red centaur

8/2/2019 Red Centaur

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ICARUS 135, 389407 (1998)ARTICLE NO. IS985997

The Composition of Centaur 5145 Pholus

D. P. Cruikshank1

NASA Ames Research Center, MS 2456, Moffett Field, California 94035-1000

E-mail: [email protected]

T. L. Roush

NASA Ames Research Center, MS 245-3, Moffett Field, California 94035-1000

M. J. Bartholomew

Sterling Software and NASA Ames Research Center, MS 245-3, Moffett Field, California 94035-1000

T. R. Geballe

Joint Astronomy Centre, 660 North Aohoku Place, University Park, Hilo, Hawaii 96720

Y. J. Pendleton

NASA Ames Research Center, MS 2453, Moffett Field, California 94035-1000

S. M. White

NASA Ames Research Center, MS 234-1, Moffett Field, California 94035-1000

J. F. Bell, III

CRSR, Cornell University, Ithaca, New York 14853

J. K. Davies

Joint Astronomy Centre, 660 N. Aohoku Place, University Park, Hilo, Hawaii 96720

T. C. Owen1

Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, Hawaii 96822

C. de Bergh1

Observatoire de Paris, 5 Place Jules Jannsen, 92195 Meudon Cedex, France

D. J. Tholen1

Institute for Astronomy, 2680 Woodlawn Dr., Honolulu, Hawaii 96822

M. P. Bernstein

SETI Institute and NASA Ames Research Center, MS 2456, Moffett Field, California 94035-1000

R. H. Brown

Lunar and Planetary Laboratory, Univ. of Arizona, Tucson, Arizona 85721

1 Guest observer at the United Kingdom Infared Telescope (UKIRT). UKIRT is operated by the Joint Astronomy Centre on behalf of the UKingdom Particle Physics and Astronomical Research Council.

389


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390 CRUIKSHANK ET AL.

K. A. Tryka

Dept. of Physics, Northern Arizona University, Flagstaff, Arizona 86011

and

C. M. Dalle Ore

SETI Institute and NASA Ames Research Center, MS 245-6, Moffett Field, California 94035-1000

Received April 29, 1996; revised June 29, 1998

The short dynamical lifetime of Pholus in its prorbit (e.g., Levison and Duncan 1993), plus its red We present a new spectrum of the Centaur object 5145 Pholus

between 1.15 and 2.4 m. We model this, and the previously and spectral properties, strongly suggest that it recpublished (0.4- to 1.0-m) spectrum, using Hapke scattering entered the planetary zone from the Kuiper Disk of ptheory. Seen in absorption are the 2.04-m band of H2O ice tesimals. Because the Kuiper Disk is the source oand a strong band at 2.27 m, interpreted as frozen methanol low-inclination comets, and because of the composiand/or a photolytic product of methanol having small molecular similarities of Pholus to comets, it is reasonable to conweight. The presence of small molecules is indicative of a chemi- that Pholus is a primitive body that has not experiecally primitive surface, since heating and other processes re-

large-scale sublimation or chemical processing thmove the light hydrocarbons in favor of macromolecular carbon heating by the Sun. Instead, it has spent all but theof the kind found in carbonaceous meteorites. The unusually106 to 107 years of its existence in the frozen state bered slope of Pholus spectrum is matched by fine grains of aabout 30 AU from the Sun.refractory organic solid (tholin), as found previously by M.

While the orbit of Pholus is comet-like, its dimenHoffmann et al. (1993, J. Geophys. Res. 98, 74037407) andP. D. Wilson et al. (1994, Icarus 107, 288 303). Olivine (which are an order of magnitude greater than those of a tywe model with Fo 82) also appears to be present on Pholus. comet nucleus and are more similar to many asteWe present a five-component model for the composite spectrum Pholus does not clearly fall into either classificatioof all spectroscopic and photometric data available for 5145 deed, both classifications have become rather indistiPholus and conclude that this is a primitive object which has not recent years, with the realization from orbital dynayet been substantially processed by solar heat. The properties of that some objects long considered asteroids are inaPholus are those of the nucleus of a large comet that has never

comets (e.g., Hartmann et al. 1986) and the detectibeen active.

1998 Academic Press cometary activity in some objects designated as asteKey Words: asteroids, composition; comets, composition; ices,

(Meech and Belton 1990). Further, recent discovercentaurs; prebiotic chemistry.numerous small bodies beyond the orbit of Neptune (Jand Luu 1995) challenge the traditional definitions of

1. INTRODUCTION comets and asteroids.At the time of this writing, no spectroscopic or d

1.1. Discovery imaging evidence for cometary activity has been deton Pholus.On 9 January 1992, the Spacewatch asteroid search proj-

ect discovered object 1992 AD, now designated 5145 Pho-1.2. Photometric and Spectrophotometric Observatio

lus, in a highly eccentric orbit that passes from a perihelionpoint just inside the orbit of Saturn to well outside the Photometric and spectrophotometric observations

shortly after the discovery showed that Pholus waorbit of Neptune at aphelion, with a period of 92.7 years(Scotti 1992, Marsden 1992). Pholus is one of the family reddest object in the Solar System measured to that

(Mueller et al. 1992, Fink et al. 1992, and Binzel 199of bodies known as Centaurs, that is, objects with orbitscrossing those of the outer planets. Measurements of the this context, red means that, with the color of th

removed, the spectral reflectance of the body incrthermal flux (Howell et al. 1992) indicate that the diameterof Pholus is at least 140 km and the albedo is 0.08 at toward longer wavelengths over the interval 0.31

Each of the observing groups referenced above ind0.55 m. Similar thermal flux measurements (Davies et al.1993a) suggest a minimum diameter of about 190 km and dently proposed that the surface of the object is cov

at least in part, by organic solids which impart the exalbedo (at 0.55 m) of 0.04. Photometric observations(Buie and Bus 1992, Hoffmann et al. 1993) yield a rotation dinary red color, and low reflectance at wavel

0.55 m. Refractory organic solids produced in experiod of 9.989.99 h (corroborated by Davies et al. 1998)and suggest that the object is irregular in shape. ments in which energy is deposited in gases and in


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COMPOSITION OF 5145 PHOLUS

exhibit a red color similar to that of Pholus (e.g., Thompson carbon molecules indicating a more chemically primsurface than exhibited by the D- and C-type asteroidet al. 1987, 1991, Strazzulla and Johnson 1991). However,

it is difficult to establish an identification of a specific sub- comets. Luu et al. (1994) also observed the 2.27-m Pband; however, they pointed out the dissimilarity tstance on the basis of the reddening characteristic alone.

Organic solids were found in the dust of Comet Halley by bitumen band in terrestrial tar sand. At this time, wnot see evidence of a match between bitumen anexperiments on three spacecraft (e.g., Kissel and Krueger

1987); the dust particles contain about 1535% organic Pholus 2.27-m band, although the final identificatithe band will likely be a substance which is rich insolid, while the remaining inorganic fraction is primarily

Mg-rich silicates (pyroxene and/or olivine) (Schulze et al. soluble hydrocarbon.Two of the three observational data sets for the p1997, Fomenkova 1997). Greenberg (1982) and others pre-

dicted the refractory organic component of comet dust visual spectral region (Fink et al. 1992, Binzel 1992) good agreement with each other, with relatively mfrom studies of interstellar dust.

