Kuiper Belt – an ESO Large Programme

Exploring the Icy World of the Edgeworth- Kuiper Belt – An ESO Large Programme

H. BOEHNHARDT (Max-Planck-Institut für Astronomie, Heidelberg, Germany) A. BARUCCI, A. DELSANTI, C. DE BERGH, A. DORESSOUNDIRAM, J. ROMON (Observatoire de Paris, Meudon, France) E. DOTTO (Osservatorio Astronomico di Roma, Italy) G. TOZZI (Osservatorio Astronomico di Arcetri, Firenze, Italy) M. LAZZARIN, S. FOURNISIER (Osservatorio Astronomico di Padova, Italy) N. PEIXINHO (Observatorio Astronomico de Lisboa, Portugal, and Observatoire de Paris, Meudon, France) O. HAINAUT (European Southern Observatory, Santiago de Chile) J. DAVIES (Royal Observatory Edinburgh, Great Britain) P. ROUSSELOT (Observatoire de Besançon, France) L. BARRERA (Universidad Católica del Norte, Antofagasta, Chile) K. BIRKLE (Max-Planck-Institut für Astronomie, Heidelberg, Germany) K. MEECH (University of Hawaii, Honolulu, USA) J. ORTIZ (Instituto de Astronomia de Andalucia, Granada, Spain) T. SEKIGUCHI, J.-I. WATANABE (National Astronomical Observatory, Mitaka-Tokyo, Japan) N. THOMAS (Max-Planck-Institut für Aeronomie, Katlenburg-Lindau, Germany) R. WEST (European Southern Observatory, Garching, Germany)

The Edgeworth-Kuiper Belt and Kuiper Belt is believed to represent the sical disk objects. Other TNOs were the Formation of the Solar most original remnant from the forma- captured in resonant orbits with System tion period of our planetary system. It Neptune forming the dynamical class of was in this region where within less ‘Plutinos’ (named after Pluto, the most The first object in the Edgeworth- than 20 million years some 4.6 billion prominent representative in the 3:2 res- Kuiper Belt was observed in 1930, years ago, icy planetesimals formed onance orbit). Gravitational scattering when Clyde Tombaugh discovered and grew to larger bodies, nowadays at Neptune has produced the class of Pluto at a distance of 43 AU (1AU = one observed as TNOs, Centaurs, short-pe- ‘Scattered Disk Objects (SDOs)’ in very astronomical unit, the mean distance riod comets and some icy satellites of eccentric orbits with semi-major axes between Earth and the Sun = 149.6 mil- the major planets. Although of the same between 50 and several 1000 AUs. The lion km), i.e. beyond the orbit of origin and most likely containing the ‘Centaurs’, presently in orbits between Neptune. About 20 years later Edge- same primordial material, these small Neptune and Jupiter and named after worth and Kuiper started to speculate bodies, as we observe them now, have half-human-half-horse demigods in about the existence of another asteroid experienced different evolution over the Greek mythology, are also considered belt at the edge of the planetary sys- age of the solar system, both dynami- to be scattered TNOs, cascading to- tem. Another 30 years later this specu- cally and physically. Nevertheless, they wards the Sun by repeated gravitation- lation became an hypothesis when contain the most primordial material al interaction with the major planets. At Fernandez and Ip argued for the exis- from the formation of the Sun and its the end, i.e. typically after some 10 mil- tence of an Ecliptic-oriented reservoir planetary system that we can observe lion years in intermediate orbits, Jupiter of icy bodies beyond Neptune as a today. will either capture them as short-period source for short-period comets, the re- comets (thus, some of them even be- cruitment of which was difficult to ex- The Global View of the come accessible for spacecraft explo- plain by gravitational capturing of Oort Edgeworth-Kuiper Belt and its ration) or will expel them into the region Cloud comets through planets when Population of Objects of the long-period Oort Cloud or even approaching the inner solar system. interstellar comets at the edge of the The hypothesis became reality in 1992, The dynamical ‘zoo’ of Cubewa- solar system. A few TNOs may get when, during a search campaign for nos, Plutinos, Centaurs and SDOs: stranded as satellites around the outer distant asteroids, Jewitt and Luu found According to current dynamical scenar- gas giants (the most prominent candi- 1992 QB1 (now numbered: 15760) at a ios, the TNO population remained in date is Triton, the largest moon of distance of 41 AU from the Sun. the region of its formation, the Edge- Neptune). In the meanwhile, after the detection worth-Kuiper Belt, over the age of the The original formation disk, more of more than 700 objects beyond the solar system. However, their orbits be- ‘Plutos’ and TNO binaries: The drop orbit of Neptune over the past decade, came perturbed by the presence of in the number of objects with semi-ma- a new class of solar system bodies has Neptune. The gravitational interaction jor axes larger than 50 AU may indicate been discovered. These Transnep- with this giant planet has ‘pumped’ ob- either a basic characteristic of the for- tunian Objects (TNOs) are the largest jects into so-called ‘excited’ (i.e. non- mation disk around our Sun (a ‘truncat- members of the Edgeworth-Kuiper Belt. circular and/or inclined), but non-reso- ed’ disk?) or may just mark the chal- They have caused a complete revision nant orbits creating the dynamical class lenge to detect the ‘cold’ and very colli- of our knowledge and understanding of of ‘Cubewanos’ (named after the proto- mated formation disk of even smaller the outer solar system: the Edgeworth- type object 1992 QB1), also called clas- planetesimals that escaped the gravita-

