Searching for water ice on 47171 1999 TC36, 1998 SG35, and 2000QC243: ESO Large program on TNOs and Centaurs�

E. Dotto,a,* M.A. Barucci,b H. Boehnhardt,c J. Romon,b A. Doressoundiram,b N. Peixinho,d

C. de Bergh,b and M. Lazzarine

a INAF—Osservatorio Astronomico di Roma, Italy; INAF—Osservatorio Astronomico di Torino, Italyb LESIA—Observatoire de Paris, France

c European Southern Observatory ESO, Santiago de Chiled LESIA—Observatoire de Paris, France; CAAUL, Observatorio Astronomico de Lisboa, Portugal

e Astronomical Department of Padova, Italy

Received 9 August 2002; revised 2 December 2002

Abstract

Transneptunian objects and Centaurs are supposed to be among the most pristine bodies of the Solar System. To investigate their physicalproperties and their surface composition, an ESO large program at the Very Large Telescope was carried out. In this paper we presentphotometric and spectroscopic near-infrared data of two Centaurs (1998 SG35 and 2000 QC243) and one transneptunian object (47171 1999TC36). For 47171 1999 TC36 and 1998 SG35 visible photometry is also presented. Models of the surface composition of these objects arepresented and discussed. By including a small percentage of water ice in our geographical mixtures, we obtain a better agreement with theobservations in the H and K bands.© 2003 Elsevier Science (USA). All rights reserved.

Keywords: Centaurs; Transneptunian objects; 2000 QC243; 1998 SG35; 47171 1999 TC36; Spectroscopy; Photometry; Visible; Near-infrared

1. Introduction

The Edgeworth–Kuiper Belt (EKB), located beyond theorbit of Neptune, is believed to contain remnant materialfrom the formation of the outer planets. In this region, icyplanetesimals have formed and have grown to larger bodiesas transneptunian objects (TNOs) and Centaurs. Centaursare located between Jupiter and Neptune and have chaoticorbits with very short lifetimes (�106 years). They aresupposed to have escaped from the EKB. Although from thesame origin, TNOs and Centaurs have experienced differentdynamical and physical evolutions over the age of the SolarSystem. Until now, 44 Centaurs and 745 TNOs are known.

Among TNOs three different dynamical classes have beenidentified: classical objects (or cubewanos), plutinos, andscattered objects.

The physical properties of these bodies are far frombeing understood. Several TNOs have red spectra, but neu-tral objects are also present. This colour diversity could bedue to different collisional evolution states and differentdegrees of surface alteration due to space weathering pro-duced by high-energy radiation. It is known that bombard-ment by high-energy radiation of mixtures of CH3OH, CH4,H2O, CO2, CO, and NH3 ices, produces a “radiation man-tle,” which is hydrogen-poor, carbon-rich, and dark (Straz-zulla 1997, 1998) and has red spectra. Several mechanismshave been proposed to explain the neutral colour of someTNOs: it could be due to fresh material coming from theinterior or an impactor after major collisions (Luu andJewitt, 1996; Jewitt and Luu, 2001) or recondensation of gasand dust after a temporary activity (Luu et al., 2000;Hainaut et al., 2000). Doressoundiram et al. (2002) found a

� Based on observations obtained at the VLT Observatory CerroParanal of the European Southern Observatory, ESO, in Chile within theframework of Program 167.C-0340.

* Corresponding author. Fax:�39-06-9447243.E-mail address: dotto@mporzio.astro.it (E. Dotto).

R

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significant correlation of the colours of classical TNOs withinclination, eccentricity, and perihelion distance, but notwith semimajor axis and absolute magnitude, concludingthat probably space weathering and impacts are responsiblefor the colour diversity. The observed variety of coloursamong TNOs and Centaurs is so far unexplained: probablyit results from the mixing of all or some of the above-mentioned processes.

The stony ingredients of the original TNOs should besilicate in nature. Even if their detection is extremely diffi-cult, since they may be covered by ice and/or organicmantles, a signature seen in the spectrum of the CentaurPholus between 0.6 and 1.6 �m is attributed to silicates(Cruikshank et al., 1998). Near-IR observations suggestedthe presence of organic materials on the surface of TNOsand Centaurs. Absorption bands at 1.5 and 2 �m typical ofwater ice have been identified in the reflectance spectra ofthe Centaurs 2060 Chiron (Luu et al., 2000) and 10199Chariklo (Brown et al., 1998; Dotto et al., 2003), and theTNO 1996 TO66 (Brown et al. 1999), while frozen meth-anol and/or some product of methanol may be present on thesurface of 5145 Pholus (Cruikshank et al. 1998). The pos-sible presence of signatures of water ice in small amountshave been found in one spectrum of the Centaur 2001 PT13by Barucci et al. (2002). Conversely, the surfaces of 8405Asbolus (Barucci et al., 2000, Romon-Martin et al., 2002)and several TNOs do not show any presence of water and/orhydrocarbon ices.

