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Project Progress

Continuing work completed in previous years on Earth-like planets around F, G and K stars, this year we submitted a paper to Astrobiology that describes a coupled photochemical-climate model for Earth-like planets around M stars. For two active M-stars (Eps Eridani and AD Leo) we constructed absolute FUV- to far-IR spectral energy distributions using optical and near-infrared observations. These spectra, and spectral models of 3 quiescent M stars were input to the climate-chemistry model to estimated the thickness of the ozone layer and the abundance of various biomarker gases (CH₄, N₂O, and CH₃Cl). We calculated visible to mid-infrared spectra for the resulting planetary environments. All of the M-star planets modeled exhibited observable ozone layers, with ozone column depths on the two active M-stars within a factor of 2 of Earth’s (Fig. 1), whereas ozone column depths for planets around quiescent M-stars were up to 8 times smaller. Abundances of reduced biomarker gases were higher on the M-star planets than on Earth, given the same assumed surface fluxes, due to the low near-UV flux from the parent stars. CH₄, for example, had a predicted concentration of ~500 ppmv on the active M-star planets, compared to 1.7 ppmv for Earth (Fig. 2). Little radiation is emitted from Mstars in the 200-300 nm range and the production of O(¹D) and the corresponding abundance of OH radicals in the troposphere of an M-star planet is much lower than on Earth. The result is that both O₂ (or O₃) and reduced biomarker gases have a higher probability of being simultaneously observed on such planets, if present, (Fig. 3), providing a strong indicator for life.

High-CO₂ planetary atmospheres were also simulated using a 1-D photochemical model to address whether O₂ and O₃ abiotic abundances can be in false positives for life. Complete FUV-far-IR spectra were assembled for six sun-like stars and spectrum of the object with the highest stellar UV flux (EK Dra) was used. A paper with the results is being prepared.

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