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The kinetic temperature of the Martian seasonal caps is controlled by the partial pressure of atmospheric CO2at the surface. When carbon dioxide condenses, typically near the poles, light non‐condensable species (Ar, N2, CO, etc.) accumulate in the atmosphere, resulting in a decrease of the CO2partial pressure and depressing the local frost point temperature. The buoyant air should mix laterally and vertically within the polar vortices. Observations show that the Martian seasonal caps' kinetic temperatures are ∼0–4 K below the expected CO2frost point, depending on latitude and season, indicating atmospheric CO2gas depletion at the surface/atmosphere interface. In the North and South, we find relatively similar non‐condensable peak enhancement factors (e.g., EFNC∼ 6–8, up to ∼8.7 in the North) at most latitudes, confirming the efficient meridional mixing within the polar vortices, despite steep surface condensation gradients. In the South, this surface enhancement is similar to column‐integrated values derived from Gamma Ray Spectrometer data, indicating efficient vertical mixing. But in the North, the surface depletion is much larger than in the entire column, suggesting poor vertical mixing. Reduced infrared emission of the seasonal caps stemming from CO2depletion is not a major energy balance factor. This work illustrates how the atmosphere's composition at the surface can be significantly different from column‐integrated values. The condensation and sublimation of the Martian seasonal CO2caps remove and then reinject massive amounts of carbon dioxide in the atmosphere at different seasons, thus changing the relative composition of the air in the polar regions, as non‐condensable species remain in gaseous form. Consequently, the CO2partial pressure and therefore the frost point temperature vary over the course of a year in a measurable manner. Using Mars Climate Sounder surface observations, we leverage this connection between ice temperature and air composition to constrain the properties of the surface/atmosphere interface in the polar regions where few observations are generally available. We find that in the South, the air composition at the surface is similar to the average composition of the atmospheric column, whereas in the North, more CO2‐depleted gas remains close to the ground without efficiently spreading upward. These findings have multiple implications: on the energy budget of the caps as the ice is generally slightly colder than usually modeled, on the modeling of the polar vortices and their dynamics, and on the uniqueness of near‐surface atmospheric conditions and composition. Mars Climate Sounder observations of the seasonal caps are used to derive the kinetic temperature of the ice and atmospheric CO2pressureThe temperatures of the seasonal caps are consistent with non‐condensable gases enrichment by factors 6–8The composition of the northern near‐surface atmosphere differs significantly from the column‐integrated average, in contrast with the South Mars Climate Sounder observations of the seasonal caps are used to derive the kinetic temperature of the ice and atmospheric CO2pressure The temperatures of the seasonal caps are consistent with non‐condensable gases enrichment by factors 6–8 The composition of the northern near‐surface atmosphere differs significantly from the column‐integrated average, in contrast with the South

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.