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[GreatJustice]

Oh wait I was just doing this.
 
That's awesome. I JUST had tests on all the Dalton's law, Charles law, Gay-lussac's law and the like. I neve rthought I'd ever be asked anythign like that again.

Already tried that. Unfortunately, those only apply to ideal gasses (hence, ideal gas law) of which SF6 is not one.

However, this might be of some use, possibly.

[Virex]

Oh wait I was just doing this.
 
That's awesome. I JUST had tests on all the Dalton's law, Charles law, Gay-lussac's law and the like. I neve rthought I'd ever be asked anythign like that again.

Already tried that. Unfortunately, those only apply to ideal gasses (hence, ideal gas law) of which SF6 is not one.

However, this might be of some use, possibly.

That's still using the ideal gas law, which is a fine approximation if you don't mind a 10% deviation.

[Eagle_eye]

Well, unless he's actually doing terraforming, that's probably preferable to the absurd amount of computation it would take to get a perfectly accurate answer.

[misko27]

Well, unless he's actually doing terraforming, that's probably preferable to the absurd amount of computation it would take to get a perfectly accurate answer.

Well don't be rude and assume things.
 
ARE you terraforming? And where specifically? Perhaps we can offer more tailored advice.

[RedKing]

That's classified information. Also, if Curiosity picks up anything unusual, it's JUST AN ANOMALY. Nothing going on up on Mars. Nope.

(I had a bit of fluff text in an Aurora fic about Mars getting terraformed with SF₆, and how one lasting effect was that long-time Martian colonists all had really deep voices due to the SF₆ hanging around in their tracheal air column, and this formed the basis for in-group/out-group social selection. Decided to expand that into a story setting and wanted to do the research.)

[Sheb]

Actually, we're all missing a really important point here. He's operating on a planet, so volume is not constant. The pressure is only going to be a function of the amount of gas in the athmosphere, equal to the weight of the gas column above a 1 sq. m surface. When it heats, the athmosphere will swell, but pressure will remain constant.

[RedKing]

Ah. So in adding gas to the atmosphere, the additional heat capacity won't change pressure, just make the atmosphere layer go to a higher altitude?

[Sheb]

Well, pressure in altitude would change with heat (As the whole athmsopher expand, more of it is above that point, so pressure increase), but at ground level it would not.

Basically, since space is exerting 0 pressure on your athmosphere (being space and all that), the only thing holding your gas molecules is the wieght of all the molecules above it (And gravity, but that's irrelevent for one molecule). If pressure get higher than the force exerted by those molecule, your lower layer of athmosphere will expand until those forces are equal again. Same if the gas pressure is lower than that wieght. Hence we just demonstrated that it must be equal to that weight.

(Of course, the "weight" I used is actually a pressure (it's the weight of the gas column per unit area), but I didn't want to be confusing.

[Another]

Actually, we're all missing a really important point here. He's operating on a planet, so volume is not constant. The pressure is only going to be a function of the amount of gas in the athmosphere, equal to the weight of the gas column above a 1 sq. m surface. When it heats, the athmosphere will swell, but pressure will remain constant.

Unless you take into account planet surface curvature relative to characteristic height of the gas column. The effects of said column not being of constant cross-section area and gravity reducing with height are usually not very significant though.

[RedKing]

(Of course, the "weight" I used is actually a pressure (it's the weight of the gas column per unit area), but I didn't want to be confusing.

....

I think you failed on that count. 

I think I'm going to need to find some simulation software.

[Siquo]

No, Sheb is right. Adding temperature won't increase the pressure at "sea level", but will somewhat increase the pressure at mountain-height.

[Sheb]

Nah, he meant I failed at not being confusing.

