Contribution from SOIR/VEX to the updated Venus ...

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Advances in Space Research xxx (2015) xxx–xxx

Contribution from SOIR/VEX to the updated VenusInternational Reference Atmosphere (VIRA)

A.C. Vandaele a,⇑, S. Chamberlain a, A. Mahieux a,b, B. Ristic a, S. Robert a, I. Thomas a,L. Trompet a, V. Wilquet a, D. Belyaev c, A. Fedorova c, O. Korablev c, J.L. Bertaux d

aPlanetary Aeronomy, Belgian Institute for Space Aeronomy, 3 av. Circulaire, 1180 Brussels, BelgiumbFonds National de la Recherche Scientifique, Brussels, Belgium

c IKI, Moscow, RussiadLATMOS, 11 Bd d’Alembert, 78280 Guyancourt, France

Received 7 May 2015; received in revised form 8 July 2015; accepted 17 August 2015

Abstract

The SOIR instrument on-board Venus Express is an infrared spectrometer covering the 2.2–4.3 lm spectral region. This instrumentallows the detection of several key species of the Venus atmosphere, including CO2, CO, H2O/HDO, HCl, HF and SO2. From the CO2

density measurements, temperature is inferred giving information on the thermal structure of the atmosphere. Here we described the kindof data (profiles, latitudinal average, etc.) that will be provided to the updated VIRA compilation.� 2015 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: Venus; Reference atmosphere; Solar occultation; Vertical profiles; Trace gases; Structure

1. Introduction

The Venus International Reference Atmosphere (VIRA)was compiled by Kliore et al. (1985) presenting a synthesisof the best available data on the neutral atmosphere andthe ionosphere of the planet. This was the first attempt tosummarise all the knowledge about Venus, providing acommon standard reference through tables and averages.The original VIRA chapter dealing with the structure ofthe atmosphere from the surface to 100 km of altitude(Seiff et al., 1985) included tables of the vertical tempera-ture and density considering no day-night, latitudinalor synoptic variations in the lower atmosphere, butallowing for latitudinal variations in the middle layers(33–100 km). Particulate matter present in the Venus

http://dx.doi.org/10.1016/j.asr.2015.08.012

0273-1177/� 2015 COSPAR. Published by Elsevier Ltd. All rights reserved.

⇑ Corresponding author.E-mail address: [email protected] (A.C. Vandaele).

Please cite this article in press as: Vandaele, A.C., et al. Contribution from S(VIRA). Adv. Space Res. (2015), http://dx.doi.org/10.1016/j.asr.2015.08.0

atmosphere included in the VIRA model has been dis-cussed in Ragent et al. (1985), in which plots and tablesgave indications on the altitude dependence of the scatter-ing properties, the size and number densities, as well asmass loadings and optical depths. The three recognisedaerosols’ modes are already fully described and charac-terised. One chapter (Keating et al., 1985) was devoted tothe structure and composition of the upper atmosphere.It gathered tables with temperature, total density, and den-sities of CO2, O, CO, He, N, N2 for altitudes from 100 kmto 250 km, considering no latitudinal variations. Data from100 to 150 km were given only for noon and midnight con-ditions only, while those for 150–250 km were provided fordifferent local times. Chapter V of the VIRA compilationlooked at the known composition of the Venus atmospherebelow 100 km altitude (von Zahn and Moroz, 1985).

Since then many missions have yielded new and valuableinformation. A first attempt to update the VIRA model

OIR/VEX to the updated Venus International Reference Atmosphere12


mailto:[email protected]



2 A.C. Vandaele et al. / Advances in Space Research xxx (2015) xxx–xxx

was put forward by Moroz and Zasova (1997). They con-sidered new data provided by the VEGA 1 and 2 UV spec-trometers (Bertaux et al., 1996; Linkin et al., 1986), Venera15 infrared spectrometer and Venera 15 and 16 radio occul-tation experiments (Moroz et al., 1986; Oertel et al., 1985;Zasova, 1995; Zasova et al., 1996) and radio occultationexperiment (Yakovlev et al., 1991), as well as NIMS obser-vations obtained during the Galileo fly-by of Venus in 1990(Carlson et al., 1993; Collard et al., 1993; Grinspoon, 1993;Roos et al., 1993; Taylor, 1995). They also included re-analysis of previous data (from previous Venera missionsand Pioneer Venus) and some ground-based observationsfor trace gases abundances (Bezard et al., 1990; de Berghet al., 1995; Pollack et al., 1993). The structure of the Venu-sian atmosphere was updated by Zasova et al. (2006) tak-ing into account measurements of the verticaltemperature profile by the VEGA spacecraft and balloons,the radio occultation measurements of Magellan, Venera15, and Venera 16, as well as the temperature profilesderived from the Venera15 IR spectrometer. They pro-posed a model of the atmosphere in the altitude range from55 to 100 km consisting of tabulated temperature depen-dent on local time for 5 latitudinal regions (<35�, 35–55�,50–70�, 70–80�, 85�). Nothing similar was ever done forthe composition of the Venusian atmosphere except foran updated version of the initial Table 5-1 of von Zahnand Moroz (1985) given in Moroz and Zasova (1997)and the compilation of observations performed below100 km (de Bergh et al., 2006) which led to a better knowl-edge of the vertical profiles of water vapour and of sul-phuric species. However, the data corresponding to thehigher altitudes (above 100 km) that were investigated inKeating et al. (1985) have never been updated.