22 Figure 1: Colour-colour diagrams of the first 28 objects observed within the Large Programme. The two panels show the B-V versus V-R (left) and R-I versus V-R (right) colours of the objects measured. The symbols are: filled circles = Cubewanos, filled triangles = Plutinos, open circles = SDOs, open triangles = Centaurs. The colour of the Sun is indicated by the open star symbol. The lines indicate the direction of increasing reddening slopes from –10 (lower left end) to 70%/100 nm (upper right end) in steps of 10%/100 nm. The distribution of the measured objects seems to be rather uniform in the typical reddening range. The shift of the objects below the line of constant reddening in the plots involving R-I colour is due to the spectral slope change of red objects towards the near-IR end of the visible spectrum. tional influence of the outermost plan- sion products to micron size dust that and their relatives seems to be reason- ets because of their large distance from the solar radiation pressure has re- ably well understood, just a very vague the Sun. The overall mass in the moved from the Edgeworth-Kuiper Belt picture of their physical properties and Edgeworth-Kuiper Belt seems to be region. The existence of binary TNOs chemical constitution exists. The faint- very small (0.15 Earth masses) and (about 10 objects so far) supports the ness of the objects (the vast majority cannot explain the existence of large idea of a serious collision environment are fainter than 23 mag) makes them TNOs like Pluto/Charon or Varuna. in the belt during and possibly beyond difficult to observe even with the largest Instead modelling results suggest that the end of the formation period of plan- telescopes: the brightest representa- the original formation disk in the TNO etesimals. Ongoing search pro- tives (about 20 mag) require easily one region was much heavier (10–100 grammes using imaging surveys and full observing night at an 8-m VLT unit Earth masses), that it should have pro- high-resolution imaging techniques at telescope for a spectroscopic study of duced more Pluto-size bodies and that large telescopes world-wide try to solve decent signal-to-noise. most of its mass got lost by gravitation- these key questions on the Edgeworth- Despite their faintness, the observed al scattering at the large planets as well Kuiper Belt. TNOs and Centaurs are only the as by mutual collisions of TNOs and Physico-chemical properties: largest members of the belt population subsequent down-grinding of the colli- While the dynamical history of TNOs with diameter estimates ranging be-

Figure 2: Photometric “reflectivity spectra” from BVRI magnitudes of TNOs and Centaurs. The three panels show the photometric “reflectivity spectra” of the first 28 objects observed with our programme. The relative reflectivity per photometric band (normalized to unity for V band) is plotted versus central wavelength of the filters used. It is a measure of the intrinsic reddening of the objects since the slope of solar spectrum is removed from the data. The broken lines indicate the numerical spectral gradient fits for the individual objects. These plots are very useful to check the consistency of the photometric data since “bad” measurements and intrinsic variability of the objects will result in notice- able deviations of the photometry from a “smooth” spectral slope in these “spectra”. If found, further investigations may be needed to clarify the nature of the scatter.

