A new model suite to determine the influence of cosmic rays on (exo)planetary atmospheric biosignatures

Abstract

Context. The first opportunity to detect indications for life outside the Solar System may be provided already within the next decade with upcoming missions such as the James Webb Space Telescope (JWST), the European Extremely Large Telescope (E-ELT) and/or the Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL) mission, searching for atmospheric biosignatures on planets in the habitable zone of cool K-and M-stars. Nevertheless, their harsh stellar radiation and particle environment could lead to photochemical loss of atmospheric biosignatures. Aims. We aim to study the influence of cosmic rays on exoplanetary atmospheric biosignatures and the radiation environment considering feedbacks between energetic particle precipitation, climate, atmospheric ionization, neutral and ion chemistry, and secondary particle generation. Methods. We describe newly-combined state-of-the-art modeling tools to study the impact of the radiation and particle environment, in particular of cosmic rays, on atmospheric particle interaction, the influence on the atmospheric chemistry and the climate-chemistry coupling in a self-consistent model suite. To this end, models like the Atmospheric Radiation Interaction Simulator (AtRIS), the Exoplanetary Terrestrial Ion Chemistry model (ExoTIC), and the updated coupled climate-chemistry model are combined. Results. Besides comparing our results to Earth-bound measurements, we investigate the ozone-production and -loss cycles as well as the atmospheric radiation dose profiles during quiescent solar periods and during the strong solar energetic particle event of February 23, 1956. Further, the scenario-dependent terrestrial transit spectra, as seen by the NIR-Spec infrared spectrometer onboard the JWST, are modeled. Amongst others, we find that the comparatively weak solar event (i) drastically increases the spectral signal of HNO 3 , while (ii) significantly suppressing the spectral feature of ozone. Because of the slow recovery after such events, the latter indicates that ozone might not be a good biomarker for planets orbiting stars with high flaring rates.

Proxima Centauri b: A Strong Case for Including Cosmic-Ray-induced Chemistry in Atmospheric Biosignature Studies

“…Tropospheric values are set to constants based on the lowermost stratospheric values. Rates for H and N( 4 S) are similar to Earth-like values of 1 and 0.6 respectively (compare, e.g., Herbst et al 2019b;Sinnhuber et al 2012), while values, e.g., for NO and OH are different. The NO formation rate is smaller than on Earth (<0.5 compared to ∼0.58), with values for the 20% CO 2 atmosphere being smaller than for the 5% CO 2 case, indicating that this difference is due to the change in bulk atmosphere.…”

“…We performed a habitability study of Prox Cen b assuming an Earth-like atmosphere, focusing on the influence of SEPs from stellar flares upon spectral transmission features. We applied our extensive model suite discussed in Herbst et al (2019b), which includes: the calculation of SEP fluxes and spectral energy distributions; their precipitation through a planetary magnetosphere and atmosphere; cosmic-ray-induced atmospheric ionization; ion redistribution within the atmosphere; and atmospheric climate and neutral composition. Our results suggest a few words of caution for the analysis of spectra from future observations of such planets around M-dwarf stars.…”

“…We apply the comprehensive model suite described in Herbst et al (2019b) to study the habitability of Prox Cen b as influenced by the strong stellar activity of its host star.…”

“…We perform a habitability study of Proxima Centauri b assuming an Earth-like atmosphere under high stellar particle bombardment, with a focus on spectral transmission features. We employ our extensive model suite calculating energy spectra of stellar particles, their journey through the planetary magnetosphere, ionosphere, and atmosphere, ultimately providing planetary climate and spectral characteristics, as outlined in Herbst et al (2019b). Our results suggest that together with the incident stellar energy flux, high particle influxes can lead to efficient heating of the planet well into temperate climates, by limiting CH 4 amounts, which would otherwise run into anti-greenhouse for such planets around M-stars.…”

Energetic electron precipitation and their atmospheric effect

“…Atmospheric ionization rates are transferred to ions per cm -3 s -1 on a geopotential altitude grid and scaled with a constant factor 10 11 leading to maximal ionization rates of slightly above 10 4 cm -3 s -1 for the exponential power spectrum with characteristic energy of 3 MeV. Model calculations with the 1D stacked box model of the neutral and charged atmosphere ExoTIC [20] are carried out starting from a base neutral atmospheric composition provided by the HAMMONIA chemistry climate model for local noon on December 13, 2009 at 53°N/0°E, with a constant ionization rate forcing for one hour from 4-5 pm. Results at the end of the ionization period are shown in Fig.…”