User blog:Cerne/Colour of the sky

Yeah, it has been three months since my last entry...

Yeah, I said I would publish another entry very shortly after that last one...

Yeah, this trend of scarcely-published mega-entries should probably stop...

Yeah, I have said all of this before...

I think it is time for a few more resolutions. Resolutions that may not fulfill current expectations or encourage any future ones but will be easier to keep. From now on, I am no longer going to say when my next entry will be published. It will be published when it gets published, and that will be that. As well, I will no longer say how long a current or future entry will be, or what it will be about. I may mention what I want to bring up in future entries from time to time but exactly when that will be shall remain uncertain, as not even I will really know until it does get published. This entry is sort of a testament to that; it started out being part of Update Part 2 but the topic eventually got so long that I decided to publish it as its own entry. What I have decided to do is to lop off the old beginning for this entry and put in what you are reading right now, and then the old beginning will precede the upcoming Update Part 2.

Before I continue with the entry, though, I would like to bring up colour-coding. Yes, I already know how to do that now, thanks to Super Warmonkey. Anyway, in two previous entries I used red to signify new text that had been inserted into an older body of text. Upon thinking about that now, making text red might be misleading; red typically implies a warning or a high-priority notice or alert. I don't really want that for what I am going to do in what (hopefully) will be the near future. I am thinking I may use green or blue instead. Yellow might be too hard to read and purple would look too wierd. I may use some other colour, too. It just won't be red. I may use red sometime, but not unless the implied situation calls for it.

ANYWAY, on with the entry.

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If I asked you what your first indication would be of actually standing on another planet (for the sake of this entry let’s make it terrestrial), what would your answer be? The gravity, perhaps, but then this is determined by factors you cannot perceive. All you are perceiving is the end result: the force that is pulling you down. What about the things that are living on that planet? That is, assuming you could tell they were alive, and could actually find them in order to make that assertion. Temperature could be a good indication, but then our own planet Earth is variable enough in this regard. There are temperatures on Earth that could kill you on both extremes, so assuming you were still alive on this indiscernible planet after five minutes or so, you would have no way of telling whether it really was Earth or not without dying soon afterward. Atmosphere is much the same way; you are either on Earth, or you’re dead. Finding out whether this questionable planet you were standing on really was Earth or not, after finding out you could breathe on it, would come much later.

No, the first indication you would most likely have would be the sky. You would see the planet’s sun(s) in the sky if it were daytime, you would see neighboring stars in the sky if it were nighttime, you would see constellations, aurorae, other astronomical phenomena like galaxies, etc. You would see clouds on a cloudy day, and “rain” (depending on what you would call “rain”) when there is heavy precipitation. But most of all, you would see the colour of the sky. And that is one aspect of my planet I would need to work out in order to remind audiences this is indeed another planet I am referring to. Please note I am not saying a planet’s sky needs to have a different colour in order to be a different planet, but as I’ve described earlier, this is a good place for the more pragmatic world-builders (such as myself) to start. And like practically everything else that is immediately noticeable when you do imagine yourself on another planet, the colour of the sky is determined by several characteristics of the planet itself.

During my research into all the different kinds of metals I could build my planet with, I ran into a number of elements that would – as a consequence of what I used to make the planet with – affect what would wind up in my atmosphere. Elements like sulphur, and molecules like carbon dioxide. I typed in earlier entries that in addition to lighter gravity I also wanted a denser atmosphere. Partly because I wanted external heat to stay on the planet’s surface longer, partly because I wanted a way for some form of airborne travel to evolve in some of the planet’s life forms – and jointly for terrestrial locomotion to not be as strenuous or as dependent on weight – and partly for aesthetic reasons (I thought a hazy fog-like surface would look cool and exotic). I also had this assumption that denser air meant more oxygen…but this turned out to be more a factor of gravity than atmospheric density, and the latter could be caused by the former, hence my likely misled assumption.