23 Figure 3: Spectral gradient histograms of the full sample of 96 objects in our analysis database. The histograms show the number of objects per spectral gradient interval for each dynamical class as given in the table at the bottom of the figure. The spectral gradients are calculated from the BVRI photometry of the objects.

temporary dust coma seen around these objects. Surface evolution scenarios: The red colour of TNOs and Centaurs is usually attributed to the effects of surface aging and darkening due to high- energy radiation and ion bombardment. Blue surface colours could be produced by major collisions through deposits of fresh icy material from the body interior or from the impactor. The estimated tween 30 and 1200 km (assuming a low different researchers arrived at different time scale for both types of colour albedo of 4 per cent). Search pro- and partially contradictory conclusions, resurfacing are of the order of 10 million grammes suggest a cumulative mass for instance a bimodal colour distribu- years. The observed colour range can function with power exponent between tion versus a continuous one with ‘out- be modelled by computer simulations 3.5 and 4. The albedo of less than 10 lyers’ in both scenarios. Through a involving both effects. However, these objects is coarsely determined: TNOs Kolmogorov-Smirnov analysis of the results are unfortunately not discrimina- and Centaurs seem to be dark objects sample it became clear that the identi- tive for conclusions on the physical na- with albedo mostly around and below fied colour populations among TNOs ture of TNO and Centaur surfaces. Re- 10 per cent. Only Pluto, Charon and and Centaurs are at best to be con- surfacing on much shorter time scales Chiron have high (around 50 per cent sidered as trends, but they lack clear could happen due to ice re-condensa- for Pluto and Charon) to moderate statistical significance because of the tion from a temporary atmosphere pro- albedo, possibly caused by fresh ice low number of objects in the available duced by intrinsic gas and dust activity. due to intrinsic atmospheric activity sample. This process is quite efficient on Pluto (see below). The rotation period of Since formed in the outer solar neb- and possibly Charon, but does certain- more than 20 objects is measured with ula, TNOs and Centaurs should be ly not work for all objects since crust results ranging from 5 and 12 h (cer- made of icy and stony compounds. formation may prevent the develop- tainly biased since long periods are Except for the larger members that may ment of surface activity and/or the heat more difficult to measure). With the ex- have undergone constitutional modifi- source in the bodies may not be strong ception of Pluto and Charon, it is un- cations due to radioactive heating in enough to cause such activity. clear whether the measured variability their interior, and those that come clos- is due to body shape or surface reflec- er to the Sun and thus get heated up in tivity or both. the surface layers, the original material ESO Large Programme on Until 2001 the taxonomic characteri- remained unchanged in the body interi- ‘Physical Studies of TNOs and zation of the population was based on or. Nevertheless, as outlined below, the Centaurs’ a sample of ~ 50 TNOs and ~ 10 surface layers of the bodies are all Centaurs, most of them observed with modified over the age of the solar sys- Programme concept and history: broadband colours in the visible wave- tem. N2 and H2O ices should be most After a number of uncoordinated pre- length range only. Nevertheless, abundant in TNOs and relatives, fol- cursor programmes at ESO telescopes colour-colour diagrams and reddening lowed by CO, CO2, CH4 and NH3. at the end of the last decade, a consor- distributions provide a first glance of Signatures of N2, CO, H2O, CH4 and tium of scientists (see the list of authors similarities and diversities in the sur- NH3 ices are indeed found in spectra of of this paper) proposed a comprehen- face taxonomy of the objects. A wide Pluto/Charon. Water ice absorptions sive observing and analysis project to range of colours (in other words were detected in the Cubewano 1996 be performed within the framework of spectral gradients or ‘reddenings’) TO66 and the Centaurs 5145 Pholus an ESO Large Programme: ‘Physical ranging from slightly bluer than the and 2060 Chiron, objects that represent studies of TNOs and Centaurs’. The Sun (~5%/100 nm) to extremely red basically the fully colour range found in project was accepted to be executed at (~50%/100 nm) in the visible wave- the total sample. The original ‘stony’ in- Cerro Paranal and La Silla during ESO length range is found in this sample. In gredients in TNOs and Centaurs should period 67 to 70, i.e. from April 2001 un- the infrared, the photometric results in- be primarily of silicate nature. Their de- til March 2003. The main goal of the dicate only minor reddening to neutral tection, however, is extremely difficult project is the development of a taxo- slopes of the surface reflectivity. For a since they may be covered completely nomic classification scheme of these finer analysis, the overall object sample with ice mantles. There is only one icy bodies in the outer solar system, the contained some adverse culprits like case, the Centaur 5145 Pholus, in identification of evolutionary tracks and bad number statistics for all dynamical which a very wide and close to margin- their relationships with dynamical classes, insufficient photometric accu- al absorption feature (ranging from 0.6 classes, the exploration of the surface racy of the data, incomplete wave- to 1.6 µm) is tentatively attributed to sil- chemistry of the objects and its correla- length coverage (7 objects had only 1 icates. The only other supportive argu- tion with taxonomic classes. The pro- or 2 colours measured) and large ment for the presence of dust in these posed observing campaigns at the differences in the results for the same objects comes from the Centaur 2060 ESO Very Large Telescope (VLT) and objects obtained by different ob- Chiron and of course their relatives, the the New Technology Telescope (NTT) servers. It is thus not surprising that short-period comets, i.e. through the comprise multi-colour broadband filter