Photometric and spectroscopic investigation of TNOsand Centaurs in the visible and near-infrared spectra is anessential tool to investigate their surface composition. InApril 2001 we began at ESO a large program devoted to thephysical study of TNOs and Centaurs, with the aim ofinvestigating the surface composition of a wide sample ofobjects, by carrying out visible and near-infrared photome-try and spectroscopy (Boehnhardt et al., 2002, 2003; Ba-rucci et al., 2002). In the framework of this ESO largeprogram we obtained the results presented here: near-infra-red spectroscopy and photometry of two Centaurs (1998SG35 and 2000 QC243) and one transneptunian object(47171 1999 TC36), and visible photometry of 47171 1999TC36 and 1998 SG35. Table 1 reports the orbital andphysical parameters of the observed objects.

2. Observations

The data presented here have been obtained at the ESOVery Large Telescope (VLT). FORS1 at unit telescope (UT)3 (Melipal) has been used to carry out visible photometricobservations (available at www.eso.org/instruments/fors1/).To perform near-infrared photometry and spectroscopy, weused the infrared-cooled grating spectrometer ISAAC atUT1 (Antu) (see www.eso.org/instruments/isaac/). Our datahave been obtained during two runs in September and Oc-tober, 2001 (Table 2): during the September run only near-infrared spectroscopy was performed, while during the Oc-tober run we carried out near-infrared spectroscopy and B,V, R, I, J, H, and Ks photometry.

2.1. Photometry

Visible photometry was performed in visitor mode onOctober 15, 2001, using broadband Bessel B, V, R, and Ifilters. Unfortunately, 2000 QC243 was observed during ahigh peak of sky absorption and it was impossible to per-form the absolute magnitude calibration and to compute itsB, V, R, and I magnitudes. The data were reduced usingIRAF’s CCDRED package. We followed the standard tech-niques of debiasing and flatfielding the images using medianbiases and median high signal-to-noise sky-flat fields takenat the begining and end of the night. Synthetic aperturephotometry was performed using IRAF’s PHOT routine.Both objects, 47171 1999 TC36 and 1998 SG35, were

Table 1Orbital and physical characteristics of the observed objects: perihelion distance q, aphelion distance Q, semimajor axis a, eccentricity e, inclination i,diameter D (computed assuming an albedo between 0.05 and 0.25), the dynamical class, the rotational period P with its references

Object q(AU)

Q(AU)

a(AU)

e i(deg)

D(km)

Class P(h)

47171 1999 TC36 30.581 48.313 39.447 0.225 8.4 260–590 Plutino �8a

1998 SG35 5.818 11.012 8.415 0.309 15.6 15–34 Centaur —2000 QC243 13.168 19.827 16.497 0.202 20.8 85–190 Centaur 9.14b

a Peixinho et al., 2002.b Ortiz et al., 2002.

Table 2Aspect data during the observations: heliocentric distance r, geocentricdistance �, and phase angle �

Object Date r(AU)

�(AU)

�(deg)

47171 1999 TC36 09 Sept. 2001 31.42 30.46 0.5410 Sept. 2001 31.42 30.45 0.5115 Oct. 2001 31.41 30.49 0.71

