The Astrophysical Journal, 618:L57–L60, 2005 January 1
䉷 2005. The American Astronomical Society. All rights reserved. Printed in U.S.A.

SEARCH FOR NONMETHANE HYDROCARBONS ON PLUTO
Takanori Sasaki and Ai Kanno1
Department of Earth and Planetary Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan; takanori@eps.s.u-tokyo.ac.jp

Masateru Ishiguro
Department of Infrared Astrophysics, Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency,
3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan

Daisuke Kinoshita
National Astronomical Observatory of Japan, National Institute of Natural Sciences, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japanand
Ryosuke Nakamura
Lunar Exploration Division, Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency,
2-1-1 Sengen, Tsukuba, Ibaraki 305-8505, Japan
Received 2004 September 27; accepted 2004 November 18; published 2004 November 29

ABSTRACT
We have carried out infrared high-resolution spectroscopy of the Pluto-Charon system in the L band with theadaptive optics system on the Subaru telescope. The spectrum is dominated by the strong and broad absorption
features of methane but includes some additional features. Comparing the spectrum with model calculations, wesuggest that absorption features around 3.1, 3.2, and 3.35 mm could be an indication of nonmethane hydrocarbons
on Pluto’s uppermost surface. Implications of the estimated mass ratio between hydrocarbons for the formationand evolution of Pluto are discussed.
Subject headings: infrared: solar system — Kuiper Belt — planets and satellites: individual (Pluto)
the higher spectral resolution, we detected several new absorption features. In order to examine the possible carriers, we
performed simple model calculations based on Hapke’s bidirectional model, which are presented in § 3. NMHCs are themost plausible candidates, and in § 4 we discuss several scenarios for their origin.

1. INTRODUCTION

Trans-Neptunian objects (TNOs) are thought to be the remnants of icy planetesimals, and Pluto is one of the largest objects
among them. Pluto is exceptional, not only in its size but also
in its surface composition of N2, CH4, and CO (Cruikshank et
al. 1997). The hydrodynamic escape of Pluto’s atmosphere(Trafton et al. 1997) implies that there has been significant
surface loss by sublimation for the age of the solar system. In
this scenario, surface volatile ices must have been replenishedby some reservoir, such as geologic activity as seen on Triton(Stern & McKinnon 2000) and/or infalling TNOs. It has been
predicted that nonmethane hydrocarbons (NMHCs) can be generated from methane through photochemical reactions in thetenuous atmosphere (Krasnopolsky & Cruikshank 1999) andcosmic-ray irradiation of the surface (Moore & Hudson 2003).
If NMHCs could be detected, then their relative abundances
would provide important clues to the primordial volatile inventories of Pluto or the subsequent in situ alteration processes.
Hydrocarbons generally have a fundamental C–H stretchingmode in the L band, the absorption of which is much stronger
than the overtones in the J, H, and K bands. However, it isdifficult to make precise spectroscopic measurements in the Lband owing to the high background and variable telluric extinctions. Using SpeX, attached to the Infrared Telescope Facility, Grundy et al. (2002) found CH4 ice absorption featuresat 3–4 mm as well as other features around 2.9 and 4.1 mm,
which were attributed to H2O, CO2, and/or SO2, respectively.
The spectral resolution of their data, however, was insufficient
to examine the absorption features of hydrocarbons. In thisLetter, we present L-band spectra obtained by the Subaru telescope using its adaptive optics (AO) system. The observations
and data reduction procedures are described in § 2. Owing to

2. OBSERVATIONS AND DATA REDUCTION

A spectroscopic observation of Pluto was carried out on
2002 May 28 UT near the opposition (R.A. p 17 h 05 m 33s,
decl. p 12⬚39  25 ) using an infrared camera and spectrograph(Tokunaga et al. 1998; Kobayashi et al. 2000) with the 8.2 m
Subaru telescope in conjunction with its AO system (Takami
et al. 1998; Gaessler et al. 2002). We employed a mediumresolution grism with a slit of 0⬙. 3 width to obtain spectra witha resolving power of approximately 400 at the L band. Pluto
itself was used as the reference source for the AO system. The
sub-Earth longitude was 40⬚–50⬚, and the separation from
Charon was 0⬙. 9. The slit was set along the position angle of
Pluto and the satellite Charon to acquire them simultaneously.
The spectra were recorded by nodding the telescope by 3⬙ along
the slit to subtract the sky emission. The typical seeing size
was 0⬙. 3–0⬙. 4 during the observation, and the total on-source
integration time was 2600 s. A nearby G3 V star SAO 141540
s(R.A. p 17 h10 m 0.1, decl. p 8⬚31 26 ) was observed as aspectroscopic standard. Special attention was paid to minimize
the air mass difference between the Pluto and standard observations and the effect of temporal variation of humidity. Pluto
was observed within 1.5 hr before/after the standard star, andthe air mass difference was less than 0.035 throughout observations. We discarded all of the data obtained on the next night,
as they suffered from rapid and large humidity variations. Afterbad-pixel correction, sky subtraction, and flat-fielding, we obtained one-dimensional spectra of Pluto by using the NOAO
Image Reduction and Analysis Facility aperture extraction
package. Since Pluto was clearly separated from Charon with1
Current address: Development Technology, Service Business Promotion,
Industrial Solution Center, IBM Japan, Ltd., 9-21, Hakozaki, Nihonbashi, Tokyo 103-0015, Japan.

