Comparison of atmospheric temperatures obtained through infrared sounding and radio occultation by Mars Global Surveyor

Mars Global Surveyor monitors temperatures in the Martian atmosphere through infrared sounding by the Thermal Emission Spectrometer (TES) as well as Radio Science (RS) occultation experiments. We report results from extensive comparisons of TES nadir and RS temperature profiles, focusing on data obtained under conditions that allow nearly simultaneous measurements at the same location and local time. We analyzed data from 13 intervals with durations of 7-19 sols that sample latitudes of 62°-85°N during midspring through early autumn. Our results generally confirm the accuracy and reliability of both sets of observations. We find particularly good agreement between the zonal variations of temperature observed by the two instruments. Zonally averaged temperature profiles agree to within ~2 K at pressures <400 Pa. Results are less satisfactory at 610 Pa, where the TES retrievals are persistently warmer by a margin of 2-8 K. These discrepancies arise primarily from a wide disparity in vertical resolution. The RS measurements reveal subtle but significant vertical variations of temperature, particularly at low altitudes, that the TES is unable to resolve. For the range of latitudes and seasons considered here, the most serious consequence is that vertical gradients of temperature deduced from TES profiles are unreliable in the lowest scale height above the surface. Our analysis also shows that the RS profiles from selected intervals might contain a low-altitude bias due to the unmodeled effect of water vapor. These results should contribute to a more confident characterization of Martian climate.