Sulphur dioxide and ozone in the atmosphere of Venus
Overview of sulphur dioxide research
Sulphur compounds play a key role in the chemistry of the atmosphere of Venus. They are formed as a result of volcanic activity on the planet and participate in the formation of clouds consisting of concentrated H2SO4 acid droplets. Moreover, the unknown UV absorber in the upper part of the cloud is also associated with sulphur compounds, inexplicably reducing the albedo of Venus in the UV range (Frandsen et al., 2016; Krasnopolskiy, 2018). Sulphur dioxide is the third most abundant gas making up 0.015% of Venus’s atmosphere. Thus, any major changes in SO2 content occurring in the upper atmosphere can be indicators of photochemical processes and dynamics on Venus. The sulphur dioxide is destroyed by chlorine and hydroxyl radicals as well as by an effective photodissociation on the day side. The catalytic processes are noticeable on the night side, where fluxes of atoms and molecules are brought by the global SSAS circulation at altitudes of about ~100 km.
Temporal and spatial distributions of sulphur dioxide
The entire history of observations of sulphur dioxide on Venus showed significant variations in its amount at the cloud top (~70 km) presented in Figure 2.1. SO2 gas was first detected in the cloud tops by a ground-based ultraviolet telescope with a content of 0.02-0.5 ppm (Barker, 1979). The SO2 abundance was mainly measured at the altitude corresponding to the cloud top level on the day side, that is about 70 km. Several instruments aboard the first spacecraft to Venus showed a global decrease of the SO2 volume mixing ratio of (VMR) in the upper atmosphere in the 80s and 90s (Figure 2.1). During this period some local peaks of the gas abundance were obtained. This result mainly came from ten years of monitoring by the ultraviolet spectrometer (UVS) on board the Pioneer Venus Orbiter (PVO). The reduction of the amount was steep at the beginning, and then it was followed by a gradual decline in the SO2 mixing ratio. It was confirmed by several other instruments working in the UV. The International Ultraviolet Explorer (IUE) observed Venus in 1979 (Conway et al., 1979) and 1987-1988 (Na et al., 1990; Na et al., 1993), when the corresponding mixing ratios changed from 380±70 ppbv to 50±20 ppbv. Lower abundance of sulphur dioxide was also obtained based on observations by the Venera-15 Fourier spectrometer in the IR range (Moroz et al., 1990; Zasova et al., 1993). Its data originally corresponded to 60-62 km but the value was scaled to an altitude of 70 km in order to compare with the UV observations (Zasova et al., 1993). Rocket observations in 1988 and 1991 measured SO2 equal to 80±40 ppbv and 120±60 ppb respectively (McClintock et al., 1994). The further observations by IUE and ground based facilities (Barker et al., 1992; Esposito et al., 1997) showed the local maximum of the SO2 content in 1991-1992. In 1995, the Hubble Space Telescope (HST) completed the first era of continuous SO2 exploration on Venus and gave a value of 20±10 ppbv (Na et al. 1995), which was 2-5 times less than previous measurements.
Figure 2.1. The timeline of sulphur dioxide observations at the cloud tops on the day side of Venus. The references are given in the legend. The SPICAV UV observations are presented with the corresponding variability of obtained values within 4-month intervals. The full range of obtained values is represented by the grey area which is accompanied by a median curve (grey solid line).
This behaviour was puzzling. The sulphur compounds are rather associated with geological activity. So, the main hypothesis to explain the observed SO2 change has considered a link with possible episodic volcanic eruptions on Venus. It was assumed that one event would inject sulphur compounds into the atmosphere, and the vertical transfer would distribute them over clouds. Then SO2 depleted and a monotonic trend of its VMR was observed (Esposito, 1984).
For spatial variations sulphur dioxide was studied in the IR range (7 µm, 8 µm and 19 µm) using a Fourier spectrometer on board the Soviet spacecraft Venera-15. The study of latitudinal distribution revealed an increase in gas content from the equatorial zone and middle latitudes to the poles of the planet. At a pressure level of 150 mbar (corresponding to ~ 62 km), the amount of SO2 at low latitudes was 0.3-0.5 ppm, and 1-2 ppm in the polar regions. The same picture was observed at the level of the cloud top (40 mbar, ~ 69 km).
