# Initial conditions for a fluorine hellscape

I'm curious what it would take to get a planet with so much free fluorine that you can have an atmosphere of predominantly or purely fluorine. I realize how improbable a planet like this is, so I'm looking for the long set of perhaps contradictory conditions that would lead to a hellscape planet like this. How do I get a planet sized ball of fluorine ash products and an atmosphere of fluorine to go with it?

Planet parameters:

1. Earth-like in mass.
2. Surface atmosphere of one bar
3. Atmosphere is >70% fluorine

Points in a good answer :

1. Organization and evolution of the proto planetary disk.
2. Processes required to concentrate enough fluorine in one place to get a fluorine planet and fluorine atmosphere.

## Out of Scope

1. Questioning the question :) I know this planet is crazy and highly improbable. Don't tell me why it can't work. Tell me why it can and what it would take.
2. Life. Whether this planet will support life doesn't matter to this question.
3. Evolution of the planet past initial formation.
• Oxygen is very reactive, but fluorine is so crazy reactive that it even reacts with the "completely" nonreactive noble gasses. Thus, it won't work. – RonJohn May 11 '17 at 4:02
• – JDługosz May 11 '17 at 6:49
• @green I'm sure it somehow ends in a planet with a fluorine like atmosphere ;) – Totumus Maximus May 11 '17 at 13:40
• @TotumusMaximus Wouldn't that dissolve the turtles, tho... – kingledion May 11 '17 at 13:45
• You can't question the question as Green already questioned the question in this question, making further questions of the question in question both circular AND redundant. – Gio May 11 '17 at 18:57

## Here goes nothing

Firstly, fluorine is not particularly abundant in the universe so you need a large source in order to get an atmosphere of it. Additionally, fluorine is easily consumed in stellar nucleosynthesis so it's hard to encounter a significant concentration for your planet forming. Anyway, let's begin!

## A Star Dies

In a Red Giant star at the end of its life, it's inside is stratified with different layers of fusion occurring. In these different layers, you get different elements.Lighter in the outer layers, heavier in the center. The Star Dies in a core collapse supernova or Type II blasting its layers outward and releasing a ridiculous amount of neutrinos.

Relevant XKCD comic. Now the outflow of neutrino radiation is so great that it may be enough to knock a proton off Neon or Neutron to make Neon-19 which decays into fluorine. This process is currently speculation. The result is an expanding fluorine rich shell of gas. For the sake of argument, let's say there is a whole bunch of it. The whole system falls into a disk around the supernova remnant.

## Planet-forming

Planets start forming during your standard process: accretion. These planets form in the heavy element-rich layers with no abnormalities. Except two relatively large planetesimals form and collide with each other. These two planets were not so dissimilar from Earth during the early Haden Eon. So violent is the collision that the metal-rich cores of the planets are knocked out and form a new planetesimal rich in metal.

## The burning

Our new planet drifts out into the fluorine rich part of the disk and begins gathering it due to its gravity. The planet has a surface rich in nickel and iron with much fewer silicates than earth. As the planet begins to gather fluorine the surface burns. But the metal rich surface reacts with the fluorine and produces a nonreactive passive layer. The silicates burn and react and form their own layers or there is not enough silicate to react with the fluorine. As such the planet is able to keep a fluorine atmosphere for a geological period of time.

## Results

A dense (why not venus atmospheric density for good measure) fluorine rich atmosphere with a whole bunch of other nasty compounds on its surface. Even More Hardcore it's also orbiting a neutron star or black hole.

• See my edit: don’t fake breakheads by using an all-bold paragraph. Use the header markup, which you can see on the toolbar. – JDługosz May 11 '17 at 6:44
• Regarding "the metal rich surface reacts with the fluorine and produces a nonreactive passive layer", I don't think you are going to get that. Iron III flouride is hygroscopic and Iron II flouride readily forms a tetrahydrate. If there is any water at all on your planet, then you will end up with a souply HF ocean saturated with dissolved iron. The dissolved iron would be constant precipitating out of solution and falling to the sea floor, while other iron would be getting dissolved by the HF. – kingledion May 11 '17 at 12:21
• @kingledion HF oceans are the clearest indicator, from space, that "here there be dragons". A foolhardy explorer wouldn't even make it to the lower atmosphere. – Green May 11 '17 at 13:43
• @JoeKissling could you clarify what you meant by "metal-rich cores of the planets were knocked out"? Knocked out to where? – Green May 11 '17 at 13:50
• @kingledion that is why I chose this method of planet-forming to keep the amount of water on the world very low. The hot cores of planets should have very little to begin with and residual heat should bake off most of what arrives back during re-accretion. The ocean, (lake) of HF will be small and the iron, HF reactions will get to an equilibrium between what's on the floor and what is dissolving in. Or the shores of said lake are nickel. I mean the passive layer as an average for the whole surface, as in the surface is not going to consume the entire atmosphere. – Joe Kissling May 11 '17 at 14:49

