Sulfur Dioxide Variability in the Venus Atmosphere




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Project description


Scientific rationale

Sulfur dioxide was first detected in the Venusian upper atmosphere from Earth-based ultraviolet observations with a mixing ratio 0.02–0.5 ppm at the cloud top [3]. Space-based identifications of SO2 UV absorption followed soon from Pioneer Venus orbiter. These observations as well as those of Venera-15 showed a steady decline of the cloud top SO2 content from 500 ppb down to 20–50 ppb about 2 years later [4]. The decline of the SO2 amount was rather fast over the first year, and much slower later on (Fig. 1). This behavior was interpreted as a massive injection of SO2 into the Venus middle atmosphere by a volcanic explosion” [4]. Current observations performed by SPICAV-UV show that the high levels of SO2 are again observed [2, 5].

Fig. 1: More than thirty years of SO2 measurements at Venus’s cloud top. Black: previous measurements [1]. Red: 8-month moving average of SPICAV data. Error bars are 1 sigma random uncertainty, and dotted red error bars represent measurement dispersion in each temporal bin. Figure from [2].

Detailed thermochemical modeling indicates that (i) volcanic outgassing is the most probable primary source of sulfur dioxide in Venus’ lower atmosphere, even if no volcanic activity has been directly observed so far, and (ii) OCS and H2S are likely formed via pyrrhotite oxidation [6]. The exchange of SO2 from the lower to the upper atmosphere is not fully understood, but undoubtedly involves convective transport, presumably in conjunction with Hadley cell circulation. Transport to the upper atmosphere may also result from direct volcanic ejection, though this possibility is controversial. Photochemical oxidation efficiently removes SO2 from Venus’ upper atmosphere leading to the formation of the sulfuric acid droplets making up the clouds and haze enshrouding the planet, and as result the SO2 abundance drops to ppb levels; SO2 located below the cloud layer is effectively sequestered from photochemical reactions.

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 activities

Photochemical 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.

Fig. 2: SO2 and SO from SPICAV-UV (above 85 km) and SOIR (below 80 km) [7].

SO2 has been observed by VIRTIS [17], SPICAV-UV [5] and SOIR [18] instruments on board Venus Express. SOIR observations provide vertical information for SO2 above the cloud deck [7]. However SO2 absorption is weak in the IR and in some cases, this would result in the determination of detection limits rather than true positive abundance measurements. A critical analysis and definition of objective criteria for true detection is being implemented. It will certainly gain by being discussed and compared with other technical and observational techniques. SOIR is sensitive to CO2 (and indirectly to temperature) and to many trace gases, which are of interest for the current study [19]. SPICAV-UV is able to detect SO2 either in nadir mode [2, 5] or in solar occultation [7]. Cross-sections in the UV are much stronger than in the IR. SPICAV-UV occultations indicate the presence of an enhanced layer of SO2 at 85-100 km height. Detailed discussions are required to explain this layer which is not predicted by models. Weak signatures of SO were also detected confirming the interconnection between SO and SO2. In nadir mode, only SO2 column densities above and within cloud are obtained. Yet the extended horizontal coverage and relative accuracy make these nadir observations highly valuable. Latitudinal and temporal (from day to several years) variability was observed, hinting to long-term changes in Venus atmosphere. VIRTIS was able to measure SO2 on the night side, but the relative accuracy was quite poor due to the narrow spectral signature of SO2. VIRTIS was only able to confirm the average value for SO2 mixing ratio near 30 km (150 ppmv).

Observation using the Hubble Space Telescope

Analysis of HST/STIS observations (See Fig. 2) obtained on Dec 28, 2010, Jan 22, 2011 and Jan 27, 2011 [20] indicates that the cloud top SO2 density was highest in Dec 2010 and decreased by a factor of ~10 one month later, but stayed within a factor of 2 between the two January observations. The range of SO2:CO2 gas mixing ratio inferred from the HST data is ~ 10-350 ppb consistent with abundances obtained from SPICAV and SOIR [2, 7]. The HST/STIS observations showed a general trend of the SO2 gas density decreasing as the latitude increased. Analysis also indicated that the SO2 density decreased with local time from the morning terminator towards noon, as expected for a photochemically controlled species. Variation in SO with latitude paralleled that of SO2, indicating that the latitudinal distribution of SO was NOT solely controlled by SO2 photolysis.

A new HST observing program has been submitted for the time period extending from Oct 20-Oct 29, 2013. We plan to obtain both HST/STIS spectral data and HST/WFC3 images of Venus’s pole-to-pole dayside atmosphere. The goal of the new program is to observe Venus’ cloud properties and their precursors in a systematic way on a day to day basis, over 2 full cycles of Venus’ H2SO4 cloud rotation period within a 10 day period; tracking the gas and cloud properties at the cloud top level, while documenting the lower (50-60 km) cloud structure.

Fig. 3: Left: 0.1” HST/STIS slit footprint for Dec 28, 2010 observation (paths 0&1) and Jan 22, 2011 and Jan 27, 2011 paths (2&3). Binning spatially along the slit every 6 pixels, a total of 56 individual spectra were obtained per day providing continuous limb-to terminator data on Venus at ~ 150 km resolution. A sample of the spectra obtained on Dec 28, 2010 is plotted on the right, showing spectra obtained at four different SZA values ranging from 40-80°, covering latitudes 25N to 6 S. The SO and SO2 gas absorption bands are denoted by blue and red lines, respectively.

Observations from the Earth

Ground-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 observations

The 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 plan

Within the project, the proposed ISSI team will:
  • Summarize the different observation techniques used to measure abundances of SO2 and related species (SO, OCS, H2S, CO etc); Understand their limitations (accuracy, altitude ranges probed etc);
  • Compare the results (abundances, vertical profiles, latitudinal and time variations) from these different techniques; Reconcile past and present observations; Make use of the observation campaign to maximize the scientific outcomes;
  • Provide a general picture / model for the variability of SO2 (and related species);
  • Understand which photochemical and dynamical processes are at play to constrain the SO2 variability and distribution scheme; Put constraints on the existence of volcanism.
The proposed International Team will synthesize past and current observations of SO2 and related species from multiple platforms into a coherent observational picture. Photochemical and dynamical models will be used to interpret the observed distributions and variability (geographic, vertical, and temporal) based on known and speculative photochemical schemes. The expected outcomes of this project will be
  • a cross-validation of measurement techniques;
  • analysis of the sulfur gases evolution through time (link to past missions)
  • building of a model atmosphere relative to the sulfur family
  • consolidation of the sulfur photochemical scheme above and below the clouds.
Our results will provide a more accurate and quantitative description of the sulfur oxidation cycle, improved understanding of the photochemical and dynamical factors causing the observed short- and long-term variability of sulfur oxides, and better quantification of the links between the sulfur oxidation cycle and cloud formation.



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