Pandoran atmosphere hypotheses.

[Prometheus]

I have to say that I don't follow you here. The percentage of its orbit that a moon could be in its primary's shadow increases with decreasing distance, with the limit being 50% of its orbit if the moon were skimming just above the primary's atmosphere. (Actually, the situation is more complex if the moon's rotation is tidally locked. The portion of the moon's surface that never sees the moon's primary would get the full stellar insolation, while the portion of the moon's surface that has the moon's primary directly overhead would get considerably lower direct insolation due to being blocked by its primary for some fraction of the time the star is above the local horizon. I imagine that there would be ferocious winds redistributing energy between the sub-primary and anti-primary hemispheres, with their strength depending on how much of the sky the moon's primary fills.)

From what I recall from the movie, Polyphemus takes up about 30 degrees in the sky, and hence (assuming that Alpha's apparent movement takes it directly behind the center of Polyphemus as viewed from Pandora) reduces Pandora's total globally-averaged insolation by 1/12th (plus whatever Polyphemus contributes in IR and reflected visible light).

The closer in the moon orbits, the faster its orbit. So, even though it may be totally covered by the shadow of the Polyphemus when it is behind it, it doesn't stay there for too long. It's never in the shadow long enough to really cool it down to any extent...and with 18% CO2, the heat distribution and retention throughout the atmosphere would be very effective. The moon wouldn't lose much heat at all. However, if it was at a distance from Polyphemus where the moon was eclipsed for a considerable period of time, because of its slow orbital motion, it may cool somewhat. Polyphemus covers about 20% of the moon's sky, which would put it at about 3.41 million kilometres. At that distance, it would orbit the planet in about 6 days or so. At it's orbital velocity, it wouldn't stay any longer than 30-36 hours behind the planet before it emerged into bright sunlight. That length of time wouldn't be enough for the moon to cool considerably with such a CO2 rich atmosphere. Not only that, Pandora does rotate but the rotation is not locked to its orbital motion (spin-orbit coupling), so the moon never has any one side facing the planet all of the time.

The moon would never get as close as the top of the planet's atmosphere... that would be well within the Roche Distance of the planet-moon system and Pandora could be pulled to shreds (if it orbited permanently that close). Actually, given Pandora's size, it would most likely plunge into the giant intact and the resulting collision (as it would penetrate right to the core of Polyphemus...given its size) would give the giant planet a lot of grief. It would seriously disrupt the giant.

For Pandora to cool significantly in the shadow of Polyphemus, it would have to linger in that shadow for a week or more. I could work out how far away it would have to be, exactly, but I don't feel like doing it right now However, it would have to be about 20-30 million kilometres from the giant, at least, for it to linger for a week or so in its shadow. That far away from Polyphemus would probably put Pandora in the gravitational influence of Alpha's outer giant planet because of orbital resonances and such. That would make Pandora's orbit unstable and the moon would've either crashed into the giant or got flung out of the system altogether.

[Prometheus]

* 530–1000 ppm causes strong stimulation of the central nervous system and rapid breathing, leading to loss of breathing;
* 800 ppm is the lethal concentration for 50% of humans for 5 minutes exposure(LC50).
*Concentrations over 1000 ppm cause immediate collapse with loss of breathing, even after inhalation of a single breath.

That means for most of Earth's history....right up until very recently, we'd be stuffed. We wouldn't be able to survive in its atmosphere because CO2 levels were way too high, especially if given a modern atmospheric composition. Although, the higher O2 levels in the past would've partially compensated for this.


Like I said before...18% CO2 is ridiculously high. JC might not have thought this one through well enough. I'd be happier at 8000-10000ppm, but I'd want no loss of O2 levels either...maybe even an increase, to counter the CO2.

[Mongo]

Actually, I think that information about 1,000 ppm leading to immediate collapse must refer to H2S, not CO2. I know that people can work in greenhouses with CO2 concentrations up to 3,000 ppm (0.3%) for 8 hours at a time with no reported side effects. The U.S. Occupational Safety and Health Administration says that average exposure for healthy adults during an eight-hour work day should not exceed 5,000 ppm (0.5%) -- so I infer that 5,000 ppm is okay as well, for at least that length of time.

