Conclusions from Gaseous Fog Research
Evening, CMDRs!
I've spent the last few days studying the conditions under which airless planets will develop the mist/fog that we'll sometimes see, and I believe I have a pretty good understanding of when and how it happens.
To summarize:
Planets with significant rock content, when heated by sunlight, give off gas, which collects in craters, but can also form a thin layer over the surface in general. In the absence of solar heating, this gas disspates into space fairly quickly. You will find this gas on the day side of a given planet, but the night side will have usually cooled enough that the fog is gone.
MOST IMPORTANTLY, THIS MEANS YOU WILL GENERALLY NOT FIND FOG ON THE DARK SIDE OF A PLANET TIDALLY LOCKED TO ITS STAR. If you're searching the dark sides of tidally locked planets because of the trailer footage alone, I'd suggest finding a new line of inquiry.
The Details:
I began my investigation in Merope. Here's a screenshot (which I already posted here before my results were conclusive) taken from above the day/night line on Merope 2D:
If you look at the horizon at the edge of the planet, on the right you can see a significant layer of beige-colored fog; on the left, as it darkens, this fog thins out and eventually disappears.
The main breakthrough, though, came entirely by accident, when I visited Shriver Landing (or whatever it's called) in the Ovid system:
That's both pitch black, AND super foggy. So what's going on here? Luckily, that moon is tidally locked to the Earth-like planet that it orbits, and its' orbital period is only
4.8 hours.
I decided to wait a while, and see how long the fog stuck around:
After thinking it over for a while, I've worked out what I think the day/night fog cycle is, on both this and other airless worlds.
DAY/NIGHT/FOG CYCLE
I. Sunrise. No/minimal fog present. Solar heating gradually causes fog to form during the "morning" of whatever part of the body you're on.
II. Eclipse (if a moon). This particular moon passes into its parent planet's shadow at the end of its "morning" period. This was when my "foggy" screenshot was taken - the beginning of the daily "eclipse" period. The Eclipse period starts out dark and foggy, but the fog dissipates a bit from the lack of sun.
III. Re-emergence. The sun comes back out from the other side of the parent body. Fog is present but diminished; heating begins again.
IV. Sunset. The angle of light gets low enough that the fog often dissipates well in advance of the "sunset line" (see my Merope 2D horizon shot above). After sunset, no fog is present.
Obviously, if a planet is not a moon, the Eclipse/Re-emergence phases won't occur.
In Conclusion:
You won't find fog on a tidally-locked night side of a planet. I suggest finding something new to search for.
These foggy areas are definitely where I'd expect to find a strange life form on a dead airless moon, but I don't have a clear enough picture here to help narrow down the search much (yet).
Happy Sciencing CMDRs!
Edit: Realized the horizon shot was actually Merope 5C, and not 2D. Whoops!