Evidence for Methane Segregation at the Surface of Pluto

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<ul><li><p>Icarus 142, 421444 (1999)Article ID icar.1999.6226, available online at http://www.idealibrary.com on</p><p>Evidence for Methane SegregationS. Doute,1 B. Schmitt,2 and E.</p><p>Laboratoire de Glaciologie et Geophysique de lEnvironement, 54 rue MolE-mail: sdoute@igpp.ucla.ed</p><p>T. C. Owen4</p><p>Institute for Astronomy, University of Hawaii, 2680 Woodlaw</p><p>D. P. Cruikshank4</p><p>NASA Ames Research Center, MS 245-6 Moffett Field</p><p>C. de Bergh4</p><p>Observatoire de Paris, 5 Place Jules Janssen 921</p><p>T. R. Geballe5</p><p>Joint Astronomy Center, 660 North Aohoku Place, Univer</p><p>and</p><p>T. L. RoushSan Francisco State University and NASA Ames Research Center, MS 2</p><p>Received February 1, 1999; revised Aug</p><p>In May 1995, a set of spectrophotometric curves of the systemPlutoCharon were recorded with the UKIRT telescope equippedwith the spectrometer CGS4. The spectra cover the near-infraredrange between 1.4 and 2.55 m with a resolution of approximately700. The existence of solid methane is confirmed by numerous ab-sorption bands, and carbon monoxide and nitrogen ices are iden-tified by their respective signatures at 2.35 and 2.15 m. We havemodeled the spectrum of May 15 that corresponds to the maximumof Plutos visible lightcurve using a radiative transfer algorithmdealing with compact and stratified media. A geographical mixtureof three distinct units is required to explain all the significant struc-tures of the analyzed spectrum. The first unit is a thin, fine-grainedlayer of pure CH4 covering a compact polycrystalline substratum</p><p>1 Present address: Institute of Geophysics and Planetary Physics. UCLA, 405Hilgard, Box 951567, Los Angeles, CA 90095.</p><p>2 Present address: Laboratoire de Planetologie de Grenoble, Bat. D dePhysique, B.P. 53, 38041 Grenoble Cedex 9, France.</p><p>3 Present address: Institut dAstrophysique Spatiale, Bat 121, UniversiteParis XI, 91405 Orsay, France.</p><p>4 Guest observer, United Kingdom Infrared Telescope, Joint AstronomyCenter, Hawaii, USA.</p><p>5 Present address: Gemini Observatory, 670 North Aohoku Place, UniversityPark, Hilo, HI 96720.</p><p>of N2CHof CH4 ancovers abodeposits thunit is eithor (b) a uthe N2CH20% of theneutral anpure N2. Talso foundremainderbetter undsublimatioPluto duri</p><p>Key Woradiative t</p><p>Pluto, tSystem, ra spacecr</p><p>421at the Surface of PlutoQuirico3</p><p>ie`re, BP 96, 38402 St-Martin-dHe`res, Franceu</p><p>n Drive Honolulu, Hawaii 96822</p><p>, California 940351000</p><p>95 Meudon, France</p><p>sity Park, Hilo, Hawaii 96720</p><p>45-6 Moffett Field, California 940351000</p><p>ust 23, 1999</p><p>4CO, which are in a molecular mixture (concentrationsd CO of the order of 0.5 and 0.10.2% respectively). Itut 70% of the observed area and corresponds to volatileat are sublimating under solar illumination. The seconder (a) a single thick layer of pure large-grained methanenit with large-grained CH4 forming a substratum and</p><p>4CO mixture a superficial layer of fine grains coveringsurface. Finally, the third unit is bright and spectrally</p><p>d is first modeled as a layer of very fine grains of nearlyholin, suggested to explain the red slope in the visible, isto be spectrally compatible with this unit. It covers theof the surface (about 1015%). All these results allow a</p><p>erstanding of the processes of deposition, metamorphism,n, and transport affecting the different ices detected onng its climatic cycles. c 1999 Academic Pressrds: Pluto, surface; ices; spectrophotometry; charon;ransfer.</p><p>1. INTRODUCTION</p><p>he ninth and the last identified planet of the Solaremains the only one that has not yet been visited byaft. However, our knowledge about this object has</p><p>0019-1035/99 $30.00Copyright c 1999 by Academic Press</p><p>All rights of reproduction in any form reserved.