Pluto's lower atmosphere structure and methane abundance from high-resolution spectroscopy and stellar occultations

0.3 MB·English

by  E. Lellouch

Astronomy&Astrophysicsmanuscriptno.pluto˙ch4 (cid:13)c ESO2009
January30,2009
Pluto’s lower atmosphere structure and methane
abundance from high-resolution spectroscopy
9 and stellar occultations
0
0
2 E.Lellouch1,B.Sicardy1,2,C.deBergh1,H.-U.Ka¨ufl3,S.Kassi4,andA.Campargue4

n
a
J 1 LESIA,ObservatoiredeParis,5placeJulesJanssen,92195Meudon,France

0 e-mail:[email protected]
3
2 Universite´PierreetMarieCurie,4placeJussieu,F-75005Paris,France;seniormemberofthe

] InstitutUniversitairedeFrance
P
E 3 EuropeanSpaceObservatory,Karl-Schwarzschild-Strasse2,D-85748GarchingbeiMu¨nchen,
. Germany
h
p 4 LaboratoiredeSpectrome´triePhysique,Universite´ JosephFourier,BP-87,F-38402St-Martin
-
o d’He`resCedex,France
r
t
s ReceivedJanuary,9,2009;revisedJanuary27,2009,accepted,January29,2009
a
[

ABSTRACT
1
v
2 Context.Plutopossessesathinatmosphere,primarilycomposedofnitrogen,inwhichthedetec-
8
tionofmethanehasbeenreported.
8
4 Aims.The goal is to constrain essential but so far unknown parameters of Pluto’satmosphere
.
1 suchasthesurfacepressure,loweratmospherethermalstucture,andmethanemixingratio.
0
Methods.Weuse high-resolution spectroscopic observations of gaseous methane, and a novel
9
0 analysisofoccultationlight-curves.
:
v Results.Weshowthat(i)Pluto’ssurfacepressureiscurrentlyinthe6.5-24µbarrange(ii)the
i
X methane mixing ratiois0.5±0.1%, adequate toexplain Pluto’sinverted thermal structure and
r ∼100 K upper atmosphere temperature (iii) a troposphere is not required by our data, but if
a
present, it has a depth of at most 17 km, i.e. less than one pressure scale height; in this case
methaneissupersaturatedinmostofit.Theatmosphericandbulksurfaceabundanceofmethane
arestrikinglysimilar,apossibleconsequenceofthepresenceofaCH -richtopsurfacelayer.
4
Keywords.Solarsystem:general;Infrared:solarsystem;KuiperBelt
1. Introduction
Since its detection in the 1980s (Brosch, 1995, Hubbard et al. 1988, Elliot et al. 1989), stellar
occultationshaverevealedremarkablefeaturesofPluto’stenuous(µbar-like)atmosphere.Pluto’s
upperatmosphereisisothermal(T∼100Kataltitudesabove1215kmfromPluto’scenter)andhas
undergonea pressureexpansionbyafactorof2from1988to2002,probablyrelatedtoseasonal
cycles,followedbyastabilizationover2002-2007(Sicardyetal.2003,Elliotetal.2003,2007,E.
Youngetal.2008).Belowthe1215kmlevel,occultationlightcurvesarecharacterizedbyasharp
2 E.Lellouchetal.:Pluto’sloweratmospherestructureandmethaneabundance
drop(“kink”)influx,interpretedasduetoeithera∼10km-thickthermallyinvertedlayer(strato-
sphere)orabsorptionbyalow-altitudehazewithsignificantopacity(>0.15inverticalviewing).So
far,observationsofstellaroccultationshavenotprovidedconstraintsontheatmosphericstructure
atdeeperlevels,noronthesurfacepressure.
While Pluto’s atmosphere is predominantly composed of N , the detection of methane has
2
been reportedfrom 1.7 µm spectroscopy (Younget al. 1997),leading to a roughestimate of the
CH column density (1.2 cm-am within a factor of 3-4). Even before its detection, methane had
4
beenrecognizedtobethekeyheatingagentinPluto’satmosphere,abletoproduceasharpthermal
inversion (Yelle and Lunine, 1989, Lellouch 1994, Strobel et al. 1996). The large uncertainty in
thedataofYoungetal.,however,aswellastheunknownN columndensity,didnotallowoneto
2
determinetheCH /N mixingratio.