# Problem

The problem here is not the reactivity of fluorine - you can make a planet out of fluorine compounds that will be stable wrt fluorine by definition. The problem is how we concentrate enough of the stuff in one place to make it dominant.

# Concentrating F

Now, what we could imagine is a highly magnetic object acting as a particle accelerator and also banding the paths of released particles - not dissimilar to a mass spectrometer on earth. So imagine a spinning neutron star ploughing through a cloud of dust released by a supernova, throwing off atoms at different angles according to their mass. These particles then impact a dense region of dust and gas, forming zones of differently concentrated elements.

# Making a Planet

Now we have a freakishly fluorine-rich area, which collapses into a star and protoplanetary disk by the normal mechanisms; there is further differentiation within this disk - because the F-rich minerals have chemical properties that have them condensing at a certain distance from the new star (just as the solar system has an ice line) so we concentrate the F-containing compounds into a smaller band; this turns into a planet with a very high percentage of fluorine. We can also deliver more fluorine from icy objects outside of this band.

# Planet Composition

Common elements such as silicon, carbon, and hydrogen wil be present; we'd want to minimize oxygen. We might propose that there is an iron core and silicate layer at depth in this planet, with a fluorosilicate layer above that, an ocean of HF and an atmosphere of F2.

Sorry but this is in a real danger of becoming bad fiction. You don't let a baby win the Olympic marathon in a book either. Fluorine isn't created in large quantities by stars so you won't find large quantities in space. In addition to that, fluorine reacts with basically anything - even some noble gases. It's way more reactive than oxygen. Look, you not only need a rare element to form a planet, you also need common elements to be absent from said planet - like hydrogen for example (or else you would have an hydrogen fluoride atmosphere). Let's not say it was created like oxygen on this planet because 70% is way too high for that. I had a whole passage about why life can't exist on that planet and this option is out of the question, but I deleted it - felt out of place. Basic thing is: It can't happen.

Just wanting it to work doesn't make it a good idea. So here are possible solutions:

The planet has been created by a super advanced civilization as a joke and any possible life has been designed by them as well with materials we do not know of yet.

The planet was inhabited by a civilization that created fluorine as industrial waste, even collecting it throughout their star system. This ended in catastrophe.

• Just because we're so fascinated with non-carbon and non-oxygen based lifeforms is IMHO enough for our own civilization to try it out when we'll have the means to do so. – Agent_L May 11 '17 at 10:59
• @Agent_L your comment ignorantly implies that life on Earth just randomly used carbon & oxygen, and that -- for example -- silicon, fluorine & arsenic would do just as well. That is far from true. Carbon's four valence bonds allow for stupendously rich chemistry, and both carbon and oxygen are stable enough to not instantly react (like fluorine does) but reactive enough to allow for the reactions we call "life". – RonJohn May 11 '17 at 15:33
• What would be wrong with a hydrogen fluoride atmosphere in this example? It'd be 90% fluorine, just bound with hydrogen. It'd definitely make a hellscape! – Doktor J May 11 '17 at 18:54
• By your definition, HGttG is "bad fiction". – Gio May 11 '17 at 19:03
• @Glo in the universe there is 10^4 more carbon and oxygen than there is fluorine. But that has no bearing on my comment. – RonJohn May 11 '17 at 19:17

There is a superadvanced race that is able to manipulate space to build what is in effect a cavatron that can process a whole star at once (Obviously it's working on something other than an electric current!) They haul the machine to stars that are just about to die and toss them in, the matter is scattered across space by it's charge and atomic mass.

They scoop up everything of value. They do not consider the fluorine of value.

(For those who haven't heard of them: You ionize the input material, subject it to a fixed voltage, deflect it with a magnet and catch it. The deflection is based on charge/mass, so long as you are precise enough and the buckets are placed appropriately it's quite capable of doing things like U-235/U-238 separation--and with a machine a good engineer might be able to build in his garage. We aren't buried in engineers building nukes because the power requirements are incredible--we even tried that route in the Manhattan project, it was abandoned as impractical.)

