Doubly charged positive ions (dications) is 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 and only atomic dications have been detected so far. The Neutral Gas and Ion Mass Spectrometer (NGIMS) measurements made on board the recent Mars Atmosphere and Volatile Evolution provide the first opportunity for a decisive detection of molecular dications, CO2 ++ in this case, in a planetary upper atmosphere. The NGIMS data reveal a dayside 2 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 the double photoionization of CO2 below 190 km and the single photoionization of CO2 + at higher altitudes, whereas CO2 ++ destruction is dominated by the natural dissociation but the 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 the CO2 + density. 1. Introduction Despite their low abundances, the study of doubly charged
O++ is an interesting species in the ionospheres of both the Earth and Venus. Recent measurements made by the Neutral Gas and Ion Mass Spectrometer (NGIMS) on board the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft provide the first firm detection of O++ in the Martian ionosphere. This study is devoted to an evaluation of the dominant O++ production and destruction channels in the dayside Martian ionosphere, by virtue of NGIMS data accumulated over a large number of MAVEN orbits. Our analysis reveals the dominant production channels to be double photoionization of O at low altitudes and photoionization of O+ at high altitudes, respectively, in response to the varying degree of O ionization. O++ destruction is shown to occur mainly via charge exchange with CO2 at low altitudes and with O at high altitudes. In the dayside median sense, an exact balance between O++ production and destruction is suggested by the data below 200 km. The apparent discrepancy from local photochemical equilibrium at higher altitudes is interpreted as a signature of strong O++ escape on Mars, characterized by an escape rate of 6×1022 s–1.