The spectrum of Pluto 040ndash093

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Citation Lorenzi V et al ldquoThe Spectrum of Pluto 040ndash093 μ M I Secularand Longitudinal Distribution of Ices and Complex OrganicsrdquoAstronomy amp Astrophysics 585 (2016) A131 copy 2016 ESO

As Published httpdxdoiorg1010510004-6361201527281

Publisher EDP Sciences

Version Final published version

Citable link httphdlhandlenet17211106193

Terms of Use Article is made available in accordance with the publisherspolicy and may be subject to US copyright law Please refer to thepublishers site for terms of use

AampA 585 A131 (2016)DOI 1010510004-6361201527281ccopy ESO 2016

Astronomyamp

Astrophysics

The spectrum of Pluto 040ndash093 microm

I Secular and longitudinal distribution of ices and complex organics

V Lorenzi1 N Pinilla-Alonso2 J Licandro34 D P Cruikshank5 W M Grundy6 R P Binzel7 and J P Emery2

1 Fundacioacuten Galileo Galilei-INAF Rambla Joseacute Ana Fernaacutendez Peacuterez 7 38712 Brentildea Baja TF Spaine-mail lorenzitngiaces

2 Department of Earth and Planetary Sciences University of Tennessee 1412 Circle Drive Knoxville TN 37996 USA3 Instituto Astrofiacutesico de Canarias IAC C viacutea Laacutectea sn 38205 La Laguna TF Spain4 Departamento de Astrofiacutesica Universidad de La Laguna Avenida Astrofiacutesico Francisco Saacutenchez sn 38200 La Laguna TF Spain5 NASA Ames Research Center Moffett Field CA 94035 USA6 Lowell Observatory 1400 W Mars Hill Rd Flagstaff AZ 86001 USA7 Massachusetts Institute of Technology 77 Massachusetts Ave Cambridge MA 02139 USA

Received 31 August 2015 Accepted 23 November 2015

ABSTRACT

Context During the past 30 years the surface of Pluto has been characterized and its variability monitored through continuous near-infrared spectroscopic observations But in the visible range only a few data are availableAims The aim of this work is to define Plutorsquos relative reflectance in the visible range to characterize the different components of itssurface and to provide ground based observations in support of the New Horizons missionMethods We observed Pluto on six nights between May and July 2014 with the imagerspectrograph ACAM at the William HerschelTelescope (La Palma Spain) The six spectra obtained cover a whole rotation of Pluto (Prot = 64 days) For all the spectra wecomputed the spectral slope and the depth of the absorption bands of methane ice between 062 and 090 μm To search for shifts inthe center of the methane bands which are associated with dilution of CH4 in N2 we compared the bands with reflectances of puremethane iceResults All the new spectra show the methane ice absorption bands between 062 and 090 μm Computation of the depth of the bandat 062 μm in the new spectra of Pluto and in the spectra of Makemake and Eris from the literature allowed us to estimate the Lambertcoefficient at this wavelength at temperatures of 30 K and 40 K which has never been measured before All the detected bands areblueshifted with respect to the position for pure methane ice with minimum shifts correlated to the regions where the abundanceof methane is higher This could be indicative of a dilution of CH4N2 that is more saturated in CH4 The longitudinal and secularvariations in the parameters measured in the spectra are in accordance with results previously reported in the literature and with thedistribution of the dark and bright materials that show the Plutorsquos color maps from New Horizons

Key words Kuiper belt objects individual Pluto ndash methods observational ndash methods numerical ndash techniques spectroscopic

1 Introduction

The surface of Pluto is covered with ices of varying de-grees of volatility and even at its low average temperatureof sim40 K there is an exchange of molecules between thesurface and the thin atmosphere Because of Plutorsquos highobliquity (1196) and the eccentricity of its orbit (perihe-lion 2966 AU aphelion 4887 AU) there are pronounced sea-sons over the 2477-year orbital period These factors ensurethat Plutorsquos surface undergoes changes on a seasonal time scaleSecular changes may also occur owing to the long-term evolu-tion of the chemical components of the surface and atmosphereunder the influence of solar and cosmic ray irradiation On thetime scale of Plutorsquos 64-day rotation period clear changes in thedistribution of ices across its surface are seen

Indeed on secular and rotational time scales changes inPlutorsquos overall (global) brightness and in the near-infrared spec-tral signatures of the three principal ices N2 CH4 and CO havebeen detected and quantified (Grundy et al 2013 hereafter G13Cruikshank et al 2015 and references therein) Because Plutohas only been observed over one-third of its full orbit secular

changes might be confused with seasonal variations with the re-sult that at the present time there is no unequivocal evidence ofany observable secular changes

In addition to the ices on Plutorsquo surface there is another ma-terial present that imparts colors as seen in the extended visi-ble spectral region 030ndash100 μm As deduced from multi-colorphotometric measurements and images the colors vary fromlight to medium yellow red-brown to black (Buie et al 2010b)The ices detected to date do not produce these colors and it isgenerally accepted that photochemical reactions in Plutorsquos at-mosphere and in the surface ices create colored macromolecu-lar matter that contains or is made of complex organic chemi-cals derived from carbon nitrogen and oxygen (eg Matereseet al 2014) The coloring properties of these materials are wellestablished (eg Khare et al 1984 Cruikshank et al 2005Imanaka et al 2012 Materese et al 2014) The coloring agent isnot uniform over Plutorsquos surface changing noticeably over thedwarf-planetrsquos 64-day rotation

The best way to quantify the color of Pluto and to searchfor diagnostic characteristics is to record the reflectance proper-ties of the surface in the photovisual or extended visual spectral

Article published by EDP Sciences A131 page 1 of 12

AampA 585 A131 (2016)

Table 1 Details of the observations

2014 date Time am (Pluto) Solar analog am (SA) Lon (deg) Lat (deg) pvMay 13 0438 15 L112-1333 16 1037 512 044

L107-684 23May 14 0340 16 L112-1333 17 497 512 048

L107-684 15June 6 0150 17 L112-1333 15 1979 509 059

L107-684 12June 7 0157 16 L112-1333 15 1413 509 050

L107-684 13June 18 0023 18 L112-1333 23 2452 507 057

P041C 15July 17 2240 17 L112-1333 17 3589 500 052

P041C 14

Notes Time = mid exposure time am (Pluto) and am (SA) = mid exposure airmass of Pluto and the solar analogs respectively Lon = Plutosub-Earth longitude Lat = Pluto sub-Earth latitude pv = Geometric albedo of Pluto at 056 μm

region (030ndash100 μm) Only a few such measurements havebeen reported in the literature beginning with observations madein 1970 by Fix et al (1970) and followed by Benner et al (1978)Bell et al (1979) (data published in Cruikshank amp Brown 1986)Barker et al (1980) Buie amp Fink (1987) and Grundy amp Fink(1996) Individually or together these studies do not define awell-sampled range of rotation and seasonal change or a fullsample of wavelengths in the desired range at adequate spectralresolution The history of color measurements with ground-based telescopes and the Hubble Space Telescope (HST) is re-viewed by Cruikshank et al (2015) who note that the color mea-sured in the (B minus V) photometric index was constant from 1953to 2000 within a few tenths of a percent but then changedabruptly in 2002 Buie et al (2010b) give the details of the HSTobservations that showed a global change in color to a redder(B minus V) index that persists to the present day

The variation in the relative amounts of ices and dark ma-terial have been mapped before (Buie et al 2010b G13) butare even more obvious now in light of the preliminary new im-ages obtained by the New Horizons mission The maps combin-ing the reflectance and colors of Plutorsquos surface clearly show adiverse palette from the very dark region on the equator infor-mally named Cthulhu Regio to the ldquoclear peach orangerdquo regioninformally named the Tombaugh Regio (Stern et al 2015)

