Article available at http://www.europhysicsnews.org or http://dx.doi.org/10.1051/epn/2009601

ethane is present in significant quantitiesin the atmosphere of various extraterres-trial objects: the giant planets of the SolarSystem (Jupiter, Saturn, Uranus and Nep-

tune), but also Titan (satellite of Saturn), Triton (satelliteof Neptune), Mars, Pluto and, further away, browndwarves, some « cold» stars and giant exoplanets.e principal method for the determination of thechemical composition and physical conditions of theseplanetary atmospheres is spectroscopy. is opticaldiagnostic technique allows chemical species to beidentified from the light they absorb or emit at diffe-rent wavelengths.e understanding of a planetary atmospheric spec-trum requires the ability to correctly model that of itsdifferent compounds. In many cases, strong methaneabsorption bands dominate the spectrum of suchbodies. us, the detection of minor compounds(complex organic molecules, trace gases, etc…) or thedetermination of other parameters such as the surface

properties, involves the subtraction of the CH4 spec-trum. is, in turn, requires an extremely reliable modelvalid over a wide spectral range (from microwaves tonear infrared). Moreover, even at low or moderateresolution, the spectral absorption profile depends onthe underlying fine structure. Its modeling thus requiresthe analysis of high-resolution laboratory spectra,which involves the study of a huge number of quan-tum states and the identification of a huge number ofspectral lines.

Methane on TitanTitan, Saturn’s main moon, has a 5150 km diameter andpossesses a thick atmosphere mainly composed ofnitrogen, N2 (98 % in average), that does not absorblight, but also of an important quantity of methane(about 1.5% in the neutral atmosphere), a little oxygen,as well as various other organic molecules, the sign of acomplex chemical activity.Titan’s surface temperature is a low –179 °C and watercan only exist there as ice. On Titan, CH4 plays a rolesimilar to water on Earth. It is present in the gaseousform in the atmosphere, forms clouds and there is evi-dence for methane rains leading to mixed methaneand ethane rivers and lakes on the surface.

EPN 40/4 17

METHANE IN TITAN’S ATMOSPHERE:FrOM FUnDaMEnTal SPECTrOSCOPy TO PlanETOlOgy1

* Vincent Boudon1, Jean-Paul Champion1, Tony gabard1, Michel loëte1, athéna Coustenis2, Catherine De Bergh2,Bruno Bézard2, Emmanuel lellouch2, Pierre Drossart2, Mathieu hirtzig2, alberto negrão3 and Caitlin a. griffith4,

* 1 Institut Carnot de Bourgogne – UMR 5209 CNRS-Université de Bourgogne, Dijon, France

* 2 Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique, Observatoire de Paris-Meudon, Meudon, France

* 3 Istituto di Fisica dello Spazio Interplanetario, Via del Fosso del Cavaliere, I-00133 Roma, Italie et Faculdade de Engenharia da Universidade do Porto,

Porto, Portugal

* 2 Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA

* DOI: 10.1051/epn/2009601

The methane molecule (CH4) is relatively abundant in the Universe and in particular in our SolarSystem. On Earth, it is the main compound of natural gas and is also the second greenhouse gasof anthropic origin. On Saturn’s satellite Titan it plays a role similar to water on Earth and leadsto a complex chemistry.

1 An extendedversion of thispaper was firstpublished inFrench in May2008 in uB Sci-ences, volume3, the researchjournal of theUniversité deBourgogne. AFrench short-ened versionsimilar to thepresent onewas then pub-lished inReflets de laPhysique, vol-ume 11, pp.13–16 (2008),the journal ofthe SociétéFrançaise dePhysique.

note

M

� This image of Titan was taken by the Cassini spacecraftduring its first flyby of the largest moon of Saturn on July 2,2004. It shows a thin, detached haze layer floating abovethe orange atmosphere. © NASA/JPL/Space Science Institute

III

fEaturEs mEtHaNE iN titaN’s atmospHErE

is conception of Titan mainly comes form theobservations and measurements made by spacecralike Voyager 1 in 1980 and, essentially by the Cassini-Huygens mission (NASA/ESA/ASI) which, since July2004, has revolutionized our knowledge of Saturn’s sys-tem including Titan. One of its main features was theEuropean Huygens probe descent in Titan’s atmosphereand its landing on the surface on January 14, 2005 aera two and a half hour descent. e Cassini orbiter conti-nues to regularly flyby Titan and the other kronian

satellites with a host of different instruments (cameras,spectrometers, radar, …), supplementing observationsmade from Earth orbit (Hubble Space Telescope, ISOsatellite) or from the ground, oen at higher spectralresolution.A series of large and regularly spaced absorption bandsdue to methane dominate the Titan spectra recorded bythe DISR (Descent Imager/Spectral Radiometer) of theHuygens probe during its descent, and by VIMS (Visualand Infrared Mapping Spectrometer) on the orbiter.Images taken during the Huygens descent combinedwith radar images from the Cassini orbiter providevaluable information. Fluvial networks cover around1 % of the surface (see Figure 1). Large smooth areas,interpreted as methane and ethane lakes or seas, coverimportant parts of the polar regions.Furthermore, methane decomposition in the upperatmosphere leads to a series of chemical reactions pro-ducing various organic compounds such as ethane(C2H6) and other more complex hydrocarbons. Nitro-gen (N2) dissociation and its recombination withmethane leads to the formation of nitriles like hydrogencyanide (HCN). Polymerization of some compoundsproduces a complex material, which constitutes thesolid particles of the orange haze that fills the atmos-phere. ese particles become condensation cores forethane and other gases and continuously fall on Titan’s