1998 SG35 08 Oct. 2001 9.28 8.31 1.5009 Oct. 2001 9.28 8.30 1.4210 Oct. 2001 9.28 8.30 1.3415 Oct. 2001 9.27 8.29 1.05

2000 QC243 09 Oct. 2001 19.28 18.51 1.9110 Oct. 2001 19.28 18.52 1.94

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bright enough to be measured without aperture (growthcurve) corrections. With seeing conditions of 0.6�–0.7�, weused an aperture radius three times the FWHM of theimages to collect the object flux. The sky level has beenestimated as the median value on a concentric ring aroundthe object with a width of 1.5 FWHM. For 1998 SC35 thering was at a distance of five times the FWHM, while, dueto the presence of a close faint background object, for 471711999 TC36 the ring was at 10 times the FWHM. Fluxcalibration was made using the zero points from the obser-vation of standard stars (Landolt 1992) for the correspond-ing night and extinction coefficients and colour terms com-puted automatically using the FORS1�2 pipeline (seewww.eso.org/observing/dfo/quality/FORS/qc/zeropoints/zeropoints.html). Magnitude errors were estimated as thesquare root of the quadractic sum of the photometric andcalibration errors. The near-infrared observations have beencarried out in visitor mode with UT1-ISAAC, using thejitter imaging technique. A combined image is generatedusing the jitter routine from the ECLIPSE package, and thedata processing routines are described in Romon et al.(2001). The calibration was performed by the observation ofseveral faint infrared standard stars from Hunt et al. (1998)and Persson et al. (1998). Photometric J, H, and Ks mea-surements (centred at 1.25, 1.65, and 2.16 �m) have been

performed before and after the spectra. During the Septem-ber run the nonphotometric sky conditions did not allow usto carry out absolute calibrations for 47171 1999 TC36. Thefields were observed again during the run of October and thestandard calibration has been carried out. The analysis of thefield and standard stars confirms the good quality of theobtained data. In Table 3 are reported the details of photo-metric observations.

2.2. Spectroscopy

Near-infrared spectroscopy has been carried out in visi-tor mode. We used ISAAC in its low-resolution spectro-scopic mode, with a slit of 1” and with the grating at threedifferent central wavelengths corresponding to J, H, and Kbands. The obtained spectral resolution is about 500. Theobservations were made by nodding the object along the slitby about 10” between 2 positions A and B. The A and Bimages have been combined using the ECLIPSE packageand following the procedure already described by Romon etal. (2001) and Barucci et al. (2002). The spectra have beendivided by the solar analogs corresponding to similar air-masses. As calibrators, we used C-type asteroids 511Davida and 70 Panopaea and solar-type stars Hyades64,Hyades142 (Hardorp, 1978), HD209847, and Hip0007040.

Table 3Photometric observations and results (UT time at start exposure)

Object Date UT Filter Ex. time (s) Mag.

47171 1999 TC36 15 Oct. 2001 06:17 B 500 21.57 � 0.0215 Oct. 2001 06:27 V 240 20.48 � 0.0315 Oct. 2001 06:13 R 200 19.75 � 0.0215 Oct. 2001 06:32 I 300 19.15 � 0.0209 Sep. 2001 02:18 J 180 18.20 � 0.0509 Sep. 2001 02:26 H 90 17.84 � 0.0509 Sep. 2001 02:35 K 90 17.88 � 0.0509 Sep. 2001 08:58 J 120 18.15 � 0.0609 Sep. 2001 09:04 H 60 17.86 � 0.0510 Sep. 2001 07:33 J 180 18.23 � 0.0610 Sep. 2001 07:41 H 90 17.88 � 0.0510 Sep. 2001 07:50 K 90 17.89 � 0.06

1998 SG35 15 Oct. 2001 07:40 B 600 21.69 � 0.0315 Oct. 2001 07:51 V 200 20.91 � 0.0215 Oct. 2001 07:36 R 200 20.35 � 0.0315 Oct. 2001 07:56 I 200 19.98 � 0.0308 Oct. 2001 04:35 J 180 19.08 � 0.0408 Oct. 2001 04:52 K 90 18.51 � 0.0409 Oct. 2001 06:14 J 180 19.90 � 0.0409 Oct. 2001 06:22 H 90 19.54 � 0.0510 Oct. 2001 07:37 J 180 19.04 � 0.0510 Oct. 2001 07:45 K 90 18.49 � 0.04

2000 QC243 08 Oct. 2001 23:58 J 180 19.36 � 0.0609 Oct. 2001 00:24 J 180 19.38 � 0.0509 Oct. 2001 00:06 H 90 19.05 � 0.0510 Oct. 2001 01:28 J 180 18.94 � 0.0410 Oct. 2001 01:36 H 90 18.59 � 0.0510 Oct. 2001 01:45 K 90 18.45 � 0.04

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The edges of each spectral region have been cut to avoid toolow S/N spectral regions. Table 4 reports the details of thespectroscopic observations.

3. Discussion

The infrared spectra, obtained after division by the solar-type star spectra observed at similar airmasses, are pre-sented in Fig. 1.