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Fig. 1.—Reflectance spectrum of Pluto with the previous low-resolution
data (Grundy et al. 2002) and synthetic spectra of intimate ternary mixture of
N2-CH4-CO with the mass ratio 1 : 0.01 : 0.002. The solid curve represents
our observation smoothed by taking a running average of 31 pixels (0.08 mm).
A data point at 3.316 mm was removed where the frame-by-frame fluctuation
was particularly large owing to very strong telluric absorption. The dotted anddashed curves indicate synthetic spectra for pure CH4 and for CH4 diluted in
the solid molecular nitrogen, respectively. The grain diameter is 3 cm for N2
and 100 mm for CH4 and CO. We normalized all the data at 3.58 mm, and thesynthetic spectra were convolved to the same wavelength resolution as theobservation.

the FWHM as sharp as 0⬙. 4, the resulting spectrum includes nocontamination from Charon. We obtained argon lamp frames
at the end of the night to determine the linear dispersion forwavelength calibration. The offset for each frame was correctedby shifting the strongest telluric absorption line at 3.316 mm
to that of the standard star. The uncertainty in the wavelength
calibration is smaller than 0.003 mm.
3. RESULTS

The spectrum of Pluto is shown in Figure 1 along with theprevious low-resolution data (Grundy et al. 2002) and synthetic
spectra of a simple ternary intimate mixture of N2-CH4-CO.
As in Nakamura et al. (2000), we employed Hapke’s bidirectional model (Hapke 1993) to calculate the spectrum of a uniform half-infinite layer covering the whole surface of Pluto. Itshould be noted that detailed model calculations are beyond
the scope of this Letter because we cannot account for therealistic solid solutions, vertically layered structure, and spatially segregated patches, unlike some previous approaches(Douté et al. 1999; Grundy & Buie 2001). It appears that theshape of the synthetic spectra is dominated by CH4 because N2
and CO have no significant absorption features in the wavelength range. Grundy et al. (2002) found a decrease in reflectance below 3.2 mm and above 3.95 mm with respect to theirmodel prediction. They interpreted the result as the contribution
from nonvolatile ices, such as H2O for the former and SO2 andCO2 for the latter. While we cannot confirm the putative bandof SO2 and CO2 owing to the limited wavelength coverage, ourspectrum shortward of 3.2 mm is consistent with their results.
Our observations show lower reflectance around 3.45 mm andadditional absorptions around 3.1, 3.2, and 3.35 mm. In orderto reproduce the features, we incorporated some NMHCs,
namely, C2H2, C2H4, C2H6, and C3H8, into the model calculations. Assuming their mass ratio to CH4 as 10%, we computed
the model spectra in Figure 2. The optical constants were mea-

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Fig. 2.—Modeled spectra including NMHCs as the fourth component. The
fourth components are C2H2, C2H4, C2H6, and C3H8. The model parameters of
the basic ternary mixture are the same as those in Fig. 1, and the mass fraction
to CH4 and the grain diameter of the fourth component are 0.1. and 30 mm,
respectively.

sured by Quirico et al. (1999), and the imaginary part is indicated in Figure 3. In Figures 2 and 4, CH4 is assumed to bediluted in the solid molecular nitrogen. It was found that C2H2,
C2H4, and C2H6 would produce the 3.1 mm absorption if theirrelative concentration to CH4 is approximately 10 weight percent. This value should be regarded as the upper limit for C3H8,
as the absorption at 3.0 mm was not seen in the observed
spectrum. Figures 2 and 3 indicate that C2H4 and C2H6 could
be associated with the absorption features around 3.2 and3.35 mm. We plotted these two synthetic spectra in Figure 4
for a detailed comparison with the observed spectrum. Adding
C2H4 as the fourth component, we obtained a better agreementwith the observations than the simple ternary mixture model,
but there was still a discrepancy between 3.4 and 3.55 mm.
While C2H6 improved the fit in this range, there was no clear
absorption around 3.65 mm in the observed spectrum. In bothcases, the observed absorptions around 3.2 and 3.35 mm shift
toward shorter wavelengths as compared to the model spectra.
Quirico et al. (1999) has shown that the dilution of solid N2
ice causes the shift of methane’s absorption. It is possible that
other hydrocarbons are affected in a similar way. More laboratory experiments are needed to explore the dilution effectFig. 3.—Imaginary part of the optical constants of various hydrocarbons(Quirico et al. 1999) used in Fig. 2.