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� Fig. 1:“Dried”

methanerivers imaged

by the Huygensprobe during

its descentonTitan.

© NASA/JPL/Space Science

Institute.

� Fig. 2:Methane’sspectrum

complexity.Horizontal linesrepresent vibra-

tion energylevels.The blackcurve gives the

number ofvibrational

sublevels foreach polyad.

The names cor-respond to

the differentabsorption

bands. Differentspectral regions

are illustratedby images and

spectra: in pink,a simulated

spectrum forlower polyadsand, in red, an

example ofthe spectra

recorded onTitan by the

Huygens probe.© NASA/JPL/

Space ScienceInstitute).

mEtHaNE iN titaN’s atmospHErE fEaturEs

surface. All this allows one to build up the scheme of atrue “methane cycle” on Titan that mimics the watercycle on Earth.e main question that arises about methane is that ofits origin because this molecule is efficiently destroyedin the upper atmosphere by solar radiation and the pro-cesses we described in the previous paragraph. So itshould have completely disappeared long ago. eremust then exist methane sources capable of re-fuelingthe atmosphere. According to recent models, lakesalone (unless very deep), are not sufficient to explainthe observed quantities (around 5 % of methane nearthe surface and 1.5 % in the stratosphere).Another pos-sibility would be the presence of methane trapped in icecrystals or formed by other processes (such as serpen-tinization) in the interior of the satellite. It could thenslowly find its way up to the surface where it would bereleased through cryovolcanic eruptions.

Methane spectrumCH4 has several remarkable spectroscopic properties.First, this molecule is highly symmetrical, the fourhydrogen atoms placed at the vertices of a regular tetra-hedron. A second essential characteristic of themethane spectrum is related to the ordering of itsenergy levels. e four characteristic vibrational fre-quencies of the atoms in the molecule display simpleratios between each other. e consequence is that thevibrational energy levels are grouped into sets calledpolyads that are regularly spaced. e more the energyincreases, the bigger is the number of levels in eachpolyad (see Figure 2). is grouping is responsible formethane’s large absorption bands.In environments like a planetary atmosphere contai-ning a few percent of methane and in which sunlightcrosses several hundred kilometers, even highly excitedpolyads, although extremely weak when observed in thelaboratory, can absorb almost all light at the correspon-ding wavelengths. Figure 2 shows a Saturn image takenby the Cassini spacecra on which the dark zones cor-respond to the absorption of light in the region of ahighly excited polyad. is absorption of light bymethane is also responsible for the opacity of Titan’satmosphere. However, transparency windows betweenpolyads exist. ey allow observing other compoundsin this atmosphere and, moreover, constitute the onlymeans to glimpse at Titan’s surface remotely. Again inFigure 2, another Titan image taken by the Cassini spa-cecra through one of these spectral windows revealsthe topography of the Saturnian moon.us, the modeling of the absorption of light in suchatmospheres necessitates considering a huge number of

excited states. But the complexity goes further! erotational structure of the spectrum superimposes ontothe vibrational one. Because of the evenhigher number of rotational states (seeFigure 3), it is mandatory to undertakehigh-resolution studies in order to finelyreproduce the profile of the observedbands, since this depends on the under-lying structure. Moreover, intensitycalculations should be as precise as possible, in view ofconcentration measurements. Intensities also dependon the temperature. Reciprocally, the precision of theintensity model conditions that of the temperaturemeasurement. Finally, a reliable model provides amuch bigger flexibility than an interpolation from labo-ratory measurements, which is necessarily limited.

recent resultsCalculated methane spectral line lists have recently allo-wed significant contributions to the analysis of somemeasurements concerning Titan.For instance, using these lists, it was possible to interpretISO satellite data from 1997 in the 2.4 – 4.9 μm spec-tral range [1]. e advantage of using a wide spectralrange instead of a window-by-window study (as inprevious works) is to allow for a better determination ofthe spectral behavior of Titan and hence a betterconstraint over the aerosol model for Titan, as well as adetermination of the methane abundance in the loweratmosphere (below 3 %). Furthermore, albedo measu-rements of Titan’s surface, obtained simultaneouslythrough several infrared windows aer the atmosphericsubtraction, allowed scientists to constrain Titan’s sur-face composition, shown to be compatible with thepresence of water ice and organics.

