The chemistry of protonated species in the martian ionosphere

Abstract

The Mars Atmosphere and Volatile Evolution Mission (MAVEN) spacecraft entered orbit around Mars on 22 September 2014. The Neutral Gas Ion Mass Spectrometer (NGIMS) instrument on this spacecraft will measure the ion densities with unprecedented sensitivity of ∼1cm-3 above the nominal periapsis altitude ∼150km. The upper region of the martian ionosphere consists of a suite of ions that have been modeled many times, and several protonated species, for which there are fewer models. We model here the density profiles for 14 major ions that have been modeled previously and 10 additional protonated species, which include the most important 4 species, HCO+, OCOH+, OH+, and N2H+, with smaller densities of several minor species, including H2+,H3+,HO2+, HNO+, ArH+, and CH+. To the ∼220 reactions that were already in our ionospheric models, we add ∼75 reactions that are of relevance to protonated species, including ion–molecule reactions and dissociative recombination reactions. These reactions also largely complete the chemistry of thermospheric H and H2, which we also show. We first discuss the chemistry of protonated species in general, and then we justify our choices for the rate coefficients of the reactions that we add here. We then describe the major production and loss mechanisms for the four major protonated species. Finally we present density profiles for all 10 protonated species that we consider here for low and high solar activity models, and show that most of their peak densities are anticorrelated with solar activity. This will confer an advantage to the MAVEN mission, which will enter the ionosphere during a period of declining solar activity. We compare our model to two previous models, and show that there are significant differences between them. The results presented here will provide a guide for the mass settings on the NGIMS instrument.