The escape of CO2+ and other heavy minor ions from Mars

Next to its main constituent O₂⁺, the Martian ionosphere consists of several other ion species, like CO₂⁺, O⁺, CO⁺, HCO⁺, N2⁺, etc. The ionospheric escape is dominated by O₂⁺ and O⁺ ions, and as a result the escape of these species is well studied. The other, minor ion species are more difficult to measure in the escaping plasma, because their contribution is typically obscured in the mass spectra of ion instruments by the more abundant O₂⁺ peak.

In this study we use data from the SupraThermal And Thermal Ion Composition instrument (STATIC) on board MAVEN to investigate the escape of these ions. We use a peak-fitting method to separate the contribution of several ion species, including O₂⁺, CO₂⁺, O⁺ and ions with a mass between 28-30 AMU. Our method is validated against Neutral Gas and Ion Mass Spectrometer (NGIMS), also onboard MAVEN, and results in the ionosphere agree qualitatively very well.

We apply this method to STATIC data from January 2016 until May 2019 to perform a statistical study examining the escape of low energy (<100 eV) heavy (>=16 AMU) ions throughout the Martian magnetosphere and its surrounding. We find that CO₂⁺ ions do escape through the tail but at a very limited rate, namely at less than 1% of the O₂⁺ escape rate. Ions with a mass between 28-30 AMU, however, are found to constitute a significant part of the ionospheric outflow, with an escape rate 30% of the O₂⁺ rate and 15% of the total heavy ion escape.