Average volume mixing ratio profile of ozone for established positive detections.

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The SPICAV instrument

The SPICAV/SOIR instrument was a set of three spectrometers (Bertaux et al., 2007a; Vandaele et al., 2008; Korablev et al., 2012) that was designed by Laboratoire Atmosphères, Milieux, Observations spatiales (LATMOS, France) in collaboration with the Space Research Institute of the Russian Academy of Sciences (IKI, Russia), and the Royal Belgian Institute for Space Aeronomy (BIRA-IASB, Belgium). All of them operated around Venus during the 8 years of the Venus Express mission, from April 2006 to December 2014, however, the full scientific phase of observations began on May 14, 2006.
The SPICAV part consisted of the UV (118-320 nm) and the IR (0.65-1.7 μm) channels that could observe the atmosphere in different modes (Bertaux et al., 2007a). For the first time, three instruments performed solar occultations of the upper atmosphere of Venus, and the SOIR spectrometer was particularly dedicated to this mode of observations. This technique of observations enabled one to study in detail the vertical distribution of CO2, SO2, H2O and other trace gases and the cloud haze on the Venus terminator (Fedorova et al., 2008; Mahieux et al., 2015; Chamberlain et al., 2020; Luginin et al., 2018).
The technique of stellar occultation was also accomplished for the first time around Venus. It provided information about the vertical distribution of CO2, SO2 and O3 gases in the night mesosphere. SPICAV UV also measured the night airglow of nitrogen monoxide (NO) and the Lyman-alpha emission at 121.6 nm. The Ly-α emission monitoring gives access to the distribution of atmospheric neutral hydrogen in the exosphere. The distribution of CO2, SO2, and O3 gases in the night-time mesosphere was studied using the stellar occultation regime, also for the first time realized from the orbit of Venus during the mission (Bertaux et al., 2007a).
Daytime nadir observations in the IR and UV ranges were used to study gases at the upper boundary of the cloud layer. Absorption of SO2, O3 were able to be detected in UV (Marcq et al., 2013; Marcq et al., 2019). The IR range was used to track water vapour abundance and to study the altimetry of clouds (Fedorova et al., 2016). The SPICAV IR measured the polarization of reflected solar radiation to study the properties of cloud aerosol particles (Rossi et al., 2015). At the night side the nadir and limb observations allowed to study IR and UV emissions originated from different atmospheric sources. The UV channel observed the night-time atmospheric emission of nitrogen monoxide (NO) (Stiepen et al., 2013; Royer et al., 2016). The scattered Lyman-alpha emission was used to restore the distribution of atmospheric neutral hydrogen in the exosphere (Chaufray et al., 2015). SPICAV IR tracked the oxygen emission at 1.27 μm originating at ~96 km. SPICAV IR was the only channel of the SPICAV/SOIR set that was able to sound below the clouds. Thermal emission spectra of the hot lower atmosphere and the surface of Venus were measured in nadir around transparency windows of 1.0, 1.1, 1.18, 1.28, and 1.31 μm (Bézard et al., 2011; Fedorova et al., 2015).

Ultraviolet channel of the SPICAV spectrometer

The UV channel of the spectrometer (SPICAV UV) was the only instrument in capacity to observe stars and perform stellar occultations. It is a spectro-imager, which works in the 118-320 nm spectral range. The measurements were taken with two main geometries: nadir and limb observing modes. The latter mode is the one employed for stellar occultations. The instrument was commanded by its main electronic unit, which interacted with the main control unit of the spacecraft.
The optical scheme of the UV channel of SPICAV is shown in Figure 1.15. The device consisted of a telescope focusing the incoming radiation to the input of the spectrometer. A flat mirror was used to orient the line of sight (LOS) of the channel. The instrument also allowed the use of a slit, narrowing the SPICAV field of view. Decomposition of the incident light into a spectrum was achieved by a concave holographic grating. The spectrum was recorded by a Thomson TH7863 TE CCD, cooled at 270 K with a Peltier, having a size of 288×408 pixels. Only the 288×384-pixel area corresponded to the actual field of view of the spectrometer, while 8 and 15 pixel rows at respectively the left and right edges of the CCD had a technological purpose, being covered for dark current monitoring. Spectra were decomposed into 384 spectral pixels (henceforth spectels). The rows of the detector (288 pixels) display the spatial distribution of the observed scene. An «image» appears on the CCD with the vertical axis representing the spatial dimension (288 pixels) of the field of view while the 384-pixel wide horizontal axis represents the spectral dimension. The maximum resolution of the spectrum reached 1.5 nm, which corresponds to approximately 3 pixels, each being ~0.54 nm wide. Spectra of a point source, like stars, can achieve this resolution regardless of the use of the spectrometer slit. For spatially extended sources however, achieving the native spectral resolution requires to put the steerable slit in place. This slit has two parts. A wide one (~0.2º) located in the upper portion, while the remainder consists of the narrow part (~0.02º). The latter was designed to fulfil the Shannon sampling criterion in the imaging plane, providing the optimal optical configuration for achieving the native spectral resolution of the SPICAV spectrometer. During stellar occultations, the slit was either retracted or the star was placed into the wide part of the slit. Both options permitted to alleviate the constraints on pointing as even tiny variations of the star position resulted in large spurious fluctuations of the signal when the slit was in place and the star located in the narrow portion.

