[Artifexian]: Good morning, Interweb! Question: do skies have to be blue and plants green?
[Artifexian]: Answer: not even slightly
[Artifexian]: Quick recap: starlight is a mixture of various different wavelengths of light.
[Artifexian]: When light hits stuff in an atmosphere, these wavelengths get scattered.
[Artifexian]: If that stuff is really small, shorter wavelengths of light are scattered more efficiently.
[Artifexian]: We call this Rayleigh Scattering. If that stuff is relatively big, all wavelengths of light gets scattered equally.
[Artifexian]: We call this Mie Scattering. Our sky is blue because of Rayleigh Scattering and our clouds white,
[Artifexian]: because of Mie Scattering.
[Artifexian]: The warm-coloured skies of Twilight occur because the more atmosphere light passes through,
[Artifexian]: the more the shorter wavelengths of light are scattered away,
[Artifexian]: leaving only the longer, redder wavelengths of light to hit our eyes.
[Artifexian]: Oh! And speaking of eyes, unless otherwise stated if I say something is a certain colour,
[Artifexian]: I mean that with respected to the human eye in Earth-like, Nitrogen-Oxygen atmosphere.
[Artifexian]: Got it? Cool. Let's world-paint.
[Artifexian]: Take an earth-like, atmosphere add warm coloured gases to it-- boom! Warm-coloured skies.
[Artifexian]: Extremely high concentrations of Methane and Ozone will tend to sky a hazy, orangey-reddish colour.
[Artifexian]: Nitrogen-Oxide would produce an orangey brown sky.
[Artifexian]: Sulfur gas will make for yellow skies or red skies, depending on the temperature of the atmosphere.
[Artifexian]: And there's probably a whole ton more warm-coloured gases out there.
[Artifexian]: Unfortunately, most, if not all of them, will be super, super toxic to carbon-based life-forms.
[Artifexian]: Alternatively, we could suspend coloured particles in our atmospheres.
[Artifexian]: In 2017, Saharan dust and Spanish smoke blew over the UK, turning her skies orange and her sun blood-red.
[Artifexian]: Obviously, a temporary phenomenon.
[Artifexian]: But for something more lasting, we can look to Mars.
[Artifexian]: Winds on Mars continually blow a fine Iron-Oxide dust into its skies. This dust absorbs blue light,
[Artifexian]: so the Martian sky is butterscotch each early day and pinky-red at twilight.
[Artifexian]: Interestingly, around the setting sun,
[Artifexian]: the sky is blue, which is like anti-Earth? Again, a consequence of the dust in the air.
[Artifexian]: So there's two methods: coloured gases, coloured particulates.
[Artifexian]: But we can also let Scattering alone do the job.
[Artifexian]: Take an Earth-like atmosphere,
[Artifexian]: make it significantly thicker-- boom! Warm-coloured skies.
[Artifexian]: By increasing the thickness of an atmosphere, and by extension the pressure,
[Artifexian]: we're essentially taking twilight conditions
[Artifexian]: and applying them to the whole sky.
[Artifexian]: We can very, very roughly estimate the thickness needed to achieve this
[Artifexian]: by doing a bit of trig on the angle subtended by the twilight zone-- don't worry,
[Artifexian]: I'm not gonna bore you with the maths. There's a spreadsheet in the description.
[Artifexian]: The important point here is that as atmospheric thickness increases, the sky will get redder,
[Artifexian]: brightness will go up, and saturation will go down.
[Artifexian]: That is to say, very, very thick atmospheres will yield pale, de-saturated, reddish skies.
[Artifexian]: Lastly, the easiest solution is perhaps to vary the star.
[Artifexian]: A sky colour is also dependent on the standard temperature.
[Artifexian]: Cool, low-mass stars will output most of their light in the red,
[Artifexian]: Massive, hot stars will output most of their light in the blue.
[Artifexian]: So, the bigger the star, the bluer, brighter, and more vivid the sky.
[Artifexian]: The smaller the star, the redder and dimmer the sky.
[Artifexian]: Basically this:
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[Artifexian]: That said, some argue that this is wrong, asserting that regardless of stellar class, skies will always be some shade of blue
[Artifexian]: given how dominant blue Scattering is.
[Artifexian]: Kind of like this:
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[Artifexian]: Basically, choose at your own peril here.
[Artifexian]: Assuming the first scenario, the transition zone between K and M stars will yield white-ish skies.