4 2
We here report on high-quality spectroscopic observations of gaseous CH on Pluto, from
4
which we separately determine the column density and equivalent temperature of methane.
Combining this information with a novel analysis of recent occultation lightcurves, we obtain a
precise measurement of the methane abundance, as well as new constraints on the structure of
Pluto’sloweratmosphereandthesurfacepressure.
2. VLT/CRIRESobservations
Plutoobservationswereobtainedwiththecryogenichigh-resolutioninfraredechellespectrograph
(CRIRES, Ka¨ufletal. 2004)installedonESO VLT(EuropeanSouthernObservatoryVeryLarge
Telescope)UT1(Antu)8.2mtelescope.CRIRESwasusedinadaptiveopticsmode(MACAO)and
witha0.4”spectrometerslit.TheinstrumentconsistsoffourAladdinIIIInSbarrays.Wefocussed
on the 2ν bandof methane,coveringthe 1642-1650,1652-1659,1662-1670and1672-1680nm
3
ranges, at a mean spectral resolution of 60,000,almost five times better than in the Young et al.
(1997) observations. Observations were acquired on August 1 (UT = 3.10-4.30) and 16 (UT =
0.55-2.20), 2008, corresponding to mean Pluto (East) longitudes of 299o and 179o respectively.
(WeusetheorbitalconventionofBuieetal.(1997)inwhichtheNorthPoleiscurrentlyfacingthe
Sun). Pluto’stopocentricDopplershift was +20.0and +24.8km/s (i.e. ∼0.11and ∼0.14nm) on
thetwodatesrespectively,ensuringproperseparationofthePlutomethanelinesfromtheirtelluric
counterparts.Oneachdate,wealsoobservedonetelluricstandardstar(HIP91347andHIP87220,
respectively).WeemphasizeheretheAugust1data,whichhavethehighestquality.
3. InferencesonPluto’sloweratmospherestructureandmethaneabundance
Theobservedspectrum(Fig. 1)showsthedetectionofnolessthan17methanelinesoftheP,Q
andRbranchesofthe2ν band,includinghighJ-levellines(uptoR7 andQ8),aswellas, more
3
marginally,thepresenceofafewweakerlinesbelongingtootherband(s)ofmethane(seebelow).
Thisspectralrichnessmakesitpossible,forthefirsttime,toseparatetemperatureandabundance
effectsinthePlutospectra.
Spectraweredirectlymodelledusingatellurictransmissionspectrumcheckedagainstthestan-
dardstarsobservations,a solar spectrum(FiorenzaandFormisano,2005)anda line-by-linesyn-
theticspectrumofPluto.ThethreecomponentswereshiftedaccordingtotheirindividualDoppler
shifts,andthenconvolvedtotheinstrumentalresolutionof60,000,determinedbyfittingthewidth
E.Lellouchetal.:Pluto’sloweratmospherestructureandmethaneabundance 3
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Fig.1. Black: Pluto spectrum observed with VLT/CRIRES. Red: Best-fit isothermal model (90
K, 0.75 cm-am CH ), including telluric and solar lines. The general continuum shape is due to
4
absorptioninthe2ν +ν and2ν bandsofsolidmethane(seeDoute´etal.1999)
2 3 3
ofthetelluriclines(andcorrespondingtoaneffectivesourcesizeof0.33”).FormodellingthePluto
spectrum,weusedarecentCH linelist(Gaoetal.2009),basedonlaboratorymeasurements(po-
4
sitions and intensities) at 81 K, and including lower energy levels for 845 lines, determined by
comparison with the intensities at 296 K collected in the HITRAN database. Although the tem-
peratureof laboratorydata issimilar to Pluto’s, we used onlylinesforwhich energylevelswere
available, in order to avoid dubious extrapolation towards lower temperatures. These data show
that,inadditiontotheJ-manifoldsofthe2ν band,thespectralrangecontainsotherlinesoflow
3
energylevel(e.g.J=2near6085.2cm−1,seeFig. 2),whichappeartobemarginallydetectedin
thePlutospectrum(seeFig.3).
