Sensitivity of Biosignatures on Earth-like Planets orbiting in the Habitable Zone of Cool M-Dwarf Stars to varying Stellar UV Radiation and Surface Biomass Emissions

摘要

We find that variations in the UV emissions of cool M-dwarf stars have apotentially large impact upon atmospheric biosignatures in simulations ofEarth-like exoplanets i.e. planets with Earths development, and biomass and amolecular nitrogen-oxygen dominated atmosphere. Starting with an assumedblack-body stellar emission for an M7 class dwarf star, the stellar UVirradiation was increased stepwise and the resulting climate-photochemicalresponse of the planetary atmosphere was calculated. Results suggest aGoldilocks effect with respect to the spectral detection of ozone. At weak UVlevels, the ozone column was weak (due to weaker production from the Chapmanmechanism) hence its spectral detection was challenging. At strong UV levels,ozone formation is stronger but its associated stratospheric heating leads to aweakening in temperature gradients between the stratosphere and troposphere,which results in weakened spectral bands. Also, increased UV levels can lead toenhanced abundances of hydrogen oxides which oppose the ozone formation effect.At intermediate UV (i.e. with x10 the stellar UV radiative flux of black bodyPlanck curves corresponding to spectral class M7) the conditions are just rightfor spectral detection. Results suggest that the planetary O3 profile issensitive to the UV output of the star from about(200-350) nm. We alsoinvestigated the effect of increasing the top-of-atmosphere incomingLyman-alpha radiation but this had only a minimal effect on the biosignaturessince it was efficiently absorbed in the uppermost planetary atmospheric layer,mainly by abundant methane. Earlier studies have suggested that the planetarymethane is an important stratospheric heater which critically affects thevertical temperature gradient, hence the strength of spectral emission bands.