Model calculation of production rates, ion and electron densities in the evening troposphere of Mars at latitudes 67°N and 62°S: seasonal variability

Abstract

We have developed a model to calculate the production rates and densities of positive and negative ions in the troposphere of Mars at evening. Using this model, the densities of 35 ions (Ar⁺, O₂⁺, CO₂⁺, O₂⁺CO₂, NO⁺, H₃O⁺, H₃O⁺H₂O, H₃O⁺(H₂O)₂, H₃O⁺(H₂O)₃, H₃O⁺(H2O)₄, H₃O⁺HO, CO₂ ⁺CO₂, CO⁺, C⁺, N₂ ⁺, NO⁺CO₂, N⁺, O₂ ⁺(CO₂)₂, O₂ ⁺H₂O, O₂ ⁺(H₂O)₂, O₄ ⁺, O⁺, CO₃ ^-H₂O, CO₃ ^-(H₂O)₂, CO₃ ^-, CO₄ ^-, NO₂ ^-, NO₂ ^-H₂O, NO₂ ^-(H₂O)₂, NO₃ ^-, NO₃ ^-H₂O, NO₃ ^-(H₂O)₂, O₂ ^-, O₃ ^-, O^-) are estimated in summer and winter at latitudes 67°N and 62°S for solar zenith angles 80° and 85°, respectively. The impact ionization source is taken as galactic cosmic rays. The model atmospheres at these locations are constructed from air density measured by radio occultation experiment aboard Mars Global Surveyor. The maximum electron densities in southern winter and northern summer are obtained at altitudes ~25 km and ~30 km, respectively, due to high efficiency of electron attachment to O_x molecules. Of the 35 ions considered in the model, the chemistry of 9 major ions (H₃O+(H₂O)_n for n = 1, 2, 3, 4 and CO₄ ^-, CO₃ ^-, NO₂ ^-H₂O and CO₃ ^-(H₂O) _n for n = 1, 2) are discussed in this paper. The ion densities are changed by factors of 5 to 10 between these two seasons.