During 2014 we conducted a new observational study in or-der to provide a high-quality data set of spectrophotometric datain the 040ndash093 μm spectral range These observations enablethe definition of Plutorsquos reflectance and the investigation of weakmethane ice absorption bands shortward of 100 μm The datahave hemispheric spatial resolution over a whole rotation of thePlutorsquos longitudes The new data are compared with earlier ob-servations to detect any changes in spectral reflectance that mayhave occurred in the past few decades This comparison also pro-vides a basis for modeling the spectrum with radiative transfermodels that account for the known components of Plutorsquos sur-face as well as additional components that are suspected to oc-cur The observations presented here include the reflected lightof both Pluto and Charon These observations are also in supportof the New Horizons mission that observed the Pluto system inJuly 2015 and are part of a long-term campaign to follow thesecular evolution of Plutorsquos surface properties

In the next section we describe the observations In Sect 3we analyze the obtained spectra we calculate the slopes of therelative reflectances we calculate the depth and the area of theabsorption bands of methane ice and we measure the shifts of

their centers In Sect 4 we discuss the results and in Sect 5 wereport our conclusions

2 The observations

We observed Pluto on six different nights between May 13 andJuly 17 2014 using the low-resolution spectroscopic mode ofACAM (auxiliary-port camera Benn et al 2008) ACAM is animagerspectrograph that is mounted permanently at a folded-Cassegrain focus of the 42 m William Herschel Telescope at theObservatorio del Roque de los Muchachos (ORM) La PalmaWe used the V400 disperser and the 15primeprime slit providing a res-olution of sim300 at 055 μm (the dispersion is 33 Aringpx) Theslit was oriented at the parallactic angle and the tracking of thetelescope was set to compensate for Plutorsquos proper motion

To cover a whole rotation of Pluto we originally scheduledseven observations equally spaced in time to observe Plutorsquossurface every sim50 degrees in longitude Unfortunately we dis-carded one of those observations because of technical problemsThe six that we kept cover 310 degrees in longitude The detailsof the observations are described in Table 1 We define longi-tudes following the IAU2009 model that corresponds with theright-hand rule Every night we obtained three consecutive spec-tra of Pluto of 300-second exposure times each which providesa SN gt 200 in most of the wavelength range and even betterafter averaging the three spectra

Data was reduced using the Image Reduction and AnalysisFacility (IRAF) standard procedures (Tody 1993) Preprocessingof the CCD images included bias and flat field correction Flatfield images were obtained using calibration lamps (tungstencontinuum lamp) in the afternoon Wavelength-calibration lamps(CuAr and CuNe) were obtained before and after the set ofPlutorsquos spectra maintaining the same position angle to avoid theeffect of instrumental flexures in the wavelength calibration

To correct the spectra for telluric absorptions and to obtainthe relative reflectance we observed every night two differentLandolt stars (Landolt 1992) that have proven to be good solaranalogs (Licandro et al 2003) We observed them at airmassesthat are very similar to that of Pluto (see Table 1 for the details)These solar analogs are bright (10 lt Vmag lt 124) so the expo-sure times were always shorter than 60 s To reduce the numberof bad pixels and artifacts we obtained three spectra of each ofthe stars as we did for Pluto The reduction of the spectra ofthe solar analogs was done following the same procedure as for

A131 page 2 of 12

V Lorenzi et al The spectrum of Pluto 040ndash093 μm I

Fig 1 Relative reflectances of Pluto corresponding to hemispheres cen-tered on six different sub-Earth longitudes All the reflectances are nor-malized at 060 μm and shifted by 015 in relative reflectance for clar-ity The labels are ordered (top to bottom) in the same order as thereflectances The vertical dashed lines show the telluric absorptionsSome of these telluric absorptions have not been completely removedThe absorption at 052 μm is an artifact and is present on the spectraof the solar analogs Its center corresponds to the magnesium triplet(05169ndash0 5183 μm) that is present in G stars

Pluto All the calibration images the solar analogs and the Plutoimages were obtained in about one hour

The extinction of the sky is dependent on the wavelengthwith shorter wavelengths experience greater extinction To mini-mize the spectral extinction effect from the difference in airmassbetween the stars and the target we applied color correction tothe spectra of the object and the stars We used the mean extinc-tion coefficient for the Observatorio del Roque de los Muchachos(King 1985)

We finally divided the color-corrected spectra of Pluto bythe color-corrected spectra of the stars to obtain Plutorsquos relativereflectance We used the sky lines at this step to refine the wave-length calibration Before averaging all of them into a ldquofinalrdquorelative reflectance we visually inspected the 18 reflectances foreach night obtained by dividing the three spectra of Pluto bythe three spectra of each solar analog and discarded those thatshowed problems due to the systematic errors (sim20 of the to-tal) We averaged the rest of them into a final relative reflectancefor each date of observation

In Fig 1 we present the six reflectance spectra obtained inthis observing campaign These are the average of the Pluto +Charon spectra divided by the spectra of the solar-type starsThis division cancels the solar color and the solar and tel-luric absorption lines The spectra have been normalized to 10at 075 μm to enhance the differences in the slope of the contin-uum below 070 μm The contribution of Charon has not beenremoved in this plot Nevertheless the spurious light of Charonis not a problem because Charon is two magnitudes fainter thanPluto and its color is neutral at these wavelengths (as we explainin detail later)

In Fig 2 we present the data from Fig 1 smoothed using anaverage filter of five pixels and plotted in geometric albedo Totransform from reflectance to albedo we used the magnitudesof Pluto alone from Fig 1 of Buie et al (2010a) in which theyplot the lightcurve of Pluto resolved from Charon The values in

Fig 2 Same spectra of Fig 1 smoothed using an average filter of5 pixels and represented in geometric albedo The symbol plus indi-cates for each spectrum the global geometric albedo of Pluto alone at056 μm calculated for the corresponding date of the observation

the figure are shown as mean opposition magnitudes corrected tophase angle α = 1 We have taken the Buie et al (2010a) mag-nitudes at α = 1 for the longitudes we observed and have calcu-lated the geometric albedos corresponding to those longitudesUsing a radius of 1184 km for the dwarf planet (Lellouch et al2015) we calculated the global geometric albedo of Pluto aloneat the V wavelength (056 μm) for each date that we observed(listed in Table 1) using the equation

minus25 log pV = VSun minus VPluto minus 5 log r + 5 log(R times Δ)

where VSun = minus2675 r = Plutorsquos radius (in AU) R = Plutorsquosmean heliocentric distance (39482 AU) and Δ = Plutorsquos geo-centric distance (38482 AU) A small revision of Plutorsquos radiusfrom the New Horizons investigation makes a negligible changeto the albedo calculations

We made no correction for the contribution of Charon tothe color of (Pluto + Charon) that we observed assuming thatCharonrsquos color is neutral gray From observations of the mutualtransit and occultation events of the mid-1980s Binzel (1988)found a nearly neutral gray color for Charon with B minus V =070 plusmn 001 ((B minus V)Sun = 065) More recently Buie et al(2010a) have derived (BminusV) = 07315plusmn00013 using HST dataAt the photometric B band (044 μm) Tholen amp Buie (1990) givethe geometric albedo of Pluto as 044ndash061 and 038 for Charon

3 Results and analysis

For each spectra we computed four parameters (1) the spec-tral slope in order to quantify the color of Pluto at different lon-gitudes and thus search for possible variations in the amountof solid complex organics over its surface (2) the integratedarea of the absorption bands of methane ice at 062 μm inorder to determine if this weak CH4 band is detected as inother trans-Neptunian objects (TNOs Licandro et al 2006bAlvarez-Candal et al 2011) (3) the depth of the methane icebands between 062 and 090 μm in order to search for possiblevariations in the abundance of CH4-ice on the surface of Plutowith longitude (4) the shift in the center of the absorption bands