In Figs. 2–4 the infrared spectra are shown along withthe visible spectra from Lazzarin et al. (2003). To improvethe S/N ratio, the resulting spectra were smoothed by Gauss-ian filtering of � � 5 pixels, providing a final spectralresolution of 250 in J, H, and K. The different spectralranges have been adjusted using the photometric observa-tions in V, J, H, and K, transformed in reflectivity using

solar values (Hardorp, 1980 and Hartmann et al., 1982). Theobtained colour indices are reported in Table 5. Most ofthem are comparable within the error bars with the visiblecolour indices available in the literature (Doressoundiram etal., 2001, 2002; Peixinho et al., 2002; Boehnhardt et al.,2001; Delsanti et al., 2001). Some differences have beenfound only in the case of 1998 SG35. A radiative transfermodel (Doute and Schmitt 1998) has been used to model thespectral behaviours in order to investigate the possible com-position of the surfaces of these bodies. We consideredseveral geographical mixtures of water ice, carbonaceouscompounds (tholins, amorphous carbon, kerogen), and oli-vine. For each combination of compounds and percentageswe obtained synthetic spectra. The suggested models are notunique and depend on many unknown parameters. Thesuperimposed solid lines in Figs. 2–4 represent for eachobject the combination of compounds and percentageswhich best reproduces the observed spectral behaviour.

Table 4Spectroscopic observations (UT time is related to the object and at start exposure)

Object Date UT Spectral range(�m)

Ex. time(min)

Airmassobject

Solar analog Airmass solaranalog

47171 1999 TC36 09 Sep. 2001 08:04 1.12–1.35 43 1.26–1.52 Hip0007040 1.4409 Sep. 2001 06:40 1.42–1.80 67 1.08–1.25 Hip0007040 1.4509 Sep. 2001 03:13 2.03–2.26 120 1.31–1.05 Hip0007040 1.4610 Sep. 2001 08:31 2.03–2.26 50 1.39–1.85 HD209847 1.32

1998 SG35 08 Oct. 2001 05:17 1.45–1.79 160 1.11–1.70 Hyades64 1.3710 Oct. 2001 08:06 2.04–2.30 60 1.53–2.33 Hyades64 1.51

2000 QC243 09 Oct. 2001 00:38 1.42–1.72 150 1.04–1.13 Hyades64 1.3910 Oct. 2001 02:07 1.93–2.35 132 1.04–1.38 HD209847 1.02

Fig. 1. Spectral reflectances of 47171 1999 TC36, 1998 SG35, and 2000QC243. The J spectra are normalised at 1.25 �m, the H at 1.65 �m, and theK at 2.16 �m. The J, H, and K spectra of 47171 1999 TC36 on the bottomwere obtained on September 9. The lower K spectrum of 47171 1999TC36, obtained on September 10, has been shifted by 0.25 units in reflec-tance. The H and K spectra of 1998 SG35 have been shifted by 0.5 unitsin reflectance. The H and K spectra of 2000 QC243 have been shifted by1.1 units in reflectance for clarity.

Fig. 2. Spectral reflectance of 47171 1999 TC36 in the V, J, H, and Kranges. The spectra have been adjusted using the photometric B, V, R, I, J,H, and Ks colours and have been normalised to 1 at 0.55 �m. Thecontinuous line represents the model composed of 57% Titan tholin, 25%ice tholin, 10% amorphous carbon, and 8% water ice with an albedo of0.13.

411E. Dotto et al. / Icarus 162 (2003) 408–414

3.1. 47171 1999 TC36

Fig. 1 shows the J, H, and K spectra we obtained inSeptember 2001. The two K spectra obtained during twodifferent nights of observation show a very similar behav-iour. This object, belonging to the dynamical class of plu-tinos, has been found to have a companion at about8000-km distance (Trujillo and Brown, 2002). In the clas-sification given by Boehnhardt et al. (2002) on the basis ofthe visible spectral gradient, this object belongs to the veryred class (Lazzarin et al., 2003). Peixinho et al. (2002)found no variation in the lightcurve in 8 h of observations.

The colour indices, reported in Table 5, have been com-puted as the mean values of the obtained measurements.