3.1.Isothermalfits
Wefirstmodelledthedataintermsofasingle,isothermalmethanelayer.Becausecollisionalbroad-
eningisnegligibleatthelowpressuresofPluto’satmosphere,resultsatthisstepareindependent
of Pluto’s pressure-temperaturestructure. Scattering was ignored, as justified below. The outgo-
ing radiation was integrated over angles, using the classical formulation in which the two-way
transmittanceisexpressedas2E (2τ),whereτisthezenithalopticaldepthoftheatmosphere.A
2
least-squareanalysisofthedatawasperformedinthe(temperature(T),columndensity(a))space.
Fig. 3showsthatthebestfitoftheAug.1,2008dataisachievedforT=90K.Toolow(resp.too
high)temperaturesleadtoanunderestimate(resp.overestimate)ofthehigh-Jlinesandanoveres-
timate(resp.underestimate)ofthelow-Jlines.Basedonleast-squarefitting,weinferredT=90+25
−18
4 E.Lellouchetal.:Pluto’sloweratmospherestructureandmethaneabundance

-1 m) 0.003
c
K (
1 J=2 J=1
8
1 torr, 0.002 ν,
@ J=2 2 3 J=7
nt
e
ci
effi
o 0.001
c
n
ptio J=3
or J=3
s
b J=3
A
0.000
6083 6084 6085 6086 6087 6088
-1
Wavenumber (cm )
Fig.2.Laboratoryspectrumofmethaneat81K inthe6083-6088cm−1 range,demonstratingthe
existenceof strong,lowJ-level,linesin additiontothe R-branchmanifoldsof the2ν band.The
3
J-levelfortheselinesisdeterminedbycomparisonoftheirintensityat81Kandatroomtemper-
ature(seeGaoetal.2009).TheJ=2doubletnear6085.2cm−1 ismarginallydetectedinthePluto
spectrum(1643.4nm,seeFig. 3).
K and a = 0.75+0.55 cm-am forthe data of August1, and similar numbers(T = 80+25 K and a =
−0.30 −15
0.65+0.35cm-am)forAugust16.
−0.30
3.2.Combinationwithinferencesfromstellaroccultations
The above inferred methane temperatures, much warmer than Pluto’s mean surface temperature
(∼50 K, Lellouch et al. 2000) are inconsistent with the existence of a deep, cold and methane-
richtroposphere,suchasthe ∼40kmtroposphereadvocatedto matchestimatesofPluto’sradius
fromthePluto-Charonmutualevents(Stansberryetal.1994).Toquantifythisstatement,wecom-
binedourspectroscopicdatawithanewassessmentofstellaroccultationlight-curves.Besidesthe
isothermalpartandthe“kink”featurementionedpreviously,recenthigh-quality,occultationcurves
(Sicardyetal.2003,Elliotetal.2003,2007,E.Youngetal.2008,L.Youngetal.2008)exhibita
numberofremarkablecharacteristics:(i)alowresidualfluxduringoccultation,typicallylessthan
3 % of the unattenuatedstellar flux (ii) the conspicuousabsence of caustic spikes in the bottom
part of the light-curves(iii) the existence of a central flash, caused by Pluto’s limb curvature,in
occultationsinwhichtheEarthpassednearthegeometriccentreoftheshadow.
To determine the range of Pluto’s thermal structures that can account for these features, we
performedray-tracingcalculationsforavarietyoftemperature/pressureprofiles,expandingupon
theworkofStansberryetal.(1994).Forthistask,weassumedaclearatmosphere.Thisisjustified
by(i)theabsenceofcolourvariationinthecentralflash(L.Youngetal.2008)and(ii)thedifficulty
forhazestobeproducedphotochemicallyattherequiredopticaldepthinatenuousatmospherelike
Pluto’s(Stansberryetal.1989).Wethusadoptedthe“stratosphericgradient”interpretationofthe
light-curves,andexploredabroadrangeofsituations,varyingthevalueofthisgradient,thelevel
atwhichtheinversionlayerconnectstoatroposphere(i.e.thetropopausepressure),andthedepth
andlapserateofthistroposphere(Fig.4).