Highly reactive species, like oxygen or fluorine, need to have some sort of cycle freeing them back in the atmosphere.

Therefore if there was some form of life which used oxydation by fluorine as energy source and some sort of plants using photosynthesis based on fluorine instead of oxygen, you could get quite some free fluorine in the atmosphere.

• In this question, the answer provided by Andrew Dodds takes a good poke at that. – Gio May 11 '17 at 19:05

This answer is going to be long and rambly because I'm writing it on my phone and I just wanted to throw all the thoughts I had out there. Maybe it should be edited later though to make it easier to read and/or to add sources for claims. Also, I may stray a bit away from your specifications for the planet at times.

First of all, O2 is a high energy molecule that can only be present in the Earth's atmosphere because it is created by life. Life is kind of like the opposite of chemical equilibrium and often pumps systems away from chemical equilibrium when it stores energy in itself by methods like photosynthesis. If a planet has an F2 rich atmosphere, it's probably biogenic, because otherwise it would have to just have so much fluorine that it ran out of things to react to (which is really weird) or it would have to be at extreme temperature-pressure condition where that was part of the equilibrium (?really high temperatures or low pressures, but not enough for atomic F), or there is some non-biogenic process pumping this disequilibribrium, like ultrahot volcanic eruptions, lightning, or radiation breaking things up.

Personally, I think the idea that a life-form might release F2 into the atmosphere is quite plausible, but only if fluorine is particularly common in it's environment in some other form. Thus, large parts of your planet need to be rich in both fluorine (probably fluorides of some kind), and energy that is accessible to autotrophs (likely sunlight, but not necessarily). The key issue is getting enough fluorine on the surface, in any form.

There are probably astrophysical processes that could produce particularly fluorine-rich parts of the universe, but I don't know how fluorine rich. Fluorine has only one stable isotope: Fluorine-19. F-19 is generally easily destroyed in environments where fusion can occur and it is not directly produced by the alpha-process, (which makes the even numbered elements from carbon to nickel). F-19 is produced in a couple variants of the CNO cycle that are somewhat important in heating large stars (CNO-III and HCNO-III), but it is quickly and easily destroyed by the following fusion reactions in those cycles. Apparently the fact that there is as much fluorine as there is is a bit of a mystery, so that leaves hope that there might be some process that isn't well studied that's very good at making it. Also, I really don't know at all, but maybe some kind of neutron- or proton- capturing process that either happened really quickly and the stopped or happened in a low energy environment might make a slightly more favorable amount of fluorine. Look up the s-process, p-process, r-process, and rp-process to see if any work. I know special astrophysical environments can maje some extremely strange balances of isotopes, like how the r-process of simulated neutron-star mergers makes more noble metals, elements around caesium, and lanthanides than anything else and a weird balance of the lighter elements.

I doubt astrophysics will solve all your problems, but it could be a good first step.

My first guess as to what the most important factor in getting a fluorine-rich environment would be is that some chemical process is concentrating fluorine near the surface, pulling it out of the mantle. I believe that fluorine is slightly enriched in the Earth's crust, but there is still a lot more of it in the mantle that we could take out. In fact, the increase in the amount of fluorides in water is one of the major threats of volcanic eruptions, and HF and SiF4 are notable components of volcanic gasses.

It's also important to just remember that planets have way more mass in there interior than near their surface. I've heard that even gold could completely cover the surface of the Earth as a thin layer of coins if you took all of it out of the Earth's interior. Enrichment based on chemical and physical properties already do wonders towards making rare elements common in some encironments. Consider how silicon is the 2nd most common element in rocky planets while helium barely present at all, how the Earth's oceans have so much chlorine (as chloride) in them because it concentrates on the surface and in water rather than in the mantle, how the Earth's atmosphere is mostly nitrogen because close to half of all the Earth's nitrogen is in the atmosphere while most of the oxygen, carbon, hydrogen, and sulfur are in in the crust, mantle, core, and oceans, or how ore deposits and crystals of rare elements like HgS deposits can form.

I'm not sure what could preferentially enrich fluorine so much to the exclusion of other elements. Fluorine is usually found as HF in the interstellar medium, which is more volatile than H2O, but not as volatile as NH3. On Earth and probably on other planets, it exists mostly in the form of metal fluorides like CaF2 and Ca5(PO4)3F (the latter of which contains most of the fluorine in Earth's crust). I think that fluoride salts are not very soluble in silicate mantles like that of the Earth, since it is enriched in the crust relative to the upper mantle and is thus classified as a "lithophile" element.