Apart from methane, specific organic molecules in the differences in the steep red slope. The spectral reflecderived by Mueller et al. (1992), however, is less stsolid state have not been identified by remote sensing ob-

servations of the surfaces of other Solar System objects, sloped toward the red. While some of these differmay result from the different solar phase angles at theprimarily because it is possible to observe only a small

portion of the infrared spectrum where diagnostic absorp- of the observations (Wilson et al. 1994), systematic ein the flux calibration, induced by the standard stars tions occur. The situation is further complicated by the

vast number of possible molecular combinations of even may also contribute. For present purposes we regar

spectral region 0.31.0m sufficiently well determinea small number of carbon and hydrogen atoms. One can,however, search for evidence of functional groups, such as do not offer any new observations. We adopt the r

slopes of the data by Fink et al. (1992) and Binzelmethyl (CH3) and methylene (CH2), as revealed by thefrequencies of the stretching and deformations of their (1992).

In this paper we describe the detailed modeling ochemical bonds. Unfortunately, the fundamentals of thesebonds occur in the infrared at 3m, while our observa- 0.4- to 2.5-m spectrum of Pholus using the Hapke sc

ing theory and astrophysically and chemically plational window for faint Solar System bodies is currentlyconstrained (by technology) to 2.5m. We must there- materials. The origin of these components in the inte

lar medium and the effect of various processing mfore search for molecular identifications through relativelyweak overtone bands (of the fundamental frequencies) as nisms (UV photolysis, ion bombardment, and heat) ar

cussed.well as combinations of stretching, bending, and scissoringfrequencies of the various chemical bonds that hold a given

2. ANALYSIS OF THE INFRARED SPECTRUMmolecule together. Within the framework of this poorlyconstrained problem, ambiguous and nonunique identifi-

2.2. New Spectral Datacations are to be expected.

Our first task has been to obtain improved spectrafor Pholus in the near-infrared (1.22.5 m) in ord

1.3. Spectroscopic Observationsprovide the basis for a more specific identification of on the surface. Because Pholus is very faint, we haveAn infrared spectrum (1.42.4 m) of 5145 Pholus ob-

tained by Davies et al. (1993b) in 1992 showed a strong able to obtain only parts of the spectrum during seobserving sessions. In this paper we present a compabsorption band centered near 2.27 m, absorption at

2.0 m, and other less distinct structure at shorter wave- spectrum derived from our own observations and thoLuu et al. (1994).lengths. Davies et al. (1993b) noted a resemblance to an

absorption band in the tholins described by Sagan and The improved spectral data (1.152.4m) reportedwere obtained on several occasions in 19921995,Khare (1979; reflectance spectra are shown in Cruikshank

et al. (1991)); however, a firm identification of this organic the United Kingdom Infrared Telescope and the CGrating Spectrometer-4 (CGS4) (Table I). CGS4 is asolid could not be made. Wilson et al. (1994) explored the

comparison with tholins more thoroughly and on the basis genic spectrometer using a diffraction grating and adimensional InSb array detector. We normally useof calculated reflectance spectra proposed mixtures of a

specific tholin, HCN polymer, and H2O and NH3 ices. We system in the low-resolution mode (first order of tlines/mm grating, 150 mm focal length camera), whreturn to this work below. Cruikshank et al. (1993) noted

the similarity in the position and shape of the 2.27-m 1.6 m gave resolution R / 250, and at 2.R 340. Until early 1995, CGS4 used a 58 62Pholus band to that in terrestrial organic solids which are

rich in light soluble hydrocarbons (bitumen). The authors detector array; with the grating and camera configurnoted, a single setting of the grating gave a spectral inproposed that Pholus surface contains small (light) hydro-


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TABLE I sphere, which affects the ability to remove strong teSpectra of Pholus absorption lines.

Uncertainties in the absolute flux of Pholus were prRegion

ily caused by inferior seeing conditions when the imaR Delta i covered Quality PrimaryDate (UT) (AU) (AU) (deg) (m) (Weight)a standard Pholus did not entirely fall within the slit of the spectr

ter. Broadband photometric measurements (at V1992 Mar. 26.3 8.724 8.323 6.14 1.45 2.43 Good (2) BS2890 wavelengths; see Table II) were used to combine the1993 Jan. 20.5b 8.932 7.994 2.03 1.432.45 (3)

ous components of the spectrum observed over the 41993 Feb. 4.4 8.959 7.982 0.94 2.042.46 Excellent (4) BS40301994 Apr. 19.2 9.595 9.088 5.29 1.141.82 V. Good (1) BS4027 period. JHK photometry was obtained by Davie1995 May 11.2 10.378 9.973 5.21 1.60 2.27 Good (1) BS4345 Sykes (1992), Davies et al. (1993b), and Howell (19951995 May 13.2 10.382 10.006 5.27 1.882.50 V. Good (3) BS4345

Table II), and most recently by Davies et al. 19981995 May 15.2 10.387 10.040 5.33 1.88 2.50 Good (3) BS4345photometric measurements by Howell (1995) were

a Weighting factor used in compiling the composite spectrum. cially important in linking the near-infrared spectrb Luu et al. (1994). ported here to the visible-region brightness of Pholu

cause she obtained JHK measurements with one telewhile V measurements were being made simultanewith another telescope at the same observatory sitesimultaneity of the VJHK measurements obviateof 0.405 m on the array. Some of the data, particularly

those in the 1.2-m region, were taken with the grating in problems of corrections for phase angle and, espe

the brightness variation of Pholus as it rotates. Wethe second order, giving R

383 at 1.2

m and spectralcoverage of 0.202 m with a single setting. The data ob- that Davies and Sykes (1992) and Davies et al. (1used estimated values of V for the times of theirtained in 1995 were taken with a new InSb detector array

of 256 256 pixels, giving 0.66-m spectral coverage with observations for the calculation of the V Jcolor. Theiis not substantially different from that observed direca single setting of the grating in the first order. At 2.2 m

with the new array, the resolution is R 850. Howell (1995), and we therefore average the two detnations by Davies et al. (1996) with the Howell valThe wavelength calibration for CGS4 is determined from

spectral lines produced by Ar and Kr lamps within the obtain VJ 2.50 0.08, and the other colors, as in Table II. (Davies et al. (1998) find slightly diffspectrometer each time the position of the grating is

changed. The absolute wavelength calibration has been colors in their most recent paper.) In calculatinadopted average in Table II we make no correctionchecked against the OH emission lines in the night sky,

and the rms accuracies from quadratic fits to several lines phase angle because of the lack of direct informati

phase-color effects for an object with the highly unin each spectral interval are typically 0.0001 m. Stars ofnear-solar spectral type and known flux are observed at colors of Pholus.