SO2 monitoring on Venus was resumed in 2006, when the Venus Express spacecraft began its orbital mission (Titov et al., 2006) with a set of three SPICAV/SOIR spectrometers on board. UV and IR instruments covered SO2 absorption bands at 190-300 nm (for SPICAV UV) and about 4 μm (for SOIR). In this period there were several ground-based observations by instruments in various spectral intervals: STIS/HST (Space Telescope Imaging Spectrograph/Hubble Space Telescope), IRTF/TEXES (Infrared Telescope Facility/Texas Echelon-Cross-Echelle Spectrograph), JCMT (James Clerk Maxwell Telescope), ALMA (Atacama Large Millimeter/Submillimeter Array) and the CSHELL (Cryogenic Echelle) spectrograph. STIS measures in the same spectral range as SPICAV UV while CSHELL operates at 4.04 μm. These instruments observed the cloud top level. SPICAV UV in nadir mode and STIS were able to perform measurements only on the day side of Venus. Solar occultations by SOIR and SPICAV UV had limited local time and latitude coverage corresponding to the Venus terminator mainly in the northern polar area. Stellar occultations by SPICAV UV were designated for the night-time observations. Other listed instruments probe both the day and night sides. TEXES measures the thermal IR radiation from Venus at 7 μm and 19 μm. The wavelength of 7 μm corresponds to emission forming at 60-80 km while the signal at 19 μm originates from altitudes several kilometres lower. JCMT and ALMA performed microwave observations of the SO2 abundance in the upper mesosphere.
SPICAV/SOIR observed Venus in nadir mode and (for the first time) in solar and stellar occultation modes. In the nadir mode the UV spectrometer monitored the atmospheric composition at cloud tops. The occultation modes allowed to obtain the detailed vertical profiles of aerosols and several atmospheric gases including SO2 in an altitude range from 65 to 140 km depending on spectral range and an absorbing particle. Nadir UV observations by SPICAV supplemented the long-term trend of SO2 VMR (Marcq et al., 2013; Marcq et al., 2020). These long-term variations were not clearly confirmed by solar and stellar occultations over 70 km (Belyaev, Evdokimova et al., 2017). However, fluctuations in the observed values remained significant; the annual dependence of VMR showed individual sporadic maxima.
The continuous monitoring in the present era of Venus observations showed a high variability of SO2 abundance in short term. Observations made by SPICAV UV in nadir and simultaneous full-disk observations from the Earth clarified some patterns of SO2 variability. The SO2 variations are more significant at low latitudes in comparison with polar regions (Encrenaz et al., 2019). The same conclusion is done by SPICAV UV monitoring (Marcq et al., 2020). According to those data, the latitudinal behaviour depends on an average SO2 VMR. A decrease towards poles was observed in periods of large SO2 abundance. But the latitude trend was inverse in SO2 minima (Marcq et al., 2013).
At the cloud tops the sulphur dioxide mixing ratio increases on average from midday towards the morning and evening terminators (Marcq et al., 2020). The TEXES particularly observes «SO2 plumes» which are areas on the Venusian disk with high SO2 VMR values. These plumes had a lifetime shorter than 24 hours. It also signifies that SO2 geographical pattern cannot remain in long time periods. The probability of such plumes corresponds well to the SPICAV SO2 local time distribution, and the maximum probability is in 0:00-6:00 time interval (Encrenaz et al. 2019).
The stellar occultations measured the night-time abundance of sulphur dioxide being 3-4 times higher than it was observed at the terminator at the altitude of ~ 95 km (Belyaev, Evdokimova et al., 2017). The increase of SO2 abundance at night also presented in the sub-millimeter (sub-mm) observations by JCMT at 85-100 km (Sandor et al., 2010). Individual sessions of measurements by ALMA showed more puzzling behaviour. The maximum in the morning near equator and the SO2 absence at night were mainly associated with short-term variability and a low signal to noise ratio (Encrenaz et al., 2015).
Vertical profile of SO2 obtained by SPICAV/SOIR instrument on board Venus Express and ground based facilities in the mesosphere
Since the first observations at the cloud tops, SO2 variations were significant in space and time. The present knowledge about vertical distribution of sulphur dioxide is combined by various experiments for the entire period of Venus exploration (Figure 2.2). Altitudes from the surface to the mesosphere were probed by different instruments mounted on landers and orbiters as well as on ground-based telescopes. The most detailed profiles below 65 km are results from in situ observations performed by ISAV ultraviolet spectrometers on board two Vega descent probes (Bertaux et al., 1996). Only these measurements provided SO2 altitude distribution with a good vertical resolution.
Before ISAV experiments the SO2 content of 185±43 ppbv (Oyama et al., 1979, 1980) was measured only by a gas chromotograph on the PV large probe indeep atmosphere. However, the ISAV-1 and -2 showed 5 times lower values (Bertaux et al., 1996). Below 40 km, the SO2 VMR profile decrease steadily with decreasing altitude down to the surface meaning that the SO2 is transformed to another sulphur-bearing molecules. That behaviour is complicated to interpret from the theoretical point of view to the thermal chemistry in the deep atmosphere (Esposito et al., 1997).