24 photometry of 60–70 objects in the vis- typically happen about one year after grammes. Nevertheless, quasi-simulta- ible and about 25 objects in the near-IR the last object positions were meas- neous broadband photometry of the wavelength range for the taxonomy ured. For the BVRI measurements we targets is taken during spectroscopy analysis and low-dispersion spec- selected targets with V brightness of runs with the aim to verify the earlier re- troscopy in the visible and near-IR of 22.5–24mag, while for JHK photometry sults and – most importantly for the IR about 15–20 objects each for the chem- the objects need to be brighter than spectra – to cross-calibrate the contin- ical studies. The ESO Programme 22.5mag in V. The SM targets are re- uum flux in the JHK bands of the ob- Committee has allocated in total 242 quested to be observed under clear to jects as needed for the modelling of the hours of VLT time and 3 nights at the photometric dark sky conditions with spectra. Visible and near-IR spec- NTT for this Large Programme. The seeing better than 0.8″. These con- troscopy runs are scheduled within a workhorse instruments for the pro- straints together with the chosen inte- few days of each other in order to guar- posed observations are FORS1 and gration times guarantee a minimum sig- antee observations of the same object ISAAC at the VLT Unit Telescopes 1 nal-to-noise ratio (S/N) for the individ- under similar phase angles. However, (Antu) and 3 (Melipal). Broadband pho- ual objects of 30–50 in BVRI and 20-40 this scheduling usually did not allow us tometry of some brighter targets is also in JHK. In terms of fulfilment of the ob- to correlate the rotational phase of the conducted with SUSI2 at the La Silla serving requirements this part of our objects for the measurements in both NTT. programme can be considered 100 per wavelength ranges, mostly because for Observing modes and target se- cent successful. many objects the rotation parameters lection: The majority of the photometry Observing strategies: The spec- are not well known. The exposure times targets are observed in service mode troscopy part of the programme is per- of the targets aimed for a S/N of about (SM) at the VLT with the exceptions of formed in visitor mode since the on-line 3–10 per wavelength pixel element brighter ones that are either imaged at detection of these slowly moving (typi- which through wavelength binning al- the NTT or during spectroscopy runs in cally 1–2″/h) faint targets in SM was lowed improvements by a factor of 3–5. visitor mode (VM). Experience from considered risky and misidentifications Given the faintness of the objects and previous programmes has shown that may cause significant loss of observing the telescope/instrument capabilities, the SM targets have to be selected time. The targets had to be selected near-IR spectroscopy is the most time- from amongst objects that allow safe amongst brighter objects, i.e. brighter consuming part of our programme even detection in the fields of view of the in- than 22.5 mag in V for spectroscopy in under favourable sky conditions, and strument; in practice only objects with the visible wavelength range and typically 1 to 11/2 nights of observing at least 2 observed oppositions appear brighter than 20.5mag in V for the near- time are required per target. Unfortu- to be ‘reliable’ to this respect. Less well IR spectroscopy. Several of the targets nately, the visitor mode observations so observed targets have a high risk of be- had some BVRI colours measured from far have had an unexpectedly high ing missed in the SM exposures that previous observations or other pro- ‘loss’ rate of about 25 per cent due to

Figure 4: Spectral gradients versus orbital parameters. The plots show the measured (from BVRI photometry) spectral gradients of the objects versus orbital excitation E (lower left panel), inclination (upper left panel), and perihelion distance (upper and lower right panels). The bubble diagrams in the right panels use the orbital eccentricity (upper right) and the inclination (lower right) as size parameter of the bubbles, i.e. the smaller the bubble the lower the eccentricity and inclination, respectively. The colour coding of the dynamical classes is identical to Figure 2. The red Cubewanos beyond 41 AU mentioned in the text appear to cluster at the left end in the left panels and (as the tiny bubbles) to the right end in the right panels.

