The Martian ionospheric observations made by Mariner IV are interpreted in terms of an atmospheric model. The ion peak is identified as an F2 peak, that is, as a maximum whose profile is controlled by ambipolar diffusion. The principal features of the resulting atmospheric model are that the atmosphere consists mainly of carbon dioxide, the temperature is very low, and there is no thermosphere. Surface temperature is 210°K. The tropopause occurs at an altitude of 14 kilometers and has a temperature of 140°K. Above the tropopause, the temperature decreases with altitude at the rate of 0.64°C km -1 , following the solid carbon dioxide vapor-pressure curve up to 100 km, where the temperature is 85°K; at higher altitudes the temperature is isothermal.