Venus Upper Clouds and the UV Absorber From MESSENGER/MASCS Observations

Abstract

One of the most intriguing, long‐standing questions regarding Venus's atmosphere is the origin and distribution of the unknown UV absorber, responsible for the absorption band detected at the near‐UV and blue range of Venus's spectrum. In this work, we use data collected by Mercury Atmospheric and Surface Composition Spectrometer (MASCS) spectrograph on board the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission during its second Venus flyby in June 2007 to address this issue. Spectra range from 0.3 μm to 1.5 μm including some gaseous H2O and CO2 bands, as well as part of the SO2 absorption band and the core of the UV absorption. We used the NEMESIS radiative transfer code and retrieval suite to investigate the vertical distribution of particles in the equatorial atmosphere and to retrieve the imaginary refractive indices of the UV absorber, assumed to be well mixed with Venus's small mode 1 particles. The results show a homogeneous equatorial atmosphere, with cloud tops (height for unity optical depth) at 75 ± 2 km above surface. The UV absorption is found to be centered at 0.34 ± 0.03 μm with a full width at half maximum of 0.14 ± 0.01 μm. Our values are compared with previous candidates for the UV aerosol absorber, among which disulfur oxide (S2O) and dioxide disulfur (S2O2) provide the best agreement with our results.

Stationary Features at the Cloud Top of Venus Observed by Ultraviolet Imager Onboard Akatsuki

“…The scattering of the solar ultraviolet radiation by the cloud is modeled with a single‐scattering approximation (Kuznetsov et al, ). This approximation is valid for an optically thin atmosphere; a complete radiative transfer calculation including the effect of multiple scattering (e.g., Lee et al, ; Pérez‐Hoyos et al, ), which is needed for Venusian clouds, is left for future studies. The relative reflectivity normalized by the reflectivity without the ultraviolet absorber for zero incidence and emission angles is given by where n cloud and n UV are the perturbed total densities ( ), σ cloud is the scattering cross‐section of cloud particles, σ UV is the absorption cross‐section of the ultraviolet absorber, and z top (=100 km) is the upper boundary of the model where n cloud and n UV are negligibly low.…”

“…The scattering of the solar ultraviolet radiation by the cloud is modeled with a single-scattering approximation (Kuznetsov et al, 2012). This approximation is valid for an optically thin atmosphere; a complete radiative transfer calculation including the effect of multiple scattering (e.g., Lee et al, 2017;Pérez-Hoyos et al, 2018), which is needed for Venusian clouds, is left for future studies. The relative reflectivity normalized by the reflectivity without the ultraviolet absorber for zero incidence and emission angles is given by…”

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

“…Most of the contrasts display values of relative radiance >0.9 (<10%), implying changes of up to ∼40% in the cloud thickness that were also considered by Takagi and Iwagami (). Elseway, spectra from VEx/SPICAV‐IR (Korablev et al, , figure 15 therein) and MESSENGER/MASCS (Pérez‐Hoyos et al, , figure 8 therein) suggest the presence of some absorption bands that may be partially responsible for the higher contrasts observed at 900 nm. H 2 O (Cottini et al, ), the controversial CH 4 (Donahue & Hodges, ) or even new candidates may be considered as potential absorbers to explain these contrasts.…”

Long-term Variations of Venus’s 365 nm Albedo Observed by Venus Express, Akatsuki, MESSENGER, and the Hubble Space Telescope

“…According to data from descent probes the unknown absorber may be located in the upper clouds (Tomasko et al 1980;Esposito 1980), and absorbs about half of the solar radiance deposited at the cloud top level, accounting for ∼3 K/Earth day of the global mean solar heating around 65 km altitude, when the total global mean solar heating is ∼6 K/day (Crisp 1986). Many candidates have been proposed for the unknown absorber, including OSSO, S 2 O, S x , FeCl 3 , and iron-bearing microorganism (Mills et al 2007;Frandsen et al 2016;Krasnopolsky 2017;Pérez-Hoyos et al 2018;Limaye et al 2018). However, none of these species satisfy both the spectral features produced by the unknown absorber, and the lifetime and simulated vertical profile required to fit the observations (Krasnopolsky 2018;Pérez-Hoyos et al 2018).…”