Well, as it also turns out, my planet may not have a denser atmosphere at all. At least not where the organic elements are concerned, though I could have a 13% increase in CO2 the way Earth did in the Cambrian Era. But when I did some research into Earth’s atmosphere, I found out about argon. Furthermore, while I was reading a canonical reference to director James Cameron’s film Avatar, I found out that one of the reasons why the fictional moon Pandora had a dense atmosphere was because of the presence of xenon. Noble gases can make an atmosphere pretty thick if there are enough of them but they can also cause issues with respiration; they are all inert, which means they cannot be used for aerobic respiration, so taking in too much of them can basically make you “drown.”

There are two ways in which noble gases can occur naturally and remain in the atmosphere long enough – and in high enough concentrations – for us to be aware of them. The first is through beta decay from an unstable alkali metal isotope if only one proton is lost, or an unstable alkaline Earth metal isotope if two protons are lost; the second is through alpha decay from heavier radioactive elements such as uranium and thorium in which all isotopes are unstable. This mostly produces helium, though, since one alpha particle is equal to one helium-4 isotope.

Unstable isotopes of all naturally-occurring elements that go through beta decay will typically decay into the element that precedes them in the periodic table; their atomic number – determined by the number of protons they have – goes down by one, and this turns them into the element that is one step above them. For instance, gallium-71 will decay into zinc-71 if it loses a beta particle, manganese-55 will decay into chromium-55 if it loses a beta particle, indium-103 will decay into cadmium-103 if it loses a beta particle, and so on.

But not always.

Sometimes a neutron is lost (this is called electron emission – losing a proton is called positron emission) and sometimes two beta particles are lost at the same time (this is called double beta decay).

All of the argon in Earth’s atmosphere is argon-40 which decays from potassium-40. On the other hand, all of the helium in Earth’s atmosphere (helium-4) comes from the alpha decay of naturally-occurring uranium and thorium deposits; it is obtained for artificial use via the natural gas industry. Stars like our sun omit helium as part of their nuclear fusion chains, but most – if not all – of it leaves our atmosphere very quickly due to the inability of Earth’s gravity and magnetic field to keep it on the surface. And don’t ask me why none of our helium comes from lithium isotopes because I don’t know. Helium seems to be the exception when it comes to beta decay, even though it apparently can decay from lithium isotopes. According to my sources, naturally-occurring helium just doesn’t seem to come from beta decay. It all comes from the alpha decay of uranium and thorium.

Anyway, the beta decay sequences for the noble gases go something like this:

Th/U --> α --> He-4 Li --> β+1 --> He Be --> β+2 --> He Na --> β+1 --> Ne Mg --> β+2 --> Ne F --> β-1 --> Ne K --> β+1 --> Ar Ca --> β+2 --> Ar Cl --> β-1 --> Ar Rb --> β+1 --> Kr Sr --> β+2 --> Kr Br --> β-1 --> Kr Cs --> β+1 --> Xe Ba --> β+2 --> Xe I --> β-1 --> Xe Rn --> ??? (Radon goes through both alpha and beta decay - what the end results are, I do not yet know)


 * Remember that the atomic number goes down because you lose one proton but the neutron count increases because a neutron was swapped for it, so the atomic mass – represented by the isotope number – stays the same. That’s why no specific isotopes were listed here.

If Mr. Cameron (or someone working for him) had done his research, he would know that in order for Pandora to have a significant amount of xenon in its atmosphere it would probably need to have a lot of radioactive caesium, barium or tellurium isotopes in its interior. That, or a lot of radioactive iodine somewhere on its surface. The apparent absence of xenon on Earth remained a mystery until the discovery of xenon dioxide deep underground, so any free xenon on another planet (or moon) may need to be produced in a more apparent way if it is going to be that much more prevalent in the atmosphere. Most likely by expelling the radioactive precursor isotopes from the planet’s interior through volcanoes and divergent plate boundaries, and then having those isotopes decay on the surface afterward.

