SPICAM on Mars Express: A 10 year in-depth survey of the Martian atmosphere

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SPICAM on Mars Express: A 10 year in-depth survey of the Martian atmosphere

Franck Montmessin, Oleg Korablev, Franck Lefèvre, Jean-Loup Bertaux, Anna A. Fedorova, Alexander Trokhimovskiy, Jean-Yves Chaufray, Gaetan

Lacombe, Aurélie Reberac, Luca Maltagliati, et al.

To cite this version:

Franck Montmessin, Oleg Korablev, Franck Lefèvre, Jean-Loup Bertaux, Anna A. Fedorova, et al..

SPICAM on Mars Express: A 10 year in-depth survey of the Martian atmosphere. Icarus, Elsevier, 2017, 297, pp.195-216. �10.1016/j.icarus.2017.06.022�. �insu-01545690�

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ContentslistsavailableatScienceDirect

Icarus

journalhomepage:www.elsevier.com/locate/icarus

SPICAM on Mars Express: A 10 year in-depth survey of the Martian atmosphere

F. Montmessinâ^,^∗, O. Korablev^b, F. Lefèvre^c, J.-L.Bertauxâ, A. Fedorova^b,A. Trokhimovskiy^b, J.Y. Chaufrayâ, G.Lacombeâ, A. Reberacâ,L. Maltagliati^d, Y. Willameê, S. Guslyakova^b, J.-C. Gérard^f, A. Stiepenê, D. Fussen^f, N. Mateshvili^f,A. Määttänenâ,F. Forget^g, O. Witasse^h, F. Leblancâ, A.C. Vandaele^f, E.Marcqâ, B. Sandelⁱ, B. Gondet^j,N. Schneider^k, M.Chaffin^k, N. Chapronâ

aCNRS LATMOS, Laboratoire Atmosphères, Milieux, Observations Spatiales, Université Versailles St Quentin en Yvelines, Quartier des Garennes, 11 bd d’Alembert, 78280 Guyancourt, France

bSpace Research Institute, 84/32 Profsoyuznaya St, Moscow, Russia

cCNRS LATMOS, Université Pierre et Marie Curie, 4 place Jussieu, 75252 Paris, France

dCEA-Saclay, route Nationale, Gif-sur-Yvette, France

eBIRA-IASB, avenue Circulaire 3, Brussels, Belgium

fLPAP, Université de Liège, Quartier Agora, 19c allée du 6 août, Liège, Belgium

gCNRS LMD, 4 place Jussieu, 75252 Paris Cédex, France

hESA-ESTEC, Keplerlaan 1, 2201 AZ Noordwijk, Netherland

iLPL, University of Arizona, Tucson, AZ, United States

jInstitut d’Astrophysique Spatiale, Orsay, France

kLASP, Boulder, CO, United States

a rt i c l e i n f o

Article history:

Received 14 December 2016 Revised 11 May 2017 Accepted 14 June 2017 Available online 22 June 2017

a b s t r a c t

TheSPICAMexperimentonboardMarsExpresshasaccumulatedduringthelastdecadeawealthofob- servationsthat haspermitteda detailedcharacterization ofthe atmosphericcompositionand activity fromthenear-surfaceuptoabovetheexosphere.TheSPICAMclimatologyisoneofthelongestassem- bledtodatebyaninstrumentinorbitaroundMars,offeringtheopportunitytostudythefateofmajor volatilespeciesinthe Martianatmosphereoveramulti-(Mars)yeartimeframe. Withhisdualultravio- let(UV)-nearInfraredchannels,SPICAMobservesspectralrangesencompassingsignaturescreatedbya varietyatmosphericgases,frommajor(CO₂)totracespecies(H₂O,O₃).Here,wepresentasynthesisof theobservationscollectedforwatervapor,ozone,cloudsanddust,carbondioxide,exospherichydrogen andairglows.TheassembledclimatologycoverstheMY27–MY31period.However,themonitoringof UV-derivedspecieswasinterruptedattheendof2014(MY30)duetofailureoftheUVchannel.ASO₂de- tectionattemptwasundertaken,butprovedunsuccessfulfromregionaltoglobalscales(withupperlimit greaterthanalreadypublishedones).Oneparticularconclusionthatstandsoutfromthisoverviewwork concernstheway theMartianatmosphereorganizesaneﬃcient masstransferbetweenthelower and theupperatmosphericreservoirs.Thishighwaytospace,aswenameit,isbestillustratedbywaterand hydrogen,bothspecieshavingbeenmonitoredbySPICAMintheirrespectiveatmosphericreservoir.Cou- plingbetweenthetwoappeartooccuronseasonaltimescales,muchshorterthantheoreticalpredictions.