In order to derive the reflectance spectrum relatithe same or comparable airmass as the target objects, withno change in the grating position. Division of the spectrum 0.55 m, the colors of the Sun were subtracted from

average V J, V H, and V K colors of Pholus. Theof Pholus by the stellar standard removes the effects oftelluric lines and establishes the spectral flux distribution colors used were those of Campins et al. (1985): V

1.12, VH 1.43, V K 1.49. The resulting intof the target object. This procedure is subject to limitationsof seeing conditions and other systematic effects, such as colors of Pholus at the J, H, and K wavelengths

then converted to reflectances relative to V, for whithe stability of the local water vapor content of the atmo-

TABLE II

Photometry of Pholus

UT Date R Delta Phase(1992) (AU) (AU) (deg) V V-J V-H V-K S

Feb. 22.148 8.71 7.89 3.8 17.11 .07 2.44 .07 2.80 .08 2.92 .06 HMar. 15.399 8.72 8.16 5.6 17.25 2.47 .02 2.98 .02 2.93 .02 DMar. 17.281 8.72 8.19 5.7 17.28 2.59 .02 2.97 .03 2.95 .02 D

Adopted values 2.50 .08 2.92 .10 2.93 .02

a Published in Howell et al. (1992).b Published in Davies et al. (1996).


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assumed pV 0.06, a value consistent with an average of optical constants published by Khare et al. (1984aTitan tholin.the radiometric determinations mentioned above.

The material called Titan tholin is the involatile prof plasma irradiation of a gas mixture of N2 C2.2. The Composite Spectrum0.9 0.1, corresponding approximately to the bulk co

Using the available VJHK photometric points as a guide sition of Titans lower atmosphere (Khare et al. 19to continuum levels, we compiled the composite spectrum. Titan tholin consists of a variety of C1C5 hydrocaCertain differences appear in the different data sets for and nitriles, both unsaturated and saturated (Thomthe same wavelength interval. These include the absorption et al. 1991), the most abundant of which have beecentered near 1.71 m suggested in the Luu et al. (1994) served in Titans stratosphere by the Voyager infspectrum and in that of Davies et al. (1993b); in other data spectrometer (Hanel et al. 1981). Other compounsets it is not even marginally seen. Furthermore, as noted Titan tholin include alkanes, alkenes, and nitrogen hebelow, the Luu et al. spectrum shows marginal evidence cyclic molecules (Khare et al. 1984b, Ehrenfreundfor a band at 2.33 m. It is possible, perhaps likely, that 1995). Upon acid hydrolysis of Titan tholin, some 16 aas the body rotates different hemispheres of Pholus have acids are produced, with glycine the most abundant (Kslightly different spectral signatures, but the composite et al. 1986). Recent spectroscopic and chromatogrspectrum in Fig. 1 represents our best judgement in select- analysis of Titan tholin (McDonald et al. 1994) coning and weighting the various data sets. earlier work demonstrating that aromatic hydrocar

The composite spectrum (Fig. 1) consists of a weighted are a minor component. The water-soluble fraction o

(by signal precision) average of all the data in Table I. In tholin is dominated by materials of molecular wspectral regions of overlap between our data set and that200600 amu. McDonald et al. (1994) quantified

of Luu et al. (1994), all of our spectra were resampled at compositional details of Titan tholin. Overall, the elethe same wavelengths as Luu et al. so that averages could tal composition of Titan tholin is represented by C3be computed. All spectra were taken with the same spec- (Sagan etal. 1984). In our model the Titan tholin repretrometer and telescope. Large symbols in Fig. 1a represent the complex organic solid responsible for the red cothe JHK photometry in Table II. Pholus (Fig. 2).

Qualitative inspection of the spectrum of Pholus bauthors who obtained the original data (referenced a3. OUR MODELsuggested that this tholin might match the red slope

3.1. Hapke Theory factorily, and Wilson et al. (1994) demonstrated thrtheir own Hapke scattering calculations that such is in

We modeled the spectrum of Pholus using the Hapke the case. In ourprimary effort to model the spectral abtheory for diffuse reflectance from an airless planetarytion features in the near-infrared, we have concentsurface (Hapke 1993). The formulation we used has beenon other materials in addition to tholin, because whidescribed by Roush et al. (1990) and Roush (1994); in thereddish slope is explained by a tholin, neither tholinpresent context we used models of intimate compositionalthe other materials included in the Wilson et al. prefmixes of grains and spatially segregated regions of differingmodels match the band positions, band shapes, or tcomposition, as described below. Calculations using thebedo levels (1.8 m) in the new spectral data. We dHapke theory require the real and imaginary indices ofthe additional molecular components below.refraction for the candidate materials to be specified, but

such information is available for only a limited number ofplausible components of the surface of Pholus. The limited 3.3. Water Iceavailability of appropriate optical constants somewhat con-

The broad absorption band centered near 2.04 m istrained our range of model computations, but we beganmatched by absorption in solid H2O. This band is seon the assumption that specific regions of the spectrumcomparable strength in two other low-albedo obwould be most affected by specific constituents and that aComet 1995/O1 (HaleBopp) (Davies et al. 1995, final model would have to incorporate all of the compo-and Saturns satellite Phoebe (Owen et al., in preparanents that are chemically plausible.Optical constants for crystalline H2O ice at T 1derived from measurements by Ockman (1957) (R

3.2. Tholin1996) were used in our calculations, although the iPholus may in fact be structurally amorphous. The We begin by matching the red slope of the spectrum

from 0.4 to 1.0 m, for which we require a material rich mum temperature near Pholus equator at perihel118 K, while the temperature of the phase changein carbon compounds, such as the tholin organics produced

by plasma irradiation of organic-rich gases. We used the amorphous to crystalline (cubic) is in the range 142


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FIG. 1. This figure gives two presentations of the composite spectrum of Pholus (see text Section 2.2). (a) The composite spectrumrepresentative error bars. Between 0.4 and 1.0 m, three data sets are shown; the continuous line is from Binzel (1992), and the other symbidentified in the key. The solid hexagons are the average values of JHK photometric points, as discussed in the text. (b) For clarity, the comspectrum of Pholus is shown without the photometric points, error bars, or the points by Mueller et al. (1992). The dashed line is the model conof Titan tholin, olivine (Fo 82), H2O ice, CH3OH ice, and carbon black, as discussed in Section 3.

(Jenniskens and Blake 1996). Our data do not have ade- position andqualitatively in shape by a band in solid mquate signal precision to distinguish clearly between the nol (CH3OH) (Sandford and Allamandola 1993). Becrystalline and amorphous phases; the optical constants for CH3OH in a vapor state has been detected in several

amorphous ice are not available in our wavelength region, ets (Hoban et al. 1991, Bockelee-Morvan et al. 1991so we have adopted the crystalline values for the time being. Mumma et al. 1993), and because CH3OH is observthe solid phase in the dense cloud material to

3.4. Methanol protostars (Grim et al. 1991, Skinner et al. 1992), metice is a plausible component of Pholus. Optical consAs an abundant molecule in interstellar space and com-for solid CH3OH at wavelengths shorter than 2.ets, CH3OH is an important potential component of theare not available in the literature, so we determined surface of a relatively unprocessed body such as Pholus.from new laboratory spectra obtained in the laboratoBecause CH3OH is a small molecule and because it isDr. Vincent Anicich at the Jet Propulsion Labreadily produced by the oxidation of CH4, the presence oftransmission spectrum of a frozen film of 13.67methanol is indicative of the chemically primitive naturethickness was obtained at 90 K, and the real and imof a surface rich in hydrocarbons.

The Pholus absorption band at 2.27 m is matched in nary indices of refraction were calculated (see App


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FIG. 2. The spectrum of Pholus with the model, showing the spectral contributions of each of the four principal components. The spemethanol ice, water ice, olivine, and Titan tholin are computed from their complex refractive indicies ( n, k) with the Hapke scattering modthey are shown here with various scalings (of the ordinate) to fit them conveniently on the figure. Not shown is the neutral and featureless spof amorphous carbon.