The Venus Express mission has however yielded awealth of new information on the Venus atmosphere, fromthe surface up to the highest layers of the atmosphere. Inparticular a series of spectrometers have been soundingthe atmosphere to derive new data on the structure but alsoon the composition. Particularly the SOIR instrument,which is part of the SPICAV suite, is sensitive in a verywide spectral region. In the following, we will brieflydescribe the instrument and explain the data that couldbe included into the updated VIRA compilation.

2. Instrument description

The instrument has already been extensively describedelsewhere (Bertaux et al., 2007a; Mahieux et al., 2008;Nevejans et al., 2006) and will only be briefly outlined here.SOIR is an Echelle grating spectrometer operating in theIR, combined with an acousto-optic tunable filter (AOTF)for the selection of the recorded wavenumber interval. Thewavenumber range covered by the instrument extends from2200 to 4370 cm�1 (2.3–4.5 lm) and is divided into 94diffraction orders (from 101 to 194). The definition and lim-its of these diffraction orders are presented in Vandaeleet al. (2013). The bandwidth of the AOTF was originally


designed to be 20 cm-1, as measured on ground beforelaunch (Nevejans et al., 2006), to allow light from onlyone order into the spectrometer. However, the measuredbandwidth of the AOTF filter is �24 cm-1 (Mahieuxet al., 2008). Information from adjacent orders thereforeleaks onto the detector. This effect is called superpositionof orders hereafter and has to be taken into account inthe retrieval method.

The resolution of the SOIR instrument varies slightlyfrom the first to the last order, from 0.12 to 0.20 cm�1,as does the spectral sampling interval which varies from0.030 cm�1 in diffraction order 101–0.055 cm�1 in order194, increasing with the pixel number and the diffractionorder. Signal to noise ratio on the SOIR transmittancesvaries between 250 and 5000 but is typically of the orderof 2000. These SNR values are deduced from the recordedspectra as explained in Vandaele et al. (2013).

The retrieval method has been described in detail in sev-eral papers introducing the solar occultation method(Vandaele et al., 2008), deriving CO2 densities (Mahieuxet al., 2015a, 2012) and trace gases such as CO (Vandaeleet al., 2015a,b), HCl and HF (Mahieux et al., 2015d),SO2 (Belyaev et al., 2008, 2012; Mahieux et al., 2015b),H2O and HDO (Fedorova et al., 2008). Temperature andtotal density, and therefore structure, has been retrievedfrom the CO2 observations (Mahieux et al., 2015a, 2010,2015c, 2012). The SOIR spectra give also access to infor-mation on the aerosol content in the upper haze. Lightextinction due to aerosols and cloud top altitudes havebeen obtained (Wilquet et al., 2012, 2009) as well as theH2SO4 concentration in the droplets (Wilquet et al.,2014). In the following of the paper, we will describe indetail the data derived from the SOIR instrument, theircoverage in terms of latitude, local solar time and altitude.As a general comment, solar occultation observations pro-vide information on a very important region of the atmo-sphere, i.e. the terminator, the limit between the day andnight sides of the planet. As a consequence the observa-tions of SOIR correspond either to 6 am or 6 pm, even ifat high latitude the definition of time starts to be meaning-less. This region is crucial because it is a place of large lon-gitudinal gradients in temperature and density.

3. Composition derived from SOIR

The spectral range covered by SOIR allows the observa-tion of several key species of the Venus atmosphere, such asCO2, CO, HCl, HF, H2O and its isotopologue HDO, aswell as SO2. The retrieval technique to derive CO2 densitiesand trace gas densities is the same and has been describedin detail in (Mahieux et al., 2010; Vandaele et al., 2008). Allspectra recorded during one occultation are analysed alltogether in an iterative way that finally delivers the densi-ties of the targeted trace gases as well as the CO2 densityand temperature if at least one of the selected wavelengthranges contains CO2 absorption bands. CO2 has beenobserved in almost all occultations, except at the beginning



A.C. Vandaele et al. / Advances in Space Research xxx (2015) xxx–xxx 3

of the mission, when we did not realise the high variabilityof the Venus atmosphere and the need to measure CO2 dur-ing each occultation. Knowing the CO2 density allows us toderive volume mixing ratios (vmr) of all trace gases.

All observations are performed at the terminator, eitheron the day side (Local Solar Time 6 am) or night side (LST6 pm) and cover both hemispheres from 90�S to 90�N lat-itudes. Note however that the values of LST correspondingto observations performed at high latitudes can differ from6 am or 6 pm. In the following we will speak of morning/evening sides of the terminator. The instrument probes themesosphere (70–95 km) and the lower thermosphere (above95 km). Depending on the species, SOIR delivers verticalprofiles extending from the top of the clouds (60–70 km)up to 170 km.