Fig. 2 shows the combination of our infrared spectra withthe visible spectrum by Lazzarin et al. (2003). The spectralbehaviour is characterised by a very red visible part and ahigh value of the V–J colour, also found by Peixinho et al.(2002). Fig. 2 shows also a tentative model of the surfacecomposition of this object. It consists of a mixture of 57%of Titan tholin, 25% of ice tholin, 10% of amorphouscarbon, and 8% of water ice with an albedo of about 0.13.To obtain this model we tried several combinations ofminerals and ices. Tholins (Khare et al., 1984) seem to bethe most likely possibility to reproduce the unusually redslope up to 1.2 �m. Ice tholin (Khare et al., 1993) and Titantholins are synthetic compounds produced by plasma irra-diation of icy mixture of water ice (86%) and C2H6 (14%),and gaseous mixture of N2 (90%) and CH4 (10%), respec-tively. None of the considered materials allowed us to re-produce the spectral behaviour between 0.4 and 1.2 �m.Our model is just a suggestion, but the most interestingresult is that water ice could be present on the surface of thisobject.

3.2. 1998 SG35

This object, classified as a Centaur, belongs in theBoehnhardt classification to the moderately red class sinceits visible spectral gradient is of about 11.8%/100 nm (Laz-zarin et al., 2003). In Fig. 1 the infrared spectra we obtainedfor this object are shown. The infrared photometric datareported in Table 3 show a large difference in J magnitude.This could be due to the lightcurve amplitude. On the basisof J data, the lightcurve amplitude is at least 0.86 mag. Anaverage value for the J magnitude has been estimated at

Fig. 3. Spectral reflectance of 1998 SG35 in the V, H, and K ranges. Thespectra have been adjusted using the photometric B, V, R, I, J, H, and Kscolours and have been normalised to 1 at 0.55 �m. The dotted line addedto the J colour underlines the uncertainty of the V–J colour index. Thecontinuous line represents the model composed of 97% kerogen, 1%olivine, and 2% water ice with an albedo of 0.03.

Fig. 4. Spectral reflectance of 2000 QC243 in the V, H, and K ranges. Thespectra have been adjusted using the photometric V, J, H, and Ks coloursand have been normalised to 1 at 0.55 �m. The continuous line representsthe model composed of 96% kerogen, 3% water ice, and 1% olivine withan albedo of 0.04.

Table 5Colour indices

47171 1999 TC36 B–V � 1.09 � 0.04V–R � 0.73 � 0.04V–I � 1.33 � 0.04V–J � 2.29 � 0.07J–H � 0.36 � 0.06H–K � �0.03 � 0.07

1998 SG35 B–V � 0.78 � 0.04V–R � 0.56 � 0.04V–I � 0.94 � 0.04J–H � 0.36 � 0.05H–K � 0.20 � 0.04

2000 QC243 B–V � 0.67 � 0.03a

V–R � 0.44 � 0.03a

V–I � 0.91 � 0.03a

V–J � 1.37 � 0.13J–H � 0.35 � 0.06H–K � 0.14 � 0.05

a Visible colour indices and V magnitude of 2000 QC243 are fromDoressoundiram et al. (2002).

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19.47 mag. The colour indices are reported in Table 5. Dueto the large variation of the J magnitude and the nonsimul-taneous observations in V and J photometry the computedV–J colour index of 1.44 is only indicative and its error barcannot be evaluated. Fig. 3 shows H and K spectra togetherwith the V spectrum by Lazzarin et al. (2003). Spectra havebeen adjusted by using the obtained infrared and visiblephotometry. Our observing plan of October 9 included alsoJ and K spectra of 1998 SG35, unfortunately technicalproblems did not allow us to take into consideration theobtained spectra. The visible colour indices have somedifferences with the previous determinations (Doressound-iram et al., 2001; Delsanti et al., 2001), in particular in V–Rand R–I colours, but are compatible with the visible spectralbehaviour.

To model the surface composition of this object weconsidered several geographical mixtures of minerals andices. The obtained model, composed of 97% kerogen, 1%olivine, and 2% water ice, is plotted in Fig. 3 as a contin-uous line. This model gives an albedo of about 0.03. Thehigh percentage of kerogen is necessary to reproduce theslope of the visible spectrum. Water ice, in small amounts,is the only component among those we have considered thatcan reproduce the behaviour of the H and K spectra. Thesmall percentage of olivine is necessary to fit the value ofthe J filter. Kerogens are complex organic compounds thathave already been used to reproduce red spectra of darkasteroids and Centaurs (Barucci et al., 2000; Romon-Martinet al., 2002). This model is only tentative and depends onthe chosen V–J colour index. Further observations are ab-solutely needed: the determination of the rotational periodand the analysis of the lightcurve amplitude would be usefulto check the variation in the infrared colour indices and thedifferences between our visible photometry and the one byDoressoundiram et al. (2001).

