Colin Wilson

Abstract:

© 2014 Elsevier Ltd. One of the difficulties in modeling Venus' nightside atmospheric windows is the need to apply CO2 continuum opacity due to collision-induced CO2 bands and/or extreme far wings of strong allowed CO2 bands. Characterizing the CO2 continuum absorption at near-IR wavelengths as well as searching for a possible vertical gradient of minor species near the surface require observations over different surface elevations. The largest change in altitude occurs during a passage above Maxwell Montes at high northern latitudes. In 2011, 2012 and 2013 the SPICAV instrument aboard the Venus Express satellite performed three sets of observations over Maxwell Montes with variation of surface altitude from -2 to 9km in the 1.10, 1.18 and 1.28-μm windows. The retrieved CO2 continuum absorption for the 1.10- and 1.18-μm windows varies from 0.29 to 0.66×10-9 cm-1 amagat-2 and from 0.30 to 0.78×10-9 cm-1 amagat-2, respectively, depending on the assumed input parameters. The retrieval is sensitive to possible variations of the surface emissivity. Our values fall between the results of Bézard et al., (2009, 2011) based on VIRTIS-M observations and laboratory measurements by Snels et al. (2014). We can also conclude that the continuum absorption at 1.28μm can be constrained below 2.0×10-9 cm-1 amagat-2. Based on the 1.18μm window the constant H2O mixing ratio varying from 25.7+1.4 -1.2 ppm to 29.4+1.6 -1.4 ppm has been retrieved assuming the surface emissivity of 0.95 and 0.6, respectively. No firm conclusion from SPICAV data about the vertical gradient of water vapor content at 10-20km altitude could be drawn because of low signal-to-noise ratio and uncertainties in the surface emissivity.

Abstract:

The Venus Express spacecraft was launched in November 2005. This first European mission to Venus arrived at the planet in April 2006, and within a month had completed on-orbit commissioning and was returning science data to Earth. After four mission extensions and eight years, the spacecraft continues to operate successfully. The end of the mission is anticipated to be in 2014, when the on-board fuel supplies are finally exhausted and the required orbit around Venus can no longer be maintained. This paper discusses the lessons learned by the Venus Express Science Operations Centre (VSOC) over the course of the eight year mission, and briefly discusses the plans for the end of the mission.

Abstract:

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.