A131 page 3 of 12

V Lorenzi et al The spectrum of Pluto 040ndash093 μm I

Fig 1 Relative reflectances of Pluto corresponding to hemispheres cen-tered on six different sub-Earth longitudes All the reflectances are nor-malized at 060 μm and shifted by 015 in relative reflectance for clar-ity The labels are ordered (top to bottom) in the same order as thereflectances The vertical dashed lines show the telluric absorptionsSome of these telluric absorptions have not been completely removedThe absorption at 052 μm is an artifact and is present on the spectraof the solar analogs Its center corresponds to the magnesium triplet(05169ndash0 5183 μm) that is present in G stars

Pluto All the calibration images the solar analogs and the Plutoimages were obtained in about one hour

The extinction of the sky is dependent on the wavelengthwith shorter wavelengths experience greater extinction To mini-mize the spectral extinction effect from the difference in airmassbetween the stars and the target we applied color correction tothe spectra of the object and the stars We used the mean extinc-tion coefficient for the Observatorio del Roque de los Muchachos(King 1985)

We finally divided the color-corrected spectra of Pluto bythe color-corrected spectra of the stars to obtain Plutorsquos relativereflectance We used the sky lines at this step to refine the wave-length calibration Before averaging all of them into a ldquofinalrdquorelative reflectance we visually inspected the 18 reflectances foreach night obtained by dividing the three spectra of Pluto bythe three spectra of each solar analog and discarded those thatshowed problems due to the systematic errors (sim20 of the to-tal) We averaged the rest of them into a final relative reflectancefor each date of observation

In Fig 1 we present the six reflectance spectra obtained inthis observing campaign These are the average of the Pluto +Charon spectra divided by the spectra of the solar-type starsThis division cancels the solar color and the solar and tel-luric absorption lines The spectra have been normalized to 10at 075 μm to enhance the differences in the slope of the contin-uum below 070 μm The contribution of Charon has not beenremoved in this plot Nevertheless the spurious light of Charonis not a problem because Charon is two magnitudes fainter thanPluto and its color is neutral at these wavelengths (as we explainin detail later)

In Fig 2 we present the data from Fig 1 smoothed using anaverage filter of five pixels and plotted in geometric albedo Totransform from reflectance to albedo we used the magnitudesof Pluto alone from Fig 1 of Buie et al (2010a) in which theyplot the lightcurve of Pluto resolved from Charon The values in

Fig 2 Same spectra of Fig 1 smoothed using an average filter of5 pixels and represented in geometric albedo The symbol plus indi-cates for each spectrum the global geometric albedo of Pluto alone at056 μm calculated for the corresponding date of the observation

the figure are shown as mean opposition magnitudes corrected tophase angle α = 1 We have taken the Buie et al (2010a) mag-nitudes at α = 1 for the longitudes we observed and have calcu-lated the geometric albedos corresponding to those longitudesUsing a radius of 1184 km for the dwarf planet (Lellouch et al2015) we calculated the global geometric albedo of Pluto aloneat the V wavelength (056 μm) for each date that we observed(listed in Table 1) using the equation

minus25 log pV = VSun minus VPluto minus 5 log r + 5 log(R times Δ)

where VSun = minus2675 r = Plutorsquos radius (in AU) R = Plutorsquosmean heliocentric distance (39482 AU) and Δ = Plutorsquos geo-centric distance (38482 AU) A small revision of Plutorsquos radiusfrom the New Horizons investigation makes a negligible changeto the albedo calculations

We made no correction for the contribution of Charon tothe color of (Pluto + Charon) that we observed assuming thatCharonrsquos color is neutral gray From observations of the mutualtransit and occultation events of the mid-1980s Binzel (1988)found a nearly neutral gray color for Charon with B minus V =070 plusmn 001 ((B minus V)Sun = 065) More recently Buie et al(2010a) have derived (BminusV) = 07315plusmn00013 using HST dataAt the photometric B band (044 μm) Tholen amp Buie (1990) givethe geometric albedo of Pluto as 044ndash061 and 038 for Charon

3 Results and analysis

For each spectra we computed four parameters (1) the spec-tral slope in order to quantify the color of Pluto at different lon-gitudes and thus search for possible variations in the amountof solid complex organics over its surface (2) the integratedarea of the absorption bands of methane ice at 062 μm inorder to determine if this weak CH4 band is detected as inother trans-Neptunian objects (TNOs Licandro et al 2006bAlvarez-Candal et al 2011) (3) the depth of the methane icebands between 062 and 090 μm in order to search for possiblevariations in the abundance of CH4-ice on the surface of Plutowith longitude (4) the shift in the center of the absorption bands

A131 page 3 of 12

AampA 585 A131 (2016)

of methane ice at 073 079 080 084 087 and 089 μm in or-der to search for possible variations in the dilution of CH4 in N2with longitude (previously reported by Quirico amp Schmitt 1997)

31 Slope

The slope in the visible is an indicator of the amount parti-cle size or type of complex organics on the surface Complexorganics have been detected on the surface of asteroids (24)Themis and (65) Cybele (Rivkin amp Emery 2010 Licandro et al2011) and the Saturnian moons Iapetus and Phoebe (Cruikshanket al 2008 2014) and their presence has been inferred for othersolar system objects (eg TNOs and Titan Cruikshank et al2005) Recently a suite of 16 organic compounds comprisingnumerous carbon and nitrogen-rich molecules has been discov-ered on the surface of the comet 67PChuryomov-Gerasimenko(Wright et al 2015 Goesmann et al 2015) The comet presentsa red slope in the visible range (between 5 and 251000 AringCapaccioni et al 2015) This discovery suggests that organiccompounds that are the precursors of the more complex organics(such as tholins) are a ubiquitous ingredient of the minor bodiesin the solar system Not surprisingly red slopes which are typi-cally associated with the complex organics are common amongthe spectra of TNOs

Here we compute the slope of Plutorsquos visible spectrum intwo ranges from 040 to 070 μm and from 057 to 070 μmThe first range comprises the full spectral region below 070 μmcovered by our spectra The second range enables the compar-ison with previous spectra of Pluto Eris and Makemake fromthe literature To compute the slope values in both cases we firstnormalized all the spectra at 060 μm and then we fit each ofthem in the chosen range with a linear function We made thiscomputation for all the six spectra and for the average relativereflectance The slope is expressed here as 1000 Aring

To evaluate the errors in the computation of the slope wetook both the error in the fit and the systematic errors in the re-flectance into account The systematic errors depend on differentcontributions eg errors introduced by color corrections or bydividing by the spectra of the solar analogs The error introducedby dividing by the spectra of the solar analog is the dominantpart of the systematic errors To estimate this error we com-pared the six individual spectra of the stars observed each nightfollowing the procedure described in Lorenzi et al (2014) andPinilla-Alonso (2016) After discarding those spectra displayingextreme behavior we took the standard deviation of the slope ofthese spectra as the error in the slope for the night These valuesare shown in Table 2 We took the maximum value in the runas the systematic error We found that the formal error comingdirectly from the computation of the slope is an order of magni-tude lower than the systematic error so we took 081000 Aringas the final uncertainty associated to the slope calculation for allthe spectra

In any case it is important to notice that the spectra presentedin this paper are the spectra of the Pluto-Charon system Even ifthe spectrum of Charon is flat in the visible wavelength range sodoes not affect the measurements of the methane bands that areonly due to Pluto spectrum the spectral slope of the Pluto onlyspectrum will be slightly redder In fact if we consider that thespectral slope of Pluto+Charon S primeP+C calculated for example be-tween 040 and 070 μm for a spectrum normalized at 060 μm is

S primeP+C =

[FP+C(070)

F(070)minus FP+C(040)

F(040)

]times F(060)

F(060)

