Observation of CO₂⁺⁺ dication in the dayside Martian upper atmosphere

Doubly charged positive ions (dications) are an important component of planetary ionospheres because of the large energy required for their formation. Observations of these ions are exceptionally difficult due to their low abundances; until now, only atomic dications have been detected. The Neutral Gas and Ion Mass Spectrometer (NGIMS) measurements made on board the recent Mars Atmosphere and Volatile Evolution mission provide the first opportunity for decisive detection of molecular dications, CO₂⁺⁺ in this case, in a planetary upper atmosphere. The NGIMS data reveal a dayside averaged CO₂⁺⁺ distribution declining steadily from 5.6 cm⁻³ at 160 km to below 1 cm⁻³ above 200 km. The dominant CO₂⁺⁺ production mechanisms are double photoionization of CO₂ below 190 km and single photoionization of CO₂⁺ at higher altitudes; CO₂⁺⁺ destruction is dominated by natural dissociation, but reactions with atmospheric CO₂ and O become important below 160 km. Simplified photochemical model calculations are carried out and reasonably reproduce the data at low altitudes within a factor of 2 but underestimate the data at high altitudes by a factor of 4. Finally, we report a much stronger solar control of the CO₂⁺⁺ density than of the CO₂⁺ density .