A). Figure 3 gives an overview of the solid CH3OH 2956.0 cm1 (as3) cm

1, with the symmetric stretch at 2cm1 (ss1). The asymmetric deformation fundamentspectrum.

The spectrum from which we calculated the indices for at 1474.1 cm1 (ad1) and 1465.8 cm1 (ad2), and the (w

symmetric deformation at 1451.4 cm1 (sd1). The 2.2CH3OH is in good agreement with that shown by Sandfordand Allamandola (1993). In addition to the distinct band (4405 cm1) band in our model is the combination o

asymmetric stretching and deformation (as2 ad1, at 2.27 m, there is a weaker band in both spectra at2.33 m and an OH absorption centered at 2.1 m. We ad2, as3 ad1, as3 ad2), while the 2.33-m (4292

band is the combination of the symmetric modesreturn later to the assignments of these bands.Our model incorporates CH3OH to account for the symmetric fundamental modes are characteristically w

particularly in the deformation mode (sd1), resultin2.27-m band; however, methanol also has a second sharpband a 2.33 m which does not appear in the composite significantly weaker combination band. The combina

as1 ad1 and as1 ad2 do not appear in the methPholus spectrum. This pair of bands arises as combinationsof the CH stretching and deformation modes. The many spectrum.

Does solid CH3OH exist on the surface of Pholumodes of CH3OH have been identified by Barnes andHallam (1970) for monomers, dimers, and multimers in an the composite spectra we present here, only the 2.2

bandofCH3OH is clearly visible, while the weaker 2.3Ar matrix. The asymmetric stretching fundamentals lie at3005.3 cm1 (as1 [our notation]), 2961.9 cm

1 (as2), and band is not. However, in the single spectrum of this r


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FIG. 3. The spectrum of Pholus, as in Fig. 1, and our model (ordinate on the left). Also shown are the measured reflectance spectra of powhexamethylenetetramine (HMT) and a computed (geometric albedo) spectrum of CH 3OH ice at 90 K (5 m grain size). The ordinate on thcorresponds to the HMT and CH3OH spectra; the spectrum of CH3OH is offset downward by 0.1 for clarity.

published by Luu et al. (1994), which is included in our optical indices have been measured, which is not thfor other potential materials.composite, there is a suggestion of the 2.33-m band at

the level of the noise in the data. In Fig. 4 we show theLuu et al. spectrum and our model, which includes CH3OH 3.5. Nitrogen-Bearing Moleculesice, for comparison.

While solid CH3OH may occur on Pholus, in consider- Spectroscopy of the dense interstellar medium (p

stars embedded in interstellar gas and dust clouds ation of the absence of the 2.33-m band in most of theindividual spectra and in the composite, we do not claim W33A] shows the presence of ices of H2O, CO, CO

CH3OH, plus another band (4.62 m) indicative of a that we have identified it uniquely in this work. We reiter-ate, however, that methanol is an abundant component (at gen-bearing species, commonly referred to as XCN

Whittet and Tielens 1997, Lacy et al. 1984, Teglerthe level of a few percent relative to H2O) of at least someactive comets, and the interstellar medium, both of which 1993, 1995, Pendleton et al. 1998). Energetic process

mixtures of interstellar ice analogs produces a solid reare probably closely related to Pholus (Greenberg 1996,1998, Mumma et al. 1993, Grim et al. 1991). We carry that is chemically stable at room temperature and pre

(Allamandola et al. 1988, Palumbo et al. 1998). TheseCH3OH in our models because it represents small hydro-carbon molecules, such as the photochemical products of dues provide a good spectroscopic match to the 4.6

region in the spectra of embedded protostellar omethanol discussed in the next section, and because its


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FIG. 4. The normalized spectrum of Pholus obtained by Luu et al. (1994), with the model presented in this paper (offset upward). Of parnote is the apparent coincidence of the two spectral absorptions due to CH 3OH (2.27 and 2.33 m) in the model and in the Pholus spealthough the 2.33-m band in the Pholus spectrum is uncertain because of the random noise (see Section 3.4).

where CIN stretching modes dominate (Pendleton et al. HMT decomposes in the presence of heat, UV irtion, and acid hydrolysis into many cosmochemically 1998). Ion bombardment of nitrogen-containing ices not

only provides a good fit to the observational data, but also esting materials, including some of the original matfrom which it is synthesized. Acid hydrolysis proallows for the good possibility that the nitrogen is in the

form of solid N2, as in the cases of Triton and Pluto (Cruik- amino acids, ammonia, and formaldehyde, all of whicof significance in the Solar System.shank et al. 1998). Solid N2 would not be broken apart by

less energetic processing mechanisms, such as UV irradia- We have obtained reflectance spectra of HMT iregion 0.2 to 2.5 m at room temperature (see Apption. If the nitrogen is in the form of some other molelcule,

such as NH3, which requires less energy to liberate the N, B). The near-infrared portion of this spectrum is shoFig. 3 (also expanded in Fig. 5) along with the specthen mechanisms other than ion bombardment may be

significant. Here we will focus on the UV photolysis of calculated from measured optical constants for CH3OH and the spectrum of Pholus. HMT has a dispecific molecules, such as (CH2)6N4 hexamethylenetetra-

mine (HMT) as one possible scenario which could lead to absorption band at 2.27 m, just as does solid CHbut in place of the distinct secondary band of CH3Othe organics on bodies like Pholus. We note, however, that

if data for 2.5 m were available for the residue of 2.33 m, HMT shows an absorption shoulder with mspectral structure extending to about 2.40 m, whicion-bombarded N2 ices, that would also be an important

component to investigate. better match to the spectrum of Pholus than isCH3OH. We have not yet been able to model the PStudies of the UVphotolytic residue of irradiated

H2O: C H3OH:CO:NH3 ( 100 : 50 : 10 : 10) mixtures spectrum rigorously using HMT because the opticastants for this material have not yet been determin(Bernstein et al. 1994, 1995) show that the residue is com-

posed of 60% HMT, while the remaining 40% is a the spectral region of our data, and although HMT

eventually give a better match to the 2.27-m regimixture of ethers, amides, and ketones. HMT is potentiallyan important molecule because it carries four atoms of Pholus spectrum than solid CH3OH, we use the refr

indices for CH3OH in our model.nitrogen and its synthesis incorporates NH3.Bernstein et al. (1995) favor a synthesis mechanism for Because the photolysis of CH3OH results in the pr

tion of formaldehyde (HCHO) as an intermediary toHMT in which formaldehyde, first formed by UV photoly-sis of methanol, reacts with ammonia to form methylene- the formation of HMT, and because HCHO is fou

comets, we have considered the possibility that HCHimine (CH2NH). This path and the subsequent reactionsare shown in detail by Bernstein et al. (1995). The efficient responsible for the features seen in the spectrum of P

Formaldehyde readily polymerizes to two forms, the tconversion of NH3 into another product, such as HMT,might also explain the low abundance of solid NH3 in (trioxane (HCHO)3) and the linear polyoxymeth

(HCHO)n or POM). POM was reported in the comainterstellar clouds (5%, Tielens and Whittet 1997).


