Purpose of VenSpec-U

The rise and fall of sulphur dioxide in the upper atmosphere of Venus over the last 40 years, expressed in units of parts per billion by volume (ppbv). The dataset on the left is mostly from NASA’s Pioneer Venus, which was in orbit around Venus from 1978 to 1992. The dataset on the right is from ESA’s Venus Express, which has been studying Venus since 2006. A clear rise in the concentration of sulphur dioxide (SO2) concentration was observed at the start of the mission, with a subsequent decrease. The increase in sulphur dioxide can be interpreted either as evidence for volcanic activity or for decadal-scale variations in the circulation of Venus’ vast atmosphere. VenSpec-U on board EnVision will carry on SO2 (and SO) monitoring with an improved accuracy, as well as better spatial and temporal resolution in order to better understand the origin of SO2 variations, including possible volcanic outgassing.

VenSpec-U is a dual channel (low resolution, hereafter LR: 190-380 nm; and high resolution, hereafter HR: 210-240 nm) UV spectral imager designed to monitor cloud top abundances of volcanic sulphured gases (SO, SO2) as well as UV contrasts through spectral analysis of backscattered sunlight on the day side of Venus. The entrance objective is shared by both channels and consists in two lenses forming the image of the field of view (FOV) onto a common plane including two slits. Both slits (LR and HR) are parallel, and their corresponding FOV are slightly shifted in their narrow direction, equivalent to 9° (corresponding to 30 km at the cloud top level of observation). Each slit image is then spectrally dispersed by their respective toroidal holographic grating, and is formed on a shared E2V Capella CMOS detector. Therefore, the narrow-slit axis of the detector contains the spectral information, whereas the long-slit axis contains the spatial information along the 22.5° FOV of each slit. Binning on the spatial axis is performed on the CMOS, enabling a SNR ~ 100 for both channels (resulting in a coarser spatial resolution on the more dispersed HR channel). The remaining spatial direction is provided through orbital scrolling, an observational strategy known as pushbroom. Observations can be conducted in a strict nadir geometry (null emission angle), or in near-nadir geometry (emission angle < 30°).

VenSpec-U optics

Optical data for VenSpec-U (P.I. Emmanuel Marcq, LATMOS/IPSL, France)