Mariner 6 and 7 Ultraviolet Spectrometer Experiment: Implications of CO₂⁺, CO and O Airglow

Abstract

The Mariner 6 and 7 ultraviolet spectrometer experiments observed intense emissions from CO, O, and CO2+ in the Martian airglow. Analysis shows that they are excited predominantly by the absorption of solar EUV photons by CO2 and constitute a major energy-loss mechanism for the thermosphere. Models of the thermospheric temperature profile and the airglow layer that demonstrate the effects of neutral chemistry and ionospheric composition are developed. With their aid, the observed CO Cameron-band emission scale height of 19 ± 4½ km is shown to suggest an exospheric temperature of 315 ± 75°K. Consideration of other data suggests a ‘best’ value of about 350°K. The emissions from CO2+ are consistent with a topside ionosphere containing about 30% CO2+. The abundance of O necessary to convert the rest to O2+ is about 2% at 135 km, in good accord with an independent determination. Within the uncertainties in the excitation efficiencies and in the thermospheric cooling mechanisms, the observations are consistent with the measured electron density. Eddy cooling may be important if the eddy diffusion coefficient is large, but it is difficult to reconcile the heating effects of CO3 with the small observed airglow scale heights. There is no indication in the data that the ionosphere is modified by the solar wind below 200 km.