The air on Rustum

[Hans Rancke-Madsen]

Many years ago I did a writeup of a Traveller world called Forboldn for Pyramid. To explain why the population was rather low and because I thought that the idea had some excellent dramatic possibilities, I gave it an atmosphere similar to the one Poul Anderson described for his world Rustum in his books Orbit Unlimited and New America. The atmosphere was so dense that the concentration of CO2 at sea level made the air unbreathable for normal humans, restricting the population to live at high altitudes. However, some members of Rustum's population had greater tolerance for CO2 and were able to live at lover levels.

In a recent discussion I was told that the composition of the air (the proportions of the various components) would be the same at all altitudes; only the density would vary. Thus, if the oxygen was dense enough to be breathed at high altitudes where the CO2 was low enough for normal humans, it would reach toxic levels at lower altitudes, making higher CO2 tolerance insufficient to allow people to breathe the atmosphere at lower levels.

The person who told me seems to know what he is talking about and has been backed by someone else too. Personally I don't know; I'm relying entirely on Anderson's reputation for taking great care in his world-building.

1) Either Anderson got it wrong.
2) Or I misunderstood Anderson and got it wrong.
3) Or my debater is wrong.

I'd really like to know which it is and -- if it's 3) -- be able to explain how it really works to those who say otherwise.

I have two problems there. Firstly, my copies of Anderson's books are packed away, so I can't find them to quote just what Anderson said about Rustum and its atmosphere. Secondly, I have been unable to google any discussions about Rustum and the accuracy or otherwise of Anderson's science.

I'm hoping that someone who has access to the books could quote what it says about the atmosphere and how the settlers who could breathe at lower altitudes without artificial aid managed that feat. I'm also hoping that someone knowledgable on the subject could tell me who is wrong here, Anderson, me, or my debater.


Hans

[malloyd]

Originally Posted by Hans Rancke-Madsen

I'm also hoping that someone knowledgable on the subject could tell me who is wrong here, Anderson, me, or my debater.

Hard to say. The relevant phenomenon is called "scale height", H = kT/(g mu), which says the pressure of the atmosphere falls off exponentially , P = P0 e^(-z/H). In ideal gases in still air in should apply to all the gas partial pressures independently, and therefore produce some of this kind of segregation, though at survivable temperatures scale heights tend to be many kilometers, you'd need a lot of dramatic relief, or I suppose really high gravities, to get really big variations. The difference in CO2 molecular weight mu = 44, vs 32 for oxygen is not huge.

However. Here on Earth though, the atmosphere below about 100 km (the "homopause") fails to behave ideally and keeps essentially the same composition, probably because it is mixed by weather. Above that it starts behaving closer to ideally, and differences in scale height are partly why the really thin outer layers of all atmospheres tend to be dominated by hydrogen and helium molecules, atoms and ions - the other factor being composition of the solar wind.

[Fred Brackin]

Questions about the specific mechanism of the atmosphere's problematic nature could come up.

For example, a common effect of a higher than usual CO2 level is stimulating the lungs to take more breaths per minute to compensate. This might mix badly with increased pressure and lead to some sort of repiratory difficulty while still being tolerable at lower pressures.

Without knowing the ipp of oxygen it's really hard to suggest possibilities. If the atmosphere was only 70% nitrogen both the IPPs of CO2 and O2 could be above Earth-normal. Then your lungs trying to clear CO2 and that leading to too much oxygen as just one possibility.

Most of the holdover in the front of my head from doing G:Atlantis, TS:Under Pressure and G:Blue Planet is the insane complexity of the subject of variant breathable atmospheres and the ways in which they can be bad for you.

I probably couldn't answer your question even if I had all the data but we don't even have enough for a good start right now.

[Anaraxes]

(1) Anderson got it wrong.

The Earth's atmosphere has a pretty consistent composition. It doesn't sort itself out by molecular weight. (The biggest exceptions are more water vapor lower down, and more ozone up in the stratosphere.) Some of the mixing mechanics seem pretty basic to me -- Hadley cells and lesser convection, prevailing winds from Coriolis force -- so I'd find it less plausible that a few quirks of weather or geography would made a difference on an alien planet.

You'd want some sort of process that will preferentially deplete CO2 higher up and/or produce it lower down. CO2 is more stable than ozone, so the latter in particular seems tricky. Maybe you could manage with weathering or some sort of biological process. But I'm no planetary scientist.

[lwcamp]

First, a bit of data on CO2. A partial pressure of 8 kPa CO_2 will cause loss of consciousness within minutes. 5 kPa of CO_2 can cause dizziness, confusion, shortness of breath, panic attacks, and headaches. 2 kPa is mildly narcotic. 1 kPa is generally safe, but may cause drowsiness. For an 8-hour work day, the regulatory limit is around 0.5 kPa. For exposures under ten minutes, 3 kPa is considered safe, over 4 kPa can be physiologically tolerated for some time but are generally considered rapidly dangerous.

Now lets look at oxygen. Partial pressures of oxygen of around 50 kPa can cause inflammation of the lungs after several hours to about a day, with onset time decreasing as partial pressure increases. At more than 160 kPa, you start to see central nervous system toxicity as well, with several hours of exposure resulting in seizures. Low pressures are also dangerous. At partial pressures of about 16 to 15 kPa and below, you will often get altitude sickness, resulting in headaches, fatigue, and swelling. In serious cases, altitude sickness can lead to swelling of the lungs and brain which can cause death. Acclimation can help with this, but about 10 kPa of partial pressure is considered the limit of long term human habitation. 8 kPa is survivable for a few days at least, but is very taxing with digestion and sleeping becoming very difficult.

On a planet where sunlight strikes the ground, differences in absorption will lead to differences in pressure, which will in turn lead to turbulent mixing of the air - what we call weather. This means that all the gases get well-mixed and in this turbulent region the atmospheric composition will be essentially unchanged. What will change is the pressure. The ratio of carbon dioxide partial pressure to oxygen partial pressure will be near constant but the absolute values will decrease with altitude.

So we want a region where the partial pressure of oxygen is greater than 10 kPa and the partial pressure of carbon dioxide is 1 kPa. Let's assume that at some reference altitude the oxygen is at 15 kPa and the CO2 is at 0.75 kPa. This is breathable. If you descend in altitude to where the oxygen is at 20 kPa (near Earth values at sea level) the CO2 partial pressure climbs to 1 kPa. Below this altitude the CO2 starts to become toxic - but it will be quite mildly toxic unless the pressures get much higher. Still, this gives a range of altitudes in which you can breathe, and a range of altitudes below that where the CO2 starts to cause problems.

One additional complication is adiabatic heating and cooling as air rises and falls. In the above scenario, if the average temperature at our reference altitude is about 15 C (fairly comfortable if a bit warm), then where the air has a partial pressure of 20 kPa of oxygen the average temperature will be about 40 C (too warm for long term survival) and where the air has a partial pressure of 10 kPa the average temperature will be about -15 C (frozen). This is assuming dry adiabatic temperature changes - condensation will moderate it a bit, but you still have wide temperature swings. (EDIT - assuming Earth-like gravity and an Earth-like atmosphere except for about 1.5 % CO_2 concentration.)

Luke

[Hans Rancke-Madsen]

Originally Posted by Fred Brackin

I probably couldn't answer your question even if I had all the data but we don't even have enough for a good start right now.

That's why I hoped that someone with ready access to the books would quote the pertinent details. About the only thing I can remember is that Rustum had a gravity of 1.25G.

(And there I definitely dropped the ball, because I only gave Forboldn a gravity of 1.1G).


Hans