I see that further down that Wikipedia article we have this:

Adaptation to increased levels of CO2 occurs in humans. Continuous inhalation of CO2 can be tolerated at three percent inspired concentrations for at least one month and four percent inspired concentrations for over a week.

and this:

At about 8% it causes headache, sweating, dim vision, tremor and loss of consciousness after exposure for between five and ten minutes.

So the effects described in Avatar for an 18% (180,000 ppm) CO2 concentration sound about right to me in terms of more rapid loss of consciousness, especially if there are trace amounts of H2S as well. I think that James Cameron got it right.

[Prometheus]

That means for most of Earth's history....right up until very recently, we'd be stuffed. We wouldn't be able to survive in its atmosphere because CO2 levels were way too high, especially if given a modern atmospheric composition. Although, the higher O2 levels in the past would've partially compensated for this.


Like I said before...18% CO2 is ridiculously high. JC might not have thought this one through well enough. I'd be happier at 8000-10000ppm, but I'd want no loss of O2 levels either...maybe even an increase, to counter the CO2.

Oops...misread the quote :P

Those conc's were for H2S :P

[Prometheus]

What's worse is there is a small amount of HCN in the atmosphere...that'd kill you stone dead almost immediately if the conc was greater than 3500ppm. 300ppm will kill you within 10 minutes. I would hope it's somewhere in the parts per billion level. You'd smell it though...the atmosphere would smell of almonds.

[Prometheus]

Actually, I think that information about 1,000 ppm leading to immediate collapse must refer to H2S, not CO2. I know that people can work in greenhouses with CO2 concentrations up to 3,000 ppm (0.3%) for 8 hours at a time with no reported side effects. The U.S. Occupational Safety and Health Administration says that average exposure for healthy adults during an eight-hour work day should not exceed 5,000 ppm (0.5%) -- so I infer that 5,000 ppm is okay as well, for at least that length of time....


.....So the effects described in Avatar for an 18% (180,000 ppm) CO2 concentration sound about right to me in terms of more rapid loss of consciousness, especially if there are trace amounts of H2S as well. I think that James Cameron got it right.

Given those values and conc's we should be fine for most periods. Although during the "Snowball Earth" periods, the CO2 level was as low or lower than pre-industrial Holocene/Pleistocene Earth. Probably anything beyond about 1800-2000Ma would be dodgy as the O2 levels were also rather lower than now (<10% of present O2 conc). Even with those later periods, the O2 levels would be crucial as some of the periods had lower levels than now (e.g. Cambrian had O2 at 12.5%). The Carboniferous Period...well, you could've boosted CO2 to 80000ppm and it probably wouldn't matter as the O2 levels were around 40%. Soggy wood would burn under those conditions. The CO2 levels then were around 800ppm.

18% CO2 would produce very rapid loss of consciousness.

[Spock]

Its just hit me as to how benign our respiration system is. In biology what we call the optimal operating range, is very narrow when it comes to respiration, as soon as we move out of that, into the tolerance zone, call that 3% of carbon in the atmosphere we become stressed. And beyond that 5% we die. Ugh. Not good enough, through science, I hope one day we can modify our respiration system to operate under much more trying conditions.

[Human No More]

Actually, I think that information about 1,000 ppm leading to immediate collapse must refer to H2S

Yeah, that was for H2S, sorry if I didn't make that clear. 18% CO2 would be a bigger danger.

['upxare]

Does anyone knows more or less precisely the mass and size of (fictive) Pandora?
It couldn't be much smaller than earth's, otherwise it couldn't keep a dense athmosphere. In fact there seems to be no gas giant in the AC system but it is highly expected that some earth-sized planets lurk around.

Quote from wikipedia:
"...In addition, the lack of any brown dwarfs or gas giants around A and B make the likelihood of terrestrial planets greater than otherwise. Currently, technologies do not allow for terrestrial planets like Earth to be detected around Alpha Centauri (unless they are a few Earth masses and orbiting very close to the star, as in the Gliese 581 and HD 40307 systems, for example), but this is expected to change in the near future.(...)"

Back to Pandora's athmosphere: 18% CO2 seems being a bit too high for any humanoid lifeform. Even if the planet could sustain stable and ideal temperatures to support life, its organisms would look totally different. Based on the flora and fauna seen on fictive Pandora, I'd say 1,8% CO2 sounds more realistic!

[Prometheus]

Diameter = 11447kms (6868 miles)
Mass = 0.72 Earth (4.32 x 10^24kg)
Gravity = 0.8 Earth (7.84m/s)