Table 2 Estimated systematic error in the slope expressed in1000 Aring

2014 date Syst error 2014 date Syst errorMay 13 045 June 7 076May 14 070 June 18 073June 6 037 July 17 071

where FP+C(X) and F(X) are the flux of Pluto+Charon and theflux of the Sun at X μm respectively and assuming that

FC(040)F(040)

=FC(060)F(060)

(because Charonrsquos spectral slope in the visible is close to 0) wefind that the slope of Pluto alone S primeP is

S primeP =FP+C(060)FP(060)

times S primeP+C

Considering the albedo and size of Pluto and Charon and thealbedo dependence with wavelength (in particular the depen-dence of Plutorsquos albedo in the visible as Charonrsquos albedo inthe visible is almost constant) we find that the spectral slopeof the Pluto-only spectrum is between 11 and 14 larger(eg in the 040ndash070 μm region the average slope will be21ndash22 1000 Aring) We preferred not to correct for the Charoncontribution to avoid introducing errors that are an artifact of theprocessing and to facilitate the comparison with past measure-ments Nonetheless for the scientific investigations within thispaper Charonrsquos contribution is not a critical factor

To compare with other spectra in the literature we computedthe slope of those spectra in the same way as we did for the newdata presented herein For this comparison we used the spectraof Pluto from Grundy amp Fink (1996) and the spectra of two otherlarge TNOs (136472) Makemake from Licandro et al (2006a)and (136199) Eris from Alvarez-Candal et al (2011) For thespectra from the literature of Pluto Makemake and Eris thereare no estimates for the systematic errors in the original papersso we take the typical value of 11000 Aring (eg Lazzaro et al2004) for all the spectra (For these spectra the systematic errorsare also an order of magnitude greater than those associated withthe fit calculation) The results are listed in Table 3

32 Methane absorption bands

The new spectra clearly show methane ice absorption bandsat 073 078ndash080 and 083ndash091 μm In Fig 3 we show thespectrum of Pluto obtained averaging the six spectra presentedin this work with the methane ice absorption bands evidencedas vertical dotted lines All these bands have very low absorptioncoefficients 50times 10minus2 sim13ndash14times 10minus2 sim10ndash80times 10minus2 cmminus1respectively as measured in the laboratory at 40 K (Grundyet al 2002) This average spectrum also shows an absorptionaround 062 μm This band has been also observed in the spec-tra of other large TNOs dominated by methane ice like Eris andMakemake (Alvarez-Candal et al 2011 Licandro et al 2006bsee Fig 4) so its identification is not in question However theabsorption coefficient of this band is unknown since it has notbeen measured in the laboratory If we look at each particularreflectance obtained in 2014 some of them show it very clearlyin particularly the phase centered at 3589

To confirm our detection we compute the integrated areaof the absorption band For this computation we first fit a

A131 page 4 of 12

V Lorenzi et al The spectrum of Pluto 040ndash093 μm I

Table 3 Computed spectral slope between 057 and 070 μm and between 040 and 070 μm with the spectra normalized at 060 μm for thespectra of Pluto+Charon (this work Buie amp Fink 1987 Grundy amp Fink 1996) for the spectrum of Makemake (Licandro et al 2006b) and for thespectrum of Eris (Alvarez-Candal et al 2011)

Longitude (degrees) Slope (1000 Aring) 057ndash070 μm Slope (1000 Aring) 040ndash070 μm Reference497 142 199 This work1037 126 206 primeprime1413 125 193 primeprime1979 127 194 primeprime2452 107 173 primeprime3589 108 172 primeprimeAverage spectrum 123 190 primeprime

25 (1983) 57 ndash Buie amp Fink (1987)329 (1983) 51 ndash primeprime144 (1990) 77 ndash Grundy amp Fink (1996)56 (1993) 108 ndash primeprime150 (1994) 104 ndash primeprime

Makemake 129 173 Licandro et al (2006b)Eris 44 65 Alvarez-Candal et al (2011)

Fig 3 Averaged spectrum of the six spectrum presented in this worknormalized at 060 μm The positions of the absorption bands of CH4

are represented by dotted lines The features at 043 052 and 059 μmare artifacts due to small differences between the spectra of the G2 starsused as solar analogs and those of the Sun The 043 μm structure cor-responds to the absorption produced by iron and calcium the 052 μmis due to the magnesium triplet and the 059 μm is due to a sodiumdoublet

straight line to the continuum on both sides of the band be-tween 0593ndash0615 μm and 0624ndash0653μm and divide the spec-trum by this continuum (see Fig 5) A first look at the plots inFig 5 suggests a detection of the band although in some of thephases (eg 497 or 1037) it is not clear if the absorption issignificant above the noise

We then calculate the area of the absorption band af-ter division by the continuum integrating it between 0615and 0624 μm We propagate the errors taking the error associ-ated with the relative reflectance and the error in the calculationof the fit of the continuum into account in each point Finally weiterate the computation of the area randomly changing the limitsof the band and we take the mean value and the standard devia-tion as the final values for the area and the associated error Forall the spectra (even those where the detection was suspicious)the integrated area of the band is a positive value Some of theintegrated areas however have large uncertainties To confirm

Fig 4 Comparison of the spectra of Eris Pluto and Makemake Thethree spectra are normalized at 060 μm All three objects show the ab-sorption bands of methane ice The band at 062 μm is more evident onMakemake and Eris than on Pluto

the detection and discard a serendipitous result we do a secondverification

For this test we repeat the procedure on a region of the con-tinuum where no band is expected We define this continuumin a region with the same number of points between 055 and057 μm What we can expect from this test is that we obtainboth positive and negative values for the area and that the ab-solute values of the areas are smaller than the uncertainty InTable 4 we show the results of this test From the calculation ofthe integrated area on the region of the continuum we obtainedboth positive and negative values whereas we only obtained pos-itive values from the calculation on the region of the absorptionbands at 062 μm After this test we can consider the detectionof the absorption bands at 062 μm as reliable with the phasecentered at sim360 as the clearest detection

33 Band depth

To evaluate the globally averaged distribution of CH4-ice aroundPlutorsquos surface we computed the depth of the absorption bandsas D = 1 minus RbRc where Rb is the reflectance in the center of

A131 page 5 of 12

V Lorenzi et al The spectrum of Pluto 040ndash093 μm I

Table 3 Computed spectral slope between 057 and 070 μm and between 040 and 070 μm with the spectra normalized at 060 μm for thespectra of Pluto+Charon (this work Buie amp Fink 1987 Grundy amp Fink 1996) for the spectrum of Makemake (Licandro et al 2006b) and for thespectrum of Eris (Alvarez-Candal et al 2011)

Longitude (degrees) Slope (1000 Aring) 057ndash070 μm Slope (1000 Aring) 040ndash070 μm Reference497 142 199 This work1037 126 206 primeprime1413 125 193 primeprime1979 127 194 primeprime2452 107 173 primeprime3589 108 172 primeprimeAverage spectrum 123 190 primeprime

25 (1983) 57 ndash Buie amp Fink (1987)329 (1983) 51 ndash primeprime144 (1990) 77 ndash Grundy amp Fink (1996)56 (1993) 108 ndash primeprime150 (1994) 104 ndash primeprime

Makemake 129 173 Licandro et al (2006b)Eris 44 65 Alvarez-Candal et al (2011)

Fig 3 Averaged spectrum of the six spectrum presented in this worknormalized at 060 μm The positions of the absorption bands of CH4

are represented by dotted lines The features at 043 052 and 059 μmare artifacts due to small differences between the spectra of the G2 starsused as solar analogs and those of the Sun The 043 μm structure cor-responds to the absorption produced by iron and calcium the 052 μmis due to the magnesium triplet and the 059 μm is due to a sodiumdoublet

straight line to the continuum on both sides of the band be-tween 0593ndash0615 μm and 0624ndash0653μm and divide the spec-trum by this continuum (see Fig 5) A first look at the plots inFig 5 suggests a detection of the band although in some of thephases (eg 497 or 1037) it is not clear if the absorption issignificant above the noise