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FIG. 5. Laboratory reflectance spectra of hexamethylenetetramine (HMT), piperazine, and paraformaldehyde (POM) (see Section 3.5)

of Comet P/Halley from data from the Giotto spacecraft upon the nature of the irradiation (UV or ion bomment). When an ice mixture of H2O and CH3OH is (Huebner 1987, Mitchell et al. 1987, but also see Mitchell

et al. 1992). We show in Fig. 5 our reflectance spectrum barded with ions it produces acetone [(CH3)2CO] (zulla et al. 1995) instead of formaldehyde. If NH3 wof polyoxymethylene (paraformaldehyde), which shows

structure in the 2- to 2.5-m region, but which does not be included in the mixture, HMT would not form butnitrogen containing molecules might arise. As a reprmatch the structure in the spectrum of Pholus. We show

it here for completeness. tative of these, we have included in Fig. 5 piperazinamine similar in structure to trioxane, with a 1- to 2.The chemical evolution of methanol appears to depend


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spectrum similar to HMT. If N2 were included in the mix- 3.8. Our Model Fitture, UV photolysis would not break the NN bond, but

The best model fit to the data was found by a mion bombardment would. In the case of the interstellar

iterative technique, in which the principal effects ofmedium, the energy source involved in nitrogen chemistry

molecular component were first evaluated, individis not yet known, but the outcome of ongoing investigations

and in an ensemble, and then applied. Our best-fimay affect the conclusions eventually reached about the

model of the Pholus spectrum consists of two prinnitrogen in comets and on Pholus.

components. The first is an intimate mixture of 55% o

(Fo 82) in 20-m grains, 15% Titan tholin with grai3.6. Minerals1m, 15%H2O ice with grain size 10m, and 15%CHice with grain size 10 m. Within this intimate mixtuOur model incorporates 20-m grains of olivine to ac-

count for absorption in the 0.9- to 1.3-m region in the the four grain types, an incident photon encounters alkinds of grains before leaving the surface. The sespectrum of Pholus. Without this component, the model

composed of tholin, H2O, and CH3OH ices, and elemental principal component is carbon black. The two compoare modeled together as a spatially segregated mixcarbon shows a large departure from the data between 0.9

and 1.3 m; olivine brings the model and the data into that is in a checkerboard kind of pattern; a photon scing from one component does not strike the other bexcellent accord. We used the optical constants of forsterite

(Oahu Island olivine, Fo (82)), derived by Pollack et al. leaving the surface. The best-fitting mixture of theprincipal components required to achieve the albe(1994) from data of Hiroi and Takeda (1990). Other oliv-

ines with different Mg and Fe contents can also fit the Pholus at every wavelength is 61.5(5)% carbon blac38.5 5% of the olivinetholinwatermethanol mixPholus spectrum, but the abundances will be different.

The strength of the 1.1-m band complex (and to some We note that the compositional model reported here rsents the surface of Pholus and may not be reflectidegree its exact central wavelength), depend upon the rela-

tive Mg and Fe contents; the high-Mg forsterite bands are the bulk composition.We found that in order to achieve the steep red weaker than in the high-Fe fayalite olivine (e.g., Lucey

1998). We have chosen the forsterite Fo (82) because cur- at the correct albedo level in the visual spectral retholin grains of 1m were required. This small size virent indications are that the olivines in both comets (Crovi-

sier et al. 1997, Schulze et al. 1997) and in the interstellar the principle adopted for Hapke scattering models thgrain size must be at least three times the wavelenggrains (Waters et al. 1998) are Mg rich. Choice of another

olivine with higher Fe content would reduce the amount the same units). There are several ways to increasparticle sizes we use. One is to scale the optical consrequired to match the observed absorption on Pholus.

We considered minerals as a possible source of the of the tholin to retain the visual slope but to be lesorbing. This would likely result in larger abundanc2.27-m feature. Reflectance spectra of 10 minerals having

relatively narrow spectral features in the 2.2- to 2.4-m tholin contained in the mixtures. Another approach wbe to increase the internal scattering within particleregion are given by Bell et al. (1994, Fig. 7), and we have

further examined the spectra of nine chlorites and per- would have the net effect of increasing the grain diamfor all components. Instead of taking either of chlorites in the USGS Digital Spectral Library (Clark

et al. 1993). We found no plausible minerals that have the ad hoc approaches, we simply note that the tholin size violates certain conditions associated with H2.27-m Pholus band with the correct central wavelength

and bandshape. Although there may be other minerals theory.not examined by us that have a spectrum similar to thatof Pholus in the near-infrared, we do not pursue this 4. OTHER MODELSpossibility further and return our focus to volatiles andhydrocarbons. 4.1. The Wilson et al. Model

Wilson et al. (1994) and Wilson (1997) used Hapke3.7. Low-Albedo Component

tering theory to compute a synthetic spectrum to mthat of Pholus. They used optical constants of iceThe three components discussed so far have high albedos

when seen in diffuse reflectance, while the surface of Pho- hydrocarbons in various combinations for a hypothmaterial that would match the red color (0.41.0lus has a very low albedo in visible wavelengths. Therefore,

we have introduced into our models a fourth component They concentrated their study on the photovisual spregion, using the same data presented here (Finkto lower the albedo to match that of Pholus. We use carbon

black, a form of elemental carbon that has very high ab- 1992, Mueller etal. 1992, Binzel 1992). Their study incrated the original near-infrared spectrum by Daviessorption across the visible and near infrared spectrum and

is quite neutral in color. (1993b), and their models were fit to that data set.


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FIG. 6. The composite spectrum of Pholus presented in this work, with our computation of the Wilson et al. (1994) model (see Section

the real and imaginary refractive indices of a variety of ice is substituted for H2O and a pure hydrocarbon t(having no nitrogen) is substituted for the Titan thoindividual materials, they sought mixtures (termed intra-

particle mixtures) of these materials that, when the individ- In Fig. 6 we reproduce the best of the two prefWilson et al. models (HTA) for comparison with our nual n and k values of which were added in a linear way,

would reproduce the hypothetical bulk material matching derived spectrum in the near-infrared and the threeviously published data sets in the photovisual spectrthe spectrum of Pholus that they derived from the sources

noted above. gion. For Fig. 6 we have recalculated the model fro

Wilson et al. HTA recipe in terms of geometric albWe note that Wilson et al. used refractive indices of H2Oice from Warren (1984), which are for ice at T 269 K, whereas Wilson et al. worked with normalized brigh

The discrepancies between the model and the datawhereas in our models we use indices for ice at the moreappropriate temperature T 100 K, as noted before. Al- for 1.4m and are particularly evident in the mism

between the strong NH3 absorption bands (2.01 andthough the differences in the indices are slight, there is asmall shift to longer wavelengths of the ice band centers m) and the broader absorption bands in Pholus spe

at nearby but somewhat different central wavelengtand a narrowing of the bands with decreasing temperature.Wilson et al. favored two intraparticle mixtures. One We find no evidence for NH3 in the spectrum of Ph

mixture (HTW) consists of 0.15 Titan tholin, 0.1 hydrogen4.2. The Hoffmann et al. Model

cyanide polymer, and 0.75 H2O ice (particle size 10 m).The second model (HTA), giving a better fit to the spec- Hoffmann et al. (1993) calculated Hapke scattering

els using Titan tholin (grain size 3 m) and neutral cotrum, consists of the same components except that NH3


