Located at 0.72 astronomical units from the Sun, with a radius 0.95 times that of the Earth, Venus is the most Earth-like telluric planet, in size, composition and distance to its star, yet at some point in planetary history there was a bifurcation between the two : Earth has been continually habitable since the end of its formation, whereas Venus became uninhabitable.
The exploration of Venus is thus essential to understand the conditions of formation and evolution of planets of terrestrial mass, providing a natural laboratory to study the evolution of habitability and conditions of emergence of life.
Many fundamental questions remain unanswered. For example : did Venus have oceans, how has that atmosphere evolved over time, and when and why did the runaway greenhouse begin ? How does Venus lose its heat, how volcanically and tectonically active has Venus been over the last billion years ? Has Venus always had a “stagnant-lid”, or was a plate tectonics regime ever present earlier in her history ? What is the composition of the hightland terrain, are these regions the oldest rocks exposed on the Venus surface, how oxidized are those rocks and do these surfaces retain evidence of an earlier time when water was more prevalent ? Are the rapid variations in atmospheric composition of sulfur elements a consequence of current volcanic activity ?
A double legacy : Magellan and Venus Express
The EnVision mission is part of the dual legacy of NASA’s Magellan mission (1990-1994), and ESA’s Venus Express mission (2007-2014).
It will simultaneously characterize all geophysical processes operating on Venus from the inner core to the upper atmosphere.
The selection phase was conducted in several stages :
◾ Call for mission proposals issued by ESA in April 2016 ;
◾ Pre-selection in April 2018 of 3 missions out of the 27 proposed for detailed concept studies ;
◾ M5 Mission Definition Review (MDR) in December 2018 ;
◾ Assessment Study Report published in February 2021 ;
◾ M5 Mission Selection Review (MSR) in February-April 2021 ;
◾ Evaluation of study results by a panel of experts and recommendation of a mission in May 2021 ;
◾ Formal vote by the ESA Program Committee and public announcement on June 10, 2021.
The selection phase has shown that the mission concept, orbital parameters, environmental constraints, data transfer and storage, and instrument accommodation on the satellite are compatible with the constraints of the ESA Cosmic Vision M5 program.
EnVision officially becomes the 5th intermediate class mission of the European Space Agency’s Cosmic Vision program, with a budget of 610 million euros (2021).
The EnVision mission will be launched by Ariane 6.2 in the early 2030s. The scientific operations will begin after the interplanetary cruise and an aerobraking phase using the upper atmosphere of Venus for orbit circularisation and final positioning in low polar orbit.
EnVision is based on a highly integrated ESA-NASA partnership, in which NASA provides the primary instrument, an S-band synthetic aperture radar (VenSAR) studied at the Jet Propulsion Laboratory. The scientific payload consists of a second radar to probe sub-surface layers (SRS), studied by the Italian Space Agency (ASI), and a suite of three spectrometers operating in the infrared and ultraviolet (VenSpec-M, -H, -U), studied respectively by the German (DLR), Belgian (BELSPO), and French (CNES) space agencies.
The determining role of French laboratories
The mission is proposed by :
◾ the United Kingdom (R. Ghail, Royal Halloway, University of London, C. Wilson, University of Oxford)
◾ and by France (T. Widemann, Observatoire de Paris-PSL and Université de Versailles Saint-Quentin/Paris-Saclay)
EnVision will continue the European progress on Venus after the Venus Express mission (2007-2014, more than 400 scientific publications) and more particularly that of the French laboratories with VIRTIS and SPICAV (LESIA, LATMOS).
Paris Observatory - PSL was from 2013 to 2016 the coordinator of the European Union FP7 EuroVenus project, also led by T. Widemann.
Moreover, France is the world leader in climate and chemical modeling of the Venusian atmosphere (LMD) and has a very strong expertise in radar or subsurface radiometry of planetary surfaces (LATMOS, IPAG).
The French teams are closely integrated in EnVision with :
- the scientific responsibility of the UV instrument (E. Marcq at LATMOS, with participation of LESIA and IRAP) and a significant contribution to the IR multispectral imager (LATMOS, LESIA), both of which will allow to link the surface activity to the atmosphere,
- the scientific responsibility of the radio science (C. Dumoulin and P. Rosenblatt at LPG) to characterize the structure of the gravity field and the internal dynamics, as well as the study of the atmosphere by radio occultation ;
- scientific co-investigators on the globality of the mission (LESIA, LATMOS, IRAP, LPG, LAM, etc.).