The VenSpec suite on the ESA Envision mission – a holistic investigation of the coupled surface atmosphere system of Venus

The ESA EnVision mission will determine the nature and current state of Venus' geological evolution and its relationship with the atmosphere, to understand how and why Venus and Earth evolved so differently. Perched at the inner edge of the Sun habitable zone, Venus may once have been habitable, with liquid water oceans, before developing the enormous greenhouse warming which renders it uninhabitable today, thus providing a natural laboratory for studying the evolution of habitability. Venus is Earth’s closest sibling geologically: similar in size to the Earth, it has remained active into the present era, unlike the much smaller Mars and Mercury. Venus is essential for understanding the links between planetary geophysical evolution and habitability of terrestrial planets from our own Earth to terrestrial planets and exoplanets everywhere, including those which will be the subject of study by PLATO and ARIEL missions in ESA’s Space Science program.

The VenSpec instrument suite is following the holistic approach of the EnVision mission by studying the coupled system of surface and atmosphere on Venus with three complementary instruments. In combination, VenSpec will provide unprecedented insights into the current state of Venus and its past evolution. VenSpec will perform a comprehensive search for volcanic activity by targeting atmospheric signatures, thermal signatures and compositional signatures, as well as a global map of surface composition. A joined VenSpec science team across the whole suite ensures that the synergies between the instruments are fully used.

A good example for the holistic investigation nature of the VenSpec suite is the characterization of volcanic plumes. VenSpec will follow them from close to the surface by the water vapor measurements of VenSpec-M and VenSpec-H, tracing them through the middle atmosphere with VenSpec-H and finally through the clouds to the upper atmosphere with VenSpec-U and VenSpec-H.  This will be complementary with the direct detection of the thermal signature of lava flows on the surface by VenSpec-M as well as change detection by the VenSAR synthetic radar and the sub-surface radar (SRS) on EnVision.

The region above the clouds (65-75 km) will be primarily studied by VenSpec-U monitoring sulphured minor species (mainly SO and SO₂) and the as yet unknown UV absorber in Venusian upper clouds, as well as the day-side observations of VenSpec-H monitoring the SO₂ abundance. The region below the clouds (30-40 km) will be targeted by VenSpec-H with high resolution atmospheric measurements on the night side. The main objective is to detect and quantify variations in SO2, H2O and HDO abundances. The lower atmosphere (0-15km) will be target by combined nightside observation by VenSpec-H and VenSpec-M.

The overlapping altitude coverage of the VenSpec-H and -U channels allows to investigate how the upper atmosphere interacts with the lower atmosphere. Combined with the VenSpec-M information on the surface and in the lower atmosphere (0-15km) measurements of VenSpec-H and VenSpec-M this allows also to study the interaction between the surface and the atmosphere.

The surface will primarily be target by VenSpec-M which will provide near-global compositional data on rock types, weathering, and crustal evolution by mapping the Venus surface in five atmospheric windows. VenSpec-M take advantage of the improved altimetry provided by the NASA VERITAS VISAR and Envision VenSAR-derived DEMs. 

VenSpec Channel	Altitude Range	Day/Night	Expected measurements
U	above the clouds (65-75 km)	Day	SO and SO2 and the unknown UV absorber
H	above the clouds (65-75 km)	Day	SO2
H	below the clouds (30-40 km)	Night	SO2, H2O and HDO
H	near-surface (0-15 km)	Night	H2O and HDO
M	near-surface (0-15 km)	Night	H2O
M	Surface	Night	Rock type, temperature