Atmospheric thermal structure and cloud features in the southern hemisphere of Venus as retrieved from VIRTIS/VEX radiation measurements

Abstract

Thermal structure and cloud features in the atmosphere of Venus are investigated using spectroscopic nightside measurements recorded by the Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS) aboard ESA's Venus Express mission in the moderate resolution infrared mapping channel (M-IR, 1-5 µm). New methodical approaches and retrieval results for the northern hemisphere have been recently described by Haus et al. (2013). Now, southern hemisphere maps of mesospheric temperature and cloud parameter fields are presented that cover variations with altitude, latitude, local time, and mission time. Measurements from the entire usable data archive are utilized comprising radiation spectra recorded during eight Venus solar days between April 2006 and October 2008.Zonal averages of retrieved temperature altitude profiles in both hemispheres are very similar and give evidence of global N-S axial symmetry of atmospheric temperature structure. Cold collar and warmer polar vortex regions exhibit the strongest temperature variability with standard deviations up to 8.5 K at 75°S and 63 km altitude compared with about 1.0 K at low and mid latitudes above 75 km. The mesospheric temperature field strongly depends on local time. At altitudes above about 75 km, the atmosphere is warmer in the second half of night, while the dawn side at lower altitudes is usually colder than the dusk side by about 8 K. Local minimum temperature of 220 K occurs at 03:00 h local time at 65 km and 60°S. Temperature standard deviation at polar latitudes is particularly large near midnight. Temperature variability with solar longitude is forced by solar thermal tides with a dominating diurnal component. The influence of observed cloud parameter changes on retrieved mesospheric zonal average temperature structure is moderate and does not exceed 2-3 K at altitudes between 60 and 75 km. The mesospheric thermal structure was essentially stable with Julian date between 2006 and 2008.Global N-S axial symmetry is also observed in cloud structures. Cloud top altitude at 1 µm slowly decreases from 71 km at the equator to 70 km at 45-50° and rapidly drops poleward of 50°. It reaches 61 km over both poles. Average particle size in the vertical cloud column increases from mid latitudes toward the poles and also toward the equator resulting in minimum and maximum zonal average cloud opacities of about 32 and 42 and a planetary average of 36.5 at 1 µm. Zonal averages of cloud features are similar at different solar days, but variations with local time are very complex and inseparably associated with the superrotation of the clouds.

Six years of Venus winds at the upper cloud level from UV, visible and near infrared observations from VIRTIS on Venus Express

“…This global trend was derived by Ignatiev et al (2009) from VIRTIS-M spectroscopy and VMC images and was later confirmed by Lee et al (2012) by joint analysis of the VeRa and VIRTIS instruments on Venus Express. A recent detailed work based on VIRTIS data confirms this trend (Haus et al, 2014) which is also in agreement with earlier radiative transfer results (Zasova et al, 2007) and in situ measurements during the descent of the Venera and Pioneer Venus probes (Schubert, 1983) and from tracking of VEGA balloons (Preston et al, 1986).…”

“…Several radiative transfer analysis place the cloud tops at altitudes of 67-71 km at the equator with a nearly constant altitude until 45-501S and a drop of altitude poleward of 501S reaching about 61-63 km over both poles (Ignatiev et al, 2009;Lee et al, 2012;Haus et al, 2014). Detailed studies of the cloud top motions can be traced back to measurements obtained by Mariner 10 in 1974 , Pioneer Venus in 1979-1985(Rossow et al, 1980Limaye and Suomi, 1981;Limaye et al, 1982Limaye et al, , 1988Limaye, 2007) and Galileo in 1990(Belton et al, 1991Toigo et al, 1994;Peralta et al, 2007;Kouyama et al, 2012).…”

“…We favor the first model over the second one because of the similar morphology of clouds found in UV and NIR images that seem to point to images sensing very similar vertical layers. We additionally considered a synthetic altimetry model for UV clouds as the median of profiles from Ignatiev et al (2009), Lee et al (2012 and Haus et al (2014), all based on VEx data. Fig.…”

The radiative forcing variability caused by the changes of the upper cloud vertical structure in the Venus mesosphere

“…The tropopause is located at around 60 km altitude at the base of the upper cloud layer (Tellmann et al, 2009). There is a clear trend for the cloud top altitude to decrease from equator toward the poles Ignatiev et al, 2009;Lee et al, 2012;Haus et al, 2014). The cloud top is defined as the level of the unity optical thickness at 4-5 μm, is located at 67 km in low latitude region and descends to 63 km at the poles (Lee et al, 2012).…”

Development and in-flight calibration of IR2: 2-μm camera onboard Japan’s Venus orbiter, Akatsuki

“…We will derive pixel-by-pixel maps of CO absorption indices to study the hypothetical return flow of the Hadley cell (or indirect cell) (Schubert 1983), which may not be detected by cloud tracking results. Haus et al (2014) performed a comprehensive study of mesospheric temperature and cloud parameters while demonstrating usefulness of analyzing multi-window (1.74-, 2.3-, and 4.3-μm) spectra of the Venus night-side disk (VEX/VIRTIS-M). On Akatsuki, longwave infrared camera (LIR) can replace 4.3-μm observations by mapping the cloud-top temperatures at 8-12 μm (Fukuhara et al 2011), while IR2 provides 1.74-and 2.3-μm cloud opacity data.…”