The primary results deduced from the analysis of SOIRspectra are thus number densities. Conversion to volumemixing ratio (vmr) requires the knowledge of the totalnumber density, or that of CO2, if a CO2 vmr is assumed:

vmrTraceGas ¼ nTraceGas � vmrCO2

nCO2

ð1Þ

Two cases are encountered: either the CO2 density isretrieved simultaneously from one of the 4 orders dedicatedto its detection and is directly used to obtain the vmr usingEq. (1), or no information on CO2 is available from theobservation itself. In that case, the CO2 density is derivedfrom the VAST model, which has been described in detailin Mahieux et al. (2012) and updated in Mahieux et al.(2015a). In both cases, an assumption is made on theCO2 mixing ratio vmrCO2, whose values are taken fromthe VIRA model: data from Seiff et al. (1985) for altitudesup to 100 km and from Hedin et al. (1983) above that alti-tude have been used. Vmr vertical profiles averaged in thedifferent latitudinal bins are also available for the VIRAcompilation.

The vertical resolution of the profiles depends essentiallyon the distance separating Venus Express and the planet. Ittherefore varies significantly since the orbit of the space-craft was very elongated, with a periapsis of about300 km and apoapsis of 60,000 km. As a consequence,the resolution ranges from a few hundreds of metres upto some kilometres. All vertical profiles resulting from theretrieval procedure are interpolated on an altitude grid of1 km step from 60 to 200 km.

In the following, each species will be considered sepa-rately, and results discussed in terms of coverage in latitudeand local solar time. Statistics are given for each of them.The SOIR profiles have been compared to the VIRA com-pilations. In the following we will refer to VIRA1 whenconsidering the original data published in 1985 (structurebelow 100 km by Seiff et al. (1985); composition below100 km by von Zahn and Moroz (1985) and structureand composition above 100 km by Keating et al. (1985))and VIRA2 for the updates done since then (de Berghet al., 2006; Moroz and Zasova, 1997; Zasova et al.,2006). The values of the VIRA compilations considered


in this study are reported in Table 1. The correspondingvalues obtained with SOIR results are also indicated inthe same Table. The numbers reported for SOIR corre-spond to the minimum, average and maximum vmr valuesobserved at different altitudes, considering the entire dataset (all latitudes, AM/PM).

3.1. Carbon dioxide

CO2 has been observed in nearly all occultations, exceptat the beginning of the mission. The individual observa-tions are shown in Fig. 1, as a function of time. In this fig-ure, circles denote morning side measurements andtriangles the evening ones. Based on a selection of these,the VAST (Venus Atmosphere from SOIR measurementsat the Terminator) model has been compiled (Mahieuxet al., 2015a, 2012). The current VAST model is built on122 orbits (Mahieux et al., 2015a), during which severalorders (i.e. wavenumber intervals) were recorded simulta-neously allowing to probe a large range of altitudes.

However there are a lot more orbits during which CO2

was also observed but on a more restricted range. Table 2gives the number of observations for the different bins con-sidered in this study (0–30�, 30–60�, 60–70�, 70–80�, 80–90�), when considering the complete dataset as well as onlythe selection used to build the VAST reference model.Morning/evening difference has been considered, whereasno distinction has been made between the Northern andSouthern hemispheres for the building of the VAST model,albeit the analysis of the complete dataset should be possi-ble in terms of North–South difference. All individual CO2

profiles are shown in Fig. 2 for the different latitudinal binsconsidered in Table 2. The VIRA profiles for LST = 12 and24 are also plotted. The average profiles for AM and PMSOIR observation are also indicated for each bin. Theseaverages do not span the same altitudes ranges as the indi-vidual profiles because at least 10 observations at one givenaltitude (plus or minus one scale height) are required toperform the average. Averages for the 60–70� Lat binmight not be representative because of the low number ofobservations at those latitudes. In general, SOIR doesnot see large variations in density between evening andmorning data. Below 140 km, the SOIR data are in agree-ment with the midnight VIRA data. Above that altitude,SOIR data tend to diverge from these LST = 24 VIRAdata, increasing more with altitude, but remaining wellbelow the VIRA data for LST = 12.

3.2. Carbon monoxide

A few profiles of CO have been published (Vandaeleet al., 2015b) in order to study the short term variationof CO in the Venus atmosphere observed by SOIR. How-ever many more CO observations have been performedduring the VEx mission and have been investigated focus-ing on the short and long term variation and the latitudinaldependence (Vandaele et al., 2015a). Considering the same



Table 1Values considered for the VIRA and VIRA2 compilations. Also indicated the references to SOIR studies dealing with the trace gases mentioned.

