Good morning, Interweb! Let's worldbuild.
Earth's atmosphere is composed of about 78% nitrogen, 21% oxygen,
and 1% argon, with trace amounts of other gases like CO₂ and H₂O.
But, it wasn't always like this.
Earth formed about 4.5 billion years ago. Its earliest atmosphere was composed of hydrogen and helium,
but was quickly replaced by a secondary atmosphere composed of volcanic gases and cometary material.
Earth cooled, oceans formed, life evolved, and blue-green algae began producing oxygen via photosynthesis,
in effect creating Earth's third, and current, oxygen-rich atmosphere.
Which by the way is divided into five layers:
the troposphere, stratosphere, mesosphere, thermosphere, and exosphere.
Now, you'd think the higher you go, the more the temperature would decrease, right?
Wrong!
It's more like troposphere, decreasing; stratosphere, increasing; mesosphere, decreasing;
thermosphere, increasing; and exosphere is constant.
Which is weird, but you know what's even weirder?
Thanks to very high-energy UV light, the temperature of the outer atmosphere can get as high as
1500K!
Hot enough to melt lead, twice over.
But the atmosphere is so tenuous up there that the actual content of heat is functionally zero.
Mind = Blown!
Anyways, atmospheric crash course done. Let's build.
No planet, no atmosphere, so you'll need to input your planet's mass, radius, and temperature here.
For the sake of time, I'll go with Earth values.
Next you need to figure out which gases your planet can retain and which will escape into space.
A good rule of thumb is that the root mean squared speed of any given gas of weight m and temperature T
must be less than about one-sixth your planet's escape velocity.
In English, this means that the speed of a gas should be well below the speed needed to escape your planet's gravity.
Plugging and chugging for each and every gas you want in your atmosphere is tedious.
Don't worry, I got you covered.
Go back to the spreadsheet, open the stability checker sheet, and boom!
Any gas in green will hang around your atmosphere for geological timescales.
Anything in red will escape into space relatively quickly.
This is why Earth lost its original hydrogen and helium atmosphere.
At our temperatures, they're just too zippy to stick around.
Great! A 100% Krypton atmosphere it is then!
Eh, no.
I've included this list for completeness, but realistically our gas choice is limited when it comes to earth-like planets.
In fact, I'd strongly advocate keeping things the same as Earth:
oxygen, so animals can breathe; CO₂, so plants can breathe; and nitrogen, so amino acids and DNA can be created.
Even argon is kind of a gimme.
It decays from radioactive potassium-40, the main source of heat within an Earth-like planet.
With that in mind, fill in whatever percentages suit you in the composition section.
I'll go with 74.9% nitrogen gas, 23% oxygen, 1.05% argon, and 1% Carbon Dioxide,
which takes me to 99.95% in total.
I usually chop the remaining 0.05% down to unspecified trace gases.
That said, you can totally fill in a complete set of trace gases.
If you do be sure to include some water vapor.
Oh, and important side note, nitrogen gas is N₂, not N, and oxygen gas is O₂, not O.
Now, all this gas has a weight, and that weight will be felt as pressure.
We measure this pressure in a number of different ways, but for me the most intuitive unit is the atmosphere.
On Earth, the pressure at sea level is equal to one atmosphere.
The habitable range of sea level atmospheric pressures depends on the nitrogen/oxygen content of our atmospheres,
best described by this graph.
In my case, I've got about 75% nitrogen,
So I'm looking at a minimum pressure of about 5.5 psi, and a max of around 30 psi,
Which in atmospheres is roughly 0.37 to 2.1 atmospheres
Let's go with 1.7 atmospheres.
Cool.
Next, go back up the composition block and have a look at the partial pressure column
This tells us how much pressure each of our gases contribute to the overall atmospheric pressure.
For habitable planets,the partial pressure of nitrogen must not exceed 3 atmospheres.
The partial pressure of oxygen must be between 0.16 and 0.5 atmospheres,
and CO₂ must be below 0.02 atmospheres.
