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Many of you may have heard that supercritical CO2 (as in at just the right temperature and pressure to be in between gas and liquid) is a possible alternative solvent for life, like water is for us. But this brings up so many questions. Let's focus on this one for now: What wavelengths of light would go through such an atmosphere? This is important because it affects the color of the autotrophs. Let's assume the star is like our sun just for simplicity.

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    $\begingroup$ (1) "What wavelengths": all of them, for a short distance... None for more than a few hundred meters. (2) On Earth we have red, brown, green, blue-green and blue autotrophs. (3) The critical point of CO2 is at 31.04° C and 72.8 atmospheres; at that pressure both oxygen and nitrogen are also supercritical. (3) At the temperature and pressure of the critical point of CO2 water is an ordinary liquid. There are many species of bathypelagic fish which live at such depths, and they "keep their internal fluids" just fine. $\endgroup$ – AlexP Nov 8 '18 at 5:04
  • $\begingroup$ Hi Joe, Please limit your question to just a single question. Your average life form on Earth probably explode if it had to keep its internals fluids at that pressure without environmental help. As a comparison, 73 atm is roughly the same pressure you get 740 meters under the sea surface. $\endgroup$ – Shadowzee Nov 8 '18 at 5:11
  • $\begingroup$ Alright, alright. I just figured I would try spreading the net wide. If you need me to limit things, lets focus on the part about how light would pass through it. That has a strong scientific basis. Will that do? $\endgroup$ – Joe Smith Nov 8 '18 at 5:35
  • $\begingroup$ That would do. Please edit your question and its title to reflect that limitation. Thanks. $\endgroup$ – JBH Nov 8 '18 at 5:44
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    $\begingroup$ Comparatively does not mean totally. More like "thunderstormdark", or "dusky", or "chasm-dark". $\endgroup$ – Erik Nov 9 '18 at 8:20
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Such planet would be similar to Venus. And question is more about reflection of light and rayleigh scattering. Your thick atmosphere will reflect a lot of light and scatter what is left, as rayleigh scattering is dependent on wavelength of light: shorter wavelengths blue/green won't make it to the surface, most light will be orange and red wavelenghts. Sunset would be good reference. Photos of Venus: enter image description here enter image description here

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Take a look at this picture - I haven't found something similar in English, but "Kohlenstoffdioxid" is CO2, and at the bottom there is the wavelength, which is blocked by the different gases. Also take a look at "atmospheric windows".

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  • $\begingroup$ I can't make heads or tails out of this $\endgroup$ – Joe Smith Nov 8 '18 at 21:29
  • $\begingroup$ Every gas asorbs specific wavelengths. Other wavelenghts are passing through, though there may be some scattering. E.g. our atmosphere allows mostly wavelengths in our visible spectrum to pass through (plus some others). A different atmosphere would block some wavelengths, e.g. there could be no "blue" as we know it. $\endgroup$ – Erik Nov 9 '18 at 8:31
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    $\begingroup$ @JoeSmith English equivalent commons.wikimedia.org/wiki/File:Atmospheric_Transmission.png with interactive description $\endgroup$ – KJO Nov 9 '18 at 12:55

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