The Influence of Cosmic Rays on Exoplanetary Atmospheric Biosignatures
Final Report Abstract
Within this project, we investigated the impact of cosmic rays from the galactic cosmic ray background and from stellar activity onto the habitability of exoplanets and the detectability of exoplanetary life via biosignatures. In particular, we addressed the following three research questions: Q1 - What is the radiation dose due to cosmic rays (CRs) at the surface of extrasolar planets? Q2 - How do CRs and resulting photochemistry affect the abundance of potential atmospheric biosignatures such as O3 , N2 O, CH4 or CH3 Cl for a range of exoplanetary atmospheres and systems? Q3 - How do CRs affect the spectral detectability of biosignatures? To address these questions, in a first step, a combined model suite was developed to follow cosmic ray propagation and its interaction with its environment from outside the planetary atmosphere down to the planets’ surface. It contains components to estimate the stellar flux of a wide class of stars, to follow galactic cosmic ray and stellar cosmic ray fluxes through the stellar and planetary magnetospheres to the top of the planetary atmosphere, to calculate the atmospheric ionization rate for a given atmospheric composition, and to calculate the atmospheric composition and climate with and without cosmic ray forcing. In a last step, the dose rate at the planet’s surface as well as the planetary transmission spectrum are calculated. While we had models available to cover each component of this model suite at the beginning of the project, these were originally set up for modern Earth, and had to be adapted considerably, leading to the development of new model codes : AtRIS to calculate the interaction of radiation with atmospheres of a wide range of exoplanets, ExoTIC to calculate the ion chemistry and impact of ionization on the neutral atmosphere for planets with a wide range of orbital structures and base atmospheres, and 1D-TERRA to calculate the neutral composition and climate of a wide range of exoplanetary atmospheres. Within the project, the new model code was applied to two scenarios: Earth as a test case, and Proxima Centauri b as an example of an Earth-like exoplanet at the outer edge of its stars habitable zone. We find that strong stellar events can have a significant impact on biosignatures on Earth-like atmospheres even for a comparatively quiet star like the sun, both in a ’negative’ sense (e.g. by weakening spectral signals of O3 ) and in a ’positive’ sense (e.g. by affecting the abundance of radiative species hence possibly favoring habitability. For Earth-like planets around the more active M-stars, the stronger stellar activity will have a large impact on the shape and signatures of the spectra, which could potentially lead to false positives or negatives. Regarding Q1, the surface ion pair production was found to be around five orders of magnitude larger than on modern Earth for flaring conditions on Proxima Centauri b for the atmospheres assumed. Regarding Q2, the CRs led to enhanced nitrogen and hydrogen oxides in Earth-like atmospheres which led to a strong (up to a factor of 10) reduction in the O3 column via removal by catalytic cycles, whereas the increased HOx led to reduction in CH4 by more than an order of magnitude due to removal with OH. High abundances of CH4 acted as an anti-greenhouse gas so its removal by CRs led to a surface warming of up to ≈10 K. Regarding Q3, CRs led to a strong weakening in e.g. the O3 fundamental band but also led to strengthening in spectral bands due to e.g. NO2 and HNO3 in Earth-like atmospheres. Tools to calculate detectability (the number of transits required for a 5-sigma band detection with e.g. JWST and ELT) were developed in a parallel-running project and these will be applied in future.
Publications
- Effects of Key GCR and SEP shower parameters in Earth-like atmospheres, DPG pPring Meeting, 13 - 17 March (2017), Bremen, Germany
Scheucher, M., J.L. Grenfell, and H. Rauer
- Effects of Key GCR and SEP shower parameters in Earth-like atmospheres, DPG pPring Meeting, 13 - 17 March (2017), Bremen, Germany
Scheucher, M., J.L. Grenfell, and H. Rauer
- The Atmospheric Radiation Interaction Simulator, Second Advanced School on Exoplanetary Science, Vietri Sul Mare, May 21-25, 2017
Banjac, S., K. Herbst, and B. Heber
- The Influence of Stellar Variability on the Atmospheres of Exoplanets Red Dwarf Stars, Second Advanced School on Exoplanetary Science, Vietri Sul Mare, May 21-25, 2017
Schmidt, V. and Sinnhuber, M.
