Photochemical control of the distribution of Venusian water

Abstract

We use the JPL/Caltech 1-D photochemical model to solve continuity diffusion equation for atmospheric constituent abundances and total number density as a function of radial distance from the planet Venus. Photochemistry of the Venus atmosphere from 58 to 112 km is modeled using an updated and expanded chemical scheme (Zhang et al., 2010 and Zhang et al., 2012), guided by the results of recent observations and we mainly follow these references in our choice of boundary conditions for 40 species. We model water between 10 and 35 ppm at our 58 km lower boundary using an SO_2 mixing ratio of 25 ppm as our nominal reference value. We then vary the SO_2 mixing ratio at the lower boundary between 5 and 75 ppm holding water mixing ratio of 18 ppm at the lower boundary and finding that it can control the water distribution at higher altitudes. SO_2 and H_2O can regulate each other via formation of H_2SO_4. In regions of high mixing ratios of SO_2 there exists a "runaway effect" such that SO_2 gets oxidized to SO_3, which quickly soaks up H_2O causing a major depletion of water between 70 and 100 km. Eddy diffusion sensitivity studies performed characterizing variability due to mixing that show less of an effect than varying the lower boundary mixing ratio value. However, calculations using our nominal eddy diffusion profile multiplied and divided by a factor of four can give an order of magnitude maximum difference in the SO_2 mixing ratio and a factor of a few difference in the H_2O mixing ratio when compared with the respective nominal mixing ratio for these two species. In addition to explaining some of the observed variability in SO_2 and H_2O on Venus, our work also sheds light on the observations of dark and bright contrasts at the Venus cloud tops observed in an ultraviolet spectrum. Our calculations produce results in agreement with the SOIR Venus Express results of 1 ppm at 70–90 km (Bertaux et al., 2007) by using an SO_2 mixing ratio of 25 ppm SO_2 and 18 ppm water as our nominal reference values. Timescales for a chemical bifurcation causing a collapse of water concentrations above the cloud tops (>64 km) are relatively short and on the order of a less than a few months, decreasing with altitude to less than a few days.

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