[Artifexian]: The denser the atmosphere, the purer that white.
[Artifexian]: Oh, and clouds on such a world would reflect any un-Scattered light,
[Artifexian]: thus appearing yellowish or reddish.
[Artifexian]: A sky full of large, colourless particles will also appear white-ish as Mie Scattering will dominate.
[Artifexian]: Heavily polluted planets could have milky whitey, grey skies. Or more obviously,
[Artifexian]: planets with large amounts of cloud cover could have permanently overcast white, grey skies.
[Artifexian]: Black skies are easy: No atmosphere no colour.
[Artifexian]: The star would be a pure white colour and will be bright enough to limit the visibility
[Artifexian]: of background stars. The further out the planet orbits, the more background stars would be visible.
[Artifexian]: Green is...tricky.
[Artifexian]: Remember, high mass stars skew blue, low mass stars skew red.
[Artifexian]: Somewhere in the middle then there's a start of peaks in the green. So green skies, right?
[Artifexian]: Nope.
[Artifexian]: See, in human eyes green will always either combine with red to give yellows and oranges,
[Artifexian]: or blue to give cyans.
[Artifexian]: Even at peak green, there's enough of the other wavelengths about to prevent a pure green sky colour.
[Artifexian]: That said, Alien eyes could work differently.
[Artifexian]: Coloured gases are, of course, an option. Chlorine would tint the sky a yellowy-green colour.
[Artifexian]: Or at high enough concentrations a yellow-green black colour. See, chlorine is a really good absorber of light,
[Artifexian]: so the more chlorine, the darker the surface conditions.
[Artifexian]: Oh, and it's super toxic.
[Artifexian]: But, it could work. We could do a Mars, invent some green dust, and suspend them in the atmosphere
[Artifexian]: If not dust, then maybe airborne plant-life. Think sky-algae or sky-plankton.
[Artifexian]: Introduce bio-luminescence, and your sky will be a green-tinted acid trip.
[Artifexian]: Think biologically created auroras.
[Artifexian]: Auroras provide temporary coloration. When charged solar particles hit the atoms in the atmosphere,
[Artifexian]: they excite those atoms and light is given off. M stars are a class of star prone to massive solar flares.
[Artifexian]: Solar flares mean more charged particles. More charged particles means more frequent, more vibrant, and more extensive auroras.
[Artifexian]: Also, if a planet's atmosphere is packed with Neon or Sodium gas, say,
[Artifexian]: these auroras will be orangey-red or yellow, respectively.
[Artifexian]: And finally,
[Artifexian]: there's the Green Flash.
[Artifexian]: When conditions are just right, the Sun flashes green as it sinks below the horizon at sunset.
[Artifexian]: On Earth, this effect is really rare and fleeting.
[Artifexian]: But if a planet rotate at a much slower rate, this green flash could become more of a nightly green glow.
[Artifexian]: Think Pirates of the Caribbean, except way less extra.
[Artifexian]: So, shorter wavelengths of light are scattered more right? Then why is our sky blue?
[Artifexian]: I mean, violet light has a shorter wavelength than blue light.
[Artifexian]: Shouldn't our sky be violet?
[Artifexian]: Turns out it kind of is, we just can't see it.
[Artifexian]: That's 'cause the Sun emits less violet light then blue light,
[Artifexian]: and our eyes are not as sensitive to violet as they are to blue.
[Artifexian]: So where we see blue, an alien may well see violet.
[Artifexian]: In 1950, the Sun over British Columbia turned to "various shades of blue or violet" The culprit?
[Artifexian]: Forest fire smoke. Same shtick happens with literal blue moons.
[Artifexian]: Diurnal cycles aside,
[Artifexian]: Wouldn't it be cool if sky colour varied regularly
[Artifexian]: and predictably? Kind of like our Sulfur atmosphere from before: yellow skies in winter red skies in summer.
[Artifexian]: Assuming of course, the correct temperatures are hitched. But another cool solution is to have an Earth-like world orbit close, binary stars,
[Artifexian]: where each star differs significantly in mass, A.K.A., temperature.
[Artifexian]: So as they eclipse one another, the sky would suddenly change colours.
[Artifexian]: Perhaps pale blue
[Artifexian]: to white
[Artifexian]: to pale orange.
[Artifexian]: Anyways, onto plants.