E.Lellouchetal.:Pluto’sloweratmospherestructureandmethaneabundance 5
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Fig.3.Modelfitting ofthe August1,2008Plutospectrum(histograms)zoomedonfourspectral
regions.TheblackcurveisamodelwithnomethaneonPluto.The90K,70K,and120Kcurves
indicateisothermal,single-layer,fits, including0.75cm-am,1.3cm-amand0.45cm-amofCH ,
4
respectively.Therotationaldistributionoflinesindicatesthata90Ktemperatureprovidesthebest
fit.The“R=1193km”model(pink,fitalmostindistinguishabletothe90Kmodel)correspondstoa
6K/kmstratospherictemperaturegradient,a1193kmradius(7.5µbarsurfacepressure)anda0.62
% methane mixingratio. The “R=1168km” modelincludesa 6 K/km stratospheric temperature
gradient, joining with a wet tropospheric lapse rate of -0.1 K/km below 1188 km (tropopause)
andextendingdowntoa 1168kmsurfaceradius(29µbar).This20km-deeptropospheremodel,
optimized here with CH = 0.36 %, is inconsistent with the methane spectrum; for this thermal
4
profile,theminimumradiusis1172km(seeFig.4).Thewavelengthscaleisintheobserverframe.
These spectral regions are those showing maximum sensitivity to the methane temperature (or
equivalentlydepth of the troposphere),as they include low J-level and high J-level lines, but for
quantitativeanalysis,aleast-squarefitonalllineswasperformed.
Wereachedthefollowingconclusions(Fig. 4and 5):(i)thestratospherictemperaturegradient
isinthe3-15K/kmrange.Gradientssmallerthan3K/kmwouldleadtoresidualfluxesinexcess
of 3 %; gradients larger than 15 K/km produce residual fluxes lower than 1 %, and are anyway
notexpectedfromradiativemodels(Strobeletal.1996)(ii)withinthisrange,theexistenceofthe
centralflash implies a minimumatmosphericpressure of 7.5±1.2bar (iii) the absence of caustic
spikes in the region of low residual flux puts stringent constraints on a putative troposphere. In
most cases, it restricts such a troposphereto be at most shallow (2-5 km deep, dependingon its
meantemperature),andthesurfacepressuretobelessthan∼10µbar.Anexceptionisthefamily
ofthermalprofileswithintermediate(5-7K/km)stratospherictemperaturegradientsandacold(<
38K)tropopause,whichappearconsistentwithoccultationcurvesforanytroposphericdepth.In
6 E.Lellouchetal.:Pluto’sloweratmospherestructureandmethaneabundance
Fig.4. Range of possible thermal profiles (pressure-temperature (left) and radius-temperature
(right)) in Pluto’s atmosphere. From bottom to top, they have stratospheric thermal gradients of
3 and 4 K/km (red profiles), 5 K/km (one orange and one green), 6 K/km (two green), 7 K/km
(green),and9and15K/km(blue).Allprofilesarecontinuousinfirstandsecondordertemperature
derivatives.Mostoftheseprofileshavenoorverylimitedtropospheres(lessthan5kmindepth),
inordertomatchtheresidualfluxobservedduringstellaroccultationsandavoidtheformationof
strongcaustics(seeFig.5).Onlyprofilesingreenandorange,withmoderatestratospherictemper-
aturegradients(5-7K/km)andacoldtropopause(<38K)canhavesignificanttropospheres.The
lapserateinsuchtropospheresrangesfrom-0.1K/km,correspondingtotheN wetadiabat(green
2
profiles)to-0.6K/km(N dryadiabat,orangeprofile).TheCRIRESspectraindicatethatthesewet
2
and dry profilescannotextenddeeper than p ∼24µbar (1172and 1169km, respectively).In the
leftpanel,thesolidlineonthetoprightisthelocusofminimumatmosphericpressureimpliedby
theobservationofacentralflash,andthesolidlineontheleftisthevapourpressureequilibrium
ofN .Thedashed-dottedlineisthevapourpressureequilibriumofCH fora0.5%mixingratio.
2 4
Thedottedlineat50Killustratesthemaximumpossiblenear-surfacegastemperature.Theshaded
areas represent the range of possible tropospheres. If Pluto has a troposphere, methane must be
supersaturatedovermostofit.
fact,suchprofiles(greencurvesinFig.4and5)leadtomodestcausticspikesintheregionofthe
“kink”,i.e.wherespikesareobservedinactualobservations,forwhichtheycanbemistaken.
