Open AccessObservation of CO 2 ++ dication in the dayside Martian upper atmosphere
Author(s) -
Gu Hao,
Cui Jun,
Niu DanDan,
Dai LongKang,
Huang JianPing,
Wu XiaoShu,
Hao YongQiang,
Wei Yong
Publication year - 2020
Publication title -
earth and planetary physics
Language(s) - English
Resource type - Journals
ISSN - 2096-3955
DOI - 10.26464/epp2020036
Subject(s) - photoionization , physics , atmosphere (unit) , ion , atomic physics , dication , aeronomy , dissociation (chemistry) , analytical chemistry (journal) , ionization , chemistry , meteorology , quantum mechanics , chromatography
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, CO 2 ++ in this case, in a planetary upper atmosphere. The NGIMS data reveal a dayside averaged CO 2 ++ distribution declining steadily from 5.6 cm −3 at 160 km to below 1 cm −3 above 200 km. The dominant CO 2 ++ production mechanisms are double photoionization of CO 2 below 190 km and single photoionization of CO 2 + at higher altitudes; CO 2 ++ destruction is dominated by natural dissociation, but reactions with atmospheric CO 2 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 CO 2 ++ density than of the CO 2 + density .