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Sulfur Dioxide Variability in the Venus Atmosphere |
Project description
Scientific rationale
The H2SO4 clouds are formed via the sulfur-oxidation cycle, which begins with SO2 photolysis producing SO, O, O2 and S, followed by the oxidation of SO2 by O forming SO3. SO3 then reacts with H2O forming H2SO4. Additionally, the distribution and abundance of the sulfur-oxide species provides an indirect measure of the structure and the dynamics of Venus’ atmosphere. The sulfur-oxidation cycle is therefore one of the most important chemical cycles to study in detail. To tackle the many aspects of Venus’ atmosphere that are impacted by and related to the sulfur-oxidation cycle, we plan to build an international science team with expertise in observational astronomy, photochemical modeling, dynamics and circulation modeling, and modeling of the radiative balance within dense atmospheres. The team’s collective expertise will be used to understand the past and current variability of SO2 and related gases (OCS, SO, H2S, CO, etc).
Photochemistry of sulfur-bearing species: modeling activitiesPhotochemical modeling is the primary tool for defining the chemical processes that support the spatial distribution and abundance of the sulfur-oxide species, molecular oxygen (O2) and H2SO4 observed in Venus’ atmosphere. However, recently observed trends in Venus’ sulfur oxide abundance levels have only been “successfully” simulated in the photochemical models when the physical properties of the key gas species are manipulated outside of their normal limits. For example, recent attempts to model the observed layer of enhanced gas-phase SO2 in the upper mesosphere [7, 8] require super-saturation ratios much larger than are found on the Earth, H2SO4 abundances larger than the observed Venus upper limit, and/or photolysis rates significantly faster than substantiated by laboratory data [8-11]. These issues combine with the long-standing unsolved challenge of reliably replicating Venus’ O2 destruction and H2SO4 production rates [12-16]. Considered altogether, these issues suggest there may be major gaps in our understanding of Venus’ sulfur chemistry. Identifying the chemical and/or physical processes that can reliably reconcile the known discrepancies between models and observations is a key problem that must be resolved before the evolution of Venus’ atmosphere can be accurately reconstructed. Consequently, detailed study of the sulfur oxidation cycle both observationally and theoretically is both timely and needed. Our proposed International Team will facilitate these studies.
Observations from Venus Express.
Observation using the Hubble Space Telescope
Observations from the EarthGround-based submm spectroscopic observations (JCMT) provide simultaneous measurements of SO2 and SO [8, 21] mixing ratios, as well as upper limits for H2SO4 [10] in the Venus mesosphere (70-100 km), with altitude resolution of 5-30 km, depending upon signal strength and altitude. JCMT altitude sensitivity does not overlap with cloud top observations of HST [20], IRTF/TEXES [22], or SPICAV [5]. JCMT data show that the SO/SO2 ratio is different on the day vs. night sides, but cannot characterize the twilight atmosphere independently. Diurnal coverage of JCMT does not overlap that of SOIR and SPICAV data [7]. Measurements from JCMT submm spectra are thus unique in their altitude, local time sensitivity, providing knowledge of Venus SO2 behavior that is not available from any other source. SO2 has been mapped using the IRTF/TEXES high-resolution spectrometer [22] showing strong variations over the disk of Venus by factors of 5 to 10. The position of the SO2 maximum was shown to vary strongly in time with maximum mixing ratio varying between 75 ± 25 ppb to 125 ± 50 ppb on days scale for 60-80 km altitudes.
Multi-disciplinary approach: Concomitant observationsThe proposed International Team activity is timed to take advantage of a planned scientific campaign focusing on measurements of SO2 and related sulfur cycle compounds, which is scheduled to take place in September-October 2013. During this campaign, already included in the Venus Express Science Activity Plan for 2013, 100% of orbits will be dedicated to relevant measurements including solar occultation (SOIR) measurements, to obtain vertical profiles of SO2 at the terminator as well as total density and temperature, and nadir dayside measurements (SPICAV), which measure geographic variation of total column SO2 (UV channel) and of water and cloud-top altitude (IR channel). Complementary Earth-based observations are being scheduled. These include high resolution thermal IR measurements from IRTF/TEXES [22] and microwave observations from JCMT [8, 21]. High-resolution UV observations from Hubble Space telescope [20] which allow simultaneous measurements of SO and SO2 have been requested. The proposed schedule of the workshop is ideally placed to allow the joint analysis and formal intercomparison of these and past/subsequent observations, and interpretation of the results using photochemical and dynamical models.
Research planWithin the project, the proposed ISSI team will:
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