Carbon dioxide absorption at high densities in the 1.18μm nightside transparency window of Venus

The attenuation of carbon dioxide at pressures between 1 and 38 bar has been measured from 1179.5 to 1182.7 nm at room temperature (293 K) by using a cavity ring down spectrometer. The densities used in the experiment correspond with the densities in the Venus atmosphere from about 50 km down to the surface, although the experimental temperature is different from that encountered in the lower atmosphere. A binary absorption coefficient of 5.47(28)×10−10 cm−1 amagat−2 has been obtained at 1180.7 nm. A regular increase of the binary absorption is observed from 1179.5 to 1182.7 nm, likely due to the far wing of the nearby ν1+3ν3 band of CO2. The results are in good agreement with an analysis of observations by the VIRTIS and SPICAV-IR instruments on board the Venus Express spacecraft. The carbon dioxide Rayleigh scattering coefficient at 1.18 μ m has been measured for the first time by using the cavity ring down technique, and a value of 1.02(14)×10−8 cm−1 amagat−1 was found.