The Martian atmosphere during the viking mission, I - Infrared measurements of atmospheric temperatures revisited

The Viking Infrared Thermal Mapper 15-mu m channel brightness temperature o bservations (IRTM T-15) provide extensive spatial and temporal coverage of martian atmospheric temperatures on diurnal to seasonal time scales. The 15 -mu m channel was designed so that these temperatures would be representati ve of a deep layer of atmosphere centered at 0.5 mb (similar to 25 km), Our re-examination of the IRTM data indicates that the 15-mu m channel was add itionally sensitive to surface radiance so that air temperature determinati ons (nominal T-15) are significantly biased when the thermal contrast betwe en the surface and atmosphere is large. This bias is suggested by the stron g correlation between the diurnal variation of tropical T-15 and surface te mperatures for non dust-storm conditions. We show that numerical modeling o f the thermal tides provides a basis for distinguishing between the surface and atmospheric contributions to IRTM T-15 and thus allows the atmospheric component to be estimated. The resulting bias amounts to a similar to 15-K offset for midday atmospheric temperatures at subsolar latitudes during re latively 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 dur ing the Viking mission. An accounting for the surface temperature bias reso lves much of the discrepancy between IRTM and corresponding microwave obser vations, indicating that there is relatively little interannual variability in global temperatures during the aphelion season (L-s similar to 40 degre es-100 degrees), We find further support for this argument in a comparison with T-15 temperatures synthesized from Mariner 9 Infrared Interferometer S pectrometer spectra, The significantly reduced diurnal temperature variatio ns in this season are consistent with the relatively clear atmosphere that is implied by the cooler temperatures. Cooler temperatures and reduced diur nal variation will likely be of significance for the modeling of water ice cloud dynamics in this season. (C) 2000 Academic Press.