ThisisanopenaccessarticleundertheCCBY-NC-NDlicense.

(http://creativecommons.org/licenses/by-nc-nd/4.0/)

1. Introduction

The SpectroscopyforInvestigation ofCharacteristicsoftheAt- mosphere of Mars (SPICAM) instrument is a dual ultraviolet in-

∗ Corresponding author.

E-mail address: franck.montmessin@latmos.ipsl.fr (F. Montmessin).

fraredspectrometerthatwasdesignedto retrievetheabundances ofasetofmajorandminorspeciesoftheMartianatmosphere.On- boardtheMarsExpressmission,SPICAMhashadthecapabilityto performobservationswithavarietyofgeometricalconﬁgurations;

monitoringthecolumn-integratedabundancesofozone,waterva- porand aerosolsin a nadir-looking mode, aswell astheir verti- cal distribution between ∼10 and 150km using stellar and solar http://dx.doi.org/10.1016/j.icarus.2017.06.022

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Fig. 1. A compilation of ﬁve Martian years of observations by SPICAM in a nadir looking mode. The ﬁgure displays zonally averaged values as a function of solar longitude (Ls) of the retrieved (from top to bottom) water vapor, ozone, molecular oxygen singlet delta daytime emission in the near infrared, and dust as well as water ice opacity at 250 nm. The degradation of the UV channel has prevented reliable derivation of ozone and dust/clouds in the ultraviolet during MY31. The unit for each quantity, whose scale is plotted on the right hand-side, is given in brackets.

Note the high degree of repeatability in the seasonal behavior of all quantities, to the exception of dust during southern summer. Note also that displayed abundances have been averaged out of a variety of local times, from morning to afternoon.

occultations. In a dedicated limb staring mode where the Mars Expressspacecraft remains ina fixed inertial position,SPICAM is used to infer the density ofhydrogen atoms from 200 up to 10 000kmofaltitudewhilerelyingontheresonantlyscatteredsolar photonsattheLyman-alphaemissionlineat121.6nmofHydrogen atoms.Thissamemodeofobservationisusedfortheidentification ofa varietyof airglows, some ofthem beingassociatedwith the presenceofauroras,asdetectedforthefirsttimebySPICAMabove themagneticfieldcrustal anomalies(Bertauxetal., 2005a).Since MarsExpress orbitinsertion on December 23,2003, SPICAM has operated continuously, accomplishing one of the longest surveys todate ofthe Martianatmosphere(see Fig. 1),pursuing the ref- erenceclimatologyestablishedbytheMarsGlobalSurveyor(MGS) missionthatstartedneartheendofthe20thcenturyandpaving the way to the Imaging Ultraviolet Spectrograph (IUVS) onboard theMarsAtmosphereandVolatileandEvolution(MAVEN)thathas beenoperatingsinceSeptember2014.

Thepurposeofthispaperistoprovideanoverviewofthemain scientiﬁc results that have been collected thanks to the multi- annualmonitoring madeby SPICAM.Several resultsdescribed in thismanuscriptare presented ingreater depth in companionar- ticles that speciﬁcally address a particular theme of the SPICAM observations(Trokhimovskiyetal.,2015;Lefèvreetal.,inprepara- tion,2017;Guslyakovaetal.,2016).Thisarticleisorganizedaround thefollowingtopics:

• Lower atmosphere sounding including observations and map- pingofwatervapor,ozone,O₂ andaerosols.

• MiddleatmospheresoundingwithacharacterizationoftheCO₂, O₂,O₃andaerosolsfrom50to150kmaswellasNOemissions subsequenttoNandOatomsrecombination.

• Upperatmospheresoundingtoinfertheconcentrationofhydro- genandoxygenatomsabovetheexobase.

2. Instrumentstatus 2.1. Instrumentdescription

The SPICAM set-up comprises separate ultraviolet and near- infraredchannelsin oneoptical unit bothconnectedto thesame Data Processing Unit. For the interested reader, detailed techni- cal descriptions of theinstrument canbe found in Bertauxetal.

(2006)andKorablevetal.(2006a).

