The Martian atmosphere during the Viking mission. I. Infrared measurements of atmospheric temperatures revisited.
The Viking Infrared Thermal Mapper 15-μm channel brightness temperature observations (IRTM T15) provide extensive spatial and temporal coverage of martian atmospheric temperatures on diurnal to seasonal time scales. The 15-μm channel was designed so that these temperatures would be representative of a deep layer of atmosphere centered at 0.5 mb (∼25 km). Our re-examination of the IRTM data indicates that the 15-μm channel was additionally sensitive to surface radiance so that air temperature determinations (nominal T15) are significantly biased when the thermal contrast between the surface and atmosphere is large. This bias is suggested by the strong correlation between the diurnal variation of tropical T15 and surface temperatures for non-dust-storm conditions. We show that numerical modeling of the thermal tides provides a basis for distinguishing between the surface and atmospheric contributions to IRTM T15 and thus allows the atmospheric component to be estimated. The resulting bias amounts to a ∼15-K offset for midday atmospheric temperatures at subsolar latitudes during relatively clear periods and is negligible at night. The proposed temperature correction results in close agreement between the stimulated and observed patterns of diurnal variation for conditions ranging from clear to dusty. A major consequence of this work is the improved definition of the diurnal, latitudinal, and seasonal variation of martian atmosphere temperatures during the Viking mission. An accounting for the surface temperature bias resolves much of the discrepancy between IRTM and corresponding microwave observations, indicating that there is relatively little interannual variability in global temperatures during the aphelion season ( Ls ∼ 40°-100°). We find further support for this argument in a comparison with T15 temperatures synthesized from Mariner 9 Infrared Interferometer Spectrometer spectra. The significantly reduced diurnal temperature variations in this season are consistent with the relatively clear atmosphere that is implied by the cooler temperatures. Cooler temperatures and reduced diurnal variation will likely be of significance for the modeling of water ice cloud dynamics in this season.