- New Insights into Cosmic-Ray-induced Biosignature Chemistry in Earth-like Atmospheres (2018), ApJ, 863, 1
Scheucher, M., J.L. Grenfell, F. Wunderlich, M. Godolt, F. Schreier, and H. Rauer
(See online at https://doi.org/10.3847/1538-4357/aacf03) - The Influence of Stellar Variability on the Atmospheres of Exoplanets Around Flaring M-Stars, European Week of Astronomy and Space Science, Liverpool, April 3-4, 2018
Schmidt, V., Sinnhuber, M., Scheucher, M., Grenfell, L. J., Rauer, H., Banjac, S., Herbst, K., and Heber, B.
- A new model suite to determine the influence of cosmic rays on (exo)planetary atmospheric biosignatures. Validation based on modern Earth (2019), Astron. Astrophys., 631, A101
Herbst, K., J. L.Grenfell, M. Sinnhuber, H. Rauer, B. Heber, S. Banjac, M. Scheucher, V. Schmidt, S. Gebauer, R. Lehmann,and F. Schreier
(See online at https://doi.org/10.1051/0004-6361/201935888) - Absorbed dose in space due to GCR up to Z=28 - The impact of phantom size, shape, composition and solar modulation (2019), J. Space Weather Space Climate, 9, A14
Banjac S., L. Berger, S. Burmeister, J. Guo, B. Heber, K. Herbst, and R. Wimmer-Schweingruber
(See online at https://doi.org/10.1051/swsc/2019014) - From Solar to Stellar Characteristics. A new Peak Size Distribution for G-, K- and M-Dwarf Star Flares (2019), Astron. Astrophys., 621, A67
Herbst, K., A. Papaioanou, S. Banjac, and B. Heber
(See online at https://doi.org/10.1051/0004-6361/201832789) - The Atmospheric Radiation Interaction Simulator (AtRIS) - Description and Validation (2019), J. Geophys. Res. (Space Phys.), 124, 50-67
Banjac, S., K. Herbst, and B. Heber
(See online at https://doi.org/10.1029/2018JA026042) - Consistently Simulating a Wide Range of Atmospheric Scenarios for K2-18b with a Flexible Radiative Transfer Module (2020), Astrophys. J., 898, 1
Scheucher, M., F. Wunderlich, J.L. Grenfell, M. Godolt, F. Schreier, D. Kappel, R. Haus, K. Herbst, and H. Rauer
(See online at https://doi.org/10.3847/1538-4357/ab9084) - Detectability of biosignatures on LHS 1140 b, Astron. Astrophys., (2020)
Wunderlich, F., M. Scheucher, M. Godolt, J. L. Grenfell, F. Schreier, P. C. Schneider, D.J. Wilson, A. Sánchez-López, M. López-Puertas, and H. Rauer
(See online at https://doi.org/10.1051/0004-6361/202039663) - Distinguishing between Wet and Dry Atmospheres of TRAPPIST-1 e and f (2020), Astrophys. J., 901, 2
Wunderlich, F., M. Scheucher, M. Godolt, J. L. Grenfell, F. Schreier, P. C. Schneider, D.J. Wilson, A. Sánchez-López, M. López-Puertas, and H. Rauer
(See online at https://doi.org/10.3847/1538-4357/aba59c) - Distinguishing between wet and dry atmospheres of TRAPPIST-1 e, EXO III, 27-31 July 2020, Heidelberg, Germany
Wunderlich, F., M. Scheucher, M. Godolt, J.L. Grenfell, F. Schreier, P.C. Schneider, D.J. Wilson, and A. Sanchez
- On-the-fly calculation of absorbed and equivalent atmospheric radiation dose in a water phantom with the Atmospheric Radiation Interaction Simulator (AtRIS) (2020), J. Geophys. Res. (Space Phys.), 124, 9774-9790
Banjac, S., B. Heber, K. Herbst, L. Berger, and S. Burmeister
(See online at https://doi.org/10.1029/2019JA026622) - Proxima Centauri b: A Strong Case for Including Cosmic-Ray-induced Chemistry in Atmospheric Biosignature Studies (2020), Astrophys. J., 893 (1)
Scheucher, M., K. Herbst, V. Schmidt, J. L. Grenfell, F. Schreier, S. Banjac, B. Heber, H. Rauer, and M. Sinnhuber
(See online at https://doi.org/10.3847/1538-4357/ab7b74)
