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

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 CO2bands, as well as part of the SO2absorption 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. The atmosphere of Venus is fully cloud covered, and its clouds are very reflective in most visual wavelengths, with the exception of the near ultraviolet, where an absorber of unknown origin strongly absorbs solar radiation. Such a mysterious absorber provides contrast to many atmospheric features whose dynamics can be tracked from images, and it also has a substantial role in the energy budget of the planet. But, as of today, we have no clear idea what it is made of. In this work, we have analyzed spectra taken during the second Venus flyby of MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft on its route to Mercury, in 2007. Using a numerical code, we have reproduced the light reflected by the equatorial atmosphere of the planet and retrieved the distribution of particles in the upper atmosphere of Venus, with a cloud top of some 75 km above the surface. We have also retrieved the absorption spectrum of the puzzling absorber and compared it with some previously proposed candidates. While no perfect match is found, sulfur‐bearing species (S2O and S2O2) provide the best agreement. There is still a long way to undoubtedly identify Venus's UV absorber, but this work provides substantial spectral constraints. We analyze spectra of Venus's equatorial atmosphere taken during the second MESSENGER flyby on 5 June 2007Cloud tops are located at 75 ± 2 km in the equatorial atmosphere.Among all candidates proposed so far, S2O and S2O2, with subindices provide the best agreement with the UV absorption retrieved here