The ultraviolet channel uses an optical entrance of 4cm di- ameterfeedingan off-axisparabolicmirrorfocusing theobserved scene at the focal plane. The instantaneous ﬁeld of view (IFOV) is limited atthe focal point ofthe parabolic mirror by a 50μ^m

wideretractableslitthatextendsoveranangularapertureof2.8°, equivalentto about two pixels on the UV detector. Inthe upper mostportionoftheslit,a10timeswideraperture allowsforthe observations of fainter sources at the expense ofdegraded spectral resolution. Spectral dispersion is accomplished by a concave holographictoroidalgratingimprintingthediffractedordersatthe entrance of an MgF2 window covering a solar blind CsTe pho- tocathode with zero quantum eﬃciency beyond 320nm. An additional sapphireﬁlter is glued above the window and covers it

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150 200 250 300 Wavelength (nm) 0.0

0.2 0.4 0.6 0.8 1.0

Voigt Coefficients

150 200 250 300

Wavelength (nm) 1.0

1.2 1.4 1.6 1.8 2.0 2.2 2.4

Voigt FWHM (nm)

Fig. 2. (Left) Wavelength-dependent parameters of the Voigt profile H(a,u) that best fits the SPICAM UV channel Point Spread Function. The solid line refers to the damping parameter a while the dashed line is for the offset u/x. (right) Estimate of the full width at half maximum of the Voigt profile and its dependence to wavelength. The brutal variation above 250 nm is due to the effect of the sapphire window glued at the entrance of image intensifier, combined with the rapidly changing index of refraction of MgF2 at short wavelength. The absence of the window (pixels > 270) improves the focusing in the remaining range.

partially,preventingoverlappingofdiffractionordersandLyman-α

straylight. Thisdeﬁnestheentranceofthe imageintensiﬁer that consistsofaMicroChannelPlate(MCP)whosegainisadjustedby a high voltagelevel ranging from500 to 900 V. The CCD detector is a Thomson TH7863 with 288 × 384 pixels (23μ^m ^pitch)

cooled by a thermo-electric device ensuring lower dark current.

Thespectralresolutionoftheinstrumentiswavelengthdependent;

it exceeds1.25nm over theentirerange, increasingup to 2.3nm around260nm(seeFig.2).

ThefocallengthoftheUVtelescopeissuchthatoneCCDpixel covers a IFOVof0.01°× 0.01° Thenarrow partofthe slitofthe spectrometer is 0.02° wide by 1.9° long while the wide part is 0.2°wideby0.98°long.Toreduce thetelemetryburden,onlyﬁve binned areas of the detector are transmitted at the end of each acquisition. Thesebinnedareasconsist ofsummedlines ofpixels that correspond to contiguous areas in the observed scene. It is thus possible to obtain the spatialvariation ofthe UV spectrum alongtheaxisoftheslit.Thenumberoflinesperbinisdeﬁnedat commandingandisusuallychosen toaccommodatetheexpected brightnessoftheobservedscene.Forapointsourcesuchasastar, the standard binning covers 16 lines of pixels while for an extended sourcesuchastheMarssurface,thebinningisreducedto 4linesonthedetector.

The near-infraredchannel ofSPICAMhas inauguratedthefirst use of acousto-optic tunable filters (AOTF) in deep space. Built by IKI, the SPICAM near-IR channel accommodates a relatively high-resolution spectrometerinaverylightdesign(<800g).The AOTF operating principle relies on producing interferences between the incident optical beam and an acoustic wave in a bi- refringent medium(TeO₂). Theinput optical beamsustainsa vol- umediffractionthatisolates twodiffractionordersaway fromthe un-diffractedfractionoftheinputbeam. Theacousticwaveprop- agation insidethe TeO₂ crystalis generatedby a radio-frequency (RF) controllerthat synthetizesthefrequencyatwhichthetrans- ducer mustbe excited to createan acousto-opticinterferencein- side the crystal. Because there is a strict relation between the input RF frequency and the diffracted wavelength, one is able to retain a specific wavelength selected within a range equiva- lent to an octave of the RF controller. Because the energy remains concentrated within the first (+ 1 and −1) diffracted orders,suchdevicesdonotrequireadditionalfilterstorejecthigher diffraction orders. This leads to a simple and adaptable spectral analyzer (any wavelength can be accessed instantaneously).

ThereisnoslitintheAOTFspectrometer,increasingthethrough- put compared to traditional grating spectrometers. In turn the

AOTF relies on a progressive scanning of the wavelengths of in- terest to reconstructa given spectrum thus reducing the overall eﬃciency.

