COMPARATIVE TEMPERATURE RETRIEVALS BASED ON VIRTIS/VEX AND PMV/VENERA-15 RADIATION MEASUREMENTS OVER THE NORTHERN HEMISPHERE OF VENUS R. Haus (1), G. Arnold (1,2), and D. Kappel (1) (1) University Münster, Institute for Planetology, Münster, Germany; (2) DLR, Institute of Planetary Research, Berlin, Germany / Phone: ) MOTIVATION AND INTRODUCTION The observed high variability of Venus’ nightside radiances in the 4-30 μm spectral range is due to the combined influence of spatial and temporal temperature and cloud profile changes. Time- averaged retrieved temperature profiles in the middle atmosphere at similar locations should agree within a few K between different data sources. Deviations may indicate that cloud composition and altitude distribution models are not optimal. Microphysical optical parameters of H 2 SO 4 aerosols strongly differ at 4.3 versus 15 μm. A change of aerosol composition would modify the spectral features of optical parameters. This may result in different retrieved temperature profiles and cloud opacities and could eventually lead to different surface emissivity results. A first step of work has focused on comparative temperature retrievals using both VIRTIS-M-IR/VEX and PMV/VENERA-15 nightside spectral radiance data in the vicinity of strong CO 2 absorption bands located at 4.3 μm (VIRTIS) and 15 μm (PMV). Well-known prominent temperature structures like ‘cold collar’ and ‘hot dipole’ have been re-examined. SCOPEMETHOD SIMULATIONS NIGHTSIDE TEMPERATURE RETRIEVAL SUMMARY AND CONCLUSIONS CLOUD FEATURE RETRIEVAL Radiative transfer and retrieval calculations using both thermal and near-infrared radiation measurements DATA SELECTION 2 Investigation of Venus’ cloud features to improve surface emissivity retrievals VIRA Kliore, A.J., Moroz, V.I., Keating, G.M. (Eds.), 1985, The Venus International Reference Atmosphere. Adv. Space Res. 5(11), VeRa Tellman, S., et al., (2009), Structure of the Venus neutral atmosphere as observed by the Radio Science experiment VeRa on Venus Express, J. Geophys. Res. 114, E00B36, Zasova, L.V., et al., (1999), Structure of the Venus middle atmosphere: Venera 15 Fourier spectrometer data revisited, Adv. Space Res., 23(9), Piccioni, G., et al., (2007), The Visible and Infrared Thermal Imaging Spectrometer, ESA SP,1295. Selection of 32 PMV orbits (1150 spectra), northern hemisphere Selection of 18 VIRTIS orbits (1370 spectra), northern hemisphere  VIRTIS/VEX and PMV/VENERA-15 measurements were used to retrieve nightside temperature profiles in the atmosphere of Venus in the northern hemisphere at altitudes 65 (55) - 90 km.  Both temperature sets do not differ by more than 7K (<80 km).  The profiles at different latitudes are consistent with VIRA and VeRa data. Temperature differences to VIRA never exceed 10 K and are typically below 7 K.  Temperatures between 55 and 75 km are sensitive to the location of the cloud top. CT altitude in terms of unity cloud optical depth at 1 μm was determined from spectrum fits in the near wings of the corresponding CO 2 bands. Lower CT altitudes imply the use of higher total cloud column factors to keep the total cloud opacity unchanged. (The cloud bottom altitude at 48 km was not varied).  The cloud top for latitudes below 55° is located nearly constant at km, but drops down to 67 km in polar regions. Individual spectra show CT altitudes as low as 65 km.  The distant wings of the corresponding CO 2 bands were used to derive spectral changes of cloud optical depth that are required to produce optimum fits of measured brightness spectra.  These preliminary results have to be interpreted with much care. - The retrieved “optimum“ depths vary with latitude and there is no systematic trend anywhere in the spectrum. The variations may indicate spatial and temporal changes of cloud composition. - Some of the suggested spectral changes may be due to errors in the CO 2 spectral line shape profiles (sub-Lorentz, line mixing) and resulting temperature retrieval errors.  More work is underway that eventually could lead to improvements of Venus‘ surface emissivity retrieval. For more detailed information on the thermal strucutre of Venus‘ nighttime mesosphere see for example Zasova, L.V., et al. (2007), Structure of the Venus atmosphere, Planet. Space Sci., 55, Grassi, D., et al. (2010), Thermal structure of Venusian mesosphere as observed by VIRTIS-Venus Express, J. Geophys. Res., 115, E09007, 11 pp. without radiance correction VIRTIS Atmospheric temperature profiles are determined from comparisons between measured and iteratively recalculated top of atmosphere radiances and corresponding brightness temperatures (Smith, 1970) (2) Final run corrections addi- tionally include retrieval of - cloud top altitude - spectrally constant cloud column factor - broad band variation of cloud depth spectral signatures - auxiliary downshift of VIRTIS band center radiance (1) Basic run only includes temperature retrieval N