  This document is the EnVision Venus orbiter proposal, submitted in October
2016 in response to ESA's M5 call for Medium-size missions for its Science
Programme, for launch in 2029.
  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 Earth. Its original
atmosphere was probably similar to that of early Earth, with abundant water
that would have been liquid under the young sun's fainter output. Even today,
with its global cloud cover, the surface of Venus receives less solar energy
than does Earth, so why did a moderate climate ensue here but a catastrophic
runaway greenhouse on Venus? How and why did it all go wrong for Venus? What
lessons can be learned about the life story of terrestrial planets in general,
in this era of discovery of Earth-like exoplanets? Were the radically different
evolutionary paths of Earth and Venus driven solely by distance from the Sun,
or do internal dynamics, geological activity, volcanic outgassing and
weathering also play an important part?
  Following the primarily atmospheric focus of Venus Express, we propose a new
Venus orbiter named EnVision, to focus on Venus' geology and geochemical
cycles, seeking evidence for present and past activity. The payload comprises a
state-of-the-art S-band radar which will be able to return imagery at spatial
resolutions of 1 - 30 m, and capable of measuring cm-scale deformation; this is
complemented by subsurface radar, IR and UV spectrometers to map volcanic
gases, and by geodetic investigations.