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materials of high and low albedo. They found a satisfactory The comparison of terrestrial organic solids with bin space retains considerable value; a coal model omatch to the spectrum of Pholus by Fink et al. (1992),

0.41.0 m, but did not consider the near-infrared region. interstellar extinction by Papoular et al. (1989, 199is of interest because it provides a fit to the ultravWilson et al. (1994) discuss the Hoffmann et al. model in

the context of their preferred mixtures, and we do not interstellar extinction curve and other spectral regiothe infrared. As noted before, reflectance spectra of consider the Hoffmann work further here, because our

emphasis is on the near-infrared region, and neither the of asphaltites, kerites, and anthraxolites show color chteristics similar to those of the low-albedo asteroids Titan tholin (alone) nor neutral materials match that por-

tion of the spectrum. C, P, and D classes (Moroz et al. 1991, 1992), even ththe strong band in many of these materials at 2.28does not fit the Pholus spectrum. Low-albedo aste

4.3. The Asphaltite Modelshow no distinct absorption bands between 1 and 2.(e.g., Luu et al. 1994, Barucci et al. 1994, Owen et al. 1On the basis of laboratory spectra of a suite of natural

terrestrial organic solids by Cloutis (1989) and Moroz et In the photovisual spectral region, as Moroz et al. (1992) and Cloutis et al. (1994) found, the presence ofal. (1991, 1992), Cruikshank et al. (1993) proposed that the

red color and the 2.27-m absorption band of Pholus might aliphatic and aromatic groups is needed to give thcolors of the asphaltite-like organics, but variations ibe matched by similar features shown by those materials.

The solids studied were tar sands, asphaltite, kerite, an- exact shape and slope of the reflectance spectra of materials are to be expected in materials that are inhethraxolite, etc., all containing varying amount of solvent-

soluble hydrocarbons of low to medium molecular weight complex mixtures of diverse composition and struRecall that Titan tholin consists of mostly aliphatic h(500 amu), collectively called bitumen. In his search for

diagnostic spectral features for remote sensing of organic carbons.solids on Solar System bodies and on Earth, Cloutis (1989)identified near-infrared absorption bands in natural tar 5. DISCUSSIONsands with specific overtones and combinations of C, H,and O vibrational fundamental modes. Tar sand contains 5.1. Organics on Pholusmacromolecular organic material that is related to pe-

We assert in this paper that the surface of Photroleum.

rich in complex carbon compounds represented hereThe most familiar macromolecular carbon material is

specific tholin, and we show spectral evidence that skerogen. Kerogen is the fraction of organic matter which

light hydrocarbons, methanol and/or its photolytic is insoluble in the usual organic solvents and is frequently

ucts, as well as H2O ice, occur on the exposed surfacereferenced in connection with meteorite and asteroid or-mix of complex and lighter organic molecules has two

ganics (e.g., Cruikshank and Kerridge 1992, Cronin et al.sources. First, about one-half of the interstellar du

1988, Cloutis et al. 1994). In a terrestrial setting, wherethe molecular cloud from which the Solar System fo

organic solids result from thermal processing of the decayconsisted of carbon and organic-rich grains, if that mo

products of biological activity, kerogens are members oflar cloud was typical of others in the Galaxy (Tielen

the sequence of natural terrestrial carbonaceous materialsAllamandola 1987). Second, after the formation of P

representing a continuous sequence from oil to graphite.as a solid planetesimal, irradiation by ultraviolet pho

The sequence is distinguished in large part by the changingsolar wind particles, and cosmic rays continued to a

ratio H/C; oil has the highest H/C, while graphite is purethe organic chemistry of the upper several meters o

C. Asphaltites (H/C 11.6), kerites (H/C 0.61.4), and an-crust or regolith. A third, though less significant, pot

thraxolites (H/C 0.030.1) are solid nongraphitic members

of this sequence, with anthraxolite being essentially theequivalent of kerogen. The degree of aromaticity (predom- 2 Cruikshank et al. (1991) reported absorption bands in some ast

the dark hemisphere of Iapetus, the rings of Uranus, and two cinance of carbon ring structures) increases with decreasingand proposed the identification of the CN bond in solid organic maH/C, ultimately terminating in the hexagonally structuredThe astronomical data were drawn from various published and u

graphite, devoid of hydrogen. lished sources. Since the publication of that paper, spectra of Asphaltite, kerite, and tar sand show a strong absorption quality of the same (and other) asteroids (Luu et al. 1994, Baruc

1994) and the dark hemisphere of Iapetus (Owen et al. 1995) haveband centered at 2.285 um with a profile similar to that ofthat the apparent absorption bands in the earlier data were spthe 2.27-m band of Pholus. The mismatch in wavelengthThe two comets, Panther (1981 II) and Bowell (1982 I), cannotis too large to explain in terms of temperature or minorserved again, so the published spectra by Jewitt etal. (1982), which

compositional effects. Thus, we abandon for the present to show a strong absorption band at 2.26 m, must be consideredtime the proposal by Cruikshank et al. (1993) of a strong The reality of an absorption band in the rings of Uranus remai

biguous.compositional similarity.


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source arises from the interaction of Pholus with organic somewhat similar to the thermal processing of terrebiological matter toward oil, kerogen, and graphiteand other material in the interstellar medium, including

other giant molecular clouds, as the Solar System moved principal difference is that the laboratory experimentduce mostly aliphatic organics and complex polythrough the Galaxy for 4.5 Gy.

Concerning the material from which Pholus condensed, while natural terrestrial organics are dominated bymatic structures.there is growing evidence from comets, meteorites, and

interplanetary dust particles that much of the original or- Aliphatic hydrocarbons have been found along sedifferent sightlines through the diffuse interstellar meganic matter in the outer solar nebula was retained during

the planetesimal building phase of the Solar Systems ori- in our Galaxy (Sandford et al. 1991, Pendleton et al. and are possibly produced in the outflow stages ofgin, and that much of the incorporated material survived

without melting or vaporization (Bradley etal. 1997, Berna- type stars (Geballe et al. 1998, Chiar et al. 1998)availability of these materials for incorporation into ntowicz 1997, Cruikshank 1997). Thus, Pholus began with

an abundant inventory of complex organic molecules that forming planetary systems makes them important istudy. The remarkable similarity of the aliphatics iresided on the interstellar grains from which it formed. The

nature of the organic matter from which Pholus formed is diffuse ISM to the Murchison meteorite (Pendleton1994) underscores this possibility.revealed by the dust in the interstellar medium, particularly

that material in the dense molecular clouds. The dust com- Once incorporated into the planetesimals, surfacecessing must continue over the 4.5 Gy of the Solar Syponent of the clouds consists of a mix of silicate grains and

grains of organic solids composed of refractory material Even with generous estimates of surface overturn th

impact gardening of Pholus surface, there was ample(polycyclic aromatic hydrocarbons, photolytic residuessuch as (CH2)6N4 and HCHO, etc.), and volatile ices of and radiation to effect surface organic chemistry by

ultraviolet light and to a depth characteristic of the sH2O, CO, CO2, alkanes, CH3OH, OCS, H2, XCN, etc. Wefind both organic material and a silicate on Pholus. scale impact gardening process. Experience with ult

let irradiation of carbon-bearing ices in the laboratoryThompson et al. 1987) shows that as the radiation

