One of the most intriguing , long-standing questions regarding Venus ’ 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 ’ spectrum .
In this work , we use data collected by MASCS spectrograph on board the MESSENGER mission during its second Venus flyby in June 2007 to address this issue .
Spectra range from 0.3 \mu m to 1.5 \mu m including some gaseous H _ { 2 } O and CO _ { 2 } bands , as well as part of the SO _ { 2 } 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 ’ small mode-1 particles .
The results show an homogeneous Equatorial atmosphere , with cloud tops ( height for unity optical depth ) at 75 \pm 2 km above surface .
The UV absorption is found to be centered at 0.34 \pm 0.03 \mu m with a full width half maximum of 0.14 \pm 0.01 \mu m. Our values are compared with previous candidates for the UV aerosol absorber , among which disulfur oxide ( S _ { 2 } O ) and dioxide disulfur ( S _ { 2 } O _ { 2 } ) provide the best agreement with our results .