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PostPosted: Sun Dec 18, 2016 5:21 pm 
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thrash wrote:
On the other hand, Titan's equilibrium (black body) temperature is 85K, so there is some greenhouse warming going on. A world with a nitrogen-methane atmosphere, necessarily beyond the snow line, would still be warmer than an airless rockball in the same position.


Is the warming a product of simply having an atmosphere or specifically of the methane in the atmosphere?


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PostPosted: Sun Dec 18, 2016 6:16 pm 
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It's the methane. Nitrogen by itself is basically transparent to both visible (incoming) and thermal infrared (outgoing) radiation, so the net effect is negligible.


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PostPosted: Sun Dec 18, 2016 6:33 pm 
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thanks

:)


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PostPosted: Sun Dec 18, 2016 8:58 pm 
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thrash wrote:
It's the methane. Nitrogen by itself is basically transparent to both visible (incoming) and thermal infrared (outgoing) radiation, so the net effect is negligible.

Yep. Methane is very important for providing a greenhouse effect at low temperatures, because water vapour freezes at 273 K and CO2 sublimates at 194 K, whereas methane still supports a significant vapour pressure right down to its freezing point, about 112 K.

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PostPosted: Sun Dec 18, 2016 9:07 pm 
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If the planet's MWR is greater than methane's 16, you'd expect a lower greenhouse effect?

Or, is there a planetary process (assuming the absence of bovine flatulence!) that produces enough methane to have an effect on the temperature before it's lost?

How does ozone figure into this? Does it? I know it's a greenhouse gas and it's MW of 48 will perhaps mean it's present when CO2 is not but how's it normally produced? (I'll go google it but I'm interested to hear it here too!)


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PostPosted: Sun Dec 18, 2016 9:12 pm 
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OK, I have caught up, O3 is produced by sunlight interacting with O2. Oxygen is unlikely to exist in the atmosphere of a Titan like world as you need water and life in the water to produce O2.

Methane is therefore the most active greenhouse gas in planets in the outer zone?


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PostPosted: Sun Dec 18, 2016 10:39 pm 
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hiro wrote:
OK, I have caught up, O3 is produced by sunlight interacting with O2. Oxygen is unlikely to exist in the atmosphere of a Titan like world as you need water and life in the water to produce O2.

Methane is therefore the most active greenhouse gas in planets in the outer zone?

I think so. You need a molecule with at least three atoms in it to have the right sort of modes available to absorb IR, and it has to be a vapour or gas to be in the atmosphere. Helium, hydrogen, and nitrogen aren't candidates. Water and carbon dioxide freeze out. You don't get much ozone without oxygen, which depends on bootic processes mostly (though maybe you'd get enough photodissociation of water in orbit around a bluish star, if you had water vapour, which you wouldn't because of freezing…). I think that leaves methane and ammonia — ammonia's unstable in UV, but you might get it in the atmosphere of an object orbiting a UV-poor M or late K star.

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PostPosted: Sun Dec 18, 2016 10:55 pm 
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hiro wrote:
If the planet's MWR is greater than methane's 16, you'd expect a lower greenhouse effect?


Yes. Also, if the planet were cold enough that methane froze out I'd expect runaway glaciation—not because of an albedo effect but because cold –> methane snowing out –> reduced methane in atmosphere –> reduced greenhouse effect –> colder.

Quote:
Or, is there a planetary process (assuming the absence of bovine flatulence!) that produces enough methane to have an effect on the temperature before it's lost?

Something is producing methane on Titan and Mars.

It doesn't take a lot to have an effect on temperature. After all, anthropogenic greenhouse warming on Earth is over a kelvin by the addition of only 130 parts per million of a 101.4 kPa atmosphere: 13 Pa of Co2, which under 1 gee is only 1.35 kg per square metre.

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PostPosted: Sun Dec 18, 2016 11:12 pm 
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thrash wrote:
It's the methane. Nitrogen by itself is basically transparent to both visible (incoming) and thermal infrared (outgoing) radiation, so the net effect is negligible.

I have just now realised what ought to have been obvious: all else being the same, the greenhouse effect is stronger with the bluish light of hot stars than with the IR-rich light of reddish stars. With a cooler star there is less discrepancy between the spectrums of incoming sunlight and outgoing radiation from the planet's surface.

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© My posts to this board are copyright under the Berne Convention. They may be quoted on the board with appropriate attribution. They may not be reproduced beyond the board except with explicit permission from me.


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PostPosted: Mon Dec 19, 2016 12:26 am 
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Thanks for the knowledge gentlemen :)


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