Thermal structure of Venusian nighttime mesosphere as observed by VIRTIS-Venus Express

摘要

The mapping IR channel of the Visual and Infrared Thermal Imaging Spectrometer (VIRTIS-M) on board the Venus Express spacecraft observes the CO_2 band at 4.3 μm at a spectral resolution adequate to retrieve the atmospheric temperature profiles in the 65-96 km altitude range. Observations acquired in the period June 2006 to July 2008 were used to derive average temperature fields as a function of latitude, subsolar longitude (i.e., local time, LT), and pressure. Coverage presented here is limited to the nighttime because of the adverse effects of daytime non-LTE emission on the retrieval procedure and to southernmost latitudes because of the orientation of the Venus-Express orbit. Maps of air temperature variability are also presented as the standard deviation of the population included in each averaging bin. At the 100 mbar level (about 65 km above the reference surface), temperatures tend to decrease from the evening to the morning side despite a local maximum observed around 20-21 LT. The cold collar is evident around 65S, with a minimum temperature at 3LT. Moving to higher altitudes, local time trends become less evident at 12.6 mbar (about 75 km) where the temperature monotonically increases from middle latitudes to the southern pole. Nonetheless, at this pressure level, two weaker local time temperature minima are observed at 23LT and 2LT equatorward of 60S. Local time trends in temperature reverse about 85 km, where the morning side is the warmer. The variability at the 100 mbar level is maximum around 80S and stronger toward the morning side. Moving to higher altitudes, the morning side always shows the stronger variability. Southward of 60S, standard deviation presents minimum values around 12.6 mbar for all the local times.