</p></li><li><p>422 DOUT E ET AL.</p><p>significantly progressed during the last few years thanks to re-cent instrumental developments.</p><p>For(HST)the FaiStern ehemispa hugedistincare thophysic</p><p>In thinfrare1996)bilitiesobservInfraRmeasur</p><p>CGS4to 2.5 They idisks odifferemationcover tof Plutcharacand nitat 2.35band ovationsfor thean iceas are hcannotthe onlspectraother c</p><p>Thisare mo</p><p>Triton.of mos1% CHthose oQuiricoduced btowarddata avmore pCH4 anous miQuiricospectrowell as</p><p>that a segregation of methane with an enriched top layer possi-bly occurs at the surface of Pluto (Schmitt et al. 1994, Quirico</p><p>r</p><p>o</p><p>rpit</p><p>s</p><p>c</p><p>t</p><p>rbye</p><p>depe</p><p>P,e</p><p>w</p><p>er</p><p>a</p><p>iho</p><p>tthe first time, the treatment of Hubble Space Telescopeimages, produced in late June and early July 1994 bynt Object Camera (FOC) ESA in blue light, has allowedt al. (1997) to generate maps nearly covering the twoheres of Pluto with 160-km resolution. These maps showspatial variability of the visible albedo with a dozen</p><p>tive features, including a bright polar cap. The featuresught to be related mainly to changes in the chemical andal properties over the surface.e same way, the improvement of ground-based near-</p><p>d (NIR) and, to a less extent, visible (Grundy and Finkspectrophotometry has dramatically increased our capa-to determine surface properties. This new generation of</p><p>ations has come essentially from the United Kingdomed Telescope (UKIRT), Mauna Kea, Hawaii. The initialements were made in 1992 with the NIR spectrometer</p><p>(Cooled Grating Spectrometer) and covered the range 1.4m at a spectral resolution of 6.7 nm (Owen et al. 1993).</p><p>ncluded the light reflected by the spatially unresolvedf both Pluto and Charon. A spectroscopic analysis of thent reflectance spectra obtained has provided crucial infor-about the nature and physical state of the materials thathe uppermost millimeters or centimeters of the surfaceo. The presence of solid methane was confirmed by eightteristic absorption bands. In addition, carbon monoxiderogen ices were identified by their respective signaturesand 2.15 m. The analysis of the profile of the 2.15-mf nitrogen, used as a thermometer in the 1993 obser-, led Tryka et al. (1994) to propose a value of 40 2 Ktemperature of nitrogen ice on Pluto. Carbon dioxide,</p><p>that is detected on Triton, seems to be absent on Pluto,ydrocarbons like C2H2, C2H4, C2H6, etc., but water icebe excluded (Owen et al. 1993). For Charon, water ice isy compound that was recognized unambiguously by itsl signatures at 1.55 and 2.04 m (Buie et al. 1987), butomponents may also exist (e.g., Roush 1994).study also showed that, on Pluto, the methane molecules</p><p>st probably diluted in nitrogen ice but not as diluted as onThis was deduced by noting that the position and width</p><p>t of the methane bands fit those of a laboratory sample of4 mixed in N2 better than either those of pure CH4 ice orf CH4 even more highly diluted in N2 (Schmitt et al. 1992,</p><p>1995). The higher mean CH4 concentration (2.6%) de-y the model for Pluto compared with Triton also pointedan intermediate state of mixing. However, the laboratoryailable at that time were insufficient to define this staterecisely. Subsequent extensive laboratory studies on pured CO (as well as several other molecules) and their vari-</p><p>xtures with solid nitrogen (Quirico and Schmitt 1997a,b,et al. 1996, 1999, Schmitt et al. 1998) allowed a detailed</p><p>scopic analysis of the initial 1992 UKIRT observation asa set of spectra taken in 1993. This led to the suggestion</p><p>1995)diluted</p><p>In Mservatisame s</p><p>a factotroscomodelfirst atmodesice segon thegrain sidentifidata, aThe seFirst, wpresenulationof Chaferentthe phhow wresponmixturtroscointerprpaper.