Consider a planet with life forms, which are different than those found here on Earth, orbiting a star similar to the Sun. Such life forms don't use oxygen and, therefore, the planet's atmosphere is devoid of it. Since ozone is formed by diatomic oxygen photolysis, there is also not a ozone layer, but life forms need to protect themselves from their star's UVB/UVC radiation, so how would they do it? I mean, It's well known that the Earth's ozone layer prevents UVC radiation and part of the UVB (which are the most dangerous for living beings) from reaching the surface. So is there any other gas, other than ozone, that works as a filter for protection against UVB/UVC radiation?

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    $\begingroup$ Consider also that it would have to be replaced. Just evolve some sunscreen... $\endgroup$ Dec 17 '19 at 22:38
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    $\begingroup$ Consider that all gases absorb UV light to some degree. Just have the planet possess enough atmosphere. After all, Earth's own southern parts have a much thinner (or occasionnaly even absent) ozone layer, and yet Australia is a great food exporter and its people live long fullfilling lives. $\endgroup$
    – AlexP
    Dec 18 '19 at 2:45
  • $\begingroup$ Chlorine is a great UV absorber, and it could, theoretically, be used as an oxidizer, like O2, by alien life. $\endgroup$
    – Aezyc
    Jan 3 '20 at 2:48

I don't believe there is any single gas that absorbs efficiently across that entire spectrum.

$SO_x$ and $NO_x$ both absorb and remit at lower energy levels across some of the wavelengths you want.

Ethene also absorbs in part of the UV spectrum and is formed by the breakdown of hydrocarbon chains. This could give you a cycle of breakdown -> absorption -> formation from hydrocarbons to ethene back to hydrocarbons driven by UV.

If you extend your notion of 'gases' to include particulates then you open yourself up for a wider selection of materials that act as UV shields for a planet. But, your atmosphere becomes very dusty -- so to speak.


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