Compositional Variation of the Dayside Martian Ionosphere: Inference from Photochemical Equilibrium Computations

Abstract

The Martian ionosphere plays a crucial role in driving the interactions of the planet with solar photons and solar wind particles. The variations of the dayside Martian ionosphere with several controlling factors, including the solar extreme ultraviolet radiation, the background atmosphere, and the underlying thermal structure, have been the topic of extensive research in terms of electron distribution. In contrast, how the ionospheric composition varies has not been systematically investigated, a topic that is attempted in this study based on photochemical equilibrium computations performed at 100–200 km altitude, including a large number of reactions. Our calculations reveal the following compositional variations as natural outcomes of the ionospheric chemistry on Mars. (1) With increasing solar irradiance, the proportions of the majority of nonterminal ions are enhanced at the expense of reduced proportions of terminal ions, including O 2 + , HCO + , NO + , and H 3 O + . (2) At high electron temperatures, the proportion of NO + is modestly reduced, whereas the proportions of the other species are nearly unaffected. (3) The response of the ionospheric composition to the upper atmospheric composition is complicated, showing the strong negative response of many trace ions to ambient CO 2 , O, and CO, as well as the strong positive response of protonated ions to H 2 , nitrogen-bearing ions to N and N 2 , water-group ions to H 2 O, and HO 2 + to O 2 . As an application of the model results, the recent ion measurements made on board the Mars Atmosphere and Volatile Evolution are used to provide hints about the realistic composition of the Martian upper atmosphere.