Photochemistry and evolution of Mars' atmosphere: A Viking perspective

Viking measurements of the Martian upper atmosphere indicate thermospheric temperatures below 200K, temperatures much colder than those implied by remote sensing experiments on Mariner 6, 7, and 9 and Mars 3. The variability in thermospheric temperature may reflect an important dynamical coupling of upper and lower regions of the Martian atmosphere. Absorption of extreme ultraviolet solar radiation can account for observed features of the ionosphere and provides an important source of fast N and O atoms which may escape the planet's gravitational field. Isotopic measurements of oxygen and nitrogen impose useful constraints on models for planetary evolution. It appears that the abundance of N= in Mars' past atmosphere may have exceeded the abundance of CO= in the present atmosphere and that the planet also has copious sources of H=O. The planet acquired its nitrogen atmosphere early in its history. The degassing rate for nitrogen in the present epoch must be less than the time-averaged degassing rate by at least a factor of 20. -1. INTRODUCTION The entry science experiments on Viking provide a wealth of new information on the structure and composition of Mars' upper atmosphere. They may be used, in combination with remote sensing data from earlier spacecraft, to develop a reasonably consistent model for Martian aeronomy. It is clear that escape processes have played a major role in the evolution of Mars' atmosphere. Recombination of Os + in the planetary exosphere provides a significant source for fast atoms which can escape the planet's gravitational field. Compositional data inferred from the retarding potential analyzer experiment Nier et al., 1976b; W. B. Hanson, private communication, 1977 indicate that O. + is the major constituent of the Martian ionosphere and suggest an average escape rate of about 6 X 10 ? oxygen atoms cm -' s -. The chemistry of the bulk atmosphere is regulated by oxygen escape on a time scale of the order of 105 years in such a manner as to ensure an escape rate for H atoms of the magnitude of 1.2 X 108 atoms cm -' s -. Hydrogen molecules are formed in the lower atmosphere by reaction of H with HO.. Escaping H atoms are released by ionospheric reactions involving H. and CO. +. Models for the Martian ionosphere are developed in section 2 and are shown to agree satisfactorily with in situ measurements by Viking. The upper atmosphere measured by Viking is unusually cold. The scale height of CO. is about 8 km, which may be compared with scale heights in the range 15-22 km as inferred from the ultraviolet spectrometer experiment on Mariner 9 Stewart et al., 1972. It is clear that the temperature of Mars' upper atmosphere is quite variable, ranging from as low as 120K to perhaps as high as 400K. This variability may be seen also in the topside plasma scale heights as measured by Mariner 4 Kliore et al., 1965, Mariner 6 and 7 Kliore et al., 1969; Fjeldbo et al., 1970, and Mariner 9 Kliore, 1974. The general characteristics of the ionosphere may be reproduced by a relatively simple photochemical model if proper account is taken of the variability of the extreme ultraviolet solar flux. It is unlikely, however, that such a simple model can account for the observed variation in atmospheric temperature.