Carbon monoxide and temperature in the upper atmosphere of Venus from VIRTIS/Venus Express non-LTE limb measurements

The upper mesosphere and the lower thermosphere of Venus (from 90 to 150 km altitude) seems to play a transition region in photochemistry, dynamics and radiation, but is still very poorly constrained observationally. Since 2006 VIRTIS on board Venus Express has been obtaining limb observations of CO fluorescent infrared emissions in a systematic manner. This study represents the scientific exploitation of this dataset and reports new information on the composition and temperature at those altitudes. This work is focused on the 4.7 μm emission of CO as observed by VIRTIS, which contains two emission bands, the fundamental and the first hot of the main CO isotope. A specific scheme for a simultaneous retrieval of CO and temperature is proposed, based on results of a comprehensive non-LTE model of these molecular emissions. A forward model containing such non-LTE model is used at the core of an inversion scheme that consists of two steps: (i) a minimization procedure of model-data differences and (ii) a linear inversion around the solution of the first step. A thorough error analysis is presented, which shows that the retrievals of CO and temperature are very noisy but can be improved by suitable averaging of data. These averages need to be consistent with the non-LTE nature of the emissions. Unfortunately, the data binning process reduced the geographical coverage of the results. The obtained retrieval results indicate a global distribution of the CO in the Venus dayside with a maximum around the sub-solar point, and a decrease of a factor 2 towards high latitudes. Also a gradient from noon to the morning and evening sides is evident in the equator, this being smaller at high latitudes. No morning-afternoon differences in the CO concentration are observed, or are comparable to our retrieval errors. All this argues for a CO distribution controlled by dynamics in the lower thermosphere, with a dominant sub-solar to anti-solar gradient. Similar variations are found with the Venus Thermospheric General Circulation Model (VTGCM), but the VIRTIS CO is systematically larger than in the model. The thermal structure obtained by VIRTIS presents a hint of local maximum around 115 km near the terminator at equatorial latitudes, but not at noon, in clear contrast to VTGCM predictions and to an upper mesosphere in pure radiative balance. A few tentative ideas to explain these model-data discrepancies are discussed.