What I want is something heavier and rarer than argon but not as rare or unpredictable as xenon. Skeptic’s pessimism is also keeping me from choosing to have a lot (1% or more) of xenon in my planet’s atmosphere. So I am going to go with krypton. At least as much krypton as Earth has of argon, but maybe more. There will be other noble gases like xenon and argon, but due to my planet’s weaker gravity and magnetic field there will be less of them Not sure about helium, though.

When I started thinking about the noble gases, the idea of a thicker and cloudier atmosphere was initially on my mind but I soon learned about another interesting feature. Nitrogen, oxygen, carbon dioxide, and atmospheric H2O are all colourless gases, so…where does the colour of our sky come from? In an earlier entry, I brought up Rayleigh Scattering whereby faster shortwave solar radiation is refracted by atmospheric particles while slower longwave radiation gets through. If a planet had smaller and/or more loosely scattered particles, more shortwave radiation would get through. But what if there was more to it than that? Thus going back to the noble gases: if there were a lot more of the heavier gases in the atmosphere – albeit maybe more loosely spaced – would even less shortwave radiation reach my planet’s surface?

Maybe.

Argon was the first noble gas to be discovered; in the latter half of the 16th century an English chemist by the name of Henry Cavendish ran an electric current through a sample of air in an attempt to separate nitrogen from oxygen. No less than 1% of the air remained unaccounted for. About one hundred years later, English physicist Lord Rayleigh and Scottish chemist William Ramsay finally isolated and identified argon. Ramsay and his assistant, English chemist Morris Travers, would go on to identify krypton, then neon and xenon, using this same method. These discoveries were made by watching for the spectrum of light that each subsequent gas would produce upon isolation. It should be no surprise, then, to learn that the “neon signs” which are still used today actually do use neon. They also use other noble gases: the bright colour you see is what happens when the gas is “excited” by an electric current travelling through it.

In addition to being hit by solar radiation, Earth’s upper atmosphere – and that of any other planet for that matter – is continuously charged with electrical currents. These currents are what produce the lightning we see in the sky during a thunder storm. As well, they may be largely responsible for the colour of the sky. After all, argon is a light cyan or bluish colour once it has been charged up by an electric current.

Keep in mind this association of noble gases like argon with the colour of the sky is all purely theoretical so far because I have not looked into the topic area a whole lot yet, but it is something I feel I can go on when trying to determine what my sky might look like. It has already been suggested elsewhere that my planet’s sky would look very cloudy, and that this cloudiness would look “closer” or lower to the ground than it would on Earth. Furthermore, since my planet’s sun has a lower luminosity than Earth’s sun does, the colour of my planet’s sky may not be as bright.

After being excited by an electric current, krypton becomes a dark indigo colour. This is still higher on the visible portion of the EM spectrum than I would expect to see being reflected anywhere on the surface of my planet, even by the plants, so I combine this with the colour that is being reflected by plants and the resulting colour I get should be the colour of my planet’s sky. Since its sun has a lower luminosity, I expect this to be somewhere between yellow and green. Maybe a lime or olive colour. For reference, on Earth, green is the highest colour on the EM spectrum that plants cannot absorb and hence the highest colour they will reflect. Plants on my conworld receive less light so they will need to absorb a lower colour and – in turn – reflect a lower colour still. Yellow-green and indigo (blue-purple) will make a green-blue colour known as turquoise or teal. Add in a higher percent of water vapour in the air, and you get a dark, dull, cloudy turquoise colour with a low and perhaps less distinct horizon at sunrise and sunset.

The question of where the electric currents come from remains uncertain. Maybe I just need to do more research on this topic. It may be as simple as emulating what we have here on Earth.

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That is the end of my entry. Yes, this last bit I am typing was added on the spot, too. Expect everything to fit better chronologically in the following entry. I don't know what else to type, which prings me to my final resolution: From now on, when I want what I am typing at that moment to be published at that moment, I am not going to care about what shows up in the text box. No more sitting around, waiting for the right word or sentence to pop up or come back to me. If I remember what I wanted to say at that time, I will bring it up in a later entry.

Thanks for reading.