title: 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 CO(2) density there must be of the order of 10(11) 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 CO(2) below 70 kilometers. The absence of an F2 region can be explained even if the density ratio of O to CO(2) is 100 at 230 kilometers on the basis of the rapid conversion of O(+) to O(2) by CO(2). Thus a model with an exospheric temperature of 400 degrees K, a modest degree of CO(2) dissociation, and diffusive separation above 70 kilometers is possible.
authors.forename: T M
authors.initials: TM
authors.lastname: Donahue