Correlations between cloud thickness and sub-cloud water abundance on Venus

摘要

Past spacecraft observations of Venus have foundconsiderable spatial and temporal variations of water vapourabundance above the clouds. Previous searches forvariability below the clouds at 30-45 km altitude foundno large scale latitudinal gradients, but lacked the spatialresolution to detect smaller scale variations. Here weinterpret results from the VIRTIS imaging spectrometer onVenus Express, remotely sounding at near-infrared 'spectralwindow' wavelengths, as indicating that the water vapourabundance at 30-40 km altitude varies from 22 to 35 ppmv(±4 ppmv). Furthermore, this variability is correlated withcloud opacity, supporting the hypothesis that its genesis islinked to cloud convection. It is also possible to fit theobservations without requiring spatial variation of waterabundance, but this places a strong constraint on the spectraldependence of the refractive index data assumed for thelower cloud particles, for which there is as yet no supporting evidence.