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alaskanflyboy
09-15-2008, 11:59 PM
I've tried searching the forums and the web as well, but I haven't found any information that would help me determine just what changes would come about in changing the composition of a planets atmosphere. Are there any calculations, or even an application that can help? I'm basically wanting to be close to realistic in converting things such as CO2 into O2 and what the resulting percentage changes would be as well as the affect on temperature and atmospheric pressure.

Likely such things do not exist and I may be trying to hard to stay with plausible science, but I thought I'd ask.

waldronate
09-16-2008, 02:07 AM
The effects on pressure would depend largely on the change in composition. In Earth's atmosphere, for example, changing all of the CO2 to O2 would have a negligible effect on air pressure. It would have an effect on temperature through reduction in greenhouse warming. How much of an effect is, of course, a subject of much debate between various entrenched groups.
The magnitude of a greenhouse effect depends on the spectrum of the host planet's star as well as atmospheric composition. On Earth, for example, we have a star with a spectral peak right around 500 nm ("green"). Where we are relative to the sun ends up at an equilibrium temperature right around 11 nm (far infrared) so items are absorbing the sun's energy in the visible (and UV and IR) and reradiating it as far IR.
The primary greenhouse gases in the atmosphere are water vapor and methane, both of whic have an absotrption spectrum that's relatively transparent across optical wavelengths but absorbs and re-radiates stringly in the far IR, which is the classical greenhouse effect. CO2 and other gases also perform this behavior, but they are at much lower concentration than the main ones.

I recommend the book World-Building by Stephen Gillett. It discusses many of the basics of atmospheres and world-building in general. It's for science fiction writers but is a good book nonetheless.

jezelf
09-16-2008, 06:49 AM
Theres some threads over at Science Fiction Fantasy Chronicles Forum (http://www.sffchronicles.co.uk/forum/37961-custom-planet-needed-for-novel.html) that have explored planet building and various atmospheres. There might be some information for you there.

Redrobes
09-16-2008, 06:51 AM
Would you not get other effects happening too which would be more important. I was under the idea that if the O2 rises then there would be many more fires on the planet which would put up a lot of smoke into the atmosphere and cause it to heat up that way.

waldronate
09-16-2008, 11:42 AM
When you add life into the mix then things get all unpredictable. Cutting down trees or draining marshes, for example, raises the overall CO2 levels, giving you a warmer climate. If you have a world with a race that plants trees instead then you might expect to see a drop in CO2 and a cooler world. However, the total amount of CO2 in the atmosphere is tiny, around 0.04%. Converting all of that to O2 wouldn't have much effect on total O2 concentration, which starts out at 20.95%.

Amount of fires wouldn't change much with such a small change in O2 concentration. You need to get whole percentage points of difference to make a significant difference. http://www.pnas.org/content/103/29/10861.abstract states that charcoal concentrations in the fossil record changed as O2 concentrations varied from 13% to 30% from Devonian to Permian. Big O2 swings, big fire differences.

Those aerosols from fires aren't the only nasties, though. If you get sulfates into the upper atmosphere then you get massive reflection of sunlight and huge cooling effects. If enough snow gets dropped on land then it can take years to recover.

The albedo (amount of light reflected) change of snow vs. soil vs. ocean is dramatic so more snow means more light reflected and more dark stuff like land and ocean means more heat.

The biggest overall climate impact on Earth is orbital fluctuations. There are several models which suggest that if we weren't doing interesting things to the atmosphere we would be coming up on an ice age again.

I'll stop rambling now. It's a nasty habit.

alaskanflyboy
09-16-2008, 02:07 PM
Thats definitely more info than I had hoped for, and a book geared toward science fiction writing is perfect since I'm working on planets in AS and FT for a scratch build sci-fi canon to hopefully use in future writing projects of mine as well as possibly making a role-play campaign setting later on. Right now, the planets I'm looking at have massive CO2 concentrations. As an example, GJ 388 B II has this for an atmosphere: 3.67 atm (N2 44%,CO2 35%,O2 10%,SO2 3%)

The goal of course is to bring that O2 up to reasonable levels while reflecting the drop in the other stuff in a realistic fashion, then try and sort out how it would alter the climate and temperature from the greenhouse gas and pressure changes, if any. Of course, then you have the albedo change as a result of the climate change when terraforming since some planets will go from light colored rock to dark green forests.

