically prove possibility of existence (just find one example) than impossibility of existence when we don't know what precisely we are looking for. $\endgroup$
    – Mołot
    Oct 19, 2016 at 13:24
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    $\begingroup$ You state that your planet is like Venus, then ask 'can such a planet form naturally.' Can you clarify the question to be either 'Can a planet like Venus with sulfuric acid oceans exist?' or 'Can silicon-chlorine-flourine metabolic pathways develop on a venus-like planet?' I think it is two broad to talk geology and biochemistry at the same time. $\endgroup$
    – kingledion
    Oct 19, 2016 at 13:48

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Honestly, if there is enough chlorine around to form a chlorine world at all, I would expect vitriolic worlds to also be chlorine worlds by default. Why? Because chloride salts are unstable in sulphuric acid. So once you have conditions that allow for the formation of a sulphuric acid sea, those are the same conditions that will result in the production and release of chlorine compounds into the atmosphere.

Even on Venus, hydrogen chloride and hydrogen fluoride are both present in the cloud layer in detectable amounts, in addition to the famous sulphuric acid.

So, what conditions are necessary for a vitriolic chlorine planet? The same conditions necessary for a vitriolic planet, plus a bunch of chlorine in the primordial cloud from which the planet condensed- which is what you need for a chlorine planet anyway.

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  • $\begingroup$ This is a good point. It seems most chlorine in the crust will eventually become hydrogen chlorine gas over geological time. $\endgroup$
    – kingledion
    Oct 19, 2016 at 19:33
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Firstly, chlorine is not a very common element, so the chlorine world seems far-fetched. Second, there is a general lack of chlorine-sulfur compounds and chemical cycles involving chlorine and sulfur. Without processes that can 'buffer' the concentration of sulfur and chlorine in a geologically active crust/ocean/atmosphere, it is tough to imagine such a world.

Chemistry is not my forte, so I can't interpret this too well, but here is a report on a speculative pathways for a chlorine-sulfur dioxide cycle that could produce such a world.

I recommend you might try for a world with a better understood set of processes, such as the sulfur-iodine cycle which does take place at high temperature and pressure.

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Since chlorine is quite a heavy element (heavier elements are produced in lesser quantities by stars), the star which produced enough chlorine to create a whole chlorine world/planet must have been a 9 solar masses+ blue supergiant star. This star exploded in a supernova and from its remains (the nebula) this planet could have formed.

Chlorine gas is not a very good greenhouse gas, meaning that your planet would not be a blistering world unless it was quite close to the parent star. Or if you prefer to have CFCs in the atmosphere instead of pure chlorine, then yes, you do get a highly greenhouse atmosphere with a large heat reserve.

A planet with an atmosphere primarily composed of chlorine is indeed possible, considering that there is nothing which makes it impossible to form. However, such a planet cannot be of a gas giant size. It would be a terrestrial planet (most probably Mars sized) with vast amounts of chlorine-containing chemicals in its crust. These chemicals either suffer decay or are processed (by organic processes) by microbes to release the chlorine in them.

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Your planet couldn't exist, because it's too hot for sulfuric acid oceans.

If your planet is Venus-like, it can't have sulfuric acid oceans. Liquid sulfuric acid only exists in the upper portions of the Venusian atmosphere. Droplets do collect into rain, but this evaporates and then dissociates into other molecules as it enters the lower portions of the atmosphere. Your planet would need to be significantly cooler for oceans of sulfuric acid to form.

EDIT: The question changed since I answered it, so I'll come back and fix this answer at some point.

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  • $\begingroup$ How cooler would it need to be? If I calculated correctly under 90 atm the boiling point of sulfuric acid is 770C (significantly higher than 462C). For water this is 279C: I cannot go below that and allow liquid water on the planet. $\endgroup$
    – Anonymous
    Oct 19, 2016 at 16:39
  • $\begingroup$ I disagree. Sulfuric acid is liquid at standard pressure between 10C and 337C. I can't find corrected values for 90 atm, but I suspect that the boiling point is much higher under pressure, just as waters is. The linked vitriolic world's surface temps of 250C-330C seem about right. $\endgroup$
    – kingledion
    Oct 19, 2016 at 17:42
  • $\begingroup$ I'm not entirely sure what temperature/pressure would work for having sulfuric acid oceans. I know that, on Venus, sulfuric acid boils/breaks down before it reaches the surface, but it has liquid sulfuric acid higher in the atmosphere. If your planet is basically Venus minus the first few kilometers of atmosphere, you could probably have oceans of liquid sulfuric acid. There will be some water in your oceans, as well, since sulfuric acid is water soluble and will absorb some water from the atmosphere. $\endgroup$
    – ckersch
    Oct 19, 2016 at 18:30

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