We then calculate the area of the absorption band af-ter division by the continuum integrating it between 0615and 0624 μm We propagate the errors taking the error associ-ated with the relative reflectance and the error in the calculationof the fit of the continuum into account in each point Finally weiterate the computation of the area randomly changing the limitsof the band and we take the mean value and the standard devia-tion as the final values for the area and the associated error Forall the spectra (even those where the detection was suspicious)the integrated area of the band is a positive value Some of theintegrated areas however have large uncertainties To confirm

Fig 4 Comparison of the spectra of Eris Pluto and Makemake Thethree spectra are normalized at 060 μm All three objects show the ab-sorption bands of methane ice The band at 062 μm is more evident onMakemake and Eris than on Pluto

the detection and discard a serendipitous result we do a secondverification

For this test we repeat the procedure on a region of the con-tinuum where no band is expected We define this continuumin a region with the same number of points between 055 and057 μm What we can expect from this test is that we obtainboth positive and negative values for the area and that the ab-solute values of the areas are smaller than the uncertainty InTable 4 we show the results of this test From the calculation ofthe integrated area on the region of the continuum we obtainedboth positive and negative values whereas we only obtained pos-itive values from the calculation on the region of the absorptionbands at 062 μm After this test we can consider the detectionof the absorption bands at 062 μm as reliable with the phasecentered at sim360 as the clearest detection

33 Band depth

To evaluate the globally averaged distribution of CH4-ice aroundPlutorsquos surface we computed the depth of the absorption bandsas D = 1 minus RbRc where Rb is the reflectance in the center of

A131 page 5 of 12

AampA 585 A131 (2016)

Fig 5 Continuum removed in the region of the spectra around the062 μm absorption band of methane The blue line represents the con-tinuum The gray shadow represents the range of wavelengths used tocalculate the area of the bands

Table 4 Integrated area of the absorption band (in unit of relative re-flectance value relative to continuum per microns) at 062 μm and teston the continuum between 055 and 057 μm measured as explained inthe text

Longitude Band at 062 μm Cont 055ndash057 μm(times10minus4) (times10minus4)

497 031 plusmn 030 007 plusmn 0701037 039 plusmn 021 051 plusmn 0541413 029 plusmn 013 030 plusmn 0351979 033 plusmn 014 ndash011 plusmn 0412452 037 plusmn 014 026 plusmn 0463589 066 plusmn 016 ndash057 plusmn 031Average spectrum 035 plusmn 009 013 plusmn 017

the band and Rc the reflectance of the continuum at the samewavelength Because several of the absorption bands of CH4 inour spectra are a superposition of different absorption bands itis not possible to measure the integrated area or the equivalentwidth of each single band We calculated the band depth that canbe isolated 062 073 079 080 084 087 089 and 090 μmfor our six spectra of Pluto To evaluate the uncertainty in thecalculation of the depth we propagated the errors taking intoaccount the uncertainties associated both with the value of therelative reflectance in the center of the band and with the valueof the continuum at the same wavelength In Table 5 we list thevalues obtained for the depths (in ) of the six spectra of Plutoand the corresponding errors

Fig 6 Band depth (on natural logarithmic scale) vs Lambert coeffi-cients (on natural logarithmic scale) for Pluto (top panel) Makemake(middle panel) and Eris (bottom panel)

34 Lambert coefficients

The appearance of an intrinsically weak band like the oneat 062 μm shown in Sect 33 demonstrates a very long (severalcm) path length of sunlight in the nitrogen ice in which CH4 isincorporated as a trace constituent This in turn helps to set thesize scale for the individual elements of polycrystalline solid N2that is characteristic of the surface of Pluto

A big effort has been made to estimate the absorption co-efficients of methane ice by means of laboratory experiments(eg Grundy et al 2002) However these experiments do nottypically cover wavelengths below 070 μm since large slab ofmethane is needed to produce that absorption We used this bandobserved in the spectrum of Pluto Eris and Makemake to esti-mate the absorption coefficient of CH4 ice at 062 μm For thispurpose we plotted the estimated band depth for the bands inthe 073ndash090 μm range on a logarithmic scale for the spectraof Pluto (for each band we used the average of the six valuesobtained for the six spectra) as a function of the correspondingLambert coefficients at T = 40 K from Grundy et al (2002) Weused the Lambert coefficients for this temperature because theavailable ones are given in steps of 10 K and 40 K is closer tothe temperature of Pluto Finally we fit a one-degree polynomial(ie straight line) to the points with a least squares fit (see Fig 6top panel)

We did the same for the spectrum of Makemake fromLicandro et al (2006b) and for the spectrum of Eris fromAlvarez-Candal et al (2011) In the case of Eris we used the

A131 page 6 of 12

V Lorenzi et al The spectrum of Pluto 040ndash093 μm I

Table 5 Band depth () and one-sigma errors for eight methane bands at each of the six longitudes observed

Longitude 062 μm 073 μm 079 μm 080 μm 084 μm 087 μm 089 μm 090 μm

497 116 plusmn 096 497 plusmn 074 257 plusmn 123 252 plusmn 110 149 plusmn 163 707 plusmn 159 1735 plusmn 134 1019 plusmn 0811037 089 plusmn 074 427 plusmn 103 097 plusmn 058 112plusmn 068 154 plusmn 059 569 plusmn 164 1603 plusmn 140 998plusmn 0551413 050 plusmn 048 408 plusmn 090 133 plusmn 120 150 plusmn 104 159plusmn 156 513 plusmn 198 1577plusmn 234 985 plusmn 0541979 075 plusmn 041 516 plusmn 046 093plusmn 057 158 plusmn 053 105plusmn 046 668 plusmn 264 1674plusmn 117 989 plusmn 1442452 089 plusmn 067 499 plusmn 104 177plusmn 044 237 plusmn 071 190 plusmn 105 628 plusmn 187 1733plusmn 153 1101plusmn 0733589 113 plusmn 038 629 plusmn 086 185 plusmn 086 300 plusmn 061 175 plusmn 069 847 plusmn 149 1913 plusmn 141 1041 plusmn 109

Table 6 Band depths () with respective errors and Lambert coefficients (Grundy et al 2002) used for extrapolating the Lambert coefficientat 062 μm

Object 062 μm 073 μm 079 μm 080 μm 084 μm 087 μm 089 μm 090 μmPlutolowast 09 plusmn 03 50 plusmn 08 16 plusmn 06 20 plusmn 07 16 plusmn 03 66 plusmn 12 171 plusmn 12 102 plusmn 04Makemakelowastlowast 100 plusmn 13 288 plusmn 25 171 plusmn 38 167 plusmn 22 191 plusmn 58 332 plusmn 37 583 plusmn 73 394 plusmn 47Lambert Coef (40 K) (223 plusmn 123) times 10minus3 503 times 10minus2 129 times 10minus2 141 times 10minus2 103 times 10minus2 706 times 10minus2 510 times 10minus1 169 times 10minus1

(288 plusmn 085) times 10minus3

Erislowastlowast 46 plusmn 10 149 plusmn 17 99 plusmn 19 94 plusmn 10 68 plusmn 17 179 plusmn 32 427 plusmn 35 251 plusmn 17Lambert Coef (30 K) (297 plusmn 146) times 10minus3 522 times 10minus2 142 times 10minus2 151 times 10minus2 112 times 10minus2 745 times 10minus2 550 times 10minus1 179 times 10minus1