[Starver]

Hmmm, ignoring for now surface perturbation from the sphere (both mountains/valleys and the generally ellipsoid nature of a rotating planet), we're talking about differentiating (or integrating, depending on which way round you're wanting to do it) thin spherical shells of gas of increasing diameter and reducing density (and thus weight according to gravitational influence, this contributing to the pressures upon the lower shells) up until the point that all the 'gas mass' is used up (so that it fades to zero density at that level).  Or, if there is still "gas to give" at the point where the pressure at any level is greater than (or significantly approaches?) the gravitational attraction by the planet below, you're going to find yourself sloughing off the extra gaseous mass until you're about equal again[1]


It sounds like a catenary-like calculation (or, at least, half-a-catenary) with an extra dimension or two to it and a varying 'force' along its length.  I's been a while since I've done this sort of thing, but I might see what I can dredge out of my rustier parts of my brain, for this one, if nobody beats me to it.


(Though the above isn't counting other issues like solar wind whipping around the upper levels.  Or, right now, stratification of gas-types or even of atmospheric temperatures, which we know from Earthly meteorology and the like doesn't even change constantly on its way to the "too tenuous to really matter" layers.)


[1] Although gas giants exist, so maybe the extra gas means extra pressure, thus denser lower layers that 'use up' more gas-mass so that the whole atmosphere 'drops' into the well where gravity exceeds the local pressure.  Maybe it's like black-holes, in that you need enough mass to create the self-same mass-holding situation, but be on the 'too little' side of the worst-case scenario and you don't have enough.  It could just be the low temperatures, except that there's extrasolar 'hot Jupiters'...  Again, modelling needed.  Though I imagine that this has been extensively done already, even if only with partial understanding to get theory to explain observation.

[MaximumZero]

I have no idea what the hell any of you just said. Good job. I'm posting to re-read until I understand, watch, and nod my head occasionally as if I get it.

[RedKing]

[1] Although gas giants exist, so maybe the extra gas means extra pressure, thus denser lower layers that 'use up' more gas-mass so that the whole atmosphere 'drops' into the well where gravity exceeds the local pressure.  Maybe it's like black-holes, in that you need enough mass to create the self-same mass-holding situation, but be on the 'too little' side of the worst-case scenario and you don't have enough.  It could just be the low temperatures, except that there's extrasolar 'hot Jupiters'...  Again, modelling needed.  Though I imagine that this has been extensively done already, even if only with partial understanding to get theory to explain observation.

I understand this part, which is why SF6 makes an ideal terraforming gas. It's incredibly dense, so it's far less likely to fly off into space at low gravity (although it still doesn't deal with lack of magnetosphere, where solar wind would blast some of the atmosphere off). That would allow for a steady increase in atmospheric pressure and a BIG increase in thermal capacity and greenhouse effect, which would (I thought) further increase atmopheric pressure and allow for lighter gases like O2 and N2...especially N2) to be inserted without experiencing a lot of "bleed-off". Mars' own ice caps could contribute significantly, because the CO2 would begin sublimating as the ambient temp increases, triggering a positive feedback cycle. Part of what I'm trying to figure out is just how much SF6 it would take to really kickstart the CO2 feedback cycle.

Let's see...CO2 sublimates at 194.7 K, at 1 bar pressure (can't seem to find data for boiling point at very low pressures, but it should be similar or even lower). Average planetary temp is just a few degrees above that, while the winter polar temps are a good 50 degrees below that.

Crap. I'd need to figure out what the critical temperature would be to melt enough higher latitude polar ice to trigger an increase in temperature sufficient to keep the region of winter deposition smaller than the region of sublimation (in other words, get to a point where more polar dry ice evaporates in the summer than refreezes in the winter). At that point, you have a self-sustaining "global warming" process which should eventually liberate all the polar ice as atmospheric CO2 (assuming no loss of atmosphere). According to Wikipedia, that would boost the atmospheric pressure to around 0.3 bar, though I can't find any estimates as to what it would raise the temperature to.

...I'm starting to wonder if there's somebody at NASA that has already crunched all these numbers, and if they'd indulge me.