5.2. Processing of Surface Materialsincreases, an initially neutral-colored and high-albedbecomes red as the violet albedo decreases. Further iAmong those processes affecting the chemistry of Pholus

after its formation, we consider only solar heat, the ultravi- ation gradually reduces the albedo at all wavelengthultimately the material becomes very dark, neutral inolet solar radiation and the flux of galactic cosmic rays

(GCR); the solar wind was likely to have been blocked by and spectrally featureless.Galactic cosmic rays (GCR) have their maximumthe heliopause, if Pholus original position in the Kuiper

Disk was beyond about 50 AU. in the energy range 10 MeV to 10 GeV. The total edeposition will be about 9 103 ergs cm2 sSolar ultraviolet at the heliocentric distances of bodies

in the Kuiper Disk is weak (for 1500 A, the flux is 1.3 1015 ergs cm2 over the age of Solar System. GeV GCR, the deposition of the energy will occur103 erg cm2 sec1), but the time scale is long; in 4.5 Gy

1.5 1014 erg cm2 was deposited. Energetic processing about the upper 10 m of surface (depending upodensity), for an accumulated flux of 1.3 1012 ergs is clearly capable of inducing chemical changes in organics

and in ices. The details of the changes depend upon the The efficiency of production of complex molecules is 1 103 eV1 (Schutte 1988). Using this efficiency fradiation source (UV, ions, particles, etc.) and exposure

time, the ice composition, temperature, etc., but the basic and following the model for production of the comorganic molecules (50 amu) in the crust and interprocess involves the break-up of individual molecules into

fragments that recombine to form more complex molecules Comet P/Halley (Mitchell et al. 1992), in 4.5 Gy, the

ber fraction of these molecules relative to H2O is and polymers. The resultant materials have strong charge-transfer electronic bands that absorb most in ultraviolet, the surface and 0.0005 at a depth of 100 m. Mitch

al. (1992) observed 103 complex molecules per Hbut extend to visible wavelengths. Hydrogen is removedduring the irradiation, and the end state is a mass of macro- the coma dust of P/Halley, and reasoned that product

these molecules by GCR after the accretion of the comolecular carbon that is depleted of hydrogen. Titan tholinis produced initially in this way (corona discharge) from nucleus was insufficient to account for this number de

The complex molecules therefore were included ia mixture of gases, but the chemical processing is grosslysimilar to that in an ice, and the tholin is an intermediate material from which the nucleus accreted.

The same reasoning pertains to Pholus and other pstep in the production of what, with continued irradiation(especially with a higher energy component), would be- tesimals still in the Kuiper Disk and the Oort Cloud

organic inventory was largely set by the material come mostly carbon (Khare et al. 1989, 1993). In this sense,the extended irradiation processing of hydrocarbon ices is which they accreted, and although it has been alter


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some depth by the GCR flux in 4.5 Gy, the basic inventory none of the reddest asteroids are even approximatred as Pholus (Mueller et al. 1992). Similarly, the chas not changed.

What, then, causes the difference between Pholus and of comets in various stages of activity are never as rPholus (Hartmann et al. 1986, Mueller et al. 1992, Jthe low-albedo asteroids and comets in the inner Solar

System? The only remaining known factor, after UV and and Meech 1988, Luu 1993). The colors of a numbKuiper Belt objects are comparably red to Pholus (TGCR are accounted for, is the ambient heat provided by

the present Sun and the heating events in the inner Solar and Romanishin 1998, Jewitt and Luu 1998).The Centaur 7066 Nessus (1993 HA2) has an oSystem from the early Sun. The presence of differentiated

asteroids and the hydrothermal chemical alteration of similarity to Pholus, with perihelion beyond Saturnaphelion inside Plutos orbit, suggesting a similar oother asteroids, all revealed in the meteorite record, dem-

onstrates the reality of those early heating events, although and dynamical history. Nessus is somewhat fainterPholus, but only slightly less red in color (Daviesthe exact physical causes continue to be debated. In any

event, objects in the Kuiper Disk did not experience the 1996, 1998). The Davies et al. (1998) colors of Newhich can be compared with the Pholus photomeheating experienced by objects in the inner planetary zone.

Do other Solar System bodies show evidence for organic Table II, are V J 2.29 0.04, V H 2.57 0.1V K 2.57 0.1. Spectroscopy of Nessus in the nesolids? Both Triton and Pluto have solid CH4, and the low

albedo material that darkens portions of their surfaces may has not yet been achieved, but its color and dynasimilarities to Pholus invite speculation that its surfbe refractory complex organic matter (Cruikshank et al.

1997, 1998). McCord et al. (1997) have reported spectral also rich in primitive organic molecules.

Centaur 2060 Chiron, which has an orbit similar toevidence (at 3.4 m) from the Galileo spacecraft for spa-tially isolated deposits of organic material on Ganymede of Pholus, has a coma of variable intensity (Hart

et al. 1990, Meech and Belton 1990) and a low-alband Callisto. The dark hemisphere of Iapetus (Owen etal., in preparation) shows both a red color and a strong spectrally featureless continuum reflectance out to 2

(Luu et al. 1994). The spectrum (1.42.4 m) of Ceabsorption band at 3 m attributed to tholin in combina-tion with H2O ice. Near-infrared spectra of the Kuiper 1997 CU26 (Brown et al. 1998), which crosses the or

Uranus, shows clear evidence of H2O ice, but no Disk object 1993 SC (Brown et al. 1997) shows strongabsorption bands bearing similarity to the spectrum of absorption bands. Thus, Centaurs appear to fall in

least three compositional classes, of which Chiron, PhTriton, but with notable differences. The presence of hy-drocarbons (perhaps CH4) is strongly indicated on 1993 and 1997 CU26 are the defining examples. Jewitt and

(1998) report a range in colors among the KuiperSC (Brown et al. 1997).The low albedo and neutral or red colors (0.31.5 m) objects and the Centaurs, including some as red as Ph

Tegler and Romanishin (1998) define two distinct of comets and certain classes of asteroids (C, P, and D)have long been thought to indicate the presence of macro- classes (neutral and red) for the Centaurs and Kuipe

objects; the diversity among these objects appears molecular carbon-bearing molecules in their surface mate-rials (e.g., Cruikshank 1987, 1989, Owen et al. 1995), but greater when the spectra are examined.they do not show spectral absorption features diagnosticof the composition of those carbon-bearing molecules. In 6. CONCLUSIONSparticular, the band we see in the Pholus spectrum at 2.27m is not seen in any other plausible candidate object to Theextraordinary red color and the rich near-IR sp

signature of Pholus are explained by the presence odate (e.g., Luu et al. 1994, Barucci et al. 1994). It is reason-able to conclude that the organic signatures in the nuclei common silicate olivine, a refractory solid complex

ganic molecules (tholin), ices of H2O and CH3OH of comets and the surfaces of C, P, and D asteroids have

been removed by heating and devolatilization, leaving very similar molecule), and carbon on its surface. The redof Pholus is thus thought to have been produced blarge molecules (500 amu) and elemental carbon, both

of which have very high opacity and no diagnostic spectral energetic processing (solar ultraviolet, corona dischor ion bombardment, alone or in some combinatiobands when seen in diffuse reflectance in the accessible

spectral regions. simple molecules (CH4, H2O, CO, NH3, N2, CH3OH)photolytic products of these simple molecules are lmolecules (300 amu) which are relatively refra