I'm not sure how deep the depletion in the mantle goes, though. I think it may be that only the upper mantle is strongly depleted in lithophile elements and the lower mantle is not. (I'll have to check back with data on the composition of magmas from plate boundaries vs magmas from hot-spots, the latter likely being volatile-rich magma from deeper in the mantle.) If this is true, then increasing the level of convective mixing between the upper and lower mantles of a planet might help, but be careful, because heating up mantle so more of it is molten (the most obvious way to do this) might increase the solubility of fluorides in it.

Somthing you may want to look into is the chemical speciation of fluorine in the interiors of carbon planets. (The term "carbon planet" generally means terrestrial planets that have more carbon than oxygen, but there is also an in-between category, as rocky planets with >~1% carbon will likely have graphite crusts according to a paper I read most of.) Ionic compounds like fluoride salts can't dissolve very well into solid carbon, and carbon is likely to be solid all the way down to the core in a carbon planet, due to its hot melting curve, high thermal conductivity (cooling the interior) and the fact that carbon planets have too much carbon to react to form more volatile compounds, though it will likely form other refractory carbides like SiC if there is more C than O. If fluoride salts are lighter than graphite, and crucially are able to collect into separate fluid or maybe even solid phases that are lighter than graphite (noting that solids can sometimes act somewhat fluid over geologic time-scales), then these fluoride salts might reach the surface as diapirs. From there, they may dissolve in water or, on a very hot planet, form lakes or seas of molten salt.

In addition, SiF4, fluorocarbons (and hydrofluorocarbons, chlorofluorocarbons, etc.) like CF4, HF, and sulfur fluorides like SF6 are all quite volatile substances (gasses at room temperature, except for some fluorocarbons, though HF melts at 19.5°C). These could be formed in some highe temperature low-pressure environments (like how SiF4 and HF can be formed from Earthly volcanism) and could help spread fluorine into the atmosphere. I think that carbon planets, with substances like elemental carbon, SiC, and sulfides floating around, which are more reactive than oxides and oxoanion minerals like silicates and SiO2, might be more conducive to the generally endothermic reactions that produce these volatile fluorides from the relatively more refractory ionic fluoride salts.

Melting points and density are one way to enrich the surface of a planet in certain elements. Perhaps the most useful property to exploit, though is solubility. The Earth's oceans are full of sodium chloride, despite the fact that chlorine is not much more common than fluorine, and actually less common in the Earth's crust (though that's probably because the ocean is not counted as part of the crust). Why is there so much chloride rather than sulfates, carbonates, silicates, etc, because chlorides are much more soluble in water than those things. Many fluorides are also very soluble. The problem is calcium (and somewhat other Alkali Earth metals like strontium and barium). There is more calcium on Earth's surface than fluorine, and fluorine prefers to bond to Ca to make CaF2 and Ca5(PO4)3F than to bond to anything that's as common, and alkali-Earth fluorides like this have extremely low solubilities in water until you get to the extreme pressure-temperature conditions when all sorts of things, including silicates, are highly soluble in water (ruining the ability of solubility to enrich fluorine effectively). If a planet had way more potassium than Earth, however, than the fluorine might form highly soluble KF. Similarly, if a planet had less calciu than fluorine on it's surface (perhaps due to being particularly deficient in calcium), then the excess fluorine might mostly bond to sodium to make highly soluble NaF. Either way (or both ways), you could get planets where fluorides partitioned into the oceans as much as chlorides, potentially leading fluorine being similarly or perhaps more common than chloride in the world's seas. This enrichment of Na and K and/or depletion of Ca, need not be due to astrophysical processes (although forming a planet out of material that the alpa-process or at least it's late stages did not contribute much to could easily help), but could be due to the melting points of Na and K compounds vs Ca ones. This exact composition difference is part of what differentiates felsic from mafic rocks on Earth, and similar differences could be formed due to the lower melting points of Na2S and K2S vs CaS and I think Na2O and K2O are less stable than CaO. (Note, enriching a planet in sulfur relative to Earth is easy by just having it form further out in the protoplanetary disc where FeS can form, c.f., Io. Also, perhaps collisions with S-containing comets might be common furthur out.)