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NONMETHANE HYDROCARBONS ON PLUTO

Fig. 4.—Reflectance spectrum of Pluto with the modeled spectra. The solid
curve shows our observation as shown in Fig. 1. The dashed and dotted curves
represent the synthetic spectrum with C2H4 : CH4 p 1 : 30 and C2H6 : CH4 p
1 : 10, respectively. While C2H6 gives better agreement with the observation
between 3.4 and 3.55 mm, the narrow absorption feature at 3.65 mm is absent.

more quantitatively. Figure 4 demonstrates that another component is needed to strengthen the absorption around 3.1 mm.
Moore & Hudson (2003) found that HCN forms in the solid
phase, with infrared absorption features at 3.04 mm, when nitrogen ices containing methane are irradiated by cosmic rays.
Unfortunately, we cannot make quantitative model calculations
because the optical constants of HCN are not available. We
added C2H2 as an alternative fifth component in the model
calculation. The resulting spectrum matches the observed spectrum quite well, as shown in Figure 5.
If we simply extend the wavelength coverage for the model
calculations, the resulting synthetic spectra would exhibit detectable absorptions in the J, H, and K bands. So far, however,
NMHCs have not been definitely identified in the J, H, or K
bands. This discrepancy could be explained by a strong concentration of NMHCs at the uppermost surface. While previous
observations in the J, H, and K bands have “seen” the underlying materials rather than the very thin surface layer, our Lband observations probe a shallower surface with deep absorption, resulting in enhanced contribution from the surface
layer. Nondetection of the spectroscopic signatures of NMHCs
at shorter wavelengths implies that they exist as a veneer. It is
worth noting that Nakamura et al. (2000) suggested the existence of solid ethane from their K-band spectra taken at asimilar sub-Earth longitude to our observations in the L band.
The veneer of NMHCs might be related to the lateral surface
inhomogeneity of Pluto’s surface (Buie et al. 1997; Grundy &Fink 1996; Douté et al. 1999; Young et al. 2001a).
4. DISCUSSION

Among numerous TNOs, Pluto and Triton have a distinctive
surface composition, including solid N2, CH4, and CO, whichreflects the fact that only these two objects are large enoughto retain such very volatile ices (Brown & Calvin 2000). On
the other hand, Krasnopolsky (1999) estimated that a 500 m
thick N2 layer has been lost from the surface since the formationof the Pluto-Charon system. Although N2 could be the dominant
form of nitrogen in the outer protoplanetary disk (Womack et
al. 1992; Owen et al. 2001), it is unlikely that such pristine N2
survived the accretional heating and formed such a thick layer

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Fig. 5.—The solid and dotted curves are the same as in Fig. 4. The dashedcurve shows the spectrum of a mixture of C2H2 : C2H6 : CH4 with mass ratios
1 : 1 : 10.

near the surface. The surface of Charon is covered by ammoniawater ice mixtures (Brown & Calvin 2000). The absorption
features of water ice have been detected on Triton (Cruikshanket al. 2000) and suggested for Pluto (Grundy et al. 2002). It
is generally accepted that Triton’s subsurface layer is composed
mainly of water ice or ammonia-water ice mixtures (Ruiz 2003)
and that Pluto has a similar underlying structure (Busarev etal. 2003) based on its density and solar abundance. Pluto may
have a deep internal reservoir that can produce N2 from morerefractory nitrogen compounds and supply it to the surface
along with CH4 and CO. The mass ratio between hydrocarbons
derived from our observations would give strong restrictions
on the differentiation and migration processes.
Alternatively, Pluto would have acquired the surface volatiles as a late-accreting veneer. Durda & Stern (2000) estimatedthat approximately 100 10 km sized TNOs have struck Pluto
over the last 3.5 Gyr, which corresponds to a uniform layer of
3 m thickness on Pluto. If we decrease the number by a factorof 25, as suggested by recent Hubble Space Telescope observations (Bernstein et al. 2004), there has been an inadequate
number of impactors to replenish the volatile loss. However,
it is likely that impact rates were much higher for the firstbillion years after solar system formation. Assuming little modification through impacts, we may link cometary compositions
with the pristine inventory of Pluto’s surface. TNOs may havedelivered the volatiles observed in cometary comae, as they
are thought to be precursors of short-period comets.
It is well known that comets contain both CO and CH4, as
on Pluto’s surface, although CO shows large comet-to-cometvariation (Gibb et al. 2003), possibly owing to selective sublimation prior to accretion. In the protoplanetary disk and theinterstellar cloud that preceded it, the gas-phase ion-molecule
reactions could have yielded C2H2. The reaction of H with C2H2
on cold dust grains could have produced C2H6 more effectively
than C2H4 (Hiraoka et al. 2000). In fact, C2H2 and C2H6 havebeen found in the comae of Oort Cloud comets (Mumma etal. 1996; Gibb et al. 2003; Kawakita et al. 2003), but detectionof C2H4 has not been reported to date. The C2H6/CH4 ratio
estimated from Figure 5 is consistent not only with Oort Cloud
comets but also with the value of the short-period comet 21P/
Giacobini-Zinner (Mumma et al. 2000) and the upper limit
for interstellar materials (Boudin et al. 1998). Moreover, theL60