Infrared channel of the SPICAV spectrometer

The infrared channel of the SPICAV spectrometer (Korablev et al., 2012) is based on the IR channel of the SPICAM spectrometer created for the Mars Express space mission. The instrument was developed and prepared for flight at IKI RAS, then integrated to the SPICAV module and calibrated in Service d’aéronomie du CNRS (now LATMOS/IPSL). The main element of this instrument is an acousto-optic tunable filter (AOTF) which is a narrow-band variable filter based on the Bragg’s diffraction of light on an ultrasonic acoustic wave excited in a birefringent crystal. Piezoelectric transducers are attached to the TeO2 crystal, and they generate radio frequency acoustic waves in the range of 80-250 MHz. Depending on the frequency of the signal, you can adjust the filter to a required wavelength.

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.

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).

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Photochemistry: a review of atmospheric models for SO2 and O3

A study of the mesosphere holds the key of fundamental components of the main chemical cycles determining the current atmospheric content. The major position is occupied by carbon dioxide and sulphur dioxide cycles. The carbon dioxide is dominant on Venus that intensifies a role of the CO2 cycle in the mesosphere. The great issue of theoretical comprehension is the stability of the CO2 mixing ratio being equal to 0.965 (von Zahn et al., 1983). Main processes of the cycle are a photolysis of CO2 on the day side, a production of O2, and a recombination of CO and O2 with a formation of CO2. The mixing ratio of CO has been observed at 20-30 ppmv at 36 km (Pollack et al 1993). A theoretical estimation shows that photolysis should produce this amount of the gas in a short period, 50 ppmv demand about 200 years (McElroy et al., 1973). The large volume of CO2 and correspondingly low abundance of CO also coincide with the very small upper limit of oxygen in its ground state. The ground-state oxygen amount was obtained to be lower than 2 ppm above 60 km from observations of the mid-latitudinal region of Venus (Mills, 1999). This limit indicates that the production of CO2 is approximately balanced by its loss via photolysis. However, the existence of oxygen emission at 1.27 μm observed on Venus argues the fast formation of O2 in the ground state (Connes et al., 1979; Crisp et al., 1996). The reproduction of CO2 from CO and O2 is a slower process (Nair et al., 1994; Mills, 1998). Thus, the stability of CO2 requires another process different from a simple oxidation. There might be a net of catalytic reactions (Yung and DeMore, 1982), however, it is not evidently resolved which process is the general one.
The sulphur cycle drives the cloud formation of a thick sulphuric acid droplets layer enshrouding Venus globally. A central position in the net of reactions is occupied by the sulphur dioxide. SO2 is the third most abundant gas in the Venusian CO2-atmosphere (<150 ppm) after the dominant carbon dioxide at 96.5% and nitrogen (3.5%). Its oxidation is a primary process leading to a formation of the H2SO4 acid. The latter further condenses at the cool cloud tops region and descends until its evaporation and dissipation becomes prevailed. The resulting SO2 and H2O are transferred upward supplying the mesospheric SO2 content. Sulphur acid synthesis is a prevailing process independent of the local time, and it results in a sharp decrease of SO2 in the upper cloud layer (Figure 2.2).

The stellar occultation technique: retrieving the atmospheric composition from transmittance spectra

Stellar occultation occurs when a star is tracked through the atmosphere of a planet while a spacecraft is moving along its orbit. On board the spacecraft, an instrument is collecting a sequence of spectra of the stellar light partially absorbed by the atmospheric species while the star is rising or setting behind the planet (Figure 3.1A). It is an efficient way to study the vertical distribution of atmospheric absorbers such as gas molecules or aerosol particles. Venus has a very dense atmosphere and very opaque clouds. This makes the method of stellar occultation usable only for studying the atmosphere above them. Occultations can probe up to an altitude where absorption features in the spectrum are overwhelmed by signal noise (Figure 3.2). The line of sight (LOS) of the instrument gradually crosses layers of the atmosphere, sensing atmospheric gases with a varying density. Absorption by the upper haze particles is what eventually limits the depth of sounding. As a result, the UV stellar occultation technique is only effective in the upper mesosphere and the lower thermosphere. SPICAV starts to be sensitive above 83-85 km (pressure ~1 mbar) up to 145 km (pressure ~10-6 mbar) (Bertaux et al., 2007a) and allows one to study the vertical structure of the planet’s night-time atmosphere. On the dayside the scattered solar light does not allow one to observe weaker sources like stars. It should be noted that at the considered altitudes, atmospheric refraction can be neglected due to the low gas concentrations.

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
1.5.1. Composition
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 mesosphere
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 Lyman-α emission Airglow of nitric oxide Solar radiance in the stellar occultation spectra
3.2.5. Wavelength- to-pixel registration
3.2.6. Spectral inversion Cases of positive gas detection Upper detection limits for two gases. 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 Method #1 Method #2 Comparison of methods Atmospheric transmission and error bars estimation.
3.2.10. Altitude assignment
3.3. Summary
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 Local time and latitude distribution Variations with a solar zenith angle Establishing independence from topography
4.4. Discussion.
4.4.1 Rapid changes in the SO2 content
4.4.2. Global patterns in the SO2 behaviour
4.5. Summary
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
5.6. Summary
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
6.6. Summary
ANNEX 1. Positive detections of SO2 presented individually
ANNEX 2. Positive detections of O3 presented individually
ANNEX 3. Parameters of stellar occultation sessions
ANNEX 4. Weighted mean
ANNEX 5. Estimation of an impact of diffrent stray light types
ANNEX 6. Résumé de la thèse en français


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