Notes The values of the coefficient for the band at 062 μm in bold format have been estimated in this work The upper one for 40 K is calculatedusing the spectrum of Pluto and the lower one is calculated using the Makemake spectrum The value for 30 K is calculated using the spectrum ofEris The other Lambert coefficients are taken from Grundy et al (2002) (lowast) Indicates average of the values computed for the six spectra shown inTable 5 (lowastlowast) band depth measured for the spectrum of Makemake from Licandro et al (2006b) and for the spectrum of Eris from Alvarez-Candalet al (2011)

Lambert coefficients corresponding to T = 30 K because thistemperature is more similar to what is estimated for the sur-face of this object (see Fig 6 bottom panel) In both caseswe measured the band depth of methane ice in the spectrumof Makemake and Eris from the literature following the sameprocedure described in the previous section for Pluto

In all the plots in Fig 6 the blue line is the least squares fitand the horizontal discontinued line corresponds to the measuredband depth at 062 μm with its corresponding error (horizontaldotted lines) The intersection between the line fitting the pointsand the measured band depth value for 062 μm band (red point)represents the extrapolated value of the Lambert coefficient forthe 062 μm band and its error (red line) In Table 6 we listed theband depths () with the respective errors and the value of theLambert coefficient for each band as taken from the literature

The value of the extrapolated Lambert coefficientsat 062 μm for Pluto is 288times 10minus3plusmn 123times 10minus3 cmminus1 while thevalue obtained for Makemake is 223times 10minus3plusmn 085times 10minus3 cmminus1The two values agree within the errors

The value obtained for Eris is 297times 10minus3plusmn146times 10minus3 cmminus1The difference due to the temperature is not significant within theerror in the calculation

35 Shifts of the center of the bands

The centers of the absorption bands of methane ice provideinformation about the mixing ratio of methane and nitrogen(Quirico amp Schmitt 1997) When methane is diluted in nitrogenice in small proportions the centers of the methane bands areshifted toward bluer wavelengths (Quirico amp Schmitt 1997) andthe shift is greater as the content of nitrogen increases (Brunettoet al 2008) Some works (Quirico amp Schmitt 1997 Quiricoet al 1999 Douteacute et al 1999 Merlin et al 2010) discuss theexistence of regions formed of pure CH4 segregated from theN2CH4 solution This effect was first reported for Pluto by

Owen et al (1993) However this conflicts with both alternativemodels for explaining Plutorsquos atmosphere According to Trafton(2015) and references therein the surface of Pluto is insteadcovered by a solid solution of these ices N2CH4 with differ-ent mixing ratios Moreover two dilution regimes one saturatedin CH4 and the other saturated in N2 coexist on the surface ofPluto The dilution regime saturated in CH4 is the responsiblefor the relatively unshifted part of the Plutorsquos spectra which waspreviously attributed to pure CH4

We measured the shifts in the center of the bands by com-paring our spectra with theoretical relative reflectances of puremethane ice The synthetic relative reflectances were calculatedusing a Shkuratov model (Shkuratov et al 1999) and the opticalconstants of methane for T = 40 K from Grundy et al (2002)

For comparison we obtained a suit of relative reflectancesconsidering different grain sizes then we shifted each modelin wavelength until we found the best comparison We used achi-square test to choose the best combination of grain size andshift To evaluate the error associated with the measurement werepeated the process several times randomly excluding somepoints of our spectra and changing the limits of the bands Theobtained shifts are listed in Table 7 In some cases the lowsignal-to-noise or to the presence of some artifacts that appearduring the reduction process and change the shape of the bandmeans that it has been impossible to measure the band shift

4 Discussion

Previous works have shown that Pluto presents a surface with azonal (longitudinal) heterogeneity (Grundy amp Fink 1996 Buieet al 2010ab G13) This heterogeneity is clearly seen in pro-cessed images of Pluto from the HST (Buie et al 2010b) andin the recently released images of the New Horizons mission1

1 httpwwwnasagovmission_pagesnewhorizons

A131 page 7 of 12

AampA 585 A131 (2016)

Table 7 Shifts (Aring) of the center of the absorption band of CH4

Longitude 06190 μm 07296 μm 07862 μm 07993 μm 08442 μm 08691 μm 08897 μm 09019 μm

497 ndash ndash74 plusmn 46 ndash ndash ndash ndash ndash76plusmn 15 ndash1037 ndash ndash156 plusmn 27 ndash ndash ndash93 plusmn 17 ndash139 plusmn 22 ndash139 plusmn 18 ndash1413 ndash ndash ndash ndash ndash ndash ndash ndash1979 ndash ndash121 plusmn 14 ndash ndash148plusmn 25 ndash69 plusmn 27 ndash108 plusmn 63 ndash157 plusmn 34 ndash2452 ndash ndash57 plusmn 18 ndash ndash ndash ndash55 plusmn 26 ndash ndash3589 ndash ndash47 plusmn 27 ndash71 plusmn 20 ndash40 plusmn 30 ndash ndash31 plusmn 26 ndash88 plusmn 23 ndash

The heterogeneity is associated with an uneven distribution ofthe ices (CH4 N2 CO) and the complex organics on the surface

G13 report long time trends in near-infrared spectra obtainedbetween 2000 and 2012 from the analysis of the absorptionbands of CH4 CO and N2 present in the spectra They find thatthe absorptions of CO and N2 are concentrated on Plutorsquos anti-Charon hemisphere (sim180) unlike absorptions of less volatileCH4 ice that are offset by roughly 90 degrees from the longitudeof maximum CO and N2 absorption On the time scale of theirobservations they find variations in the depth and in the ampli-tude of the diurnal variation of the absorption bands of CH4 COand N2 They conclude that the geometric distribution of theseices (in particular CO and N2) on the surface of Pluto can ex-plain these secular changes although seasonal volatile transportcould be at least partly responsible

Although visible wavelength spectroscopy does not enablethe study of the N2 and CO concentrations we can discuss thelongitudinal variation within our set of observations and the sec-ular variation when comparing with previous observations of thedistribution of methane ice and of the dark-red organic materialAlso from the study of the center of the CH4 bands we canextract information on the relative amount of CH4 diluted in N2

With respect to the methane G13 find that the stronger CH4absorption bands have been getting deeper while the amplitudeof their diurnal variation is diminishing consistent with addi-tional CH4 absorption at high northern latitudes rotating intoview as the sub-Earth latitude moves north They conclude thatthe case for a volatile transport contribution to the secular evolu-tion looks strongest for CH4 ice despite it being the least volatileof the three ices

41 Longitudinal variations

We note from Fig 2 that spectra at different longitudes have dif-ferent albedos as well as different overall shapes across the fullspectral range demonstrating the longitudinal heterogeneity ofthe surface of Pluto To show this in more clearly in Fig 7 wherewe plotted the variation with longitude of the spectral slope valueobtained from our spectra and compared it with a map of part ofthe surface of Pluto (mostly the northern hemisphere that is alsothe hemisphere currently observed from Earth) obtained by theNew Horizons mission2) The highest (redder) value of the slope(at longitude sim100) is centered on the hemisphere that accord-ing to the new maps of Pluto contains the large dark regionwhereas lower values correspond to a zone that is on averagebrighter (sim240) This is compatible with a higher abundanceof solid organic material (redder in color and lower in albedothan the icy species observed in Pluto) in the darker region anda higher abundance of ices in the brighter ones