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C2H2/CH4 ratio approximately agrees with the values for Oort
Cloud comets (Mumma et al. 1996; Brooke et al. 2003). The
observed 3.2 and 3.35 mm features could be ascribed to cometary C2H6, not C2H4, and C2H2 may be responsible for theabsorption at 3.1 mm.
In contrast to the extreme deficit of N2 in some comets (Cochran et al. 2000; Cochran 2002), TNOs, formed in the colder
regions, are thought to contain larger amounts of N2 owing toentrapment in amorphous ice (Notesco & Bar-Nun 1996; Owen
et al. 1999) or in clathrates (Iro et al. 2003). Such TNOs may
have delivered intact N2 to Pluto. Owen & Bar-Nun (1995),
however, suggested that the likely maximum value for the initial
N2/CO ratio cannot account for the currently observed N2 dominance on Pluto. While the vapor pressure over multicomponent
ices is not well understood, the composition of surface frost could
be generally different from the underlying reservoir layer (Younget al. 2001b). Presumably, the different volatility and/or the escape rate of N2 and CO led to efficient concentration of N2 in
the surface frost through sublimation-recondensation. The observed relative ratios between less volatile hydrocarbons mayplace some additional constraints on the models for atmospheresurface interaction and/or the atmospheric escape.
NMHCs could be secondary products generated from methane. Krasnopolsky & Cruikshank (1999) simulated photochemical reactions in Pluto’s tenuous atmosphere and the subsequent
precipitation to the surface. C2H2 and C4H2 have the highest
precipitation rates among numerous photochemical products,
but C2H2 cannot produce the observed 3.2 and 3.35 mm features,and we do not have optical constants for solid C4H2. The precipitation rates of C2H4 and C2H6 are smaller than those of C2H2
and C4H2 by an order of magnitude. If the 3.2 and 3.35 mm
features are an indication of C2H4 and/or C2H6 produced by thephotochemical reactions, their concentrations relative to C2H2
and C4H2 must be somehow increased. Moore & Hudson (2003)
conducted a systematic infrared study of proton-irradiated N2-

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rich ices containing CH4 and CO. They found C2H2, C2H4, C2H6,
and C3H8 along with HCN and HNC in the irradiated analog
materials of Pluto’s surface. It should be emphasized that they
identified C2H6 with absorption at 3.35 mm, but the expected
absorption around 3.65 mm was not detected, as in our observation. In addition, HCN is expected to decrease the gap between the models and observation around 3.1 mm. Therefore,
we do not rule out the possibility that the observed features
were associated with NMHCs produced by in situ surface reactions induced by cosmic-ray irradiation to the original ternary
mixture of N2, CH4, and CO. In either case, thin deposits arevulnerable to micrometeorite impact. The relative mass ratio
to the parent methane, derived from our observations, could
be a key to understanding the gardening process on Pluto, suchas the poorly known dust flux (Krivov et al. 2003) and verticalmixing timescale (Krasnopolsky & Cruikshank 1999).
We have obtained a high-resolution spectrum of Pluto in the
L band, which indicates unknown absorption features around
3.1, 3.2, and 3.35 mm. Instead of the more refractory icessuggested from previous observations of low spectral resolution(Grundy et al. 2002), we added NMHCs to the model calculations in order to reproduce the observed spectrum. Although
we have no definite identification, the comparison of the spectrum and model calculations suggests the presence of a veneer
of NMHCs on Pluto, the mixing ratio of which to methane isof the order of 10%. We briefly examined several formation
mechanisms that could account for the observed species andtheir relative ratios to methane.
We thank E. Quirico for providing us with the optical constant data of various ices in electronic form and Y. Abe andY. Yamashita for their many helpful suggestions and comments.
We thank K. Ishibashi, Y. Inoue, and A. Suzuki for supporting
the data analysis. We also thank the anonymous reviewer fora constructive review.

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