Upper limits for H2SO4 in the mesosphere of Venus |
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| Authors: | Brad J Sandor R Todd Clancy Gerald Moriarty-Schieven |
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| Institution: | 1. Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301, USA;2. National Research Council of Canada, Joint Astronomy Centre, 660 North Aohoku Place, Hilo, HI 96720, USA;1. Cornell University, Dept of Astronomy, 318 Space Sciences Building, Ithaca, NY, 14853, USA;2. Observatoire de Paris, LESIA, 5 Place Jules Janssen, 92195 Meudon Cedex, France;3. University of Wisconsin-Madison, Space Science and Engineering Center, 1225 West Dayton Street, Madison, WI 53706, USA;4. Jet Propulsion Laboratory, M/S 183-501, 4800 Oak Grove Drive, Pasadena, CA 91109 USA |
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| Abstract: | Rapid temporal variability of SO2 and SO in the Venus 85–100 km mesosphere (Sandor, B.J., Clancy, R.T., Moriarty-Schieven G.H. 2007]. Bull. Am. Astron. Soc. 39, 503; Sandor, B.J., Clancy, R.T., Moriarty-Schieven, G.H., Mills, F.P. 2010]. Icarus 208, 49–60) requires in situ sources and sinks for these molecules. While many loss mechanisms are recognized, no process for in situ production is known. Observational investigations to find, or constrain other potential sulfur reservoirs offer one method toward understanding the applicable photochemistry. Here, we report upper limits for gas-phase H2SO4 (sulfuric acid) abundances in Venus’ 85–100 km upper mesosphere, derived from 16 ground-based sub-mm spectroscopic observations in the period 2004–2008. Unlike the ubiquitous sulfuric acid solid/liquid aerosol, the gas phase would be photochemically active, potentially both source and sink for SO and SO2. H2SO4 is retrieved from sub-mm lines located in the same bandpass as the SO2 and SO lines described by Sandor et al. (Sandor, B.J., Clancy, R.T., Moriarty-Schieven, G.H., Mills, F.P. 2010]. Icarus 208, 49–60). H2SO4 upper limits reported here are thus simultaneous and spatially coincident with measurements of SO2 and SO, providing for analysis of the three sulfur species collectively. The average H2SO4 abundance over 16 observations is 1 ± 2 ppb (i.e. <3 ppb). Upper limits for individual observations range from 3 to 44 ppb, where quality of the observing weather is the dominant constraint on measurement precision. The sum of H2SO4, SO2 and SO varies widely. In one comparison, the sum H2SO4 + SO2 + SO] measured on one date differs by 10-σ from the sum measured 2 months later. We conclude that upper mesospheric sulfur atoms are not conserved among the three molecules, that H2SO4 is not a significant sulfur reservoir for balancing the observed variations of SO2 + SO], and is not relevant to the (still unknown) photochemistry responsible for observed behavior of SO2 and SO. Having ruled out H2SO4, we infer that elemental sulfur is the most probable candidate for the needed third reservoir. |
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