The CO2 continuum absorption in the 1.10- and 1.18-μm windows on Venus from Maxwell Montes transits by SPICAV IR onboard Venus express

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 9 km 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, Bézard et al., 2011 based on VIRTIS-M observations and laboratory measurements by Snels et al. (2014). We can also conclude that the continuum absorption at 1.28 μm can be constrained below 2.0×10−9 cm−1 amagat−2. Based on the 1.18 μm 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 10-20 km altitude could be drawn because of low signal-to-noise ratio and uncertainties in the surface emissivity.