Species VIRA 1 VIRA 2 SOIR

Z (km) T(K)/Vmr Z (km) T(K)/Vmr Z (km) Vmr min/avg/max values

Structure temperature

<33 No Lat/LST dep [1] <55 [1] 65–175 Lat dep [5, 6, 7]33–100 Lat dep [1] 55–100 LST & Lat dep [4]>100 No Lat dep – day/night [2] >100 No Lat dep – day/night [2]

Composition

CO2 <100 0.965 [3] 65–175 Lat dep [6, 7, 8]>100 [2]

CO 12 2 � 10�5 [3] 70–140 Lat dep [9, 10]22 2 � 10�5 [3] 30 2.3 � 10�5 [5]

36 2.3 � 10�5 [4]42 3 � 10�5 [3] 40 2.9 � 10�5 [5]52 3 � 10�5 [3]65 (ct) 4.5 � 10�5 [3] 70 0.1/4.0/20 � 10�5

75 0.4–11 � 10�5 [3] 80 0.7/5.9/60 � 10�5

90 2 � 10�4 [3] 90 0.2/1.1/50 � 10�4

>100 [2] 100 0.8/3.0/14 � 10�4

120 0.7/3.3/40 � 10�3

140 0.4/3.6/14 � 10�1

HCl 65 (ct) 4 � 10�7 [3] 15–30 5.0 � 10�7 [5] 70–115 Lat dep [11]23.5 4.8 � 10�7 [4]

85 2.4/9.6/50 � 10�8

100 0.8/4.2/18 � 10�7

110 0.8/5.5/18 � 10�7

HF 65 (ct) 5 � 10�9 [3] 30–40 5 � 10�9 [5] 70–110 Lat dep [11]33.5 1–5 � 10�9 [4]

80 0.3/1.4/10 � 10�9

90 1.9/9.9/30 � 10�9

100 0.4/2.9/15 � 10�8

110 0.5/3.3/15 � 10�7

H2O 0 2 � 10�5 [3] 0 1 � 10�5 [4] H2O [12, 13]10 3–5 � 10�5 [4,5] 70–110

22 1.4 � 10�3 [3] 20 2 � 10�5 [4,5] HDO42 5.2 � 10�3 [3] 40 3 � 10�5 [4,5] 70–9545 2 � 10�4 [3]50 3 � 10�4 [3]58 3 � 10�5 [3] 55 1–40 � 10�6 [4,5] 75–95 H2O: 0.6/1.2/1.6 � 10�6

65 (ct) 1–40 � 10�6 [3] 65 1–30 � 10�6 [4,5] HDO: 5/8/13 � 10�8 [12]85 3 � 10�6 [4]

SO2 12 2.2 � 10�5 [4,5] 70–110 Lat dep [14,15]22 2 � 10�4 [3] 22 3.8 � 10�5 [4,5]58 4 � 10�6 [3] 42 1.3 � 10�4 [4,5]66 2 � 10�6 [3] 52 1.1 � 10�4 [4,5]69 1 � 10�7 [3] 62 3.0–20 � 10�6 [4]

69 0.02–2 � 10�6 [4] 70 0.9/5.6/35 � 10�8

80 1.4/7.2/33 � 10�7

90 0.7/1.4/35 � 10�7

ct: cloud top – LST: local solar time – Lat dep: latitude dependent.[1] Seiff et al. (1985).[2] Keating et al. (1985).[3] von Zahn and Moroz (1985).[4] Zasova et al. (2006).[5] de Bergh et al. (2006).[6] Mahieux et al. (2010).[7] Mahieux et al. (2012).[8] Mahieux et al. (2015a).[9] Vandaele et al. (2015b).[10] Vandaele et al. (2015a).[11] Mahieux et al. (2015d).[12] Fedorova et al. (2008).[13] Chamberlain et al. (2015).[14] Belyaev et al. (2008).[15] Mahieux et al. (2015b).


Please cite this article in press as: Vandaele, A.C., et al. Contribution from SOIR/VEX to the updated Venus International Reference Atmosphere(VIRA). Adv. Space Res. (2015), http://dx.doi.org/10.1016/j.asr.2015.08.012


Fig. 1. Location of the CO2 observations, in terms of orbit number versus latitude. Orange triangles and blue circles correspond to the evening andmorning sides of the terminator respectively. The numbers in the centre are the occultation seasons. (For interpretation of the references to colour in thisfigure legend, the reader is referred to the web version of this article.)

Table 2Statistics of the CO2 observations for the different latitude bins and on each side of the terminator. Numbers are given for the complete data sets and forthe selection used to build the VAST reference model. For the complete dataset, the numbers of observations are also separated for the Northern andSouthern hemispheres.

Complete dataset VAST

Number of observations AM(North/South)

Number of observations PM(North/South)

Number ofobservations AM

Number ofobservations PM

0–30� 19/29 17/34 15 730–60� 6/32 5/43 7 1060–70� 14/14 12/12 5 570–80� 31/9 33/9 8 780–90� 83/15 76/15 25 33Total 153/99 143/115 60 62


latitudinal bins definition and separating for AM and PMobservations, averages are also obtained for CO. Thestatistics of which is shown in Table 3.