The allowed thermal profiles were finally tested against the methane spectrum. We assumed
uniformatmosphericmixing,aplausiblecasegiventhat(i)thesourceofmethaneisatthesurface
(ii)itsequivalenttemperatureimpliesthatalargefractionofmethaneisintheupperatmosphere,
andperformedaleast-squareanalysisofthedatainthe(surfaceradius,CH mixingratio)domain.
4
Notsurprisinglyinviewoftheisothermalfits,thermalprofileshavingno(oramini-)troposphere
areallconsistentwiththemethanespectrum.Forexample,forastratospherictemperaturegradient
of6K/km,asurfaceradiusof1193km(surfacepressure=7.5µbar,i.e.theminimumrequiredby
the occultations)providesan adequatefit ofthe August1 data fora CH mixingratioof 0.62%
4
.Incontrast,profilesincludingtoodeepatropospherecanberejectedasgivingtoomuchweight
to cold methane and leading to a line distribution inconsistentwith the data. Based on such fits,
the maximum troposphericdepth is found to be 17 km (i.e. 0.85 pressure scale heights) and the
E.Lellouchetal.:Pluto’sloweratmospherestructureandmethaneabundance 7
Fig.5.Ray-tracingcalculationsofoccultationlight-curvesforrepresentativetemperatureprofiles
of Fig. 4. The shaded area near the bottom of the light-curve represents the range of residual
flux(0.00-0.032)observedintheCFHTAugust21,2002occultation(Sicardyetal.2003),witha
closestapproachtoshadowcentreof597km.(WeestimatethattheAATJune12,2006occultation
light-curve (E. Young et al. 2008) consistently indicates a 0.01-0.03 residual flux). Red: light-
curve for the thermal profile with 3 K/km stratospheric gradient of Fig. 4, extending to 9 µbar.
This“stratosphere-only”modelisconsistentwithobservedlight-curves.Blue:light-curveforthe
thermal profile with 15 K/km stratospheric gradient, and a 4-km deep troposphere at ∼36.5 K.
Thisprofileproducesanunacceptablecausticsspike,causedbythesecondary(“farlimb”)image
Green: light-curve for a thermal profile with 6 K/km gradient in the inversion layer, joining the
N saturationvapourpressurewitha∼-0.1K/kmgradientinthetroposphere.Inthiscase,modest
2
causticsarestillproduced,buttheyappearnearthelight-curvekink.
maximumsurfacepressure is24 µbar. Takingallconstraintstogether,Pluto’ssurfacepressurein
2008isintherange6.5-24µbar.Therangeofmethanecolumndensitiesis0.65-1.3cm-am.Deeper
(i.e. colder) models require larger methane columns than shallower models, but since they also
haveahighersurfacepressure,themethanemixingratioisaccuratelydeterminedtobe0.51±0.11
%. Constraints from the August 16 data are somewhat looser (a maximum surface pressure and
tropospheredepthof32µbarand23km,respectively).TheminimumPlutoradiusimpliedbythe
data is 1169-1172km (Fig. 4). This value holdsfor the nominalastrometric solutionsfor stellar
occultations,typicallyuncertainby∼10km.Giventhisuncertainty,ourlowerlimitontheradiusis
consistentwithmostinferencesfromthemutualevents(nominally1151-1178km,see Tholenet
al.1997).Thetropospheredepthisfreefromthisuncertainty,andthereforebetterconstrainedthan
Pluto’sradius.
8 E.Lellouchetal.:Pluto’sloweratmospherestructureandmethaneabundance
4. Discussion
4.1.Methanemixingratioandpossiblesupersaturation
Throughabsorptionofsolarinputinthenear-IRandradiationat7.7µm,methaneisthekeyheat-
ing/coolingagentinPluto’satmosphere,andinparticularmustberesponsibleforitsthermalinver-
sion.Detailedcalculations(Strobeletal.1996)showthat,eveninthepresenceofCOcoolingand
foranassumed3µbar“surface”(i.e.baseoftheinversionlayer)pressure,a0.3%methanemixing
ratio produces a 7 K/km “surface” gradient and a temperature increase of ∼36 K in the first 10
km.Althoughsuchcalculationswillneedtoberedoneinthelightofourresults,a0.5%methane
mixingratioisclearlyadequatetoexplainthe∼6K/kmgradientindicatedbytheoccultationdata,
furtherjustifyingourassumptiontoneglecthazeopacity.
