Venus: Key to understanding the evolution of terrestrial planets

Why are the terrestrial planets so different? Venus should be the most Earth-like of all our planetary neighbours. Its size, bulk composition and distance from the Sun are very similar to those of the Earth. Its original atmosphere was probably similar to that of early Earth, with large atmospheric abundances of carbon dioxide and water - possibly even a liquid water ocean. While on Earth a moderate climate ensued, Venus experienced runaway greenhouse warming, which led to its current hostile climate. How and why did it all go wrong for Venus? What lessons can we learn about the life story of terrestrial planets in general, whether in our solar system or in others? ESA's Venus Express mission proved very successful, answering many questions about Earth's sibling planet and establishing European leadership in Venus research. However, further understanding of Venus and its history requires several more lines of investigation. Entry into the atmosphere is required to measure noble gas isotopes to constrain formation & evolution models. Radar mapping at metre-scale spatial resolution, and surface height change detection at centimetre scale, would enable detection of current volcanic & tectonic activity. A lander in the ancient tessera highlands would provide clues as to the earliest geologic record available on Venus. To address these themes we propose a combination of an in situ balloon platform, a radar-equipped orbiter, and (optionally) a descent probe. These mission elements are modelled on the 2010 EVE M3 mission proposal, on the 2010 EnVision M3 proposal, and on Russia's Venera-D entry probe, respectively. Together, these investigations address themes of comparative planetology and solar system evolution. This document was submitted in May 2013 as a response to ESA's Call for White Papers for the Definition of Science Themes for L2/L3 Missions in the ESA Science Programme.