Atomic oxygen in the Martian thermosphere

Modern models of thermospheric composition and temperature and of excitation and radiative transfer processes are used to simulate the O I 130‐nm emission from Mars measured by the Mariner 9 ultraviolet spectrometer. We use the Mars thermospheric general circulation model calculations (MTGCM) of Bougher et al. (1988a) and the Monte Carlo partial frequency redistribution multiple scattering code of Meier and Lee (1982). We find that the decline in atomic oxygen through the daylight hours predicted by the MTGCM cannot be reconciled with the excess afternoon brightness seen in the data. Oxygen concentrations inferred from the data show a positive gradient through the day, in agreement with the original analysis by Strickland et al. (1973), although the absolute amounts are somewhat less because we use a larger photoelectron impact excitation and a somewhat larger solar flux in the 130‐nm triplet. In addition, the data suggest that the oxygen abundance increases toward high southerly latitudes, in contrast with the MTGCM prediction of high values in the northern (winter) hemisphere. It appears that solar forcing alone cannot account for the observed characteristics of the Martian thermosphere and that wave and tidal effects may profoundly affect the structure, winds, and composition.