An intercomparison of ground-based millimeter, MGS TES, and Viking atmospheric temperature measurements: Seasonal and interannual variability of temperatures and dust loading in the global Mars atmosphere

During the period October 1997 to September 1999 we obtained and analyzed o ver 100 millimeter-wave observations of Mars atmospheric CO line absorption for atmospheric temperature profiles. These measurements extend through on e full Mars year (solar longitudes L-S of 190 degrees in 1997 to 180 degree s in 1999) and coincide with atmospheric temperature profile and dust colum n measurements from the Thermal Emission Spectrometer (TES) experiment on b oard the Mars Global Surveyor (MGS) spacecraft. A comparison of Mars atmosp heric temperatures retrieved by these distinct methods provides the first o pportunity to place the long-term (1982-1999) millimeter retrievals of Mars atmospheric temperatures within the context of contemporaneous, spatially mapped spacecraft observations. Profile comparisons of 0-30 km altitude atm ospheric temperatures retrieved with the two techiques agree typically to w ithin the 5 K calibration accuracy of the millimeter observations. At the 0 .5 mbar pressure level (similar to 25 km altitude) the 30 degrees N/30 degr ees S average for TES infrared temperatures and the disk-averaged millimete r temperatures are also well correlated in their seasonal and dust-storm-re lated variations over the 1997-1999 period. This period includes the Noachi s Terra regional dust storm, which led to very abrupt heating (similar to 1 5 K at 0.5 mbar) of the global Mars atmosphere at L-S = 224 degrees in 1997 [Christensen et al., 1998; Conrath et al., this issue; Smith et al., this issue]. Much colder (10-20 It) global atmospheric temperatures were observe d during the 1997 versus 1977 perihelion periods (L-S = 200 degrees-330 deg rees), consistent with the much (2 to 8 times) lower global dust loading of the atmosphere during the 1997 perihelion dust storm season versus the Vik ing period of the 1977a,b storms. The 1998-1999 Mars atmosphere revealed by both the millimeter and TES observations is also 10-15 K colder than prese nted by the Viking climatology during the aphelion season (L-S = 0 degrees- 180 degrees, northern spring/summer) of Mars. We reassess the observational basis of the Viking dusty-warm climatology for this season to conclude tha t the global aphelion atmosphere of Mars is colder, less dusty, and cloudie r than indicated by the established Viking climatology even for the Viking period. We also conclude that Mars atmospheric temperatures exhibit their m ost significant interannual variations during the perihelion dust storm sea son (10-20 K for L-S = 200 degrees-340 degrees) and during the post-aphelio n northern summer season (5-10 K for L-S = 100 degrees-200 degrees).