Search for horizontal and vertical variations of CO in the day and night side lower mesosphere of Venus from CSHELL/IRTF [formula omitted] observations

Abstract

High-resolution (R~43,000) spectroscopic observations of both day and night sides of Venus were acquired using CSHELL at IRTF between 2202 and 2207cm−1 (4.53–4.54μm) during the latest maximal eastern and western elongations of Venus in two four-nights observing runs in August 2012 and November 2013. Their purpose was to investigate the effect of the recent (from 2007 onwards) decrease of sulfur dioxide (SO2) at the cloud top level on the spatial distribution of carbon monoxide (CO), since both species are involved in the mesospheric photochemical cycles. Observations of neighboring CO2 lines are fully consistent with the previously determined aerosol scale height in the 68–74km range. We could therefore determine CO mixing ratio on both day and night sides of Venus, taking into account both scattered solar and thermal components, as well as the overlying CO dayglow on the day side. CO spatial distribution is mostly unchanged compared to previous measurements, ranging from 25ppmv to 45ppmv at 70km. The scattered solar component reveals an increase in CO with increasing altitude with a scale height of (5±0.5)km between 70 and 76km. Horizontal variability is found to be weak, with a possible increase towards higher latitudes on the day side of about 10ppmv. Yet the accuracy of our absolute values of CO mixing ratio is limited by several assumptions, and we cannot rule out that some of the variability we measure for CO could alternatively be explained by other changes occurring in the Venusian atmosphere, e.g. changes in both dT/dz and aerosol scale height.