5.3. Other Objectsand stable, but which upon further irradiation wouldtinue to reduce to elemental carbon and macromoleLow-albedo asteroids at Jupiters heliocentric distance

(the Trojans) and in the asteroid Main Belt, have colors carbon-bearing molecules (500 amu) in the presensignificant heating. We propose that most of the or(0.52.5m) that are either neutral (flat), or red to varying

degrees (e.g., Luu et al. 1994, Barucci etal. 1994). However, material on Pholus surface represent the partially


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the diffuse surface of the integrating sphere average out the sucessed remnants of the original inventory of material,angular components of the scattered or reflected light before it rformed in the interstellar medium, from which Pholus andthe detector.

other planetesimals accreted in what is now the Kuiper Themeasured reflectance data could be corrected to obtain theabDisk. reflectanceof thesamples.This small correctionfactor, thediffuse s

reflectanceof thecoating material, accounts for absorptionof theinFurther processing of Pholus by solar UV and heat caning spheres coating material at the reference beam reflectancbe expected to remove hydrogen from the small organicHowever, data provided by the manufacturer indicate that the reflemolecules, transforming the surface into a less red-coloredof the coating material is greater than 96% at all measurement

and spectrally featureless macromolecular carbonaceouslengths, so this correction is negligible at most wavelengths, ranmass. By this reasoning, Chiron represents the more heav- at most a 4% decrease in reflectance at the wavelengths where the cis most highly absorbing.ily processed stage of the chemical evolution of such a

body.Insofar as the reflectance spectrum of Pholus is so well ACKNOWLEDGMENTS

fit by a rigorous model that includes all of the basic compo-We thank Dr. Jane Luu for providing a digital version of the spnents of comets, it seems reasonable and appropriate to

of Pholus from Luu etal. (1994), and Dr. L. V. Morozfor her contribthink of Pholus as the nucleus of a comet that has never

to an early draft of this paper. Dr. Scott Sandford kindly provided been active. As such, it is demonstrably one of the most IR spectrum of frozen methanol and together with Dr. L. J. Allamprimitive objects yet studied. gave us helpful comments. Dr. Ellen Howell kindly made availa

photometry of Pholus in advance of publication. Dr. Vincent Amade available his laboratory facilities for the measurement of the

APPENDIX A: THE METHANOL SPECTRUM of several materials used in this work. Dr. R. Binzel provided a chis spectrum in digital form. We thank Dr. G. H. Herbig for a disc

Laboratory measurements of the spectrum of solid methanol wereof the accretion of interstellar dust particles on Pholus. We are g

made at the Extraterrestrial Ice Facility, at the Jet Propulsion Laboratory.to the staff of UKIRT for its excellent support of the telescope and

Spectra were measured between 1 and 5 m at temperatures between This research is supported by NASA through Planetary Astr90 and 120 K, a range which includes the temperature at which a solid

RTOP 196-41-67-03, Planetary Geology and Geophysics RTOP phase transition occurs.

60-01, and other sources.A thin film of methanol was formed by spraying gaseous methanol

onto a sapphire window, held at T 90 K, at a rate of approximatelyREFERENCES0.07m/min. Theexactthickness of the filmwas determined by measuring

interference fringes produced by a laser whose beam wasdirected onto theAllamandola, L. J., S. A. Sandford, and G. J. Valero 1988. Photocharea where the film was forming. The film thickness for the measurements

and thermal evolution of interstellar/precometary ice analogs.presented here was 13.67 m.76, 225252.The temperature of the window was maintained by a combination of

cooling, provided by contact with a cold finger, and warming, from a Barnes, A. J., and H. E. Hallam 1970. Infrared cryogenic studheater, attached to the window. The temperature of the window was Isotopically substituted methanols in argon matricies. Trans. Fknown to an accuracy of0.5 K during the measurements. Soc. 66, 19201931.

A spectrum was taken at 90 K, and then the temperature of the system Barucci, M. A., M. Lazzarin, T. Owen, C. Barbieri, and M. Fulchwas incrementally raised to 120 K. Spectra taken at intermediatetempera- 1994.Near-infrared spectroscopy of dark asteroids.Icarus 110, 28tures show that a change in the shape of some of the spectral bands

Bell, J. F. III, J. B. Pollack, T. R. Geballe, D. P. Cruikshank, occurred between 100 and 105 K, probably corresponding to a phase

Freedman 1994. Spectroscopy of Mars from 2.04 to 2.44 m durtransition of solid methanol that occurs in this temperature range (Falk

1993 opposition: Absolute calibration and atmospheric vs mineand Whalley 1961).

origin of narrow absorption features. Icarus 111, 106123.Values of the real and imaginary indices of refraction were determined

Bernatowicz, T. J. 1997. Presolar grains from meteorites. In From Sby using the subtractiveKramersKronig analysis (e.g., Warren 1984).to Planetesimals (Y. J. Pendleton and A. G. G. M. Tielens,Astron. Soc. Pacific Conf. Series, pp. 227251. Astronomical Sof the Pacific, San Francisco, CA.APPENDIX B: THE LAB SPECTROMETER

Bernstein, M. P., S. A. Sandford, L. J. Allamandola, and S. ChangExperimental measurements of the spectral reflectance of our labora- Infrared spectrum of matrix-isolated hexamethylenetetramin

torysamples in the 0.25- to 2.5-m region were made at roomtemperature and H2O at cryogenic temperatures. J. Phys. Chem. 98, 12206using a dual-beam, dual-monochrometer, dispersive Perkin-Elmer

Bernstein, M. P., S. A. Sandford, L. J. Allamandola, S. Chang, andLambda 9 UV/VIS/NIR spectrophotometer equipped with a Labsphere

Scharberg 1995. Organic compounds produced by photolysis of rDRTA-9 integrating sphere. The integrating sphere is coated with Spec- interstellar and cometary ice analogs containing methanol. Astrtralon, a high reflectance material (the spectral reflectance is greater than

J. 454, 327344.95% over the entire wavelength region), and disks of the same material

Binzel, R. 1992. The optical spectrum of 5145 Pholus. Icarus 99, 23were used for the reference beam and the instrument background correc-Bockelee-Morvan, D., P. Colom, J. Crovisier, D. Despois, and G. Ption. The 6-inch-diameter sphere has entrance ports to admit the sample

1991. Microwave detection of hydrogen sulphide and methaand reference beams.Comet Austin (1989c1). Nature 350, 318320.Thesampleis positioned opposite theentrance portto make reflectance

measurements in an integrating sphere. The light scattered into all angles Bradley, J. P., D. E. Brownlee, and T. P. Snow 1997. GEMS andpre-accretionally irradiated grains in interplanetary dust partiof the backward hemisphere is collected by the sphere and measured by

the detector after multiple internal reflections. Internal reflections against From Stardust to Planetesimals (Y. J. Pendleton and A. G.


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Davies, J. K., N. McBride, S. L. Ellison, S. F. Green, and D. R. BallBrown, R. H., D. P. Cruikshank, Y. J. Pendleton, and G. J. Veeder 1997.1998. Optical and infrared photometry of six Centaurs. IcarSurface composition of Kuiper Belt object 1993 SC. Science 276,213227.937939.

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