</p><p>In Mof the11, 13surfac</p><p>TheUKIRTing poand thmeasu</p><p>of theensure</p><p>certainmentiodescrib</p><p>Thefor 1 hital unthrougSY()sky wiather than a horizontal distribution of patches of pure andCH4 (Cruikshank et al. 1997, Grundy and Fink 1996).ay 1995, an additional series of three PlutoCharon ob-ns were acquired with the UKIRTCGS4 system in the</p><p>pectral range but with a spectral resolution improved byof about 2.5 (2.64 nm). In this paper, we present a spec-</p><p>ic analysis of these observations as well as numericalng of one of these spectra. The scope of this work is aempt to derive the horizontal and vertical distributionof the different ices detected, to solve the problem of theregation. This study also leads to improved constraintsintrinsic chemical and physical properties (composition,izes etc.) and about the geographical extent of the solidsed so far. In the first part of the paper, the observationalwell as the relevant calibration procedures, are described.ond part is dedicated to analysis of the Pluto spectrum.e recall the contributions of the modeling approach, andthe framework and the numerical tool used for our sim-</p><p>s. Second, after modeling and extracting the contributionon to the measured spectral reflectance, we assign the dif-ands observed and analyze the methane bands in terms ofsical state in which the methane occurs. We then describetested and selected the best surface representations cor-ing to the different coexisting modes of the molecularN2CH4CO and pure methane suggested by the spec-</p><p>ic analysis. A synthesis of these results, followed by theirtation and discussion, is presented in the final part of the</p><p>2. PRESENTATION AND CALIBRATIONOF THE OBSERVATIONS</p><p>ay 1995, three nonspatially resolved reflectance spectralutoCharon system were acquired respectively on Mayand 15. The longitudes of the sub-Earth point on theof Pluto were respectively 66, 313, and 200.CGS4 spectrometer measures the flux received by thetelescope with a spectral resolution of 2.64 nm (resolv-er=dof approximately 750). The H range 1.42.1mK range 1.92.55 m were recorded separately. The</p><p>ement of the lines of an argon lamp and the observationtmospheric bands of the OH radical, both in emission,a good spectral wavelength calibration with a mean un-ty of 1 nm. The details of the calibration procedure arened in Quirico et al. (1999). The following paragraphes the way the radiometric calibration was achieved.radiative power that arrives on the detectors is integratedon average. The resulting curve S(), measured in dig-ts, is deeply marked by absorptions due to the transferthe terrestrial atmosphere. As a consequence, a spectrumf a solar-type star is recorded in the same region of theh only a short time shift. Care was taken to observe this</p></li><li><p>CH4 SEGREGATION ON PLUTO 423</p><p>star, BS5384, at an airmass close to the midpoint of the airmassrange for which Pluto was observed, leading by a simple ratioS()=SY(the disturbH2O). On1.95 mdistributiotime the rsidering twhich issurementsits absolutelescope.sured specthe blackbplanet and</p><p>Finally, thrical albed</p><p>where Dgeocentric</p><p>FIG. 1. (to form a ne te(dotted and</p><p>radius, F() is the corrected and calibrated flux as received onEarth, and Fs() is the solar flux at 1 AU, as tabulated by SmithCalibration operation applied to the 05/11/95 observation of Pluto. The H regionw coherent spectrum (solid line). This spectrum is compared with better-calibradashed lines).) to the correction of the solar spectral features anding atmospheric bands (principally those of CO2 and</p><p>ly the strongest lines of telluric H2O in the range 1.8cannot be removed accurately, since the atmosphericn of H2O varies over short time intervals. In the sameadiometric calibration of the data is performed con-he star BS5384 as a blackbody, the temperature of5900 K. In that case, the infrared photometric mea-</p><p>of Leitherer and Wolf (1984) allow us to calculatete spectral flux FB(; T D 5900 K) as received by theDividing the PlutoCharon