All individual observations are plotted in Fig. 3. We seeon Fig. 4 that at high altitudes (above 120 km), the agree-ment with VIRA is better than at lower altitudes. More-over, a morning/evening difference is also clearlyobserved, with lower values at the morning terminator thanat the evening one. This trend is however reversed at loweraltitudes. Densities are systematically lower than VIRA atlow altitudes. The scale heights are also quite different. Infact, three zones are apparent in the SOIR profiles: Below90 km, the scale height is almost constant with value rang-ing between 3 and 9 km (average value of 6 km) dependingon the particular orbit. Between 95 and 110 km, the scaleheight is slightly increasing, from values of 6–12 km, AMscale heights being somewhat lower than PM values: from8 to 10 km for AM, and from 9 to 12 km for PM. Then,above 115 km, the scale height is decreasing to reach valuesaround 6 km at 140 km. Typical values of CO vmrs are


indicated in Table 1. These values represent the full set ofdata, i.e. including all latitudes and AM/PM observations.

3.3. Water: H2O and HDO

Water vapour and its isotopologue are targeted simulta-neously in the majority of the SOIR dedicated watervapour observations. H2O is detected between 70 and110 km and HDO between 70 and 95 km of altitude. EarlySOIR water vapour observations were published in 2007(Bertaux et al., 2007b) and 2008 (Fedorova et al., 2008).Previous results show depletion in the volume mixing ratioat 85 km in both HDO and H2O and an increase in HDO/H2O ratio above the clouds. No noticeable temporal vari-ability was detected. Minimum, average and maximum val-ues reported in Fedorova et al. (2008) are also indicated inTable 1. The average ratio HDO/H2O obtained inFedorova et al. (2008) was 240 ± 25 times the terrestrialratio. This value is lower than the value reported for thelower atmosphere (157 ± 30). Numerous subsequent H2O



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Fig. 2. CO2 densities observed by SOIR for the different latitudinal bins considered. For each bin, morning (blue) and evening (orange) data are shown,with the VIRA profiles for day (LST 12, solid purple line) and night (LST 24, dashed purple line) conditions. The average SOIR profiles for the morningand evening sides of the terminator are also indicated by dark blue and red lines respectively.

Table 3Statistics of the CO observations for the different latitude bins and on eachside of the terminator considering hemispherical symmetry.

Number of observations AM Number of observations PM

0–30� 25 2430–60� 21 1960–70� 13 1270–80� 18 1780–90� 32 29Total 109 101


and HDO SOIR observations have been obtained between2007 and 2014 and with recent improvements in instrumentcalibration, data reduction (Vandaele et al., 2013) and a


long base line of data, a new analysis has been undertaken(Chamberlain et al., 2015).

3.4. Hydrochloric acid

Densities and volume mixing ratios of HCl are measuredin the 70–115 km altitude region, at the Venus terminator.All latitudes from Pole to Pole are covered (Mahieux et al.,2015d). For H35Cl and H37Cl, 180 orbits are considered;their localisation is shown in Fig. 5. Vertical profiles areusually obtained for the 70–115 km altitude region, seeFig. 6, where the VIRA values are also shown. Between90 and 120 km altitude, HCl vmr values from SOIR vary



Fig. 3. Location of the CO observations, in terms of orbit number versus latitude. Orange triangles and blue circles correspond to the evening andmorning sides of the terminator respectively. The numbers in the centre are the occultation seasons. (For interpretation of the references to colour in thisfigure legend, the reader is referred to the web version of this article.)


between 2 � 10�7 and 1 � 10�6 in agreement with the val-ues adopted by VIRA, although showing a larger variabil-ity. As reported by many authors (Krasnopolsky andPollack, 1994; Mills and Allen, 2007; Yung and DeMore,1982; Yung et al., 2008), HCl plays a key role in the chem-ical cycles, involving CO2 and the (poly)sulphuric com-pounds. The SOIR results show that further investigationis necessary for understanding the vertical variations andtrends. Comparing the SOIR results to 1D-photochemicalmodel results (for example, Parkinson et al. (2015)) wouldhelp understanding and explaining these observations.

The ability of SOIR to target both H35Cl and H37Cl iso-topologues has also been investigated. Indeed, observationsof both isotopologues are always performed simultane-ously, since absorption lines of both species are observedin the same spectral interval. Numerous concomitant den-sity profiles lead to the determination of the 37Cl/35Cl iso-topic ratio on Venus, found to be equal to 0.34 ± 0.13,which is compatible with the value found on Earth(Mahieux et al., 2015d).

Profiles have been grouped in latitudinal bins in order toinvestigate possible trends. A latitudinal symmetry relativeto the Equator has been considered, together with a longi-tudinal symmetry with regard to the terminator side, i.e. nodifference between morning and evening sides of the termi-nator were considered in Mahieux et al. (2015d). The num-ber of observations in the different latitude bins issummarised in Table 4. Both isotopologues have been con-sidered together to build these profiles, since we showedthat the 37Cl/35Cl isotopic ratio can be considered as equiv-alent to the one on Earth. Moreover, the atmosphere is wellmixed at altitudes below the homopause, located at anapproximate pressure level of 10�6 mbar (�130 km) on


Venus (Mahieux et al., 2012; von Zahn et al., 1980), andthe molecular mass difference is small between the twoisotopologues.

