A small flotilla of missions will be heading to Venus at the end of the decade: NASA’s VERITAS orbiter and DAVINCI lander; and ESA’s EnVision orbiter. Each of these three have significant US and European components, so why are all three selected, and what will be learned from EnVision specifically? First, the missions are synergistic, i.e., we gain more from them in concert than the sum of each of them alone, a strategy that has worked well at Mars. Rather like MGS and Pathfinder, VERITAS provides a global geophysical survey (gravity, topography and imagery), while DAVINCI obtains in situ data on key atmospheric volatiles and ground truth data from possibly ancient continental tessera terrain. Somewhat like MRO, EnVision is designed to obtain repeated, targeted, holistic data from the interior, subsurface, surface, lower atmosphere, clouds and upper atmosphere of Venus, from a suite of complementary instruments: an imaging radar, VenSAR; a subsurface sounder, SRS; an infrared imager, VenSpec-M; an IR spectrometer, VenSpec-H; a UV spectrometer, VenSpec-U; and radio science experiments, RSE.
The goal is to determine the geological processes and geochemical cycles that govern the planet today and whether they were different in the past. For example, VenSpec-M, -H and the RSE will track the sources of water vapor and sulfur dioxide from volcanic vents, through the lower atmosphere, and into the clouds, to discover how they are sustained. VenSpec-U will assess whether these volcanic inputs are associated with the episodic volatile spikes in the upper atmosphere. Likewise, VenSAR, VenSpec-M and SRS will map the nature and extent of surface weathering that are the sinks for these volatiles, to discover whether there is a balance through long-term recycling, or if there is only a one-way path to the atmosphere. Of course, there are many other targets of interest, from ancient tessera terrain (including the DAVINCI landing site) to plains resurfacing and crater degradation. The integrated observations from this suite of instruments will allow for the development, testing and refinement of new models relevant to Earth-sized planets everywhere, and to so help understand how and why Venus evolved so differently to Earth.