Frozen Atmospheres...

[signless]

I'm working on a halloween adventure for my players.

It's a salvage operation on the lonely, isolated, outermost planet of a backwater system. I'm building the world around a modified model of Pluto, where the world has just entered perihelion and the atmosphere has frozen solid.
I'm looking for a solid sheet of frozen methane/CO2/Nitrogen that's at least several meters thick. How big does my world have to be in order to get something like that to happen? I'd like to keep the surface gravity somewhere in the range of 1-1.5 g's if possible.
...help?

 

[Malenfant]

I think you mean "aphelion" - the atmosphere is likely to freeze when it's furthest from the star and therefore coldest (perihelion is when it's closest).

If the surface gravity is 1 - 1.5 g that far in the outer zone, your world has to be HUGE - about 12,000 km *radius*, just to have a surface gravity of 1 g. And out there, that means it can hold onto hydrogen and helium and in all probability has snowballed its atmosphere so that it is really a gas giant and not a solid planet. It'd be rather odd for a behemoth that size to have not become a gas giant. I won't say it's impossible, but it's very strange for it to be there.

It could be smaller and denser, but it'd be very odd for such a dense, rocky body to be out there. Plus that'd still have the problem of holding onto hydrogen and probably snowballing.

You could just say it's a low gravity icy world and have it being about size 3 or 4. That'd be able to hold onto CH4, NH3, or N2 (CO2 isn't as common in the depths of the outer zone) but not H2. Gravity would be about a quarter to a fifth of that on Earth.

That any use?

 

[signless]

I think you mean "aphelion" - the atmosphere is likely to freeze when it's furthest from the star and therefore coldest (perihelion is when it's closest).

I think I wrote that unclearly. The world has just entered perihelion, and the frozen atmosphere is just beginning to sublimate into gas. I'm going for an EXTREMELY swirling, foggy, cold mood in the setting. And one that poses a challenge for a salvage team...how do you free big artifacts from the grip of a frozen, highly-volatile atmosphere without blowing yourself up? I want the work to be painstaking and back-breaking. The kind where you HAVE to pay attention to detail...so MUCH attention that you don't always notice a shift in the swirling, shadowy atmosphere behind you...

You could just say it's a low gravity icy world and have it being about size 3 or 4. That'd be able to hold onto CH4, NH3, or N2 (CO2 isn't as common in the depths of the outer zone) but not H2. Gravity would be about a quarter to a fifth of that on Earth.

That any use?

Heck, yeah. Having gravity somewhere around 1-1.5 would've been nice, but not absolutely necessary. I'm thinking this is more of a "captured" wanderer, rather than a legitimate outer planet...especially if I set it well-off the system's plane of the ecliptic.

 

[Malenfant]

Oh... yeah, you were a bit unclear

. I thought you wanted the world to have its atmosphere frozen out already

Not sure if a sublimating surface would be quite so foggy and "swirly". I kinda picture it more like a comet - pockets of gas sublimating violently and make random "geysers" blowing out with no warning (seen the movie "Deep Impact"? Kinda like that, when they're walking on the comet and sun rises). Though I guess that after that process has continued for a few years there'd be enough of an atmosphere to suspend some stuff in it (and the low gravity would help keep particles in the air).

 

[Zutroi]

I'd suggest reading a little Niven as well - there's a great short story (the name escapes me at the moment. Damn Senility!!) about the effects of a fusion drive on Pluto's frozen atmosphere...

 

[signless]

Yeah. I think I remember that one...but I may be confusing it with another. I think the hero strips at the end in order to freeze himself solid...

 

[Zutroi]

That would be the second of Niven's stories about Pluto. The one I'm thinking of involves the atmosphere freezing out, stratified by freezing point, so there's a layer of nitrogen, one of oxygen, one of hydrogen, etc. The fun begins when the fusion drive has melted into the oxygen, hydrogen, methane and ammonia strata...

 

[Jeff M. Hopper]

You're thinking of the Niven novel "World of Ptavvs" where man meets up with the first Thrintum in the Known Space timeline. The singleship hovers over Pluto near the end of the story.

Damn, that was one of the first scince fiction stories I ever read, too...