3.5. Hydrofluoric acid

Densities and volume mixing ratios of HF have beenmeasured in the 75–110 km altitude region, at the Venusterminator covering all latitudes from Pole to Pole(Mahieux et al., 2015d). In this study, 44 orbits were con-sidered. Since then, additional observations have been car-ried out. The localisation of all HF observations is shownin Fig. 7 and the relevant statistics is given in Table 5.All individual HF vmr profiles are plotted in Fig. 8. Theyare in agreement with the VIRA values (1 � 10�9–5 � 10�9) which correspond to the cloud top altitude(65 km). The VMR profiles of HF, as well as those ofHCl, show a positive gradient above 70 km, which is notexpected from models (Krasnopolsky, 2012) that predictconstant vmr profiles at those altitudes. It seems difficultto explain the positive gradient at this stage but it shouldbe noted that a positive gradient has also been observedfor SO2 (Belyaev et al., 2012; Mahieux et al., 2015b;Moullet et al., 2013) and HCl and SO2 are linked by chem-istry in the Venus mesosphere.

3.6. Sulphur dioxide

SO2 has a weak absorption structure at 4 lm, fromwhich number density profiles are regularly inferred. SO2

volume mixing ratios are calculated from the total numberdensities that are also derived from the SOIR measure-ments. Since the abundance of SO2 in the Venus



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Fig. 4. CO densities observed by SOIR for the different latitudinal bins considered. For each bin, morning (blue) and evening (orange) data are shown,with the VIRA profiles for day (LST 12, plain purple line) and night (LST 24, dashed purple line) conditions. The average SOIR profiles for the morningand evening sides of the terminator are also indicated by dark blue and red lines respectively.


atmosphere is quite close to the SOIR SO2 detection limit,we developed a specific method to clearly indicate positiveobservations and performed a very detailed sensitivitystudy (Mahieux et al., 2015b). This study was limited tothe measurements recorded from the beginning of the mis-sion up to February 2013 (up to orbit 2500). This limiteddatabase counts 137 solar occultations potentially contain-ing a SO2 signature. Fig. 9 is a map of the measurement asa function of latitude and orbit number. The filled markersindicate that SO2 has been detected during the occultation,


while the unfilled ones represent non-detection. 93 positivedetections out of 137 are recorded, which represents 68% ofthe occultations.

Table 6 summarises the number of measurements in thedifferent latitude bins considered in this work, assuminglatitudinal symmetry with respect to the Equator and aLST symmetry between dusk and dawn. The latituderegions, used for binning the measurements, were chosensuch that there are sufficient positive detections in eachbin, to ensure statistic validity. Since this study, more



Fig. 5. Location of the HCl observations, in terms of orbit number versus latitude. Orange triangles and blue circles correspond to the evening andmorning sides of the terminator respectively. The numbers in the centre are the occultation seasons. (For interpretation of the references to colour in thisfigure legend, the reader is referred to the web version of this article.)

10−9

10−8

10−7

10−6

10−5

60

70

80

90

100

110

120

130

VMR HCl

Alti

tude

[km

]

Fig. 6. All individual HCl vertical vmr profiles as a function of the altitude. Morning (blue) and evening (orange) data are shown. The average SOIRprofiles for the morning and evening sides of the terminator are also indicated by dark blue and red lines respectively. The pink dashed and purple solidlines correspond to the VIRA 1 and 2 values respectively (see Table 1).

Table 4Statistics of the HCl observations for the different latitude bins and oneach side of the terminator considering hemispherical symmetry.

Latitudebin

Number of measurementsAM

Number of measurementsPM

0–30� 18 2330–60� 13 1460–70� 12 1070–80� 18 1880–90� 35 24Total 96 89



spectra were recorded that potentially contain the SO2 sig-nature. They will be analysed in near future to complementthe first available data (Mahieux et al., 2015b).

Vertical profiles usually extend from 65 to 95 km forSO2 (see Fig. 10). Most of the profiles are confinedwithin the 2 � 10�8 to 2 � 10�7 vmr range which is invery good agreement with the value of VIRA 1 andthe lowest one of VIRA 2 (0.02 � 10�6, see Table 1).Values as high as the upper values from VIRA 2 havenever been observed.



Fig. 7. Location of the HF observations, in terms of orbit number versus latitude. Orange triangles and blue circles correspond to the evening andmorning sides of the terminator respectively. The numbers in the centre are the occultation seasons. (For interpretation of the references to colour in thisfigure legend, the reader is referred to the web version of this article.)

Table 5Statistics of the HF observations for two latitude bins and on each side ofthe terminator considering hemispherical symmetry.

Latitudebin

Number of measurementsAM

Number of measurementsPM

0–60� 18 2160–90� 29 34Total 47 55

10−11

10−10

10−9

60

70

80

90

100

110

120

130

140

VM

Alti

tude

[km

]

Fig. 8. All individual HF vertical vmr profile as a function of the altitude. Mevening sides of the terminator mean profiles are also indicated by dark blue anto the minimum and maximum values in VIRA 2 (see Table 1).



