Upper Atmosphere and Ionosphere of Mars

Abstract

It is argued that the single-layer ionosphere at 125 kilometers discovered in the Mariner IV occultation experiment is an Fl region coinciding with the ultraviolet photoionization peak. The CO₂ density there must be of the order of 10¹¹ molecules per cubic centimeter. Such a density is consistent with the properties of the lower atmosphere by Mariner IV anid the temperature model of Chamberlain and McElroy if the atmosphere is mainly CO₂ below 70 kilometers. The absence of an F2 region can be explained even if the density ratio of O to CO₂ is 100 at 230 kilometers on the basis of the rapid conversion of O⁺ to O₂ by CO₂. Thus a model with an exospheric temperature of 400°K, a modest degree of CO₂ dissociation, and diffusive separation above 70 kilometers is possible.