2 httpwwwnasagovmission_pagesnewhorizons

In Fig 8 (bottom panel) we show the variation in the banddepth of the five stronger CH4 absorption bands measured in ourspectra A sinusoidal trend is clearly visible with a minimum ata longitude between 140ndash160 and a maximum at a longitudenear 360 This phase (360) is also the one with the clearestdetection of the 062-μm band The location of this CH4 max-imum agrees with the trend found by G13 (maximum CH4 be-tween 260 and 320) but appears a bit shifted to the east Inparticular G13 finds that there are subtle differences in the lon-gitude of the maximum of the absorption for different bands ofCH4 with the weaker absorption bands having the minimum andmaximum at longitudes more to the east than the stronger CH4absorption bands (see Fig 4 of G13) They interpret this differ-ence between stronger and weaker bands as due to the presenceof different terrain types with terrains with larger particles orwith greater CH4 enrichment that produce the weaker bands thatare more prevalent on the Charon-facing hemisphere Figure 8shows how this trend continues for the bands in the visible thatare even weaker than in the NIR with the shallowest bands(080 and 079 μm) having a minimum at sim180 and a maximumat sim360 The largest abundance of CH4 or the largest particlescan be found in the hemisphere centered at sim0 and is not cou-pled with either the darkest (probably higher organic abundance)or the brightest areas (probably higher volatile component)

With respect to the degree of dilution of CH4 in N2 G13found that all the CH4 bands in the infrared range show a blueshift (indicative of dilution) with a minimum blue shift corre-sponding to the Charon-facing hemisphere and a maximum blueshift corresponding to the anti-Charon hemisphere That is ex-actly what Fig 9 shows with a maximum absolute value ofthe shifts between 100 and 200 of longitude correspondingto the anti-Charon hemisphere The minimum of the shifts cor-responds to a region where the abundance of methane is higher(the Charon-facing hemisphere) This could indicate a higher de-gree of saturation of CH4 in this hemisphere (Tegler et al 2012Grundy et al 2013 Trafton 2015) Our measurements in the vis-ible show the same trend as do the estimates from the NIR ob-servations apparently indicating that the degree of dilution isuniform over different path lengths of the light

42 Secular variations

To compare our observations with earlier spectral data we showall our spectra in Fig 10 with spectra from Buie amp Fink (1987)and Grundy amp Fink (1996) overplotted on spectra obtained atsimilar longitudes (but more northern latitudes) For examplewe compare our spectrum taken at a longitude of 497 (sub-Earth latitude 51) with Grundy and Finkrsquos data for 1993 (lon-gitude 56 sub-Earth latitude sim11) and our spectrum at 3589longitude (sub-Earth latitude 51) with 1983 spectrum fromBuie amp Fink (1987) (longitude 329 sub-Earth latitude simndash10)Differences in the new spectra compared to earlier data can be

A131 page 8 of 12

V Lorenzi et al The spectrum of Pluto 040ndash093 μm I

Fig 7 Spectral slope (1000 Aring) calculated between 040 and 075 μm for spectra of Pluto normalized at 060 μm vs longitudes (top panel)and map of part of the surface of Pluto result of combination of images taken by the Long Range Reconnaissance Imager (LORRI) on NewHorizons and combined with the lower-resolution color data from the instrument Ralph on the spacecraft (bottom panel) (httpwwwnasagovmission_pagesnewhorizons) The scale of the horizontal axis is the same for the top and bottom panels The red line indicates thesub-Earth latitude at which our data have been obtained (sim51)

a result of the different latitude aspects temporal changes ora combination of both From the comparison of the slopes inthe visible (see Table 3) we can see that the spectra obtainedin 2014 are all redder than corresponding previous spectra Thiscould suggest a stronger presence of complex organics on Plutorsquossurface with respect to previous epochs

G13 reported a difference in the secular evolution betweenstronger and weaker methane absorption bands observed in thenear-infrared Grundy amp Fink (1996) and G13 attribute this toa different evolution of CH4 poor terrains (that only producethe stronger bands) and of terrains richer in CH4 (that producethe weaker bands) They find that the stronger bands increase instrength with years while the weaker bands in their sample donot If this trend extends to the visible wavelengths as alreadynoted by Grundy amp Fink (1996) we would see a secular de-crease in the depth of the bands Indeed from Fig 11 we cannote that the depths of the bands at 073 and 089 μm measuredin our spectra are lower than those reported in Grundy amp Fink(1996)

G13 also find that the amplitude of the pattern of the lon-gitudinal variation decreases with time This is consistent withthe change over the years of the sub-Earth latitude of Pluto withan increasing contribution to the observed spectrum of north-ern polar ices over the contributions from equatorial and south-ern latitudes which as seen in the color map in Fig 7 show amuch greater contrast with longitude In Fig 11 (top and middlepanels) we plotted the measured band depth of the weak bandsat 073 and 089 μm along with the depth of the same bands fromGrundy amp Fink (1996) and a sinusoidal fit for each data set Inthe bottom panel we show for comparison purposes the secularvariation of the strong band at 172 μm from G13 The depthsand the amplitude of the variations measured by us are slightlylower than those of 1983ndash1994 following the same trend of thenear-infrared bands described in G13

Fig 8 Top panel variation in the band depth (in ) with longitude forthe five stronger CH4 absorption bands measured from the spectra ofPluto presented in this work Bottom panel variation in the depth of theabsorption bands at 114 172 and 232 μm from G13 The points in thegray shadow represent replicate values (at longitude plusmn 360) to bettervisualize the variation in the band depth with the longitude

It is good to keep in mind that because Pluto has not been ob-served over a whole orbit it is especially difficult to characterize

A131 page 9 of 12

AampA 585 A131 (2016)

Fig 9 Top panel measured band shifts (Aring) with respect to the posi-tion of the pure methane from this work vs Plutorsquos longitude We con-nected the points corresponding to the bands at 073 087 and 089 μmwith lines to illustrate the variation with longitude (for the other bandsthere are too few points) At longitudes between 100 and 200 theshifts are greater thus the CH4N2 is smaller which is compatible witha lower abundance of CH4 suggested by the smaller band depths shownin Fig 8 Bottom panel variation in the shifts (Aring) of the bands at 089116 172 and 220 μm with respect to the position of the pure methanefrom G13 The points in the gray shadow represent replicate values (atlongitude plusmn360) to better visualize the variation in the band depth withthe longitude

secular variations since they might be confused with seasonalor regional (geometrical) variations The analysis of the secularvariation of the slope in the visible shows a reddening of Plutorsquossurface from 1983 to 2014 suggesting a higher degree of pro-cessing of the surface On the other hand the analysis of thesecular variation of the methane ice bands in the visible is con-sistent with the results from the NIR and indicates a decrease inthe contrasts in band depth along Plutorsquos surface In our casethe clear correspondence of these changes with the pattern seenin the color maps of Pluto obtained with New Horizons supportsthe contention that they are probably dominated by the sub-Earthlatitudinal change rather than by seasonal changes

43 Comparison with Makemake and Eris

To compare the spectral properties of Pluto Makemake andEris in Sect 3 we calculate slope and band depths for both thesame parameters as computed for Plutorsquos spectra

The spectral slope of Pluto calculated between 057 and070 μm is identical within the errors to that of Makemakefrom Licandro et al (2006b) and three times larger thanthe spectral slope of Eris from Alvarez-Candal et al (2011)(see Table 3 and Fig 4) This suggests that the surface ofMakemake contains a similar amount of solid complex organ-ics (the coloring agent) as Pluto (averaging over the entire sur-face) while Erisrsquos surface has a smaller amount of organics thanboth Plutorsquos and Makemakersquos as previously suggested (eg

Fig 10 Secular variation on Pluto reflectance Pluto-Charon spectrafrom this work shown with data at similar spectral resolution and signalprecision from Buie amp Fink (1987) and Grundy amp Fink (1996) at simi-lar longitudes All the spectra are normalized at 060 μm and shifted by05 in relative reflectance for clarity

by Lorenzi et al 2015 and references therein) This shallowerspectral slope is consistent with refreshing of the surface of Eriswith volatile ices (N2 CO CH4) deposited after the collapseof the atmosphere at some intermediate distance on Erisrsquo wayfrom aphelion to the perihelion as suggested by Licandro et al(2006a) and Alvarez-Candal et al (2011)