4. Atmospheric structure data derived from SOIR

observations

Temperature vertical profiles are derived from the CO2

density profiles using the hydrostatic equilibrium. Themethod and results have been already discussed in detailin Mahieux et al. (2012, 2015a). The statistics of the temper-ature profiles is the same as the one associated to CO2 (seeSection 3.1). The current VAST model contains 10 profilescorresponding to the different latitude bins defined earlier

10−8

10−7

10−6

R HF

orning (blue) and evening (orange) data are shown. For the morning andd red lines respectively. The pink dashed and purple solid lines correspond



Fig. 9. Location of the SO2 observations, in terms of orbit number versus latitude. Circles denote morning side measurements and triangles the eveningones. The filled markers indicate that SO2 has been detected during the occultation, while the unfilled ones represent non-detection.

Table 6Statistics of the SO2 observations for the different latitude bins.

Latituderegion

Number of positivedetections

Number ofmeasurements

0–40� 18 3440–70� 13 2570–80� 18 2480–90� 44 54Total 93 137

10−9

10−8

165

70

75

80

85

90

95

100

VMR

Alti

tude

[km

]

Fig. 10. All individual SO2 vertical vmr profile as a function of the altitude.profiles for the morning and evening sides of the terminator are also indicatedand purple solid lines correspond to the VIRA 1 and VIRA 2 minimum and



for each of the terminator sides. As for CO2, the number ofobservations when considering the complete data set (notrestricted to the VAST model) is high enough to deriveinformation for both hemispheres separately. All individualtemperature profiles are plotted in Fig. 11 for the differentlatitudinal bins considered in this work. They are all charac-terised by a succession of minima and maxima already dis-cussed in Mahieux et al. (2012, 2015a). The existence ofthese minima and maxima, which are not present in theVIRA data, can be explained by radiative cooling and

0−7

10−6

10−5

SO2

Morning (blue) and evening (orange) data are shown. The average SOIRby dark blue and red lines respectively. The pink dot-dashed, pink dashedmaximum 2 values respectively (see Table 1).



0 50 100 150 200 250 300 350 400 450 50060

80

100

120

140

160

180

Temperature [K]

Alti

tude

[km

]

Lat 0 − 30

0 50 100 150 200 250 300 350 400 450 50060

80

100

120

140

160

180

Temperature [K]

Alti

tude

[km

]

Lat 30 − 60

0 50 100 150 200 250 300 350 400 450 50060

80

100

120

140

160

180

Temperature [K]

Alti

tude

[km

]

Lat 60 − 70

0 50 100 150 200 250 300 350 400 450 50060

80

100

120

140

160

180

Temperature [K]

Alti

tude

[km

]

Lat 70 − 80

0 50 100 150 200 250 300 350 400 450 50060

80

100

120

140

160

180

Temperature [K]

Alti

tude

[km

]

Lat 80 − 90

Fig. 11. Temperatures observed by SOIR for the different latitudinal bins considered. Morning (blue) and evening (orange) data are shown, with theVIRA profiles for day (LST 12, solid purple line) and night (LST 24, dashed purple line) conditions. The average SOIR profiles for the morning andevening sides of the terminator are also indicated by dark blue and red lines respectively.


warming due to different CO2 bands active in the atmo-sphere. This has been investigated in detail in Erwin et al.(2015). Note however, from Fig. 11, that SOIR and VIRAtemperature agree at the lower altitudes covered by SOIR(below 90 km); SOIR temperatures then show a systematicmaximum at 100 km which is never seen by VIRA; finally,above 110 km, SOIR temperatures go through a minimumthen maximum but remain within the VIRA night and dayprofiles.

Since the CO2 ro-vibrational lines are resolved in theSOIR spectra, we derived rotational temperature verticalprofiles. This study was presented in Mahieux et al.(2015c), where we show that the rotational temperature


are in very good agreement with the kinetic temperaturesderived using the hydrostatic equilibrium. However, theuncertainties on the rotational temperatures are much lar-ger than on the hydrostatic temperatures, because thederivation is much more sensitive to the noise in the spec-tra. For this reason, hydrostatic temperatures only will beprovided to the VIRA update.

5. Aerosol data derived from SOIR observations

A continuum of absorption is observed in the SOIRspectra, which is primarily shaped by the light extinctiondue to the presence of aerosol particles in the upper haze



Orbit number500 1000 1500 2000 2500 3000

Latit

ude

[deg

]

-90

-45

0

45

90

2 43 5 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 286

Fig. 12. Location of upper haze observations, in terms of orbit number versus latitude. Orange triangles and blue circles correspond to the evening andmorning sides of the terminator respectively. The numbers in the centre are the occultation seasons. (For interpretation of the references to colour in thisfigure legend, the reader is referred to the web version of this article.)

Extinction [km-1]

10-4 10-370

75

80

85

90

95

100

105

Extinction [km-1]

10-4 10-3

Alti

tude

[km

]

65

70

75

80

85

90

95

100

Extinction [km-1]

10-4 10-375

80

85

90

95

100

105

110Lat: 60°N-90°N Lat: 60°S-90°SLat: 60°N-60°S

Fig. 13. Extinction profiles at 3 lm observed by SOIR for the different latitudinal bins considered. For each bin, morning (blue) and evening (orange) dataare shown.


