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

Abstract

[1] 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 mm 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 20-21LT. 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.

Venus Atmospheric Thermal Structure and Radiative Balance

“…Comparison of different maps acquired in close temporal sequence (about 60 min) suggests that short time variability is maximum (about 5 K) around the 1 mb level, in the region 65°S-75°S and on the dawn side of the night hemisphere. Grassi et al (2010) extended this analysis, considering a wider set of data and presenting average temperature fields in the local time/latitude/pressure space (Fig. 3) and their variability.…”

“…Panels a-d refer to the levels of 100, 31.6, 12.6 and 4.0 mb. From Grassi et al (2010). See also Fig.…”

“…The night side data detected only the thermal emission from the atmosphere, whereas the day side measurements contained contributions from the scattered sunlight and solar-induced fluorescence. Different groups have analyzed the day and night side observations separately - Grassi et al ( , 2010Grassi et al ( , 2014; Migliorini et al (2012); Haus et al (2013Haus et al ( , 2014 and by Garate-Lopez et al (2015) using only the night-time data acquired at moderate emission angles (> 30°, solar zenith angles < 95°). The variability of total opacity inside the CO 2 band allowed retrieval of air temperatures from the cloud top level (about 100 mb, or ∼ 65 km at intermediate latitudes) up to 1 mb (∼ 85 km), with an effective resolution in the order of 5-7 km .…”

“…Grassi et al (2014) expanded the analysis of average fields by considering all suitable measurements acquired by VIRTIS-M during its operative lifetime. With respect to Grassi et al (2010), the re-analysis also benefited from a new retrieval scheme that allows one to consistently incorporate the treatment of variable cloud deck altitude as well as the retrieval of carbon monoxide content. Results of Grassi et al (2010) are essentially confirmed: more stringent acceptance criteria resulted in a wider cold collar region (polar dipole demonstrated it is particularly difficult to model with the new retrieval algorithm), while improved confidence allowed the authors to discuss an average field at the 1.4 bar level, that turned out to be roughly symmetric around midnight.…”

Dayside temperatures in the Venus upper atmosphere from Venus Express/VIRTIS nadir measurements at 4.3μm

“…New measurements made by various instruments on the VEx spacecraft have provided important advances concerning the vertical characterization of the Venus atmospheric thermal structure, but this applies mostly below about 100 km. For example, radio occultation probed from 40 to 90 km (Tellmann et al 2009(Tellmann et al , 2012, while sensing the night-time emission at selected IR wavelengths, allows us to sense between 65 and 96 km (Grassi et al 2010;Migliorini et al 2012;GarateLopez et al 2015). Temperatures in the altitude range between 90 and 150 km have been inferred, but only on the nightside with stellar occultation (Piccialli et al 2015), and in the morning and evening terminators with solar occultation techniques (Mahieux et al 2015b).…”