Non-Thermal Processes of Nitric Oxide Formation during Precipitation of Auroral Electrons into the Upper Atmospheres of Terrestrial Planets

Abstract

Nitric oxide is a potential biomarker in the N₂–O₂ atmospheres of terrestrial exoplanets, which can be detected by space missions, including the planned launch of the Russian Spektr-UV observatory. From observations of the Earth’s thermosphere in the polar regions, it is known that important sources of formation of this molecule are the precipitation of high-energy electrons into the planet’s atmosphere, as well as the non-thermal processes accompanying them. In this paper, we study the non-thermal processes of nitric oxide formation in the polar regions of the Earth’s upper atmosphere, as well as the atmospheres of exoplanets located in the potential habitable zone of active stars. For this purpose the following models are developed: a numerical kinetic Monte Carlo model of the interaction of energetic electrons with atmospheric gas, a kinetic Monte Carlo model of the interaction of suprathermal N(⁴S) atoms formed as a result of dissociation of N₂ molecules by electron impact with the surrounding gas; a model of odd nitrogen chemistry with molecular and eddy diffusion. Based on results of calculations, it is confirmed that the process of dissociation of N₂ by an electron impact during the interaction of the stellar wind with the atmosphere of the planet is an important source of suprathermal N atoms, which contribute to a significant increase in the non-thermal formation of NO in the N₂–O₂ atmospheres of terrestrial planets (both locally, in the case of a planet’s own magnetic field, and throughout the whole planet’s surface, in case of its absence). Since the column concentration of NO during flares becomes larger, therefore, the chances of detecting of nitric oxide biomarker in the atmospheres of the terrestrial-type exoplanets located in the potential habitable zone of active stars are also become larger.