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Astronomy

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    $\begingroup$ While it has little to do with the question, I'm curious why Methane is such a strong greenhouse gas with a relatively small absorption spectrum in the chart above. Nice details by the way. $\endgroup$
    – userLTK
    Commented Oct 9, 2019 at 14:10
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    $\begingroup$ @userLTK There is not enough methane in the atmosphere to saturate those absorption bands (yet). The optical depth $\tau_{\lambda}$ at a given wavelength $\lambda$ and height $z$ is $\tau_{\lambda}=\int_{\infty}^{z} dz' n(z') \kappa(z')$, with $n(z')$ being the local number density and $\kappa(z')$ the opacity per mole, which is again a function of the temperature. Only at $\tau_{\lambda}>1$ is the photon mean-free path smaller than the atmospheric scale (which means the photon can't escape), and we see that both the absorption strength (given by the opacity) and the amount of gas contributes. $\endgroup$
    – AtmosphericPrisonEscape
    Commented Oct 9, 2019 at 14:42
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    $\begingroup$ @MichaelS: Right, this band at $15\mu m$ is opaque, so the outgoing radiation gets trapped there. But this is only a small fraction of the total black body energy, so it doesn't contribute to a strong greenhouse effect. $\endgroup$
    – AtmosphericPrisonEscape
    Commented Oct 10, 2019 at 10:18
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    $\begingroup$ It might improve this answer is you explicitly mention that the bottom graphs are Earth, the top is Mars, and the Mars absorption graph is "upside down" compared to Earth graphs (Earth graphs "absorption and scattering", while Mars measures "transmission"). Great answer! $\endgroup$
    – Yakk - Adam Nevraumont
    Commented Oct 10, 2019 at 17:22
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    $\begingroup$ @Yakk-AdamNevraumont: Thanks, but that's literally my first two sentences. $\endgroup$
    – AtmosphericPrisonEscape
    Commented Oct 10, 2019 at 18:06