Despite the emphasis on Venus of early space exploration (Mariner, Venera, Vega, & Pioneer), and the more recent Magellan, Venus Express, and Akatsuki Missions, Venus re- mains a global mystery. Sitting in our own back yard, Venus represents an unusual example of terrestrial planet formation and evolution that obviously differs from Earth and the other solid planets of the inner solar system. 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 highland tessera 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 ?
The first two questions of ESA’s Cosmic Vision plan focus on the formation of planets and the emergence of life, and on the solar system itself. Central to both of these goals is the study of the terrestrial planets if the inner Solar System. Earth, Mars, Mercury and Venus – show a wide range of evolutionary pathways, and so are key to understanding planets and their habitability everywhere, including in other stellar systems.
The NASA Magellan, ESA Venus Express, and JAXA’s Akatsuki missions to Venus have revealed a complex, dynamic planet with surprising similarities as well as differences from Earth, and yet it remains the least understood of the terrestrial planets. Further Venus exploration to find the causes of these differences is crucial to understanding how the Solar System works and the conditions for planet formation and the emergence of life. This is of particular relevance in this era of discovery and characterisation of a variety of terrestrial planets in other stellar systems and discussion on their path toward habitability.
The dramatic discovery of volcanic hotspots in Venus Express data suggest that the next stage of Venus exploration must focus on its surface, the largest in the entire Solar System : the geological interface between its dense, hostile atmosphere and its Earth-like but puzzling interior. Magellan data reveal an incredible number of volcanoes, as well as rift systems, mountain belts, and a range of features still poorly understood, on a world with a crater count indicative of mean surface age of only 500 Ma, as young as Europe.
After its successful Venus Express mission (2006-2014), the European Space Agency, on behalf of the Science Community and with the participation of NASA, selected a Venus orbiter mission as one of the three candidates for Cosmic Vision Programme’s M5 slot (launch in the early 2030s): EnVision.
EnVision will address the following specific science questions:
- Is Venus geologically active ?
- How has Venus geologically evolved through time ?
- How Venus atmosphere is linked to its geology ?
- How does Venus interior work ?
The science objective of EnVision tie within ESA’s Cosmic Vision themes:
- What are the conditions for planet formation and the emergence of life?
- How does the Solar System work ?
In that purpose, EnVision, carrying 5 instruments and 1 experiment (an S-band Synthetic Aperture Radar, a Subsurface Radar, 3 spectrometers and a radio science experiment), will investigate Venus from its inner core to its atmosphere at an unprecedented scale of resolution, characterising in particular, core and mantle structure, signs of active and past geologic processes and looking for evidence of the past existence of oceans. EnVision will help understanding why the most Earth-like planet in the solar system has turned out so differently, opening a new era in the exploration of our closest neighbour.