On the other hand the methane bands are consistently deeperin the Makemake spectrum than in the Eris spectrum and in thePluto spectrum they are even weaker (see Table 6 and Fig 4)This indicates that methane is more abundant on the surface ofMakemake than on Eris and on the surface of Eris than on Plutoas previously reported by Licandro et al (2006b)

44 Other applications

The spectra presented in this paper provide an independent cal-ibration of the MVIC (Multispectral Visible Imaging Camera)instrument (Reuter et al 2008) aboard the New Horizons space-craft and allow synthesis of a ldquogreen channelrdquo that enables thepreparation of false-color images of Pluto with the data from thespacecraft encounter Figure 12 shows an average spectrum ofPluto from the present investigation with the transmission curvesof the MVIC filters superimposed Overplotted we show thephotometric appearance of this spectrum as seen through MVIC

A131 page 10 of 12

V Lorenzi et al The spectrum of Pluto 040ndash093 μm I

Fig 11 Secular trend of the methane absorption bands variation inthe absorption band depth for the bands at 073 μm (top panel) and089 μm (middle panel) compared with the secular variation for theband at 172 μm (bottom panel) from G13 Lines in green represent asinusoidal fit of band depths from this work lines in purple represent asinusoidal fit of band depths from Grundy amp Fink (1996) The points inthe gray shadow represent replicate values (at longitude 360) to bettervisualize the variation in the shifts with the longitude

filters The vertical lines indicate the maximum and minimumvalues of these ldquoconvolved photometricrdquo points from our set ofspectra and give an indication of how the variation showed byour data could be detected by MVIC

These new spectra provide the desired color information formodeling efforts that will use optical constants for non-ice mate-rials presently being synthesized in the laboratory by the UV andelectron-irradiation of a Pluto mixture of ices known from near-IR spectroscopy (N2CH4CO 10011) The chemical analysisof the refractory residue from a UV-irradiated Pluto ice mix-ture has been published by Materese et al (2014) and resultsfrom the electron-irradiated ices are presented in Materese et al(2015)

5 Conclusions

Low-resolution visible spectra of Pluto at six differentplanetocentric longitudes between May 13 and July 172014 were obtained using the ACAM imagerspectrographof the 42 m William Herschel Telescope (Observatoriodel Roque de los Muchachos La Palma Spain) These datacover a whole rotation of Pluto with a step of sim50 and a gapat sim300

Fig 12 Longitudinal variation inferred for MVICS observations Theaverage spectrum from the 2014 observations presented in this workis presented with the filter transmission bands of the New HorizonsMultispectral Visible Imaging Camera (MVIC Reuter et al 2008) Thecolored dots show the convolution of these transmissions with the av-erage Pluto spectrum The vertical bar shows the variation among thesix reflectances (maximum and minimum values) The value at the NIRband is a lower limit since our spectra do not cover the full band pass ofthat filter

The new spectra clearly show methane ice absorption bandsat 073 078ndash080 μm and 083ndash091 μm also seen in previouslyreported spectra of Pluto (Grundy amp Fink 1996) We reportedthe detection of the spectra of the methane-ice band equiva-lent of the 062-μm CH4 gas absorption band in Pluto spec-trum This band was previously reported in the spectra of TNOsEris and Makemake (Alvarez-Candal et al 2011 Licandro et al2006b) The measurement of the depth of the absorption bandof methane ice at 062 μm in the new six spectra of Pluto al-lowed us to estimate the Lambert coefficient at this wavelengthat T = 40 K (not measured in the laboratory yet) as 288times10minus3plusmn123 times 10minus3 cmminus1 From the Makemake spectrum we obtaineda similar value within errors 223 times 10minus3 plusmn 085 times 10minus3 cmminus1From the spectrum of Eris we obtained a Lambert coefficientof 297 times 10minus3 plusmn 146 times 10minus3 cmminus1 in this case for a T = 30 K

The spectral properties of Pluto compared to those ofMakemake and Eris support that Plutorsquos surface holds a sim-ilar amount of solid complex organics (the coloring agent) asMakemake while Erisrsquos surface has a lesser amount of organ-ics than them both as previously suggested (eg Licandro et al2006b Alvarez-Candal et al 2011 Lorenzi et al 2015) On theother hand our spectra confirm that methane is less abundantin the Plutorsquos surface than on those of Eris and Makemake aspreviously reported by Licandro et al (2006b)

The longitudinal variation of the slope in the visible is inphase with the distribution of the dark and bright material asseen in the color maps of Pluto from New Horizons with thereddest slopes in the hemisphere centered at 90 and the less redmaterial in the hemisphere centered at 270

The longitudinal and secular variations of the methane bandsobserved in the visible wavelengths are consistent with the be-havior of the weak absorption bands in the near-infrared re-gion reported by G13 The maximum absorption due to methaneice is located at the phase centered at 360 (Charon-facinghemisphere) and the minimum at sim180 (anti-Charon-facing

A131 page 11 of 12

AampA 585 A131 (2016)

hemisphere) The curve is shifted from the curves of the strongermethane ice bands in the NIR The depths and the amplitudeof the diurnal variations of the methane ice in the spectra pre-sented here (54 N) are slightly lower than those of 1983ndash1994(Grundy amp Fink 1996) following the same trend of the near-infrared bands as described in G13 This trend indicates that ourobservations capture regions with a lower contrast and agreeswith the contrast in the color maps of Pluto with more dark andbright material lying in the equatorial latitudes

All of the detected CH4 bands are blueshifted with respect tothe position for pure methane ice indicating a certain degree ofdilution of the CH4 in other ice probably N2 We observed a lon-gitudinal variation with a maximum shift located at longitudesbetween 100ndash200 and a minimum shift around 360 longitudeThat the shift is minimum in the regions where the abundance ofmethane is higher could be indicative of a dilution of CH4N2that is more saturated in CH4

Acknowledgements This paper is based on data from override observationsmade with the William Herschel Telescope under program 055-WHT1114Aoperated on the island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de los Muchachos of the Instituto de Astrofiacutesica deCanarias DPC WMG and RPB were supported in part by NASArsquos NewHorizons mission JL acknowledges support from the project ESP2013-47816-C4-2-P (MINECO Spanish Ministry of Economy and Competitiveness) Wewant to thank the referee E Lellouch for his valuable comments that improvedthe manuscript

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570Benn C Dee K amp Agoacutecs T 2008 in SPIE Conf Ser 7014 6Benner D C Fink U amp Cromwell R H 1978 Icarus 36 82Binzel R P 1988 Science 241 1070Brunetto R Caniglia G Baratta G A amp Palumbo M E 2008 ApJ 686

1480Buie M W amp Fink U 1987 Icarus 70 483Buie M W Grundy W M Young E F Young L A amp Stern S A 2010a

AJ 139 1117Buie M W Grundy W M Young E F Young L A amp Stern S A 2010b

AJ 139 1128

Capaccioni F Coradini A Filacchione G et al 2015 Science 347 0628Cruikshank D P Imanaka H amp Dalle Ore C M 2005 Adv Space Res 36

178Cruikshank D P Wegryn E Dalle Ore C M et al 2008 Icarus 193 334Cruikshank D P Dalle Ore C M Clark R N amp Pendleton Y J 2014 Icarus

233 306Cruikshank D P Grundy W M DeMeo F E et al 2015 Icarus 246 82Douteacute S Schmitt B Quirico E et al 1999 Icarus 142 421Fix J D Neff J S amp Kelsey L A 1970 AJ 75 895Goesmann F Rosenbauer H Bredehoumlft J H et al 2015 Science 349

020689Grundy W M amp Fink U 1996 Icarus 124 329Grundy W M Schmitt B amp Quirico E 2002 Icarus 155 486Grundy W M Olkin C B Young L A Buie M W amp Young E F 2013

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