Changes in atmospheric chemistry on earthlike exoplanets across the habitable zone of main sequence stars

Abstract

We have used a coupled radiative-convective photochemical column model to calculate changes in atmospheric biomarkers on a planet having Earth's composition which we situated at the inner, mid, and outer Habitable Zone (HZ) for a solar, F2V and K2V star. The HZ was defined conservatively to be suitable for humans i.e. 0°C < T_surface > 30°C which led to the model calculating a narrow HZ width of (0.96-1.09) Astronomical Units (AU) for the solar-type star, (1.57-1.81) AU for the F2V star and (0.51-0.58) AU for the K2V star. Despite the narrowness of the HZ the biomarkers H2O, CH4 and CH3Cl varied by large amounts i.e. by factors 8, 11 and 10 respectively in the column on moving outwards across the solar HZ, for example. Whereas H2O decreased moving outwards across the HZ due to enhanced condensation in the troposphere, CH4 and CH3Cl increased associated with a slowing in H2O+O1D --< 2OH, hence less OH, an important sink for these two compounds. Ozone changes were smaller, around a 5-10% increase across the HZ. A source-sink analysis suggested the important process was a slowing in the O3+hv sink. We also considered changes in species which impact ozone – the so-called family species (and their reservoirs) which can catalytically destroy ozone. HCl, for example is a chlorine reservoir (storage) molecule, which increased by a factor 26 in the mid-stratosphere (32km) on moving outwards over the solar HZ. For the F2V and K2V stars, similar sources and sinks dominated the chemical biomarker budget as for the solar case and columns trends were comparable across the HZ. Ratios of biomarkers are easier to detect than are absolute concentrations. Our results imply that ratios such as (O3/H2O) and (CH4/H2O) can vary by large amounts and still be consistent with habitability, even for a conservatively-defined HZ.