Global maps of Venus nightside mean infrared thermal emissions obtained by VIRTIS on Venus Express

Abstract

One of the striking features about Venus atmosphere is its temporal variability and dynamics, with a chaotic polar vortex, large-scale atmospheric waves, sheared features, and variable winds that depend on local time and possibly orographic features. The aim of this research is to combine data accumulated over several years and obtain a global VENUS LOWER CLOUDS IN THE INFRARED ATMOSPHERIC WINDOWSObservations at different wavelengths, including ultra-violet (UV), visible (VIS) and nearinfrared (NIR) spectral ranges, and comparison of observed features are very important to investigate cloud properties. As largely demonstrated in the past and more recently shown with the Akatsuki data [Limaye 2018a], observations in the NIR bands are able to probe clouds at lower altitudes with respect to the observations in the UV because these are less sensitive to scattering by small particles. However, the NIR CO2 transparency windows can contribute to probe deeper in the atmosphere, and hence when same structures are found in the UV, VIS and NIR these might indicate that the features extend from the cloud top to a depth of one scale height and larger cloud particles are detected.The wavelengths analysed in the present paper, centred around 1.74 and 2.25μm, belong to radiance coming mainly from below the cloud layers. Radiative transfer models indicate that the radiance at these bands originates about 10-20 and 20-30 km above the surface, respectively, see [Allen and Crawford 1984, Crisp 1991, Carlson 1991 and also the recent review by [Titov 2018]. This thermal radiation from the lower layers goes up through the cloud layer and is attenuated mostly by the lower clouds with differing optical depths [Titov 2018, Sánchez-Lavega 2017]. Therefore, the features observed at these wavelengths show mainly the spatial variations in the opacity of the lower to middle clouds, around 44-48 km altitude. [Peralta 2017a] These clouds have been studied since decades by Earth telescope observations, see [Tavenner 2008] and references therein and by several space missions, starting with the fly-by of Venus from the Galileo NIMS instrument [Carlson 1991], later by VIRTIS on Venus Express [Sánchez-Lavega 2008, Hueso 2012] and more recently by Akatsuki [Nakamura 2016] with the IR2 camera as described by [Satoh 2016, 2017] and [Peralta 2018], which also includes a good overview of Venus cloud observations at 2.3μm since 1978. These clouds are known to be composed mainly of sulphuric acid covering the whole planet [Marcq 2018, Titov 2018], dynamically elongated by the strong zonal winds coming from the super-rotation and travelling towards the poles with the meridional winds caused by the Hadley cell [Sánchez-Lavega 2018]. The general structure and dynamics of the clouds have been analysed and compared extensively with General Circulation Models [Lebonnois 2010, Garate-Lopez 2018, Kashimura 2019, Ando 2019].Space Agency, the Italian Space Agency (ASI), French Space Agency (CNES) and other national agencies that supported the Venus Express mission an...

Venusian Cloud Distribution Simulated by a General Circulation Model

“…As a result, the cloud is relatively thin in middle latitude. The latitudinal distribution of the cloud thickness obtained in the present study is qualitatively consistent with the infrared measurements (Cardesín Moinelo et al, 2008Moinelo et al, , 2020Carlson et al, 1991;Crisp et al, 1991;Haus et al, 2014;Limaye et al, 2018).…”

Quasi‐Periodic Variation of the Lower Equatorial Cloud Induced by Atmospheric Waves on Venus

“…This cloud model includes condensable gases of H 2 O and H 2 SO 4 , and condensation, evaporation, and sedimentation of sulfuric acid cloud particles. Their model reproduced the latitudinal distributions of the cloud and H 2 O and H 2 SO 4 vapors consistent with the infrared and radio occultation measurements (e.g., Cardesín‐Moinelo et al., 2020; Cottini et al., 2015; Crisp et al., 1991; Kolodner & Steffes, 1998). They also found that the latitudinal cloud distribution is mainly influenced by the mean meridional circulation in low‐ and mid‐latitudes and the transient waves in high latitudes.…”

“…The spatial and temporal variation in the Venusian cloud distribution has been studied by observing the near infrared thermal radiation from Venus which illuminates the silhouettes of the cloud deck. It has been shown that the cloud is optically the thickest in high‐latitudes and the thinnest in mid‐latitudes by infrared measurements (e.g., Cardesín‐Moinelo et al., 2020; Carlson et al., 1991; Crisp et al., 1991). Crisp et al.…”

“…The spatial and temporal variation in the Venusian cloud distribution has been studied by observing the near infrared thermal radiation from Venus which illuminates the silhouettes of the cloud deck. It has been shown that the cloud is optically the thickest in high-latitudes and the thinnest in mid-latitudes by infrared measurements (e.g., Cardesín-Moinelo et al, 2020;Carlson et al, 1991;Crisp et al, 1991). Crisp et al (1991) also showed that the infrared brightness varied quasi-periodically in low-latitudes and the bright and dark regions dominated by a zonal wavenumber-1 structure rotating with Venus with a period of 5.5 ± 0.15 Earth days.…”

Winds From the Visible (513 nm) Images Obtained by the Venus Monitoring Camera Onboard Venus Express

“…Alternative explanation follows recent evidences of that the cloud structure strongly varies with latitude. Both Fourier spectrometry onboard Venera‐15 orbiter (Zasova et al., 2002, 2007) and Venus Express observations (Titov et al., 2018 and references therein; Cardesín‐Moinelo et al., 2020) suggested that in the high (>60°) latitudes, the cloud top descends by about 8 km compared to the low and middle latitudes, aerosol becomes much denser with the scale height at the cloud top decreasing to less than 1 km. In the opaque polar cloud with sharp upper boundary, all wavelengths sound approximately the same altitude close to the cloud top that could explain convergence of wind profiles at high latitudes in Figure 10a.…”