EPN 40/4 19

� Fig. 3:Simulatedmethanespectrum atincreasingspectralresolutions,showing therotationalstructure (inblue). In theupper panel,the magentacurve repre-sents alow-resolutioncalculatedspectrum. Thelower rightpanel com-pares a verysmall part ofthe calculatedspectrum witha laboratorymeasurement(in red).

On Titan, Ch4

plays a role similarto water on Earth

III

fEaturEs mEtHaNE iN titaN’s atmospHErE

Another recent major contribution was the evidence ofa vast polar ethane cloud at Titan’s North pole [2]. iswas made possible thanks to the simulation of themethane spectrum in the so-called octad region (seeFigure 2) around 2.1 to 2.2 μm. As illustrated in Figure4, it was shown that the difference between spectrainside and outside the cloud is due to diffusion bysmall particles in the 30 to 50 km altitude range, withcharacteristics indicating that this cloud is likely to becomposed of ethane. Ethane (C2H6), the main productof methane photodissociation by solar radiation, should

precipitate on the ground. e apparent absence ofliquid ethane on the ground at mid latitudes of Titanwas up to recently a mystery. ese new observationssuggest that in fact ethane seems to condensate in thistype of polar clouds and is then likely to accumulate inthe lakes that are present in these regions.Calculated methane absorption coefficients in the midinfrared allow for a correct modeling of Titan’s atmos-pheric absorption as measured by the CFHT [3], fromwhich surface albedo variations can be deduced.Finally, spectra were recorded between 2.03 and 2.40μm thanks to the VLT for different regions of Titan,including the Huygens probe landing site, as can beseen in Figure 5 [4]. Calculated absorption coeffi-cients, using the methane abundance profile, asmeasured by the Huygens probe, allow again for a cor-rect modeling of Titan’s atmospheric absorption,from which the surface albedo can be deduced in aspectral domain that was not covered by Huygens’DISR instrument.

PerspectivesAdvances in the theoretical model presented here(mainly with respect to higher polyads and spectral lineshapes), associated with experimental work at Titan'satmospheric conditions, would allow us in the future tobetter constrain the methane opacity. is would lead tothe resolution of remaining issues with the interpreta-tion of Cassini-Huygens and Earth-based near-IR Titandata, concerning essentially the vertical haze profileand the composition of Titan's surface. �

acknowledgmentse authors recognize financial support from theAgence Nationale de la Recherche, within the ANR-08-BLAN-0254 project.

references[1] Titan's 3-Micron Spectral Region from ISO High-Resolution

Spectroscopy, A. Coustenis, A. Negrão, A. Salama, B. Schulz,E. Lellouch, P. Rannou, P. Drossart, T. Encrenaz, B. Schmitt,V. Boudon and A. Nikitin, Icarus 180, 176 (2006).

[2] Evidence for a Polar Ethane Cloud on Titan, C.A. Griffith,P. Penteado, P. Rannou, R. Brown, V. Boudon, K.H. Baines,R. Clark, P. Drossart, B. Buratti, P. Nicholson, C.P. Mckay,A. Coustenis, A. Negrão and R. Jaumann,Science313, 1620 (2006).

[3] Titan’s Surface Albedo from Near-Infrared CFHT/FTS Spectra :Modeling Dependance on the Methane Absorption,A. Negrão, A. Coustenis, E. Lellouch, J.-P. Maillard, P. Rannou,B. Schmitt, C.P. Mckay and V. Boudon, Planetary and SpaceScience 54, 1225 (2006).

[4] 2-μm Spectroscopy of Huygens Probe Landing Site on Titan withVLT/NACO, A. Negrão, M. Hirtzig, A. Coustenis, E. Gendron,P. Drossart, P. Rannou, M. Combes and V. Boudon, Journalof Geophysical Research 112, E02S92 (2007).

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� Fig. 5: Upper right: VLT (Very Large Telescope) image. Bright zones correspond tohigh albedo regions. Top: Measured spectrum around the Huygens landing site witha resolving power of 700 (in red) and simulated spectrum (in green). Bottom: rela-tive difference between measurements and the best model (in blue). Above 2.12 μm,the measurement error is ± 27 %. The vertical haze profile and the surface albedo at2 μm were determined from these measurements.

� Fig. 4: left: Albedo (proportion of reflected solar light) maps of Titan at 2.8 μmobtained by the VIMS instrument of the Cassini spacecraft. Red areas have highestalbedo. The red spot near the North Pole (NP) is an ethane cloud. Upper right: Albedoas a function of latitude at 2.11 μm (in black) and 2.17 μm (in magenta), revealing thediffusion by the cloud that is visible at 2.11 μm beyond 51°N. lower right: measured(dots) and simulated (curves) spectra at two different latitudes. The differencebetween spectra at these two latitudes reflects the diffusion by the ethane cloud thatis situated around 30 to 50 km altitude. Figure from Reference [2].