And on the topic of AS, does anyone know if changing the atmosphere composition in AS has any effect on things when you have it pop up FT for the planet generation? Or is it all completely information just for show?

waldronate
09-16-2008, 11:22 PM
If I recall it's just basic things like global temperature and water percentage that come across to FT. There's nothing in FT that relates to atmospheric composition beyond a vague "greenhouse effect" parameter.

alaskanflyboy
09-17-2008, 11:23 PM
I didn't know if AS tried to do something internally with its atmosphere notes and calculate them for the greenhouse effect parameter. I suspect I'm likely complicating things more than I need to for the sake of trying to adhere to known science given the fuzziness of the concept to begin with.

EDG
12-31-2008, 09:17 PM
*bump* thought I'd add something here...

IIRC you have to get between 30 and 35% O2 in the atmosphere to have problems with fire raging out of control (at those levels, EVERYTHING catches fire at the slightest spark, even wet wood in marshes and swamps) - but it'd be self-regulating because the global firestorms would lock up some of the oxygen by converting things to oxides - so that should be the effective maximum. Conversely candles and matches have trouble lighting when the O2 level drops below 17% (or 19%, I can't recall right now).

The really important thing is Oxygen Pressure (also known as O2 ATA), which is the fraction of free O2 in the atmosphere multiplied by the pressure of the atmosphere. So in our own atmosphere the oxygen pressure is 0.21 (0.21 x 1 atm pressure = 0.21 atms). The same percentage of O2 at the surface of a world where the atmospheric pressure is 2 atms would have an oxygen pressure of 0.42 (0.21 x 2 atm = 0.42 atms). What I'm not sure about is whether the percentages I quoted above for the fires apply to O2 ATA or to absolute percentage of oxygen - I suspect they apply to the O2 ATA though. So you'd get global firestorms around the 0.35 atm O2 ATA mark, and fires would be hard to start around the 0.17 atm mark.

Oxygen starts causing longterm problems in humans if the O2 ATA goes above 0.50 atm on Earth (in medical conditions) - the lungs start getting damaged by the gas. But it's not instantly lethal at least - whereas if it drops below about 0.12 (such as on the tops of the highest mountains) then you'd be suffering from potentially lethal problems like altitude sickness, hypoxia etc very quickly.

Other gases cause problems at high pressures too - IIRC N2 causes nitrogen narcosis above N2 ATA of around 3 atms (Any divers in the audience should already be pretty familiar with all this). I think this is why heliox is used when diving at greater depths, because the helium doesn't have that problem.

It's all something to keep in mind if your planets have high pressure atmospheres though.

Redrobes
01-01-2009, 09:54 AM
Really interesting and useful numbers there. I guess for a world that has a stable but higher pressure atmosphere then the people and creatures on it would be adapted for that environment with more resilient lungs etc. I'm guessing that higher pressure atmospheres are on planets that are larger or have denser earth / core than ours.

EDG
01-01-2009, 04:21 PM
Really interesting and useful numbers there. I guess for a world that has a stable but higher pressure atmosphere then the people and creatures on it would be adapted for that environment with more resilient lungs etc.

Creatures that evolved there would, yes. Humans can acclimatise to an extent, but without actual genetic engineering we won't survive long. People live (and are born in and spend their whole lives) in low O2 ATA conditions in villages high in the Himalayas on earth, but even though they're used to it, they still have health problems. (another cool thing about low pressure - water boils at lower temperatures there. So making food there has to be adapted to the pressure in order to cook food properly - IIRC they basically use pressure-cooker type technology rather than just heating things directly with fires, which also are harder to light up there).



I'm guessing that higher pressure atmospheres are on planets that are larger or have denser earth / core than ours.

Not necessarily. If a planet can hold onto a gas, then it can hold onto as much as it likes of it. Look at Venus - it's slightly less massive than Earth and yet its atmosphere is 90 times thicker. But generally, more massive planets will have higher pressure - pressure is proportional to gravity, so if we suddenly and magically increased the earth's mass so that surface gravity was 1.5Gs (but we kept the radius the same) and didn't change our atmosphere at all, the atmospheric pressure at the surface would be 1.5 atms, not 1 atm. It all really depends on how much gas there is, which may be a lot or may be a little depending on the circumstances and history.