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

Abstract

[1] Past spacecraft observations of Venus have found considerable spatial and temporal variations of water vapour abundance above the clouds. Previous searches for variability below the clouds at 30-45 km altitude found no large scale latitudinal gradients, but lacked the spatial resolution to detect smaller scale variations. Here we interpret results from the VIRTIS imaging spectrometer on Venus Express, remotely sounding at near-infrared 'spectral window' wavelengths, as indicating that the water vapour abundance at 30-40 km altitude varies from 22 to 35 ppmv (±4 ppmv). Furthermore, this variability is correlated with cloud opacity, supporting the hypothesis that its genesis is linked to cloud convection. It is also possible to fit the observations without requiring spatial variation of water abundance, but this places a strong constraint on the spectral dependence of the refractive index data assumed for the lower cloud particles, for which there is as yet no supporting evidence. Citation:

Spatially resolved measurements of H2O, HCl, CO, OCS, SO2, cloud opacity, and acid concentration in the Venus near-infrared spectral windows

“…It can be difficult to determine whether these correlations are real or whether they result from insufficient correction of the cloud in certain regions. Species such as water vapor and CO have been reported in previous studies to be anticorrelated with cloud opacity [ Tsang et al ., ; Barstow et al ., ], but we urge caution of the interpretation of similar features reported here. Just as residual scattered light can contaminate the dark side of Venus due to the challenges of accurately removing dayside radiation on every pixel, residual cloud ghosting features may remain due to the difficulties of completely correcting the spectral effects of variable cloud properties, especially at the longest wavelength end of our spectra where cloud ghosting is strongest and toward the planet's poles where the cloud structure is not well constrained.…”

“…Table summarizes the results of previous studies. In general, these analyses suggest that water in the lower atmosphere has little variability except possibly near the base of the cloud deck, where it may anticorrelate with cloud opacity [ Tsang et al ., ; Barstow et al ., ]. Previous studies show that CO and OCS may be spatially anticorrelated: CO is enhanced at higher latitudes compared to the equatorial region, and OCS tends to display the opposite behavior.…”

Habitable worlds with JWST: transit spectroscopy of the TRAPPIST-1 system?

“…It has been extensively used to model both exoplanets and solar system worlds (e.g. Lee et al 2012;Tsang et al 2010;Fletcher 2011). Each planet is treated as though the atmosphere at the terminator, the region probed in transmission spectroscopy, has identical chemistry to Earth's present day atmosphere.…”

Contrast sources for the infrared images taken by the Venus mission AKATSUKI

“…Cloud optical thickness, cloud altitude, and temperature information may be determined separately by introducing information obtained from other wavelength regions. Future studies should consider acidity as a parameter in addition to cloud thickness, height, and temperature, as indicated by Tsang et al (2010).…”