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Thread: Why no greenhouse effect on Titan

  1. #1
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    Why no greenhouse effect on Titan

    This has bugged me because I can't figure it out. Somebody asked me why, like Venus and its hellish temperatures due to the greenhouse effect of its thick atmosphere, isn't Titan warmer?

    Is there a reason Titan's thick atmosphere, even though it's far from the sun, doesn't cause a greenhouse effect? Or does it and it would be even colder if it didn't have such a thick atmosphere?

  2. #2
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    Titan's toasty -180 degrees C is way warmer than the ambient -270.

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    My guess: it is far away from the sun, and the thick atmosphere blocks solar radiation from getting through in both directions. SO it is located in a very cold place, and in the sahdow of its own atmosphere. You need light to get in first if you want greenshouse effects. The solar flux is low there already, and with the atmosphere blocking lots of light...

    Just my guess. Obviously, some light gets through, as Huygens was able to make ohotographs (OK he had a lamp, but that wasn't on all the time).
    With sufficient thrust, water towers fly just fine.

  4. #4
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    Cool. Thanks. That makes sense that in order to fuel runaway greenhouse effects, you have to have warmth to begin with.

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    Hmm.. I hadn't thought about it before, but how much sunlight gets to Venus' surface and what wavelenghts? It seems to reflect so much in the visible wavelenghts that I always figured it must be a gloomy place.

    CJSF
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  6. #6
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    I remember reading that very little light actually makes it into Venus. Something to do with the yellow color of it's CO2 cloud covering. I want to say 90 percent or more of the light that hits it is reflected away, but I can't remember well enough.

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    Venus's bolometric albedo is approximately 60%, and its visual light albedo is even greater. Thus, despite being closer to the Sun than Earth, the surface of Venus is not as well heated and even less well lit by the Sun. In the absence of any greenhouse effect, the temperature at the surface of Venus would be quite similar to Earth. A common conceptual misunderstanding regarding Venus is the mistaken belief that its thick cloud cover traps heat. The opposite is true. The cloud cover keeps the planet much cooler than it would be otherwise. The immense quantity of CO2 in the atmosphere is what traps the heat by the greenhouse mechanism.
    http://en.wikipedia.org/wiki/Venus_(planet)


    This implies that if Venus wasn't cloudy it might become hot enough to glow red.

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    The cloud cover keeps the planet much cooler than it would be otherwise. The immense quantity of CO2 in the atmosphere is what traps the heat by the greenhouse mechanism.
    I'm confused. Aren't the clouds made of CO2?

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    Quote Originally Posted by Kebsis
    I'm confused. Aren't the clouds made of CO2?
    No, they're mostly sulfuric acid.

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    Quote Originally Posted by Evan
    This implies that if Venus wasn't cloudy it might become hot enough to glow red.
    Why doesn't that happen to Mercury? In fact, why is Venus's surface hotter than Mercury?

    The pressure of Venus' atmosphere at the surface is 90 atmospheres (about the same as the pressure at a depth of 1 km in Earth's oceans). It is composed mostly of carbon dioxide. There are several layers of clouds many kilometers thick composed of sulfuric acid. These clouds completely obscure our view of the surface. This dense atmosphere produces a run-away greenhouse effect that raises Venus' surface temperature by about 400 degrees to over 740 K (hot enough to melt lead). Venus' surface is actually hotter than Mercury's despite being nearly twice as far from the Sun.

    The Nine Planets

  11. #11
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    Quote Originally Posted by Disinfo Agent
    Quote Originally Posted by Evan
    This implies that if Venus wasn't cloudy it might become hot enough to glow red.
    Why doesn't that happen to Mercury?
    Venus is hot because it's got a CO2 atmosphere 90 times the presure of earth's. Mercury has no atmosphere, so no 400 degree K greenhouse effect.

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    Quote Originally Posted by Disinfo Agent
    Why doesn't that happen to Mercury?
    It has no atmosphere and greenhouse effect. As it rotates very slowly, its nigth side radiates the heat and it gets very cold (as low as -173°C).

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    So do the CO2 clouds make Venus hotter, or colder? :-?
    Or is it that rotation needs to be taken into consideration, too?

