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baric
2010-May-15, 05:16 PM
First, let me start with an excerpt from the wiki entry on Titan:


The conventional greenhouse effect occurs because the atmosphere is largely transparent to solar radiation, but largely opaque to infrared. In an anti-greenhouse effect, this situation is reversed (i.e. the atmosphere is opaque to solar, but lets out infrared).


ok, let me make sure I understand this clearly before asking questions...

Solar radiation comes in at specific wavelengths which are determined by the blackbody radiation curve (see: http://quantumfreak.com/wp-content/uploads/2008/09/black-body-radiation-curves.png)

A planetary atmosphere may be "transparent" or "opaque" to this radiation, depending on its composition.

If it is "opaque", then a portion of the incoming radiation is absorbed by the atmosphere and/or reflected back into space. If it is "transparent", then a portion of the incoming radiation passes through, reaches the planetary surface and warms it.

The planet then re-radiates energy back out at specific wavelengths, based on its blackbody radiation curve. Since planets are typically very cool in relation to stars, this is primarily infrared radiation.

If the atmosphere is opaque to this IR radiation, it is trapped and creates a warming effect on the planet. This is the greenhouse effect as seen on Earth & Venus. If it is not, then it passes back out without a warming effect. This is the anti-greenhouse effect as seen on Titan.

Some questions:

1) Because of its thick clouds, Venus has a very high albedo. This means that most of the solar radiation from the Sun is being reflected away rather than entering the Venusian atmosphere, correct? If so, does this imply that the fraction of solar radiation reaching the surface is primarily responsible for all of the trapped heat (due to the heavy CO2 blanket), or is another source of heat also significant (i.e. geothermal?)

2) Because of its surface temperature, our Sun emits a lot of light in the UV spectrum. If it were instead a common red dwarf star, the majority of its light would instead be in the IR. If this were the case, would standard "greenhouse" gases such as CO2 and H2O prevent the solar radiation from reaching the planet, thus resulting in an inverted warming effect as on Titan, where the upper atmosphere is warmer than the surface?

Romanus
2010-May-16, 02:03 PM
If so, does this imply that the fraction of solar radiation reaching the surface is primarily responsible for all of the trapped heat (due to the heavy CO2 blanket), or is another source of heat also significant (i.e. geothermal?)

Yep--also significant is the great mass of the atmosphere itself, which gives it considerable thermal inertia, as well as (probably) heat being reradiated by the surface, though it's surely in some sort of equilibrium. I doubt geothermal heat contributes much, if anything.

If it were instead a common red dwarf star, the majority of its light would instead be in the IR. If this were the case, would standard "greenhouse" gases such as CO2 and H2O prevent the solar radiation from reaching the planet, thus resulting in an inverted warming effect as on Titan, where the upper atmosphere is warmer than the surface?

Though I admit I'm no expert, I doubt it would have much effect. True, a red dwarf Sun would emit most of its radiation in the infrared, but its Planck curve would be broad enough to cover plenty of "windows" in which the atmosphere is transparent, and thus IR could reach the surface. On top of that, the wavelengths where a red dwarf's radiation would peak (around 1-2 microns) are already mostly transparent to the Earth's atmosphere.

Jerry
2010-May-21, 01:17 AM
Even with a red dwarf star, a majority of the transmitted energy would be found in the higher energy, higher wavelength bands; but if a body was emitting only in the infrared region; yes, large concentrations of greenhouse gases would reject most energy. Incidently, dense clouds do block A LOT of energy. One of the reasons storms are cold; and one of the reasons an incorrect argument is often heard that greenhouse effects are self-correcting.

baric
2010-May-21, 03:16 AM
True, a red dwarf Sun would emit most of its radiation in the infrared, but its Planck curve would be broad enough to cover plenty of "windows" in which the atmosphere is transparent, and thus IR could reach the surface.



Even with a red dwarf star, a majority of the transmitted energy would be found in the higher energy, higher wavelength bands

I am working on simulating this and forgot this key point. I had become fixated upon the peak in the Planck curve, forgetting that a lot of energy even in red dwarfs would be UV and also that greenhouse gases only absorb just a small fraction of the IR spectrum.

Thanks again!

Jerry
2010-May-21, 06:04 PM
As is quite common, I inverted one of the terms: I meant Higher energy, Higher frequency (lower wavelength) emissions.

neilzero
2010-May-26, 05:15 AM
Hi Baric: I think your analysis is very close to mainstream and I have no reason to doubt the mainstream on this topic. The extreme cases of transparent and opaque probably occurs nowhere in our solar system, so some energy is absorbed by the atmosphere, both incoming and outgoing. So the atmosphere is warm = more than 10 degrees k, for even kuiper belt objects if any have a significant atmosphere. This warm atmosphere (hot for Venus) sends long wave IR to the surface which warms the surface at least slightly. Likely all planets have some GEO thermal heat, but with no exceptions that I know of does the GEO thermal heat warm the surface or the cloud tops more than 20 degrees c = 36 degrees f. Exceptions would be new born planets still contracting at a brisk pace, and possibly the moon Io.
Some one said Venus reflects 90% of the sun's energy that the planet intercepts.
I will guess that 15% of our sun's energy is in ultraviolet. For an average red dwarf = M class 1% is ultraviolet. For a borderline red and brown dwarf(except a recently born brown dwarf (which can be thousands of degrees k) only one part per million is UV or visable. Occasional solar flares being the exception, if brown dwarfs produce flares or CMEs Neil