3.3. 2000 QC243

This object, dynamically classified as a Centaur, belongsto the moderately red class since its visible spectral gradientis of about 10.1%/100 nm (Lazzarin et al. 2003). In Fig. 1the H and K spectra we obtained on October 2001 areshown. Technical problems on October 9 did not allow us tohave also the spectrum in the J range. The obtained infraredmagnitudes (Table 3) show significant differences. This canbe due to the surface variation related to the lightcurveamplitude. Ortiz et al. (2002) observed this object on Au-gust 2001 and found a half rotational period of 4.57 �0.05 h, and a lightcurve amplitude of about 0.7 mag. Thedifferences in the observed magnitudes correspond to dif-ferent rotational phases seen during observations.

In Fig. 4 the H and K spectra, together with the visiblespectrum by Lazzarin et al. (2003), are shown. Unfortu-nately, this object has been observed on October 15, 2001during a high peak of sky absorption, it was not possible toperform the absolute calibration of our visible observations

and we did not obtain its visible magnitudes. The differentspectral ranges have been adjusted by using our J, H, and Kmagnitudes and the V magnitude given by Doressoundiramet al. (2002). This V point (20.31 � 0.03 mag) is simulta-neous to the Ortiz et al. (2002) observations and corre-sponds to the principal maximum of the lightcurve. Due tothe uncertainty in the rotational period determination wecannot compute where our photometric and spectroscopicinfrared observations fall on the lightcurve. Assuming thatthe lightcurve is not affected by colour variations we com-puted the V–J colour index considering the J value observedon October 10 at 01:28 UT, which is our brightest magni-tude, and the J–H and H–K colours considering the H and Kvalues observed consecutively (Table 3). We compute theerror bar in our V–J colour index as the difference betweenthe amplitude of the V lightcurve and the maximum varia-tion among our J observations.

In Fig. 4 the radiative transfer model (continuous line) isalso shown. Although the interpretation of this kind ofspectra is neither easy nor unique and different combina-tions of minerals can reproduce the observed spectral fea-tures, we suggest a surface composition of 2000 QC243dominated by kerogen (96%) with 3% of water ice and 1%of olivine. This model gives an albedo of about 0.04. Thehigh percentage of kerogen reported in Fig. 4 is necessary toreproduce the slope of the visible spectrum, while the in-clusion in our model of a small percentage of water iceimproves the fit with the infrared spectra around 1.5 and2.0 �m.

4. Conclusion

In this paper we have presented visible and infraredphotometry and infrared spectra of two Centaurs (1998SG35 and 2000 QC243) and one plutino (47171 1999TC36), while BVRI data are available for all three objects inthe literature, our near-infrared measurements can be con-sidered as the first data on these objects. The obtainedspectral behaviours have been interpreted in terms of pos-sible surface composition using a radiative transfer modelwhich allows us to reproduce the observed spectral featuresmodeling the surface by geographical mixtures of severalminerals and ices. Our models, although tentative, suggestthat the surface composition of all our targets is character-ized by some percentage of water ice. This is in agreementwith the present scenario of the formation and evolution ofTNOs and Centaurs, in which these bodies have to containwater and/or hydrocarbon ices. So far the reason the spectralfeatures of ices do not appear on every TNO and Centaurspectra is not evident. To interpret the surface compositionof TNOs and Centaurs in terms of evolutionary state, lab-oratory experiments are in progress to investigate the prop-erties of minerals and ices on the surface of these bodies andto model the alteration processes which are supposed tohave modified their pristine surfaces (Moroz et al., 2002).

413E. Dotto et al. / Icarus 162 (2003) 408–414

Moreover, further high-quality observations are needed.Since these objects are very faint and often close to the limitof what can be seen with standard observations at largertelescopes, it will be necessary to improve the observationaltechniques in order to increase the S/N and to enlarge theavailable data sample. This will allow us to carry out a moredetailed investigation of the surface compositions of TNOsand Centaurs and to evaluate possible compositional varia-tions of their surfaces.

Acknowledgments

The authors thank M. Fulchignoni for useful discussions.We acknowledge careful reviews by R. P. Binzel and S.C.Tegler that improved this manuscript. N.P. acknowledgesfunding from the FCT, Portugal (ref:SFRH/BD/2000/1094).

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