[Artifexian]: Remember, stellar output peaks in certain frequencies and colours,
[Artifexian]: so we have two floral options: plants that are the same colour as a star's peak output
[Artifexian]: or the complement to that colour, like if a star outputs predominantly in the blue, plant life could either be bluish or
[Artifexian]: yellowish. Yellow plants are kind of saying "Okay. I need starlight to live,
[Artifexian]: so let me take in the most starlight I can in the most efficient manner possible."
[Artifexian]: Whereas blue plants are saying "Look, all that peak radiation may be damaging to me,
[Artifexian]: so I'm going to block it and just feed off the other wavelengths instead."
[Artifexian]: Both strategies could potentially work.
[Artifexian]: Here's a rundown of strategy A:
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[Artifexian]: And here's a rundown of strategy B:
[Artifexian]: Also, there's a colour calculator in description, so go check that out.
[Artifexian]: In both cases,
[Artifexian]: I've made the plans whitish around high-mass stars and black around low-mass stars. Very massive stars output
[Artifexian]: so much radiation. that a plant will likely want to reflect back as much of it as possible,
[Artifexian]: lest it burn. Very low-mass stars output
[Artifexian]: so little radiation that a plant will want to absorb as much as possible, lest it starve.
[Artifexian]: Unless, of course, those stars are Flare Stars, in which case the low radiation output will be punctuated by
[Artifexian]: bursts of massively increased radiation.
[Artifexian]: Not good news for plant-life!
[Artifexian]: Especially black plant-life: those plants would need to develop an early warning system,
[Artifexian]: some sort of UV detector, perhaps.
[Artifexian]: and the ability to take cover. That might mean evolving a shell-like structure they could withdraw into
[Artifexian]: or maybe burrowing abilities or maybe they literally move,
[Artifexian]: blurring the line between plant and animal.
[Artifexian]: Or maybe plants in such a world would need to live underwater, the water acting as a shield.
[Artifexian]: Basically a dimly lit, orangey-red and black swamp world. Pretty cool, right?
[Ewa]: That is pretty cool,
[Ewa]: but I really like the idea of a seasonally changing sky.
[Ewa]: so let's say we have a large continent with a tall mountain range all along its western and
[Ewa]: southwestern flanks. The center of the continent is a giant desert,
[Ewa]: and in winter a powerful monsoon wind blows from the interior towards the coasts.
[Ewa]: There's a great city in the south and during the winter monsoon, the sand blown from the great desert fills the air
[Ewa]: and paints the sky red. The red season is also the dry season and as it continues,
[Ewa]: rivers and watering holes start to dry up,
[Ewa]: so the people of the city bring outeverything blue they own into their courtyards and into the streets
[Ewa]: to change the colour of the sky. And in the city center atop the main temple,
[Ewa]: they turn the sacred bellows north to reverse the direction of the wind
[Ewa]: Summer comes eventually and the monsoon winds start to blow from the sea to the interior,
[Ewa]: bringing with them a blue sky as well as clouds and rain.
[Ewa]: Rivers fill with water, people plant their crops, and cows give birth to calves.
[Ewa]: But along with renewing, life the southern winds also bring death.
[Ewa]: stagnant pools of rainwater turn into breeding grounds for mosquitoes,
[Ewa]: and the insects transmit malaria and dengue fever, killing many people.
[Ewa]: And so everyone brings out their red clay pots and puts on their red clothes to banish the blue sky and its clouds
[Ewa]: while the sacred bellows are turned to point south to call upon the desert winds.
[Ewa]: And with time, the winter monsoon comes and fills the air with sand,
[Ewa]: painting the world red until it's time to turn the bellows again.
[Artifexian]: Good morning interweb! A massive, massive, massive
[Artifexian]: Thanks goes out to Ewa of WorldbiuldingNotes for collaborating with me on this video.
[Artifexian]: She is an amazing creator and deserves so many more views than she currently has,
[Artifexian]: so, please, please, please go over there and blast her page. I promise you, you'll love her content.
[Artifexian]: Also a massive thanks goes out to Simon Clark, another Youtuber who you will love,
[Artifexian]: For helping me out with some of the science in this video. And finally a massive
[Artifexian]: Thanks goes out to each and every one of you for watching this video and supporting artifexian. In particular,
[Artifexian]: I wanna shout out. Isaac Silbert, Andrew Chehayl, Robin Hilton, World Anvil, Ripta Pasay, John Hooyer, and
[Artifexian]: new top-tier patron, A.E. Stephenson. You all are absolutely amazing. Until next time, Edgar Out!
