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VEGA 1 and VEGA 2 Entry Probes: an Investigation of Local UV Absorption (220-400 Nm) in the Atmosphere of Venus (SO 2 Aerosols

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VEGA 1 and VEGA 2 Entry Probes: an Investigation of Local UV Absorption (220-400 Nm) in the Atmosphere of Venus (SO 2 Aerosols VEGA 1 and VEGA 2 entry probes: An investigation of local UV absorption (220-400 nm) in the atmosphere of Venus (SO 2 aerosols, cloud structure) Jean-Loup Bertaux, Thomas Widemann, Alain Hauchecorne, V. Moroz, A. Ekonomov To cite this version: Jean-Loup Bertaux, Thomas Widemann, Alain Hauchecorne, V. Moroz, A. Ekonomov. VEGA 1 and VEGA 2 entry probes: An investigation of local UV absorption (220-400 nm) in the atmosphere of Venus (SO 2 aerosols, cloud structure). Journal of Geophysical Research. Planets, Wiley-Blackwell, 1996, 101 (E5), pp.12709-12745. 10.1029/96JE00466. insu-03132206 HAL Id: insu-03132206 https://hal-insu.archives-ouvertes.fr/insu-03132206 Submitted on 5 Feb 2021 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. JOURNALOF GEOPHYSICALRESEARCH, VOL. 101,NO. E5, PAGES12,709-12,745, MAY 25, 1996 VEGA 1 and VEGA 2 entry probes: An investigationof local UV absorption(220-400 nm) in the atmosphereof Venus (SO2, aerosols,cloud structure) Jean-Loup Bertaux, Thomas Widemann, and Alain Hauchecorne Serviced'Adronomie du CentreNational de la RechercheScientifique, Verrieres-le-Buisson, France V. I. Moroz and A. P. Ekonomov SpaceResearch Institute, Moscow,Russia Abstract.In 1985the VEGA 1 andVEGA 2 spacecraftdropped two descent probes into thenightside of Venus.On board was the French-Russian ISAV ultravioletspectroscopy experiment,consisting of a UV lightsource absorbed by atmosphericconstituents circulatingfreely into a tubeattached outside the pressurized modules. ISAV generateda wealthof absorptionspectra in the220- to 400-nmrange with an unprecedented vertical resolution(60-170 m) from62 km of altitudedown to the ground.On the basisof known instrumentproperties and a carefulexamination of the lightcurves recorded in 13 wavelengthintervals in theUV, we showthat most of therecorded differential absorption (at eachwavelength with respect to 394nm) canbe explainedby a combinationof gaseousSO2 absorption and absorption by aerosolsdeposited on the mirrorsduring the crossingof Venus'lower cloud. The spectralsignature of thisabsorber, termed X, was obtained,thanks to an unexpectedshock on VEGA 1 whichremoved this absorberfrom the mirrorsat 18 km of altitude.The UV spectralsignature of X resemblesthat of croconicacid, C505H2, whose absorbing power as a contaminantof H2SO4 dropletsat 2.5%dilution is compatible with the observations. However, the nonidentity of the spectral signature,together with stability arguments, makes this identification less plausible. Whatever its nature, the relevanceof this new absorberX is discussedin connectionwith thealbedo of Venusand the IR variableleak windows. If thisabsorber X detectedby ISAV in thelower cloud were also present in theupper cloud, it wouldbe a good candidateto explainthe UV part(• < 400nm) of theVenus albedo. Three layers of absorbingmaterial, called b, c, and d, are identified in the data of both ISAV I and 2 in the altituderange 49-43 km. The higherlayer b is insidethe lowercloud identified by the nephelometerof PioneerVenus, while the twoother layers are well below the lower•1oud boundaryas measured by Pioneer Venus. The SO2 profile (from 60 kmdown to 10km) is characterizedfor ISAV 1 by a doublepeak of themixing ratio (150 ppmv at 51.5km, 125 ppmvat 42.5 km) separatedby a deeptrough at 30 ppmvat 45.6 km. For ISAV 2 there is a singlepeak at 43 km.Both SO2 profiles are quitecompatible with recent ground-based measurements,showing 130 + 40 ppmvin the altituderange of 35-45 km [B&ardet al., 1993].Below the clouds the measured SO2 mixing ratio decreases steadily on bothprobes, downto 25 _+2 ppmvat 10km for ISAV 1,which is lowerthan previously reported valuesfrom gas chromatograph measurements (shown to be incompatiblewith ISAV measurements).The variation of SO2mixing ratio with altitude implies a strongvertical transportwhich is givenas a functionof altitude,showing the sourceand sink regions of SO2from 10 to 60 km of altitude.These data should impose severe constraints on future chemicalmodels of theatmosphere of Venus, occurring after volcanic episodes or impact crateringevents. The totalSO2 column density (0-60 km) wasmeasured to be 4 x 1022 moleculescm-2 or4.2 gcm -2 • a factorof 5 belowprevious estimates ß With an average reactionrate of 4.6x 10•ø molecules cm-2 s-1 withcalcite, CaCO3, estimated byFegley andPrinn [1989], it wouldtake only 27,000 years to getrid of SO2and associated H2SO 4 dropletsif calcitewere present all overthe surfaceof Venus.Therefore SO2 and its associatedubiquitous cloud cover may only be transientphenomena in the life of Venus. 