Martian Atmosphere: The Mariner Occultation Experiment

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Abstract
Observations of the Martian ionosphere with the Mariner IV spacecraft have indicated that the upper atmosphere of that planet differs widely from models we proposed earlier. Those models were based on the assumption that there is complete photo-dissociation of CO2 in the thermosphere, but they were otherwise the result of a straight-forward application of the heat-budget equation. A radically different semiempirical model has recently been proposed which is based on the assumptions that the ionosphere peak is, as on Earth, of the F2 type and that O+ is the dominant ion. There is no true thermosphere in this model, for the temperature of the entire upper atmosphere would have to remain below 100[degree] K to fit the observations. We have taken issue with this proposal on the grounds that its temperature profile could not be maintained. We have computed a model atmosphere, with 44% CO2 and 56% N2, that will fit the observations, provided that CO2 is not strongly dissociated. The major ionization is in an E region produced by solar x-rays; the dominant ion is probably NO+ or O2+. A physical discussion of the radiative losses by CO2 shows that a normal thermosphere develops and that the exosphere temperature (at the top of the thermosphere) is at least 400[degree] K. Absence of strong ionization peaks corresponding to the terrestrial F1 and F2 regions is plausible. If the mean molecular weight is considerably larger than that assumed in this model, the observed ionospheric peak might be an Fl region produced by solar ultraviolet radiation. It is still plausible in this case that no F2 peak will develop, and the thermal structure is not likely to differ importantly from that shown in Fig 2 in the article.