As a case study in the construction of alien atmospheres,
let's look at two planets: Niflheim and Clorox.
Both worlds feature halogen atmospheres, (atmospheres based on these elements)
but where Niflheim reimagines, Clorox tweaks. Two very different build methods.
Niflheim orbits a blue-white B8 star at a distance of 4.97 AU.
It's gravity and atmospheric pressure are the same as Earth's, but it's average temperature is about -60 degrees Celsius.
So already Niflheim is looking like a very alien world.
Its atmosphere is comprised of various fluorides of nonmetals.
A fluoride is a compound involving fluorine, and a nonmetal is any of these fellas.
Small amounts of various inert gases, a touch of free oxygen, and about four to five percent fluorine.
When it's really cold, it rains phosphorus pentafluoride.
But, under normal conditions, rain on Niflheim is hydrogen fluoride.
And the atmosphere has a slight yellowish tinge to it because of all the fluorine.
So the basic idea here is that oxygen and water have been replaced by fluorine and hydrogen fluoride.
Which, chemically speaking, works just fine as long as the planet is very cool, (Check!)
as the liquid range of hydrogen fluoride is -83 degrees Celsius to 19 degrees Celsius.
Niflheim's crust is composed of various fluorides of metals, (these guys).
Its terrain is very flat,
as a lot of the surface rocks, calcium difluoride, titanium tetrafluoride, and zirconium tetrafluoride are mechanically very weak.
No silicates about to help build giant mountains. Rocks tend to be either gray or black as a lot of fluoride metal compounds are colorless.
Colored rocks do exist, but they're rare. Interestingly, uranium occurs naturally as uranium hexafluoride,
which is super useful in terms of nuclear energy.
Rivers, lakes and oceans are made of the aforementioned hydrogen fluoride. Unlike water, hydrogen fluoride ice does not float,
so rivers and lakes freeze from the bottom up. Niflheim plant life uses photosynthesis just like terrestrial plants,
but the reaction is slightly different.
Starlight is used to power a reaction between carbon tetrafluoride and hydrogen fluoride that produces free fluorine.
This sort of reaction can only take place on a world that receives a lot of UV, (Check!)
as breaking up hydrogen fluoride and carbon tetrafluoride requires a lot of energy.
The role magnesium plays in terrestrial plant chlorophyll is played by nickel on Niflheim, and plants are a variety of colors
not just homogeneous green. As for animals, their hard tissues (bones, teeth, shells, etc.) are made of Teflon.
Titanium is a key component their blood which results in colourless arterial blood and violet veins.
Interestingly, the sea floors on Niflheim are covered in a layer of powdered Teflon,
left behind after the death of various hard-shelled marine creatures.
In short, Niflheim is a radically different planet to Earth.
A heavily fluorinated, massively corrosive, bitterly cold hell-world would be a compliment.
But it does work. Sorta. See, fluorine is a very rare element.
There's something like one fluorine atom for every 15,000 oxygen atoms in the universe.
But even if there was somehow enough fluorine knocking about to stock up the atmosphere,
it's super, super chemically active, so it probably wouldn't hang around very long.
So there's a little bit of hand-waving involved.
With Clorox, Stephen L. Gillette gets around these issues and at the same time maintains a halogen atmosphere.
Clorox features a standard terrestrial nitrogen-oxygen atmosphere,
but there's about one percent chlorine present, and trace amounts of other chlorinated organic compounds.
The justification for the presence of this rogue halogen is essentially biological warfare.
Primitive creatures on Clorox learn to use the chloride present in Clorox's oceans to produce chlorine gas as a defense mechanism.
Their predators adapted, an arms race ensued, and eventually the entire food chain packs some sort of chlorine based weaponry,
culminating in the release of free chlorine into the atmosphere by plants via their natural gas defense mechanism.
Essentially, Clorox's entire ecosystem is comprised of carbon-based oxygen breeders that possess a high tolerance for chlorine.
This tolerance comes in the form of tougher cell walls and membranes,
specifically designed to keep out the toxic chlorine. That said, organic molecules on Clorox are heavily chlorinated.
Shells, teeth, and bones are made of plastics.
Trees are made of the likes of PVC instead of cellulose, and plant leaves look like black refuse bags.
See, chlorine absorbs short-wavelength blue light, so by being black, plants absorb as much of the remaining spectrum as possible.
Oh, and things like flower perfumes and decay orders are made of compounds like PCB, chlordane, and DDT.
The sky on Clorox has a greenish yellowish cast, (remember, chlorine absorbs shorter wavelengths of light) and is very smoggy.
There's no ozone layer, as chlorine is great at breaking ozone back into ordinary oxygen,
but luckily, the chlorine itself is a good enough absorber of UV light.
Chlorine is also heavier than nitrogen, oxygen, and carbon dioxide, and as such it sinks, and accumulates in the low-lying regions.
The concentration in these areas can be so potent that even the chlorine-tolerant locals run the risk of asphyxiation in these regions.
Chlorine reacts with water to produce hydrochloric acid, so freshwater rivers and lakes are made of dilute acids.
Think swimming in a river of mild vinegar.
And, because of their salt content, oceans are a mild bleach.
Chemical weathering on Clorox is severe. Only clays and quartz are present. No limestone. It would simply fizz away.
Metallurgy is non-existent because chlorine inhibits flames, so fires only smolder,
and even if metals could be worked, they wouldn't last very long given the corrosiveness of the environment.
This all means that the locals on Clorox are trapped in the wood-stone part of the tech tree,
Despite their wood being a high-grade plastic.
So there you go! Two very alien worlds, two very different design philosophies.
The re-imagination method, and the tweaking method.
In my opinion though, the tweaking method is by far and away the best option to take, should you want to construct your own alien atmospheres.
Sure, Niflheim is a super-cool exotic alien world, but in changing literally everything about the world, the setting, in effect, becomes meaningless.
It's too alien, too unfamiliar and the technical knowledge required to construct an utterly alien world is beyond most.
Sure, if you've got a PhD, more power to you, but for the rest of us plebs, the tweaking method is best.
It takes a system we all know and understand and mods it.
Modding requires less technical knowledge, takes less time and effort, and creates an equally exotic yet still familiar setting.
But the killer feature here is the ability to be able to say that your world could have become reality.
Earth could have been Clorox had early bacteria learned to harness and weaponize the chloride present in Earth's oceans.
Our skies could have been greenish-yellow, and our trees, plastic.
That thought alone is more mind-blowing than any utterly alien world ever could be.
Good morning, interweb! In case you missed it at the start, Niflheim and Clorox are two fictional planets by H. Beam Piper
And Stephen L. Gillette respectively, links to the sources in the doobly-doo if you want some further reading.
Thank you all so much for watching,
and a special thanks goes out to the patrons Isaac Silbert, Andrew Chehayl, Robin Hilton, World Anvil,
Ripta Pasay, and new patron John Hooyer! Thank you all for making Artifexian continue to be a thing.
It means the world, you're awesome! And until next time, Edgar out!