The ISAV measurements were confirmed by other observations that sounded higher altitude layers. The Fourier spectrometers on board Venera 15&16 were sensitive to a range of 60-62 km and observed VMR about 2000 ppbv (Moroz et al., 1990). In-situ Venera 11&12 observations by gas chromatography provided values about 100 ppm at the lower cloud border that coincided with Vega’s profiles (Gel’man et al., 1979). The IR thermal emission at 2.3 μm is a way to a remote sensing of the atmosphere at 30-45 km. Such observations were done by ground based instruments (Bézard et al., 1993; Pollack et al., 1993; Arney et al., 2014) and by VIRTIS-M (Marcq et al., 2008). The most recent IR data showed 156±42 ppm in the Northern Hemisphere and 135±36 ppm in the southern hemisphere. In general, no significant variations of the SO2 content below the clouds were revealed. It is in correspondence with expecting stability of the lower atmosphere.
The gas abundance was studied over the clouds in much more detail by the SPICAV/SOIR instrument during the period of the Venus Express mission. SO2 absorption was measured in the first UV and IR observations of solar and stellar occultations (Figure 2.2). The altitude distribution retrieved from those occultations demonstrates an inversion layer of the SO2 content around 85-100 km. On average, an increase of the SO2 VMR with altitude from 8-20 ppbv at 85-90 km to 50-180 ppbv at 100 km was observed (Belyaev et al., 2012; Belyaev, Evdokimova et al., 2017). This distribution was supplemented by the SOIR data with a decrease of the SO2 abundance from 100-300 pbbv at 65 km to 50-100 ppbv at 80 km possessing large uncertainty values (Mahieux et al., 2015). The previous results of the stellar occultation data revealed the SO2 VMR rising with altitude from 10-30 ppbv at 85-95 km to 100-300 ppbv at 100-105 km (Belyaev, Evdokimova et al., 2017). However, this increase is re-investigated further in this work in the framework of the detailed calibration study.
All in all, one can see significant short-term changes in the amount of sulphur dioxide in the mesosphere that are interpreted by different models. Discussions of the models that could be compared with our occultation data are presented in Section 2.3 of this Chapter and in Chapter 4. The comprehensive overview of SO2 content variations with time, altitude and latitude retrieved from experiments and analysed by models is presented in papers of Vandaele et al. (2017a, 2017b).
Ozone in the atmosphere of Venus.
Ozone on Venus excites a great interest for researchers due to its active participation in basic atmospheric chemical processes, as well as its bio-protective function in the terrestrial atmosphere. On the Earth the large amount of the UV radiation is absorbed by the abundant O3 layer in the stratosphere where the vertical temperature profile is inverted. Here the Hartley band of ozone absorption at 240-300 nm is saturated, and the atmospheric transmission in this range is close to zero. On Venus, where the total O3 content is expected to be >103 times less than on the Earth, the Hartley band is applicable for the spectral investigation. Before this first detection, the ozone presence was estimated by different photochemical models (Yung and DeMore, 1999; Krasnopolsky, 2010a; Mills, 2007). However, so far, except for the molecular O2 airglows another oxygen species was not quantified experimentally on Venus. It constrained estimation of the ozone concentration in the atmosphere. The upper limit for the oxygen abundance was estimated to <2 ppmv that signified extremely low amounts of the Venusian ozone (Mills, 1998).
The discovery of ozone on Venus concluded a presence of this gas on three terrestrial planets: Earth, Mars and Venus. Due to the Earth, with the O3 protection of the biosphere, and Mars, with an evidence of oxygen-rich airmasses, ozone is considered in a search of possible life. It is known that such a protection from the strong UV radiation requires a significant abundance of O3. The observed values in the Earth atmosphere are around 300 DU (Dobson Units), where 1 DU corresponds to a column concentration of 2.69×1016 molecules per cm2. Mars and Venus have a tiny amount of ozone compared to the terrestrial quantity. The Martian ozone compiles <1 DU (Lefèvre et al., 2004; Lebonnois et al., 2006; Montmessin and Lefèvre, 2013; Määttänen et al., 2013), while the Venusian one is observed to be less than 0.5 DU (Montmessin et al., 2011; Marcq et al., 2019).