That's just to make the air breathable.
In truth, you'd want CO₂ to be below 0.005 atmospheres to avoid any physiological stresses.
Argon, despite being an inert gas, should be capped at about 1.6 atmospheres.
Stray outside these limits, and it's death by a myriad of grim, grim afflictions.
Seriously, don't do it.
Oh and for the completionists, here's a list of maximum partial pressures for some more noble gases.
And, here's the max pressures for various trace gases.
My planet checks out on all fronts.
CO₂ is admittedly high, but that will make for some cool local color.
Interestingly the O₂ partial pressure is close to the highest Earth has ever seen.
High oxygen worlds are really fun.
The more oxygen you've got, the worse fires will be.
Things will burn hotter, faster, and more easily.
More oxygen also makes respiration easier, so in theory creatures could get very big.
This is sort of what happened during the Carboniferous period, where insect gigantism was all the rage.
two meter long millipedes, giant dragonflies, giant scorpions, the works.
Anyways, the spreadsheet thing computes the average atomic weight of your atmosphere and its scale height.
The scale height H is the distance over which the pressure and density of an atmosphere fall by a factor of 1/e.
For every additional H the value falls by an additional 1/e.
Basically this means the larger a scale height, the more slowly an atmosphere tapers off to nothing.
With these values in the bag, we can enter in any given altitude here,
and the spreadsheet would spit out the atmospheric pressure at that altitude,
Which seems trivial, but it can lead to some really great story ideas.
Remember how the habitable range of pressures for this particular world was 0.37 to 2.1 atmospheres?
Well, given that pressure decreases as altitude increases,
we can figure out the minimum and maximum habitable elevations:
In this case, anything between about 1.5 kilometers below sea level and 12 kilometers above sea level could in theory be habitable.
That being said, at about 1km below sea level, the partial pressure of carbon dioxide will begin reaching toxic proportions,
so I could be looking at the world with habitable highlands and uninhabitable lowlands:
oppressively hot, subject to intense weather, and bathed in toxic gases.
Kinda neat, right?
If I had gone with a low-pressure at sea level,
I could have easily designed a world in which a person would require a pressurized suit to climb tall mountains.
And there's a ton of other interesting story ideas that can arise out of this altitude equation.
Oh, and FYI, when the pressure drops to almost zero you're technically in outer space,
Which happens at about 120 kilometers up on my world.
And finally, density.
The air density at sea level on my world is about 2.1 kilograms per meter³, or about 1.7 times that of Earth's,
which gives me a final cool story idea.
High air density can improve hearing because sound travels better in denser mediums.
It can create more interesting storms because of the greater mass of moving air,
And perhaps most interestingly, can make flight easier.
If the air density were high enough, and the gravity low enough, you may even be able to achieve human-powered flight.
Think Icarus, except for realsies.
In summation, by keeping everything Earth-like but slightly tweaking the percentages and pressures,
I was able to create a world full of giant creatures, rampant wildfires, toxic badlands, and intense storms.
So the take-home here is that you really don't need to mess with things too much.
Take Earth, mod it, and awesomeness will happen.
No exotica required.
Good morning, interweb! I messed up the labeling in the passive voice part of the last video.
So, just so we're all clear, the passive voice looks like this:
So it kind of works like a function where the old A argument gets dropped,
and the old P argument gets promoted to S.
Incidentally, the old A argument can be reintroduced, in which case it's called a non-core oblique argument.
“Brock was seen by Ash.”
I'll do a video dedicated to voicing somewhere down the line, but just for now, that is the correction.
Apologies.
Anyways, as always, link's in the doobly-doo to the spreadsheet, so you can go use them to your heart's content.
Thank you so much for watching, everyone. It means the world.
Special thanks go out to all the patrons who, without them, I would not be making this.
Isaac Silbert, Andrew Chehayl, Robin Hilton, World Anvil, and Ripta Pasay, you guys are all amazing.
Thank you so, so much.
And until next time, Edgar out.