Clouds and hazes of Venus

More than three decades have passed since the publication of the last review of the Venus clouds and hazes. The paper published in 1983 in the Venus book summarized the discoveries and findings of the US Pioneer Venus and a series of Soviet Venera spacecraft (Esposito et al. in Venus, p. 484, 1983). Due to the emphasis on in-situ investigations from descent probes, those missions established the basic features of the Venus cloud system, its vertical structure, composition and microphysical properties. Since then, significant progress in understanding of the Venus clouds has been achieved due to exploitation of new observation techniques onboard Galileo and Messenger flyby spacecraft and Venus Express and Akatsuki orbiters. They included detailed investigation of the mesospheric hazes in solar and stellar occultation geometry applied in the broad spectral range from UV to thermal IR. Imaging spectroscopy in the near-IR transparency “windows” on the night side opened a new and very effective way of sounding the deep atmosphere. This technique together with near-simultaneous UV imaging enabled comprehensive study of the cloud morphology from the cloud top to its deep layers. Venus Express operated from April 2006 until December 2014 and provided a continuous data set characterizing Venus clouds and hazes over a time span of almost 14 Venus years thus enabling a detailed study of temporal and spatial variability. The polar orbit of Venus Express allowed complete latitudinal coverage. These studies are being complemented by JAXA Akatsuki orbiter that began observations in May 2016. This paper reviews the current status of our knowledge of the Venus cloud system focusing mainly on the results acquired after the Venera, Pioneer Venus and Vega missions

Clouds and hazes of Venus

More than three decades have passed since the publication of the last review of the Venus clouds and hazes. The paper published in 1983 in the Venus book summarized the discoveries and findings of the US Pioneer Venus and a series of Soviet Venera spacecraft (Esposito et al. in Venus, p. 484, 1983). Due to the emphasis on in-situ investigations from descent probes, those missions established the basic features of the Venus cloud system, its vertical structure, composition and microphysical properties. Since then, significant progress in understanding of the Venus clouds has been achieved due to exploitation of new observation techniques onboard Galileo and Messenger flyby spacecraft and Venus Express and Akatsuki orbiters. They included detailed investigation of the mesospheric hazes in solar and stellar occultation geometry applied in the broad spectral range from UV to thermal IR. Imaging spectroscopy in the near-IR transparency “windows” on the night side opened a new and very effective way of sounding the deep atmosphere. This technique together with near-simultaneous UV imaging enabled comprehensive study of the cloud morphology from the cloud top to its deep layers. Venus Express operated from April 2006 until December 2014 and provided a continuous data set characterizing Venus clouds and hazes over a time span of almost 14 Venus years thus enabling a detailed study of temporal and spatial variability. The polar orbit of Venus Express allowed complete latitudinal coverage. These studies are being complemented by JAXA Akatsuki orbiter that began observations in May 2016. This paper reviews the current status of our knowledge of the Venus cloud system focusing mainly on the results acquired after the Venera, Pioneer Venus and Vega missions

Atmospheric structure and dynamics as the cause of ultraviolet markings in the clouds of Venus.

When seen in ultraviolet light, Venus has contrast features that arise from the non-uniform distribution of unknown absorbers within the sulphuric acid clouds and seem to trace dynamical activity in the middle atmosphere. It has long been unclear whether the global pattern arises from differences in cloud top altitude (which was earlier estimated to be 66–72 km), compositional variations or temperature contrasts. Here we report multi-wavelength imaging that reveals that the dark low latitudes are dominated by convective mixing which brings the ultraviolet absorbers up from depth. The bright and uniform mid-latitude clouds reside in the 'cold collar', an annulus of cold air characterized by approx30 K lower temperatures with a positive lapse rate, which suppresses vertical mixing and cuts off the supply of ultraviolet absorbers from below. In low and middle latitudes, the visible cloud top is located at a remarkably constant altitude of 72 ± 1 km in both the ultraviolet dark and bright regions, indicating that the brightness variations result from compositional differences caused by the colder environment rather than by elevation changes. The cloud top descends to ~64 km in the eye of the hemispheric vortex, which appears as a depression in the upper cloud deck. The ultraviolet dark circular streaks enclose the vortex eye and are dynamically connected to it

Retrieval of air temperature profiles in the Venusian mesosphere from VIRTIS-M data: Description and validation of algorithms

International audienceWe present here methods developed for the retrieval of air temperature profiles in the Venusian mesosphere from the absolute radiances measured by the Visual and Infrared Thermal Imaging Spectrometer (VIRTIS) on board the Venus Express satellite. The infrared M channel of the instrument acquires multispectral images between 1000 and 5000 nm. In nighttime measurements, radiance in the range 3800-5000 nm is dominated by the thermal emission and absorption by the clouds and carbon dioxide. Since the latter is the main atmospheric component, it is possible to exploit the strong variability of its opacity in this spectral range, as resolved by the instrument, to reconstruct the vertical air temperature profile as a function of pressure. In this context we decided to adopt the Twomey et al. (1977) relaxation scheme. The resulting code was extensively tested on a set of simulated VIRTIS-M data. Comparison of the known input conditions with the results of analysis code allowed us to evaluate the systematic and random errors affecting the retrievals procedures on a statistical basis. The code returns the vertical air temperature profile with an uncertainty of less than 1 K in the region between 70 and 7 mbar (66 and 77 km above the reference surface) and less than 4 K throughout the entire range 100-0.1 mbar (64-95 km). Finally, we present the first examples of the code applied to actual measured Venusian data, demonstrating its capability to achieve a satisfactory modeling of the observations and provide physically reasonable results

Variations of sulphur dioxide at the cloud top of Venus's dynamic atmosphere

International audienceA pulse of sulphur dioxide in Venus's upper atmosphere was observed by the Pioneer Venus spacecraft in the 1970s and 1980s and attributed to volcanism. Recent sulphur dioxide measurements from Venus Express indicate decadal-scale fluctuations in sulphur dioxide above Venus's cloud tops in an atmosphere that is more dynamic than expected

A chaotic long-lived vortex at the southern pole of Venus

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