UV radiation and energetic particles are harmful for life, leading to the destruction of biomolecules. But has this always been the case? What role did these high-energy processes play in the formation of the first organic molecules in the atmosphere of early Earth or other planets? We investigate the effect of stellar X-ray and UV (XUV) radiation, cosmic rays (CR), and stellar energetic particles (SEP, mainly protons) on the atmospheric chemistry of the hot Jupiter HD 189Ionospheric and Solar Terrestrialb and identify key signatures of these interactions. We choose this planet because it is one of the best studied and observed exoplanets today, allowing us to optimize the model before later applications to habitable worlds. We use 3D simulations of HD 189Ionospheric and Solar Terrestrialb’s atmosphere for the pressure-temperature profiles and XUV spectra of the host star from the MOVES collaboration. To model the chemical reactions, we use the STAND2020 network, which includes ion-neutral C/H/N/O chemistry. We study in detail the formation of the amino acid glycine and its precursors. Our results suggest that the all types of high-energy radiation enhance the formation of glycine. We identify ammonium (NH4+) as an important signature of CR and SEP influx, even though the degree of ionization of the atmosphere remains low. XUV radiation only ionizes the very top of the atmosphere. Ultimately, we show that high energy processes increase glycine and precursor production and thus may potentially play an important role in prebiotic chemistry.
In the 1950s, Stanley Miller and Harold Urey conducted their famous experiments where they fired electrical sparks in a gas mixture resembling what they thought was early Earth’s atmosphere connected to heated water to resemble the warm pond. They let the experiment run for a week and produced more than 20 amino acids. However, the atmospheric composition they chose is most likely not correct for early Earth. We conduct similar experiments but with a different atmospheric composition of varying fractions of oxygen, nitrogen, and CO2. In these experiments we focus on the effect of lightning on the production of simple organic molecules from an abiotic atmosphere, by breaking up the strong bond of the N2 molecules.
The problem is, that our experiments are confined to a small flask while Earth’s atmosphere and oceans are many orders of magnitude larger. So, in addition, we want to simulate the effect of lightning and other energy sources on the atmospheric chemistry of Earth and exoplanets with a computer model.
Therefore, we apply a chemical rate network (STAND2020) to the atmosphere of the hot Jupiter HD 189Ionospheric and Solar Terrestrialb. The network consists of more than 5000 reactions between more than 500 neutral and charged molecules and is complete up to 6 H, 2 C, 2 N and 3 O atoms. It is embedded into a code that applies the reactions of the network to the chosen planetary atmosphere. In addition, it includes the effect of external radiation sources onto the chemistry. In this study we include the effect of stellar X-ray and UV (XUV) radiation (based on observations of the host star) and stellar protons (we compare an observed X-ray flare to similar events from the sun where we know the associated proton flux) as well as galactic cosmic rays (based on measurements of the Voyage probe). The temperature profile of the atmosphere was extracted from the results of a 3D atmospheric model that also includes cloud formation. We chose two profiles at the equator (noon and midnight) as exemplary for the day- and nightside.
We find that the energetic particles (both stellar protons and cosmic rays) reach much deeper layers of the atmosphere than the less energetic XUV radiation which gets absorbed in the upper-most layers of the atmosphere. Even though the galactic cosmic rays are much more energetic than the XUV photons, their overall flux is much smaller and therefore the ionization rate is too. We also find that the stellar protons are able to ionize the upper atmosphere in a region where the cloud abundance is high, too. Both effects are necessary for lightning: the electrons produced by the protons can attach to the cloud particles with larger particles accumulating more charges. These particles separate under the influence of gravity, producing an electric field in the atmosphere. To include lightning into the model will be subject of our future work.
The network also includes the production pathways of the amino acid Glycine which was one of the products of the original Miller-Urey experiment. We find that all energy sources we studied strongly enhance the abundance of Glycine in the atmosphere. However, the abundance remains very small. Precursors of Glycine such as Ethylene and Formaldehyde which are similarly enhanced by the high-energy sources, are much more abundant, though, and potentially observable signatures of prebiotic chemistry.