1. Introduction 2 flewby planet Venus in June1985. They released two floating balloonsand two atmosphericprobes which were designed to Before their encounterwith cometHalley in March 1986, conductboth atmosphericin situ measurementsand soil anal- the two Sovietinterplanetary spacecraft VEGA 1 and VEGA ysis,similar to previousVenera missions. The twoprobes were Copyright1996 by the AmericanGeophysical Union. identical,and each carried a newtype of instrument,the ISAV, Paper number 96JE00466. a spectrometerdesigned to measurethe local atmospheric 0148-0227/96/96JE-00466509.00 absorptionand its detailedspectrum in the UV range(220- 12,709 12,710 BERTAUX ET AL.: VEGA 1 AND VEGA 2 ENTRY PROBES 400 nm). In the following, ISAV 1 designatesthe instrument interpretthis feature asdue to the depositionof cloudparticles on board the probe launched from VEGA 1, and correspond- on the mirrors of the absorptioncell, containinga contaminant ingly, ISAV 2 was on the probe launchedfrom Vega 2. for whichwe determinethe UV absorptionspectral signature. The flyby geometryat Venus was constrainedby the desire The resultsthat we presenthere are quite relevantto a number to encountercomet Halley later on, and as a result, the atmo- of important new featureswhich have emergedin the overall sphericprobes were releasedon the nightsideof Venus. This knowledgeof planet Venus since 1987. was an ideal situation to conduct an "active spectroscopy" 1. Hartleyet al. [1989]have proposed that croconicacid, Cs experimentlike ISAV, in which the UV spectrumof an artifi- O 5 H2, a polymer of carbon monoxide, if diluted as a minor cial light sourcewas continuouslymeasured through a gas cell contaminantin sulfuricacid droplets,could explain the spec- of 1.70-m path length, in which was circulated the ambient tral albedo of Venus in the wavelengthregion 350-450 nm. atmosphereall along the descentdown to the ground. Indeed we found in ISAV data an aerosol contaminant with a Useful measurements were collected from 62.5 km altitude UV spectral signaturebearing some resemblanceto that of down to the ground,with an unprecedentedvertical resolution croconic acid. However, it is fair to say here that there are (from 40 to 180 m), thanksto the relativelyhigh samplingrate argumentsagainst the existenceof croconicacid in the atmo- (one spectrumevery 4 s). This was the first time such an sphere of Venus. instrumentwas flown in an extraterrestrialplanet, and we do 2. The superb radar mapping of Magellan revealed clear not expectto have new opportunitiesto repeat it in the fore- signsof pastvolcanic activity but, to the bestof our knowledge, seeablefuture. Among other reasons,this characterof unique- no signsof presentvolcanic activity. The concentrationof SO2 nesscompels us to presenta critical analysisof the actual data near the ground of Venus is a crucial test of a state of equi- and our interpretation of these data in terms of vertical distri- librium or nonequilibrium in the chemical reactions of SO2 bution of UV gaseousabsorption (SO2 and other gases)and with the rocks(CaCO3), a nonequilibriumvalue beingpoten- aerosol absorptionspectrum. tial evidencefor injection of SO2 in the atmospherethrough A first analysisof these data was publishedearlier in the recent volcanicactivity or impact cratering. Soviet literature [Bertauxet al., 1987] (also with an uncon- 3. The discoveryof near-infrared (NIR) windowsin the trolled translationin Englishas indicatedby the orthographof nightsideof Venus byAllen and Crawford[1984] allowsscien- the authorsnames, Berto et al., in which the local UV absorp- tists to probe the gaseouschemical compositionbelow the tion was assignedto SO2 (whichwas expected to be significant) clouds,as discussedextensively by Pollack et al. [1993], and and tentatively to another gas, elemental sulfur, S8, because useful comparisonscan be made with the in situ ISAV mea- SO2 alone could not accountfor the observedspectral shape of surements.In addition, it is now generally admitted that the the absorption. leakage of IR thermal emissiongenerated below the clouds In this paper we present a new analysisof the ISAV 1 and from some areas of the cloud cover is connected to a decrease ISAV 2 data, characterizedby the following changesand im- of the lower cloud optical thicknessor number of particles provements. [Kamp and Taylor, 1990; Carlsonet al., 1991], in an altitude 1. We have a better understandingof the way the instru- region where most of the aerosols'UV absorptionwas col-
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