Withthisset-up, thenear-IR channel ofSPICAM iscapable of isolatingateachacquisitiona0.5–1.2nmbandwidthwithintheac- cessible1to 1.7μ^m^range.^In ^the^range^of^the ^water^vapor^band

the resolving power λ^/λ ^is ∼2000. Wavelengths are registered sequentiallywiththeincrementof∼0.3×λ^windowing^or^loose

samplingallows reducing the duration ofmeasurements. To bet- tercharacterizeatmosphericaerosols,thecontinuousabsorptionis alsomeasuredatseveraldistantwavelengths.TheIFOViscircular, andamountsto1°fornadirmeasurements,andto∼4whenob- servingtheSun.

2.2.Calibrationandinstrumentageing

As explained in Bertauxet al.(2006),we deﬁne the sensitiv- ityoftheSPICAMUVchannelasthesensitivityoftheinstrument fora stellar source placed atthe center ofthe spectrometer slit.

The nominalwavelengthassignment is definedaccordingly,mak- ing it validfor all extended source measurements collectedwith the slit in place. The so-called efficient area S_eff (in cm²) is a wavelength-dependentfunctionoftheactualinstrumentphotosen- sitive area collecting the photonsemitted by a point source(see Fig.3).ThedisplayedS_effcurveisobtainedwhenthestarsignalis integratedover 16CCD lines(centered onCDD line144) encom- passingroughly90%ofthetotalsignal(theremaining10%spread- ingawayfromthedispersionarea).Thedropatshortwavelength cut-off isdue to the opacity of MgF₂ window, while thedrop at longerwavelengthreflectsthedrop-off ofthephotocathode.

An updated version of the calibration procedure of the SPI- CAM UV spectra is given in Snow et al. (2013). We recall here themainexpression tobe usedforconvertingSPICAMUV detector readouts(in Analog-to-Digital Units,ADU) into physical units (photons×cm⁻²)forapointsource:

N_ADU/pixel/acquisition=g×Np×λ^×^Ti×S_{e f f}×fstar

andphotons×cm⁻²×sr⁻¹forextendedsources:

NADU/pixel/acquisition=g×Np×λ^×^Tⁱ^×^Se f f×ω

The number of ADUs N_ADU yielded by a pixel during one in- tegration is therefore proportional to the gain of the detector g, thenumberNp ofphotonsdetected per wavelengthunit andper second,thespectral samplingλ⁽∼0.54nmper pixel)andtothe integrationtime T_i(insecond).Incaseofanextendedsource,the

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Fig. 3. Updated version of the eﬃcient area Seffof the SPICAM UV channel based on the updated calibration procedure established by A. Reberac and reported in Snow et al. (2013) . The previously published version of Seff(see Bertaux et al., 2006 ) is shown for comparison in dashed line.

solidangle ω ^subtended^by ^a ^pixel⁽⁸ × 10⁻⁸sr)needs tobe in- cluded as an additional multiplicative term. The parameter fstar

referstothefractionoftheincidentphotonsfallingexactlyonthe pixel.Asindicated by Bertauxetal.(2006), abinning of16 lines of pixel contains around 90% of the stellar light (fstar=0.9). The recordedevolutionoftheinstrumentsensitivityisshowninFig.4 forthe10yearsoftheSPICAMsurvey.Theplottedquantityisthe instrumentgain g asafunction of theintensiﬁer highvoltage.It isclearthattheSPICAMUVchannelencounteredagradualdegra- dationfromlate 2011onwards thateventually led toa >10-fold reductionofsensitivity,stillallowingustomaintainacceptablesci- entiﬁcperformances.

2.3.Instrumentstatus

AsofDecember2014,SPICAMUVchannelhasceasedreturning sciencedata,sendingonlydarkframesinstead.Thissituationisthe outcomeofaprogressivedemiseoftheUV channel whoseorigin wasfirst identified 9 years before. In January 2006, the SPICAM UVchannel startedtoexhibit ananomalous image intensifierbe- havior,characterizedbymultiplesporadicandspuriouschangesof thehighvoltagesettingduringasequenceofobservation.TheSPI- CAMUV channeldatacleaning process haseventuallyallowed us toretainall healthyspectrawhileidentifyingmost(> 95%)ofthe affectedones.SPICAMLevel0Cdataintheultravioletavailableon theEuropean SpaceAgency’sPlanetaryScienceArchive havebeen producedthisway,eachrecordedspectrumbeingassociatedwith a flag parameter indicating whether the spectrum is infected or not.

For additionaldetails, the readerisinvitedto consult the Ap- pendixsectionwhereasummary isgivenofthemainchanges in dataprocessingandinourunderstanding oftheinstrumentchar- acteristicssince the ﬁrstpublication on thistopic(Bertauxetal., 2006).