25 Figure 5: JHK colour-colour plots of the first 8 objects measured in the SM photometry part of our programme. H-K versus J-H is displayed in the left panel, the right one shows H-K versus J-K. For explanations of symbols and lines see Figure 1. The objects are identified with small labels at the symbols. bad sky conditions and some technical observations had been conducted and Highlights from the First problems. Additional overheads and about 60 per cent of the data reduction Results further time loss due to weather are and analysis phase is performed. Table caused by the need to cross-calibrate 1 shows the number of objects ob- The results outlined in this summary the spectra in the three near-IR wave- served per dynamical class (Cubewa- are obtained from data collected during length regions through JHK absolute no, Plutino, SDO, Centaur) and obser- the first year of this ESO Large photometry. The availability of disper- vation type (BVRI and JHK photometry, Programme. They are described in sive optics covering more than one visible and near-IR spectroscopy). Up greater detail in various papers that are near-IR band at lower spectral resolu- to now, the photometry worked out as already published (Barucci et al., 2002, tion could have resulted in additional planned, while the spectroscopy suf- A&A 392; Boehnhardt et al. 2002, A&A benefits and higher efficiency for our fered from time losses (as explained 395) or will be published soon (Dotto et programme. above). Since the object selection for al., 2003, Icarus, in press; Lazzarin et Data reduction: The basic data re- SM observations had to be done al. 2003, AJ, in press). Another three duction products of the photometry are months in advance, some of our tar- papers with results from Period 69 are absolute broadband filter magnitudes in gets have been observed by other in preparation for submission to scien- BVRI and JHK, filter colours and spec- groups in the meanwhile, and despite tific journals in early 2003. tral gradients. As side results astromet- all efforts, duplication of observations BVRI colour-colour-plots and ric positions and a search for satellites turned out to be unavoidable. A few tar- spectral gradient histograms: The around the prime targets are performed gets were selected on purpose for database used here contains 96 ob- (so far without success). The magni- repetition of certain types of observa- jects compiled from our programme tude (equivalent to sizes), colour and tions in order to allow cross-checks of (28) and from published data. The gradient data are correlated with each our results with those of other ob- colour-colour-plots and photometric other and with orbital parameters to as- servers and to address specific un- “reflectivity spectra” of the 28 objects sociate and explore taxonomic classes solved questions related with the ob- measured by our programme in Period and their colour properties through sta- jects. The most striking constraint caus- 67 are displayed in Figures 1 and 2, re- tistical methods. This exercise is ap- ing duplication is the fact that the spectively. The colour-colour plots plied to our database alone and to an sample of near-IR targets accessible (Fig. 1) show that the objects follow – even larger dataset merging our results with standard instruments at modern with some scatter – the lines of con- with those published in the literature. 8–10-m class telescopes is rather limit- stant reddening in BVR colours, but The spectra are reduced to relative re- ed (~ 30 objects up to now). they start deviating in the transition reflectivity units that allow us to determine spectral gradients and to identify ab- Table 1. Physical studies of TNOs and Centaurs: Number of objects observed/reduced per sorption features that may be indicative dynamical class (status: end 2002). for the surface chemistry. Each spectrum covering a wider wavelength Object class BVRI JHK vis.spec. IR spec.* range is modelled through a Hapcke re- flectance modelling code to constrain Cubewanos 25/15 7/4 1/1 0/0 and, if possible, to quantify the surface Plutinos 19/10 7/4 5/4 4/3 chemistry of the targets. In a final Scattered Disk 13/7 6/5 3/2 3/2 phase of the project it is planned to cor- Centaurs 9/7 6/5 6/6 5/4 relate spectral properties with the photometric taxonomy of the objects. Total 66/39 26/18 15/13 12/9 Status of the programme: By the end of 2002, about 90 per cent of the *About 30 % of the object spectra are incomplete, i.e. JHK bands are not fully covered.

