Photochemical Control of the Distribution of Venusian Water and Comparison to Venus Express SOIR Observations

Christopher D. Parkinson1, Yuk L. Yung2, Larry Esposito3, Peter Gao2, Stephen W. Bougher1,

Mathieu Hirtzig4

1 Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan, MI, USA2 Division of Geological and Planetary Sciences, California Institute of Technology, CA, USA3 Laboratory for Atmospheric and Space Physics, University of Colorado, CO, USA4 Foundarion “La main à la pâte”, Montrouge, France

DPS 2014, Tucson, ArizonaNovember 12, 2014

Important reaction pathways connecting SO, SO2, SO3 and H2SO4

H2SO4, Aerosols, Clouds

PhotochemistryUse Caltech/JPL KINETICS model to solve continuity equation:

To obtain mixing ratio for key species usingStandard reference lower boundary values,Maximum SO2, minimum H2O values, andMinimum SO2, maximum H2O values

Lower Boundarymixing ratio

sensitivity study

• Water profiles with fixed SO2 lower boundary of 25 ppm: H2O varies between 10-35 ppm

• SO2 profiles with fixed H2O lower boundary of 18 ppm: SO2 varies between 5-100 ppm

Water and SO2 Sensitivity Study

SO2 sensitivity study: Mixing ratio of H2O at 80 km

Water sensitivity study: Mixing ratio of SO2 at 80 km

• Last slide does not explain the whole story, as only the black curves are correlated between the two figures (viz., 18 ppm H2O and 25 ppm SO2 for our std ref case).

• Figures are “anti-symmetric” and that there is a dramatic chemical bifurcation in the middle of each plot. In regions of high SO2 abundance, we have a corresponding low H2O abundance, and vice versa.

Temperature and Eddy Diffusion Profiles (from Zhang et al, 2012)

• VEx VeRa daytime temperature profile below 100 km from observations of near the polar region (71° N)

• Above 100 km the temperature is from Seiff (1983)

• Kzz = Ko (n(z)/nref)-a

Eddy DiffusionSensitivity Study

• Complicated relationship, so need to consider SO2 and H2O fluxes in order to interpret sensitivity to eddy diffusion

Conclusion (1)• H2O is modeled between 10 – 35 ppm at our 58 km

lower boundary using an SO2 mixing ratio of 25 ppm as our nominal reference value.

• SO2 mixing ratio is then varied at lower boundary between 5 and 100 ppm holding the H2O mixing ratio of 18 ppm at the lower boundary

• SO2 can control the water distribution at higher altitudes.

• SO2 and H2O can regulate each other via formation of H2SO4.

Conclusion (2)

• In regions of high mixing ratios of SO2 there exists a “runaway effect” such that SO2 gets oxidized to SO3, which quickly soaks up H2O causing a major depletion of water between 70 and 100 km. (i.e. a complete sequestration of H2O by H2SO4 aerosols).

• In addition to explaining some of the observed variability in SO2 and H2O on Venus, our work may also shed light on the observations of dark and bright contrasts at the Venus cloud tops observed in the ultraviolet spectrum.

Extra slides past this point…

• Not for talk…for possible questions asked.

H2SO4 Saturation Vapor Pressure

Stull (1947) Richardson et al. (1986) Ayers et al. (1980)

SO, SO3 & H2SO4 Profiles

Reactions Rates related to SO2

a.SO3 → SO2 + O ClO + SO → Cl + SO2 O + SO + M → SO2 + M ClCO3 + SO → Cl+ SO2 + CO2

SO + SO3 → 2SO2

b.SO2 → S + O2 SO2 → SO + O O + SO2 + M → SO3 + MClCO3 + SO2 → Cl + SO3 + CO2

Reactions Rates related to SO

a.SO2 → SO + O S + O2 → SO + O

b.SO → S + OClO + SO → Cl + SO2 O + SO + M → SO2 + M ClCO3 + SO → Cl+ SO2 + CO2

SO + SO + M → (SO)2 + M SO + SO3 → 2SO2

Reactions Rates related to SO3

a. H2SO4 → SO3 + H2O O + SO2 + M → SO3 + M ClCO3 + SO2 → Cl + SO3 + CO2

b. SO3 → SO2 + OSO3 + H2O → H2SO4

O + SO3 → SO2 + O2 SO + SO3 → 2SO2