# Emission Spectra

## Main Question or Discussion Point

Hi,

I have found several links using google to emission spectra of the Earth:

http://www.xylenepower.com/
http://spaceguard.esa.int/NScience/n...y/emission.htm

All of which indicate a temperature of 280K and peak clearly at around 18microns. However, Wien's displacement law indicates that the peak from thermal emission of the Earth should be at around 10 microns.

Many of these websites are indeed specifically about Wien's law and yet these two values do not correlate at all. What is going on?

Natski

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Astronuc
Staff Emeritus
Hi,

All of which indicate a temperature of 280K and peak clearly at around 18microns. However, Wien's displacement law indicates that the peak from thermal emission of the Earth should be at around 10 microns.

Many of these websites are indeed specifically about Wien's law and yet these two values do not correlate at all. What is going on?
How did one determine it should be 10 microns?

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Chronos
Gold Member
Zealous crackpots seeking validation on the net, natski. Is a common affliction.

Kurdt
Staff Emeritus
Gold Member
From Wien's displacement law 280K would give a peak at about 10 microns. I don't know how they came up with that graph but the theoretical black body line should not be 280K if it peaks at 18 microns.

Astronuc
Staff Emeritus
I used the calculator here -

http://hyperphysics.phy-astr.gsu.edu/hbase/wien.html#c3

And 280K corresponds to a wavelength of 10.35 microns - peak, but

a peak wavelength of 18 microns correspond to 161 K.

What's up with that?

Does this imply that the blackbody approximation is not valid?

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Kurdt
Staff Emeritus
Gold Member
http://wxpaos09.colorado.edu/radiation/background.html" [Broken] page and a couple of others I found show the peak in the correct place. All I can assume is they've taken the emission spectrum from something else and drawn the theoretical curve on and assumed it was the Earth's temperature of ~280K.

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Astronuc
Staff Emeritus
Looking at the absorbtivity plot on the page cited by Kurdt in the previous post, there is a conspicuous peak at around 18 microns for CO2 and a band where absorption by water for wavelengths above 18 microns.

The absorption in the atmosphere seems dominated by H2O with some contribution from CO2. Does this imply the emission is also dominated by H2O?

Kurdt
Staff Emeritus
Gold Member
I'm by no means an expert Astro, but from what I can gather it is this NASA image that they have adapted.

http://origins.jpl.nasa.gov/habitable-planets/invest12.html [Broken]

I don't think its a true Earth spectrum but an ideal spectrum of an Earth like planet which the ESA will be looking for with projects like DARWIN and NASA with their origins project.

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I think the emission spectra that peaks at 18 microns is correct, despite the fact it disagrees with Wien's law. The reason for this is that my supervisor, who specializes in atmospherics, showed me the bible of atmospherics and in it the Earth spectra was also peaking at 15-18 microns, labeled as 300K. (I will post the name of the book soon, just looking for it now)

I thought at first this could be due a doppler shift or something but these observations should not have a moving observer. The book in question makes very little reference to Wien's law however.

Another possible explanation is that this emission temperature is the temperature of the atmosphere and not the surface. Perhaps a surface temp of 300K implies an atmospheric temp of 200K which implies an emission peak at 18 microns?

Natski

Astronuc
Staff Emeritus