spectrum S() by the mea-trum SY() of BS5384 and multiplying the result byody flux FB(; T D 5900 K) yields for the flux of theits satellite</p><p>F() D S()=SY() FB(; T D 5900 K): (1)e astronomical spectrum is converted into a geomet-o Ag according to the expression</p><p>Ag() D D2d2 F()</p><p>R2 Fs(); (2)</p><p>and d respectively correspond to the heliocentric anddistances of PlutoCharon, R is their mean quadratic</p><p>and Gottlieb (1974).After applying this calibration procedure, it appears that no</p><p>photometric coherence is achieved between the H and K spec-tral regions as judged from the overlapping range 1.92.1m. Inthe K region the measurements of May 11, 13, and 15 give con-sistent albedo spectra while the H region displays unexpectedlyhigh and variable values. On May 11, clouds attenuated the sig-nal from the calibration star BS5384 while its H -band spectrumwas being measured. On May 13 and 15, the skies were clear,but we observed the star at H through a partially opened domebecause of concerns that the starlight would saturate the array ofthe CGS4 spectrometer. Following the data reduction proceduresdescribed above, we just multiplied the H -band spectra from thethree nights by scaling factors (0.75, 0.47, and 0.375 for 11, 13,and 15 May, respectively) to match the K -band spectra in theirregions of overlap. We note that the scaled H -band fluxes areconsistent with those measured in earlier years when a widerslit was used and a better photometrical calibration achieved.For example, a spectrum measured on April 16, 1994, corre-sponding to the visible lightcurve minimum of Pluto at 96 lon-gitude, is very consistent with most of the H and K spectrum ofMay 11, 1995, at 66 longitude (Fig. 1). On the lightcurve, thecorresponding visible albedos differ by a bit less than 5% (Fig. 2)(Buie et al. 1997). In addition, an H spectrum recorded on Maydotdash line) is adjusted to the K region by a multiplication factord spectra from 05/27/92 and 04/16/94 at similar rotational phases</p></li><li><p>424 DOUT E ET AL.</p><p>FIG. eduring thtime of t</p><p>27, 199observapercentnot matside theservatio1994 spthe surfof averthat theare corr</p><p>first expIn co</p><p>brationrescalin1994 reuncertaspectruyields ative erroof the sslope. I1.4- tothe H rK -regio2, at 51 longitude, i.e., only 15 away from the 05/11/95tion, also fits the scaled 1995 H spectrum within a few. Nevertheless, part of the two surrounding spectra doesch exactly with the 1995 measurement, essentially out-strong methane bands. The noticeable difference in ob-n conditions (15 andC30 longitude for the 1992 andectra relative to the 1995 one) or a possible evolution oface of Pluto may explain the spectral differences in termsage surface composition and/or physical state. We note</p><p>variations in the continuum around 1.5 m (Fig. 1)elated with the visible albedo (Fig. 2), thus favoring thelanation.nclusion, we believe that our initial photometric cali-is correct for all three spectra in the K region and afterg for the H -region spectra. Knowing that the 1992 andference spectra at lower resolution display an estimatedinty of 15% and considering the difference with them of May 11, we think that our calibration operationll three H and K May 1995 flux spectra with a conserva-r of20% in albedo. This uncertainty applies over mostpectral range, as multiplying factor, or a broad spectraln the following we use the calibrated and concatenated2.55-m flux spectra, except for May 13 restricted toegion due to some spurious instrumental effects in then data.</p><p>3. ANALYSIS OF THE OBSERVATION</p><p>3.1. Framework of the Modeling Approach</p><p>In the visible and near-infrared ranges, the flux that comes toEarth from the PlutoCharon system, or from any other plane-tary body in general, totally or partly arises from the reflectionand absorption of the solar light that penetrates more or le...</p></li></ul>

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