(between �70 and 90 km) of the Venus mesosphere(Wilquet et al., 2009). Vertical profiles of the extinctioncan be retrieved from virtually each diffraction order ofthe SOIR instrument, as far as the spectra are not spectro-scopically saturated in the targeted range of altitudes. Thedata set for VIRA is restricted to 3 successive diffraction


orders (148–150, around 3 lm) that were routinelyrecorded. This way, the spectral dependence of the extinc-tion is eliminated allowing a climatological study of theupper haze.

A study of the aerosol loading and its variations in termsof time, latitude, altitude and LST has been published



Table 7Statistics of the aerosol observations for the different latitude bins.

Latitude bin Number ofmeasurements AM

Number ofmeasurements PM

90�S–60�S 33 2960�S–60�N 79 8160�N–90�N 94 74Total 206 184


(Wilquet et al., 2012), demonstrating a latitudinal depen-dence of the upper haze, in addition to considerableshort- and long-term time variations. Since then, the dataset has doubled from �200 to almost 400 observations,covering all latitudes over more than 8 years (Fig. 12).

It now allows the detection of difference as a function ofthe local solar time at equatorial and mid-latitudes (middlepanel in Fig. 13). A latitudinal symmetry relative to theEquator is considered between 60�S and 60�N. The reasonfor which North and South polar latitudes are separated isbecause of the very different shape of the extinction profilesbetween both kinds of observations. However, these

Extinction [km-1]

10-4 10-3

Alti

tude

[km

]

70

75

80

85

90

95

100

105

110

Fig. 14. Model of the extinction at 3 lm as compiled from SOIR data:Mean profiles (thick lines) and corresponding standard deviations (thinlines) for polar latitudes (|60–90�|, dashed lines) and for equatorial andmid-latitudes (|0–60�|, solid lines). For each latitude bin, morning (blue)and evening (orange) profiles are shown.


differences are not atmospheric and are purely due the veryelliptical shape of the VEX orbit around Venus with theperigee being above the North Pole. The fine-structuresin the extinction profiles observed at high Northern lati-tudes (left panels in Fig. 13) are completely smoothed outat other latitudes because the vertical resolution deterio-rates with distance from the North Pole. In comparison,the extinction appears strictly decreasing with altitude athigh Southern latitudes (right panels in Fig. 13). The num-ber of observations in the different latitude bins is sum-marised in Table 7.

Extinction profiles are grouped and averaged in latitudi-nal bins and separating AM and PM observations. Anexample is presented in Fig. 14 where mean extinction pro-files for polar latitudes (|60–90�|) and mid- and equatoriallatitudes (|0–60�|) are presented for each side of the termi-nator. The mean profiles for the two latitude bins consid-ered are outside of the standard deviation of each other,for the morning as well as for the evening side and theextinction at polar latitudes is lower by a factor 5–10 thanfor the |0–60�| latitude bin. The differences between morn-ing and evening mean profiles are significant for the |0–60�|latitude bin as they are outside of the standard deviation ofeach other while this is not the case for polar latitudes.

6. Conclusions

We have shown that SOIR can provide significant infor-mation on the composition and structure of the Venusatmosphere, as well as on aerosols and clouds (opticaldepths, composition and cloud tops altitudes). Densitiesof CO2, CO, HCl, HF, H2O, HDO, SO2 are available aswell as volume mixing ratio of all trace gases, since CO2

has been either measured at the same time or is derivedfrom the VAST model.

SOIR can provide individual vertical profiles of themain atmospheric species present in the Venus mesosphereand lower atmosphere. This encompasses the region fromabove the clouds (�60 km) up to 170 km altitude. Dataare only provided at both terminator sides (6 am and 6pm). Averaged profiles are also provided for different lati-tude bins, considering or not hemispheric symmetrydepending on the species (the available number ofobservations).

Acknowledgements

The research program was supported by the BelgianFederal Science Policy Office and the European SpaceAgency (ESA – PRODEX program). The research was per-formed as part of the ‘‘Interuniversity Attraction Poles”programme financed by the Belgian government (PlanetTOPERS). The authors also recognize the support fromthe FP7 EuroVenus project (G.A. 606798) as well as thatof the HiResMIR International Scientific CoordinationNetwork (GDRI). The authors acknowledge the supportprovided by International Specialised Skills Institute,




through the organization of the 3 International Teamsaround Venus: ‘‘Sulfur Dioxide Variability in the VenusAtmosphere” (http://www.issibern.ch/teams/venusso2/);‘‘Towards a self-consistent model of the thermal structureof the Venus atmosphere” (http://www.issibern.ch/teams/venusatmos/) and ‘‘Improved understanding of Venusclouds” (http://www.issibern.ch/teams/venusclouds/). A.Mahieux would like to thank the FNRS (Belgium) for hisCharge de Recherches position.

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Contribution from SOIR/VEX to the updated Venus ...

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