The presence of methane in Pluto’s stratosphere implies that it is not severely depleted by
atmospheric condensation. Yet, a remarkable result is that for models including a troposphere,
methaneappearstobesignificantlysupersaturated(Fig. 4),byasmuchasafactor∼30fora∼38
Ktropopause.GiventhatPluto’stroposphereisatmostshallow(lessthan1pressurescaleheight),
thisplausiblyresultsfromconvectiveovershootassociatedwithdynamicalactivity,combinedwith
apaucityofcondensationnucleiinaclearatmosphere.
4.2.Theoriginoftheelevatedmethaneabundance
In agreementwith Younget al. (1997),the CH / N mixing ratio we deriveis ordersof magni-
4 2
tudelargerthanthe ratiooftheirvapourpressuresatanygiventemperature,andthe discrepancy
isevenworseifoneconsidersthatmethaneisaminorcomponentonPluto’ssurface.Twoscenar-
ios(Spenceretal.1997,Traftonetal.1997)havebeendescribedtoexplainthiselevatedmethane
abundance(i)theformation,throughsurface-atmosphereexchanges,ofathinmethane-richsurface
layer(theso-called“detailedbalancing”layer),whichinhibitsthesublimationoftheunderlying,
dominantlyN , frost,andleadsto anatmospherewith the same compositionasthisfrost(ii)the
2
existenceofgeographicallyseparatedpatchesofpuremethane,warmerthannitrogen-richregions,
andwhichundersublimationboosttheatmosphericmethanecontent.Interestingly,detailedanal-
ysesof1.4-2.5µmand1-4µmmid-resolutionspectragiveobservationalcredittobothsituations.
Itisnoteworthythatour0.5%atmosphericabundanceisidenticaltotheCH /N ratiointheN -
4 2 2
CH -COsubsurfacelayerofDoute´etal.(1999),consistentwiththedetailedbalancingmodel,and
4
agreesalsowiththesolidmethaneconcentrationinferredbyOlkinetal.(2007)(0.36%).Inthis
framework,atypical15µbarsurfacepressurecouldbeexplainediftheN -CH -COsubsurface
2 4
layerisat40.5K(consistentwiththeN icetemperaturemeasurementsofTrykaetal.,1994)and
2
overlaid by a 80 % CH - 20 % N surface layer. On the other hand, and in favour of the alter-
4 2
natescenario,thermalIRlightcurves(Lellouchetal.2000)aswellassublimationmodelsforCH
4
(Stansberryetal.1996)indicatethatextendedpureCH patchesmayreachdaysidetemperatures
4
wellinexcessof50K;thisismorethansufficienttoexplainthe∼0.075µbarCH partialpressure
4
indicatedbyourdata.
Discriminatingbetween the two cases may rely onthe time evolutionofthe N pressureand
2
CH mixingratio.Inparticular,thedecreaseofatmosphericCH withincreasingheliocentricdis-
4 4
tance is expected to lead to a drop of the CH abundance in the detailed balancing layer, which
4
E.Lellouchetal.:Pluto’sloweratmospherestructureandmethaneabundance 9
maydelay the decreaseofthe N pressure(Traftonet al. 1998).AssumingT = 100K, Younget
2
al.(1997)reporteda0.33-4.35cm-ammethaneabundancein1992.Althoughtheirerrorbarsare
verylarge,their bestfit value(1.2cm-am)islargerthanours(0.65cm-amforthis temperature).
Combinedwith the factorof ∼2 pressureincreasebetween1988and2002,thissuggeststhatthe
methane mixing ratio is currently declining. The ALICE and Rex instruments on New Horizons
will measure Pluto’s surface pressure and methane abundancein 2015. Along with the data pre-
sentedinthispaper,thiswillprovidenewkeysontheseasonalevolutionofPluto’satmosphereand
thesurface-atmosphereinteractions.
Acknowledgements. ThisworkisbasedonobservationsperformedattheEuropeanSouthernObservatory(ESO),proposal
381.C-0247.WethankDarrellStrobelforconstructivereviewing.
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