Maybe if you can get an ocean full of fluoride salts and/or a non-trivial amount of volcanically-originated fluorine compounds like the quite stable SiF4 and CF4 into the atmosphere than it might be plausible for life-forms to be surrounded by enough fluorine that storing energy through endothermic reactions that produce F2 might be viable.

About oceans, though: The planet probably shouldn't have too much liquid water to start out with. F2 reacts pretty quickly with H2O, so turning all the H2O into HF and O2 will increase it's lifetime in the atmosphere a lot. Life could perhaps adapt to using liquid HF as a solvent instead of water if the change was very slow and the conditions cold and/or high-pressure enough. (Pure HF is liquid -83.6°C~19.5°C at 1 atm.) Interestingly, apparently many organic compounds including proteins and carbohydrates can dissolve in HF without reacting (although they do react with F2) because C-F bonds aren't any more stable than H-F bonds. HF is less stable than other fluorine compounds the life-forms that originally made the F2 likely started out decomposing, so the autotrophs could probably adapt to make what water their biochemistry still needs, and store energy at the same time, by using the endothermic reaction:

4HF + O2 → 2H2O + F2.

F2 also reacts with rock-forming oxides, but since the these are solid and so are many of the reaction products (AlF3, NaF, MgF2, CaF2, KF, FeF2, FeF3, but not SiF4), they may form a thin surface fluoride layer before too much F2 is used up. Once there are no liquids or gases on the surface that react with water, and the solids are all covered with a thin layer of fluorides, F2 will have a much longer life-time in the atmosphere and will have at least a chance of accumulating.

I should also mention atmospheric loss as a way to affect a the composition of a planets atmosphere. Maybe if a lot of the lighter elements in an atmosphere are lost over the first billion years or so, then heavy fluorine compounds like SiF4, CF4, and, SF6 might a lot of what remains.

Also, I'm going to repost this comment I made here:

Have you seen either of these: Alien Atmospheres: How to Make Plastic Trees (or the fiction it referring to and links in the description) or this 2014 paper: Fluorine-Rich Planetary Environments as Possible Habitats for Life?

• This is amazing! Way more detail than I what I had expected. Thank you very much! – Green Mar 4 at 6:13
• I just had a realization about this: The planet probably shouldn't have too much liquid water to start out with. F2 reacts pretty quickly with H2O, so turning all the H2O into HF and O2 will increase it's lifetime in the atmosphere a lot. Life could perhaps adapt to using liquid HF as a solvent instead of water if the change was very slow. F2 also reacts with rock-forming oxides, but since the these are solid and so are many of the reaction products (AlF3, NaF, MgF2, CaF2, KF, FeF2, FeF3, but not SiF4), they may form a thin surface fluoride layer before too much F2 is used up. I'll add this. – H. H. Mar 19 at 20:11

Ionizing radiation can release fluorine from rocks. CF "Stinking Fluorite" which is Fluorite contaminated with Uranium, in which beta particles break apart fluorite into calcium and fluorine.

https://en.wikipedia.org/wiki/Antozonite

So your hellscape just got more hellish. To keep fluorine in the atmosphere there has to be enough beta emitters in the crust and fluorites comingled to trigger its release.

So its not just poisonous but radioactive too.

The necessary beta particles could also come solar wind. Maybe the planet has a dead core and weak magnetic, so solar radiation impinges the surface unheeded.

Even more hellish.

• Hi Daniel, welcome to Worldbuilding! Your answer doesn't really answer my question though. While I appreciate the additional information about how to make the fluorine hellscape more hellish, I'd like to hear what you have to say on how such a planet might form in the first place. – Green May 12 '17 at 0:18
• Welcome Daniel! You can edit your answer with the little grey "edit"-button. If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! – Secespitus May 12 '17 at 7:19

First problem is to get a fluorine rich planet at all. Stellar nucleosynthesis is working against this, and the remaining possibility is an engineered planet with unusual elemental composition. So as a starter, you have an advanced civilisation that builds a fluorine rich planet as kind of laboratory for planet evolution.

Second, what will the atmosphere of fluorine rich planet contain? Most probably, predominantly HF. $CF_4$ will polymerise over time given solid teflon. Probably some $SF_6$ and some $NF_3$ will be in the atmosphere, too. They are a very strong greenhouse gases leading to heating up the planet: Your hell will probably have Venus-like temperatures!

I don't expect free $F_2$ to be a major component of the atmosphere unless there is some biological activity on the planet. It will be very strange and unlikely lifeforms, but I can imagine exotic life based on carbofluorides.