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.

The VenSpec-U experiment will map distribution and spatial and temporal variations of sulfur bearing gases (SO, SO2) and unknown particulate absorber at the cloud tops. These measurements will support the search for volcanic activity by constraining variability of the species that can be attributed to the atmospheric dynamics.

Instrument description

The VenSpec-U instrument is a dual channel UV spectral imager (low and high spectral resolution channels, “LR” and “HR” hereafter). Each channel consists of an entrance baffle, an objective composed of two lenses and a stop diaphragm, and a spectrometer composed of a slit and a toroidal holographic grating. It also includes s shortpass filter to reject the wavelengths above the higher limit of both channel bands and a zero-order trap to avoid straylight due to internal reflections of the grating zero-order. Both LR and HR slits are parallel and the optical layout is such that both channels have the same instantaneous FoV, allowing simultaneous observations and calibrations. Each slit image is then spectrally dispersed by its respective toroidal holographic grating and is formed on a shared CMOS back-side illuminated detector. 

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. The spectra of LR and HR channels are dispersed one above the other on the focal plane. The remaining spatial direction is provided through orbital scrolling (“pushbroom” strategy). Binning on the spatial axis is performed on the detector. The detector will be controlled such that the integration time and the binning scheme is adjusted independently (and simultaneously) for each channel giving high flexibility and providing parameters for the optimisation of each acquisition. 

The HR channel 205-235 nm at 0.2 nm spectral resolution and spatial sampling not coarser than 24 km (12 km target) should reach a SNR of at least 100. The LR channel 190-380 nm at 2 nm spectral resolution and spatial sampling not coarser than 5 km (3 km target) should reach a SNR of at least 200. According to our forward model based on SPICAV-UV heritage, this shall be sufficient to measure the targeted species (SO, SO2, UV absorber) with an accuracy better than 25%. These measurements shall allow characterizing variability on timescales from hours to years. 

Instrument heritage

VenSpec-U is an instrument based on a strong heritage mainly provided by PHEBUS onboard BepiColombo, a double UV spectrometer covering from 55 nm to 315 nm; and SPICAV-UV onboard Venus Express, an UV spectrometer covering from 118 nm to 320 n). Both instruments were assembled and tested in LATMOS as it is expected for VenSpec-U, so the laboratory technical expertise is also a major part of this heritage. The optical scheme of the spectrometer part of VenSpec-U is based on holographic gratings and is therefore very similar to the PHEBUS and SPICAV ones. Due to the quite large field of view required, VenSpec-U is the first ultraviolet spectrometer developed at LATMOS using lens objectives as telescope.