Discovery of ozone in the atmosphere
Ozone on Venus was discovered by Montmessin et al. (2011) in SPICAV UV stellar occultation spectra. It was confidently detected in 29 night-side sessions in the beginning of the mission at altitudes ~100 km with number densities of 107-108 cm-3 (Figure 2.3). Those concentrations appeared as an episodic peak of ozone content that demonstrated some sporadic presence of the gas in the mesosphere. After the discovery the occultation processing of the whole dataset, which counts >400 sessions, was necessary in order to characterize the O3 mesospheric distribution in detail. Venusian ozone is characterized by a vertically confined and horizontally variable layer residing at a mean altitude of 100 km.
Table of contents :
CHAPTER 1. Venus and its atmosphere
1.1. The Earth’s evil twin
1.2. A new view of Venus
1.3. History of observations
1.3.1. Venus space missions
1.3.2. Venus Express
1.3.3. Future missions
1.4. The surface of Venus
1.5. The atmosphere of Venus
1.5.2. Structure of the atmosphere
1.5.3. The cloud layer
1.5.4. Transparency windows
1.5.5. Mesosphere and atmospheric dynamics
1.6. The SPICAV instrument
1.6.1. Ultraviolet channel of the SPICAV spectrometer
1.6.2. Infrared channel of the SPICAV spectrometer
CHAPTER 2. Sulphur dioxide and ozone in the atmosphere of Venus
2.1. Overview of sulphur dioxide research
2.1.1. Temporal and spatial distributions of sulphur dioxide
2.1.2. Vertical profile of SO2 obtained by SPICAV/SOIR instrument on board Venus Express and ground based facilities in the
2.2. Ozone in the atmosphere of Venus.
2.2.1. Discovery of ozone in the atmosphere
2.2.2. Ozone at the top of the clouds
2.3. Photochemistry: a review of atmospheric models for SO2 and O3
CHAPTER 3. Data processing of stellar occultation spectra
3.1. The stellar occultation technique: retrieving the atmospheric composition from transmittance spectra
3.2. Calibrations and stray light correction in the raw data.
3.2.1. Studied trace gases
3.2.2. UV Signal considerations with SPICAV
3.2.3. Estimation of errors in atmospheric transmission spectra
3.2.4. Sources of an additional emission registered by the UV channel of SPICAV
220.127.116.11. Lyman-α emission
18.104.22.168. Airglow of nitric oxide
22.214.171.124. Solar radiance in the stellar occultation spectra
3.2.5. Wavelength- to-pixel registration
3.2.6. Spectral inversion
126.96.36.199. Cases of positive gas detection
188.8.131.52. Upper detection limits for two gases.
184.108.40.206. Chlorine oxide absorption band
3.2.7. Vertical inversion problem
3.2.8. Calibration influence on the spectral inversion
3.2.9. Stray light elimination technique
220.127.116.11. Method #1
18.104.22.168. Method #2
22.214.171.124. Comparison of methods
126.96.36.199. Atmospheric transmission and error bars estimation.
3.2.10. Altitude assignment
CHAPTER 4. Sulphur dioxide
4.1. CO2 and SO2 retrievals: from column abundances to profile and its variability.
4.1.1. Carbon dioxide distribution in the upper mesosphere and the lower thermosphere.
4.1.2. SO2 in the upper mesosphere
4.2. SO2 profiles: Comparison with data from previous studies.
4.3. Variations of SO2 mixing ratio
4.3.1. Short term variations
4.3.2. Long term variations of SO2 mixing ratio.
4.3.3. Diurnal variations of SO2
188.8.131.52. Local time and latitude distribution
184.108.40.206. Variations with a solar zenith angle
220.127.116.11. Establishing independence from topography
4.4.1 Rapid changes in the SO2 content
4.4.2. Global patterns in the SO2 behaviour
CHAPTER 5. Ozone
5.1. Ozone retrievals
5.1.1. The main feature of the ozone positive detections
5.2. Ozone positive detections distribution
5.2.1. Average volume mixing ratio profile of ozone for established positive detections.
5.2.2. Spatial variations of ozone positive detections
5.2.3. Temporal variations of ozone based on positive detections
5.3. Detection limits of ozone
5.4. Review of possible correlations with other chemical compounds
5.5. Comparative analysis of ozone layers in Earth, Mars and Venus atmospheres.
5.5.1. Ozone on the Earth
5.5.2. Ozone on Mars and Venus
CHAPTER 6. O2 (α1Δg) emission in the upper mesosphere
6.1. The infrared emissions in the night atmosphere
6.2. SPICAV observations of lower atmosphere thermal emission
6.3. Modelling of the night thermal emission
6.3.1. Direct model
6.3.2. Inverse problem
6.4. Mapping water vapour and aerosols and uncertainties
6.5. Map of oxygen airglow in the night mesosphere