TheIRchannel,ontheotherhand,hasexhibitedastablebehav- iorthroughouttheMEXmission,andnonoticeabledegradationof theIRspectraqualityhastobereportedatthispoint.

2.4.Dataset

Since thebeginning ofits operationsatMars, SPICAMhas ac- complishedmorethan12,500sequencesofobservations(seeTable 1) amongwhichseveralthousandsofatmosphericproﬁlesaswell asacontinuousmulti-annualtrackinginnadirmode.

3. Loweratmospheresounding(0–50km)

ContrarilytomostNASAmissions,theESAMarsExpressorbiter does not obey a sun-synchronous orbitand thus nadir mapping coversavarietyoflocaltimesfrommorningtoafternoon.Thereis a speciﬁc interestforidentifying andstudying thelocal time dependence of thespecies behavior (suchas O₃ andH₂O), butthe MarsExpressorbitdoesnotperformaregularlocaltimesampling andsomealiasingwithseasonaltimescaleisdiﬃculttoavoid.For thisreason,localtimedependenceisnotdiscussed inthefollow- ing yetshould be the purposeof futurein-depth analysisofthis dataset.

3.1. Watervapor

•Nadirmapping

The infraredchannel ofSPICAM hasprovideda nearly contin- uous survey of the column-integrated abundance of water vapor since2004usingthewatervapor1.38μ^m^absorption^band^(as^de-

scribed inFedorova etal., 2006a). Thisdataset spanningMartian Years27–31(seeFig.1)enablestheinvestigationofboththesea- sonalandtheinter-annualvariationsofatmosphericwater.Thera- diative transfer model used for the retrieval accounts for multi- ple scatteringcaused by dust and clouds.Comparedto the orig- inal version of the retrieval reported in Fedorova et al. (2006a), thedatatreatment(Trokhimovskiyetal.,2015)usedfortheresults presentedhereincorporatesanumberofimprovementsmainlyre- latedtoabettercharacterizationoftheinstrumentcalibrationand ofthemodelinputparameters.Thesensitivityofresultstoaerosol properties,surfacealbedo,solarspectrum,andwatervaporverti- caldistributionhasbeencarefullyevaluated.Forinstance,neglect- ing theaerosolscatteringbiasestheinferred wateramountbyas muchas60–70%duringdustyevents.Watervaporretrievalisalso sensitive tocloud activityespeciallyaround aphelionin thetrop- ics. However,theeffectofcloudsanddusthavebeenincludedin theretrieval.Informationondustandiceabundanceswereextrap- olated fromthepublishedTHEMIS dataset(whichspans untilthe middleofMY30-Smithetal., 2009)down tothenear-IRrangeof SPICAM.

SPICAM observations reveal a water vapor seasonal behavior that is to a large extent in agreement withprevious monitoring (inparticularTESdataset,asrevisedfromSmith,2004);watercol- umnintegratedabundancepeaksnear60–70pr.μ^mⁱⁿ^the^north-

ernsummerpolarregionasthepolarlayereddepositsgetexposed to intensiﬁed spring/summer insolation whereas only 20pr.-μ^m

is observed during the southern summer at the south pole. The equivalentvolume oficesublimatingasvapor intheatmosphere isabout0.5km³ atthebeginning oftheyearandlater buildsup to2.3km³ atLs=100° atatime whenwatervapor abundanceis foundmaximumonaglobalaverage.

To the exception of the MY28 dust event that is discussed below, SPICAM reveals a water cycle that is stable on average throughout the terrestrial decade monitored. With the revisit of the Mars Atmospheric Water Detector dataset (Fedorova et al., 2010), the picture of a Mars’ water cycle evolving in a quasi- stabilizedstateappearsmoreandmoreconﬁrmed(Richardsonand Wilson,2002; Montmessinetal., 2004,2017). Mars’swatercycle respondsessentiallytothelargeseasonalandspatialvariationsof insolationoverthe globe.It ispossiblethat variabilitymayoccur ontimescalesgreaterthanadecade,butthecompilationofwater vapormeasurementsmadetodatesuggeststhewatercycledidnot changesigniﬁcantlyovermorethan20Marsyears.

When comparing Martian Years (Fig. 1), one is able to cap- ture the prominent dropof water abundance that occursduring the dust storm ofMY 28. This sudden decrease of water, which was alsoreported in CRISM data (Smith et al., 2009), cannot be