The Response of Atmospheric Chemistry on Earthlike Planets around F, G, and K stars to Small Variations in Orbital Distance

Kurzfassung

One of the prime goals of future investigations of extrasolar planets is to search for life as we know it. The Earth's biosphere is adapted to current conditions. How would the atmospheric chemistry of the Earth respond if we moved it to different orbital distances or changed its host star? This question is central to astrobiology and aids our understanding of how the atmospheres of terrestrial planets develop. To help address this question, we have performed a sensitivity study using a coupled radiative-convective photochemical column model to calculate changes in atmospheric chemistry on a planet having Earth's atmospheric composition, which we subjected to small changes in orbital position, of the order of 5-10% for a solar-type G2V, F2V, and K2V star. We then applied a chemical source-sink analysis to the biomarkers in order to understand how chemical processes affect biomarker concentrations. We start with the composition of the present Earth, since this is the only example we know for which a spectrum of biomarker molecules has been measured. We then investigate the response of the biomarkers to changes in the input stellar flux. Computing the thermal profile for atmospheres rich in H₂O, CO₂ and CH₄ is a major challenge for current radiative schemes, due, among other things, to lacking spectroscopic data. Therefore, as a first step, we employ a more moderate approach, by investigating small shifts in planet-star distance and assuming an earthlike biosphere. To calculate this shift we assumed a criteria for complex life based on the Earth, i.e. the earthlike planetary surface temperature varied between 0 deg C‹T_surface ‹30 deg C, which led to a narrow HZ width of (0.94-1.08) astronomical units (AU) for the solar-type G2V star (1.55-1.78) AU for the F2V star, and (0.50-0.58) AU for the K2V star. In our runs we maintained the concentration of atmospheric CO₂ at its present-day level. In reality, the CO₂ cycle (not presently included in our model) would likely lead to atmospheric CO₂ stabilising at higher levels than considered in our runs near our quoted "outer" boundaries. The biomarkers H₂O, CH₄ and CH₃Cl varied by factors 0.08, 17, and 16, respectively in the total column densities on moving outwards for the solar case. Whereas H₂O decreased moving outwards due to cooling hence 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 10% increase on moving outwards partly because cooler temperatures led to a slowing in the reaction between O₃ and O¹D. We also considered changes in species which impact ozone – the so-called family species (and their reservoirs), which can catalytically destroy ozone. Hydrochloric acid (HCl), for example, is a chlorine reservoir (storage) molecule, which increased by a factor 64 in the mid-stratosphere (32 km) on moving outwards for the solar case. For the F2V and K2V stars, similar sources and sinks dominated the chemical biomarker budget as for the solar case and column trends were comparable.