    I'm confused by the Wikipedia article, too. On one hand, it says that Venus's thick clouds raise its surface temperature above the maximum temperature on Mercury, despite the fact that Venus is twice away from the Sun as Mercury. On the other hand, it says that the clouds keep the planet's surface "much cooler than it would be otherwise". What am I missing?

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    It is the CO2 atmosphere (not clouds) that make Venus hot. CO2 is colorless in visible light but traps infrared. Were it not for the clouds most of the solar heat would be trapped. Thermal equilibrium would not be reached until the surface became hot enough to radiate sufficient heat in a wavelength not reflected back by the CO2 atmosphere. This would be in the visible spectrum which is why I said it implies it would glow red. The H2SO4 clouds prevent this as they are nearly opaque.

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    Quote Originally Posted by Disinfo Agent
    So do the CO2 clouds make Venus hotter, or colder? :-?
    Or is it that rotation needs to be taken into consideration, too?
    As near as I can gather, it just makes its surface temperature more constant. The heat that is trapped is retained even during the planet's 2796 hour long night. It also regulates how much heat is trapped in the first place. Atmospheric pressure allows for higher levels of heat transfer by conducting heat from the upper cloud deck to the ground more effectively. The planet is basically a pressure cooker.

  16. #16
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    The CO2 makes Venus hotter. The clouds in the atmosphere make it cooler.

    This has been proposed as a solution for global warming here on Earth, just increase cloud cover a few percent, and those clouds would cool Earth off and keep temperatures stable :P

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    Quote Originally Posted by Disinfo Agent
    So do the CO2 clouds make Venus hotter, or colder? :-?
    Or is it that rotation needs to be taken into consideration, too?
    Sulfur acid clouds make the planet colder as they shield the infrared light (not much, I suspect). It's the CO2 that warms the planet by blocking the infrared light escaping from the surface. CO2 is a greenhouse gas, remember.

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    O.K., I got it now. Thanks. 8)

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    Quote Originally Posted by Kullat Nunu
    Quote Originally Posted by Disinfo Agent
    So do the CO2 clouds make Venus hotter, or colder? :-?
    Or is it that rotation needs to be taken into consideration, too?
    Sulfur acid clouds make the planet colder as they shield the infrared light (not much, I suspect). It's the CO2 that warms the planet by blocking the infrared light escaping from the surface. CO2 is a greenhouse gas, remember.
    Now that's an interesting tidbit. Especially in light of SO2 being considered one of the greenhouse gasses that environmental groups want to eliminate. Now, admittedly, SO2 + H2O in clouds equals some nasty nasty acid rain, but I've also heard it referenced as a greenhouse issue.

  20. #20
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    Quote Originally Posted by Doodler
    Quote Originally Posted by Kullat Nunu
    Sulfur acid clouds make the planet colder as they shield the infrared light (not much, I suspect). It's the CO2 that warms the planet by blocking the infrared light escaping from the surface. CO2 is a greenhouse gas, remember.
    Now that's an interesting tidbit. Especially in light of SO2 being considered one of the greenhouse gasses that environmental groups want to eliminate. Now, admittedly, SO2 + H2O in clouds equals some nasty nasty acid rain, but I've also heard it referenced as a greenhouse issue.
    Weird, I've heard exactly opposite: that SO2 actually cools the atmosphere. Somebody probably knows better?

  21. #21
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    Yup, I was correct: SO2 aerosols cool atmosphere. SO2 released by major volcanic eruptions cause global cooling.

    Here's source.

    Sulfur dioxide (SO2) is the main gas released by volcanoes that can affect climate in the short term. Chemical reactions that occur when SO2 reaches the atmosphere produce tiny sulfuric acid droplets called “aerosol.” Very energetic eruptions push the aerosol up into the stratosphere, the layer of the atmosphere from 10 to 50 km altitude (around 32,000 to 164,000 ft), where it inhibits the sun’s energy from reaching and warming the earth’s surface.

    Once in the stratospheric jet stream, the aerosol quickly encircles the globe. The microscopic droplets tend to remain aloft for months to years, promoting global cooling.

