Winds in the Middle Cloud Deck From the Near‐IR Imaging by the Venus Monitoring Camera Onboard Venus Express

Abstract

For more than 8 years the Venus Monitoring Camera (VMC) onboard the Venus Express orbiter performed continuous imaging of the Venus cloud layer in UV, visible and near‐IR filters. We applied the correlation approach to sequences of the near‐IR images at 965 nm to track cloud features and determine the wind field in the middle and lower cloud (49–57 km). From the VMC images that spanned from December of 2006 through August of 2013 we derived zonal and meridional components of the wind field. In low‐to‐middle latitudes (5–65°S) the velocity of the retrograde zonal wind was found to be 68–70 m/s. The meridional wind velocity slowly decreases from peak value of +5.8 ± 1.2 m/s at 15°S to 0 at 65–70°S. The mean meridional speed has a positive sign at 5–65°S suggesting equatorward flow. This result, together with the earlier measurements of the poleward flow at the cloud tops, indicates the presence of a closed Hadley cell in the altitude range 55–65 km. Long‐term variations of zonal and meridional velocity components were found during 1,200 Earth days of observation. At 20° ± 5°S the zonal wind speed increases from −67.18 ± 1.81 m/s to −77.30 ± 2.49 m/s. The meridional wind gradually increases from +1.30 ± 1.82 m/s to +8.53 ± 2.14 m/s. Following Bertaux et al. (2016) we attribute this long‐term trend to the influence from the surface topography on the dynamical process in the atmosphere via the upward propagation of gravity waves that became apparent in the VMC observations due to slow drift of the Venus Express orbit over Aphrodite Terra.

Morphology and Dynamics of Venus's Middle Clouds With Akatsuki/IR1

“…Unlike the contrasts of up to 40% reported for UV (Belton et al, ; Lee et al, ), the NIR albedo during past missions exhibited weaker contrasts of ∼4% (Belton et al, ; Hueso et al, ; Khatuntsev et al, ). We calculated the contrast of Venus's NIR albedo ( ) using 519 photometrically corrected IR1 images acquired from 7 December 2015 to 9 December 2016 and solar zenith angles (SZAs) and emission angles (EAs) <60°.…”

“…Since photons with longer wavelengths can penetrate the Venusian clouds deeper before being reflected (Sánchez‐Lavega et al, ; Takagi & Iwagami, ), the middle‐lower clouds of Venus can be observed on the dayside at 570–680 and 900–1,000 nm, although with weaker contrast (Belton et al, ; Hueso et al, ). The altitude of these contrasts is not well constrained and different estimates have been obtained with radiative transfer calculations (51–55 km from Iwagami et al, , based on Takagi & Iwagami, ; 58–68 km from Khatuntsev et al, ) or comparing cloud‐tracked speeds with vertical profiles of the wind from entry probes (55 km from Belton et al, ; 62 km from Peralta et al, ; 50–57 km from Khatuntsev et al, ). First studies of these clouds came from polarized images at 935 nm during the Pioneer Venus mission (Limaye, ), although it was not until the analysis of Galileo in 1990 that their different morphology and slower wind speeds became evident through 986‐nm images (Belton et al, ; Peralta et al, ).…”

Circulation of Venusian Atmosphere at 90–110 km Based on Apparent Motions of the O2 1.27 μm Nightglow From VIRTIS‐M (Venus Express) Data

“…The evidence for the stationary wave above Aphrodite Terra comes from the longwave infrared camera (LIR) at 10 ± 2 μm and the ultraviolet imager onboard of Akatsuki spacecraft, which observed the bow‐shaped structure (Fukuhara et al, ). The influence of the surface topography on dynamic processes in the middle cloud deck was found from dayside near‐infrared (1 μm) images obtained by VMC (Venus Monitoring Camera) onboard of Venus Express (Khatuntsev et al, ). The influence of surface might manifest an irregular behavior due to the occasional stability of the convective layer between 52 and 57 km (Hauchecorne, ).…”

Venus Atmospheric Thermal Structure and Radiative Balance

“…If the zonal winds in the mesosphere remain in cyclostrophic balance, the observed temperature structure indicates (Limaye and Suomi 1981;Khatuntsev et al 2017;Seiff 1982).…”