Microwave spectra of Mars atmospheric CO were obtained in May (LS=197¿) and November (LS=305¿) of 1988 and January (LS=341¿) of 1989. The November observations, which include both 12CO and 13CO spectra, are analyzed to obtain the CO mixing ratio and temperature profile of the atmosphere of Mars for the 0--70 km altitude region. The resulting volume mixing ratio for CO (6¿1.5¿10-4) compares well with the 1967 infrared reflectance determination (8¿2¿10-4) by Kaplan et al. (1969). Based upon these observations and other ground-based observtions of O2, we suggest that large variations (>50%) in the global abundances of CO and O2 are not characteristic of the present Mars atmosphere. Assuming a constant CO mixing ratio, we derive atmospheric temperature profiles from the May 1988 and January 1989 12CO spectra which are quite similar to the temperature profile found for the November 1988 period. All of the derived temperature profiles in 1988 and 1989 agree within 10 K with the Viking descent measurements (Seiff and Kirk, 1977), even though the LS values for these observations are distinctly different from those for the Viking measurements (LS=100¿--120¿).

The same analysis for 12CO spectra observed in November 1975 (Kakar et al., 1977, LS=340¿), March-April 1980 (Good and Schloerb, 1981, LS=86¿), and January 1982 (Clancy et al., 1983, LS=77¿) yields Mars atmospheric temperature profiles which are distinctly different from the Viking northern summer profiles. The March-April 1980 and January 1982 profiles, in particular, are 20--40 K cooler than the Viking profile for altitudes above 10 km. We conclude that all of the periods corresponding to the microwave observations are characterized by dust-free, radiative-convective equilibrium conditions for the Mars atmosphere. We conjecture that, during such cold northern spring-summer seasons; condensation of H2O2 above 20--30 km altitude may affect the altitude distribution of O3; the base altitude of H2O clouds may decrease to as low as 5--10 km; and CO2 condensation at low latitudes may occur locally in the 30--40 km altitude region. Finally, we propose 25 K solar longitudinal (LS) variations in low-to-mid-latitude temperatures for both dusty and clear periods of the Mars atmosphere. ¿ American Geophysical Union 1990