  22. #22
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    This has been proposed as a solution for global warming here on Earth, just increase cloud cover a few percent, and those clouds would cool Earth off and keep temperatures stable
    You DARE to post this on an Astronomy board!!!??????

    I am severely photon starved now thank you very much, without add more clouds!

    Mumble.... mumble.... mumble... stomp.. stomp.. stomp...[SLAM]! :wink:

  23. #23
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    Quote Originally Posted by Doodler
    Quote Originally Posted by Kullat Nunu
    Quote Originally Posted by Disinfo Agent
    So do the CO2 clouds make Venus hotter, or colder? :-?
    Or is it that rotation needs to be taken into consideration, too?
    Sulfur acid clouds make the planet colder as they shield the infrared light (not much, I suspect). It's the CO2 that warms the planet by blocking the infrared light escaping from the surface. CO2 is a greenhouse gas, remember.
    Now that's an interesting tidbit. Especially in light of SO2 being considered one of the greenhouse gasses that environmental groups want to eliminate. Now, admittedly, SO2 + H2O in clouds equals some nasty nasty acid rain, but I've also heard it referenced as a greenhouse issue.
    The big volcanic erruption in the Phillipines (I think in the 90's) created a measurable cooling of the atmosphere because of SO2 areosoles.

    But SO2 is not a greenhouse gas and I don't recall it being called that. Environmental groups want to ban it because of acid rain, as you mentioned
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  24. #24
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    Quote Originally Posted by Kullat Nunu
    Yup, I was correct: SO2 aerosols cool atmosphere. SO2 released by major volcanic eruptions cause global cooling.

    Here's source.

    Sulfur dioxide (SO2) is the main gas released by volcanoes that can affect climate in the short term. Chemical reactions that occur when SO2 reaches the atmosphere produce tiny sulfuric acid droplets called “aerosol.” Very energetic eruptions push the aerosol up into the stratosphere, the layer of the atmosphere from 10 to 50 km altitude (around 32,000 to 164,000 ft), where it inhibits the sun’s energy from reaching and warming the earth’s surface.

    Once in the stratospheric jet stream, the aerosol quickly encircles the globe. The microscopic droplets tend to remain aloft for months to years, promoting global cooling.
    Thank you much, it was probably just me crossing contexts. SO2 is a pollution gas, but not in the same manner as CO2.

  25. #25
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    Quote Originally Posted by Christopher Ferro
    Hmm.. I hadn't thought about it before, but how much sunlight gets to Venus' surface and what wavelenghts? It seems to reflect so much in the visible wavelenghts that I always figured it must be a gloomy place.

    CJSF
    What makes a greenhouse a greenhouse? The greenhouse!!

    Yes, Venus's atmosphere reflects upwards of 90% of visible light: but its virtually opaque to infrared - and infrared is the wavelength of radiated heat (at Venus's temperatures). That's how a greehouse works: it lets in light (even if only a little) and traps heat.

  26. #26
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    Quote Originally Posted by russ_watters
    Quote Originally Posted by Christopher Ferro
    Hmm.. I hadn't thought about it before, but how much sunlight gets to Venus' surface and what wavelenghts? It seems to reflect so much in the visible wavelenghts that I always figured it must be a gloomy place.

    CJSF
    What makes a greenhouse a greenhouse? The greenhouse!!

    Yes, Venus's atmosphere reflects upwards of 90% of visible light: but its virtually opaque to infrared - and infrared is the wavelength of radiated heat (at Venus's temperatures). That's how a greehouse works: it lets in light (even if only a little) and traps heat.
    I wasn't being incredulous at the "greenhouse" effect on Venus. I was just wondering how dimly lit the surface is and what wavelengths make it through the atmosphere (or how much absorption there is).

    CJSF
    "I like the stories
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    As much as anybody else
    But when I'm seeking knowledge
    Either simple or abstract
    The facts are with science"

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  27. #27
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    Just to make sure everyone here is aware:

    Bad Greenhouse, brought to you by the Bad Meteorologist.

  28. #28
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    According to wikipedia, the atmosphere of titan is 94% nitrogen anyway, and about 45 times thinner than the atmosphere of Venus (which is almost all CO2)

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