26 Figure 6: Visible Reflectivity spectra of TNOs and Centaurs. The spectra of TNOs are displayed in the left panel, those of Centaurs in the right one. The spectra are shifted in Y direction for clarity. The dots represent the respective results from quasi-simultaneous photometry of the objects taken during the spectroscopy runs. gion to the near-IR. The latter is a con- respectively. E is an estimate for the ve- preference for neutral colours is found sequence of the slope change seen in locity of the object with respect to an- among the near-IR spectra of the ob- the spectra of red to very red objects other one at the same distance, but in jects – see Figure 5. Only 2 objects are beyond 750 nm. The reddening distri- circular orbit in the Ecliptic, and it might definitely redder than the Sun. Whether bution seems to be continuous without thus be related to the collision velocity the red objects are members of a sec- a clear indication for a bi-modality as and/or the collision probability. The ond colour population can easily be announced by other groups using plots of the spectral gradients versus E questioned. The overall interpretation smaller datasets. In the spectral gradi- and i suggest a correlation between of the visible to near-IR colour range ar- ent histogram of Figure 3 all 96 objects surface colour and collision history of gues for only minor colour changes in are used and sorted for each dynamical the objects, since the range of spectral the infrared part of the spectra, but for class in the same gradient bins of width gradients increases with increasing ex- a significant and very wide absorption corresponding to the averaged uncer- citation parameter E and decreases of variable depth for many TNOs and tainty of the data. With one exceptions with decreasing inclination. On the oth- Centaurs in the visual wavelength (1994 ES2) the spectral gradients of all er side, the plot of the spectral gradi- range. This implies that the observed objects fall between –5 and 55 %/100 ents versus perihelion distance shows reddening in BVRI is in fact the long- nm. Outlyers in previous datasets a clustering of very red Cubewanos in wave wing of a wide absorption feature turned out to be artefacts, most likely circular and Ecliptic orbits beyond ~ 41 from a yet unknown chemistry that ex- due to observations and data reduction AU. The peak for the Cubewanos in the tends beyond the visible region well (blends of background objects) or un- spectral gradient statistics of Figure 3 is into the UV. usual object behaviour (for instance caused by exactly this group of objects. Visible spectra: The spectra of 1994 EV3 shows rapid and large-ampli- Moreover, for objects with perihelia be- TNOs and Centaurs observed in the tude variability that mimics unusual tween ~ 36 and 40 AU the width of the visible are mostly featureless and show colours even in short-term photometric reddening range seems to increase the constant (though different) slopes at filter sequences as identified through closer the object approaches the Sun. wavelengths below ~ 750 nm (Fig. 6). our programme). The Cubewanos At present, these trends are not yet ful- At larger wavelength a continuous gra- show a pronounced peak for spectral ly established since more objects need dient change towards smaller slopes is gradients between 25 and 45 %/100 to be included for statistical reasons. observed in the red objects. With one nm that is caused by a population of red However, if the trends can be con- exception (possibly caused by rotation objects in circular and low inclination firmed, they would argue for different effects in the photometry) the spectral orbits beyond 41 AU. At least for the reddening scenarios, i.e. the trends gradients obtained from spectra are in Plutinos, and possibly also for Centaurs with excitation and inclination for the good agreement with the photometric and SDOs, the distribution of spectral balance between space weathering ones. It came as a surprise that wide gradients in the visual wavelength re- and collision resurfacing on one side absorption dips were found in the visi- gion seems to be distinctly different and the trend with perihelion distance ble spectra of two Plutinos, 2000 EB173 from those of the Cubewanos. for the balance between space weath- and 2000 GN171, taken in April 2001 Correlations with orbital parame- ering and intrinsic activity on the other (see Fig. 7). The absorption features ters: In Figure 4 we show the meas- side. Most puzzling is the group of red are centred around 600 and 730 nm ured photometric spectral gradients Cubewanos beyond 41 AU and there is for 2000 EB173 and at 725 nm for 2000 versus orbital excitation E, inclination no easy explanation for their existence GN171. There are two very puzzling as- and perihelion distance of the objects. at present. pects with these detections: (1) The The excitation parameter is defined as JHK colours: While at visible wave- features were not confirmed with spec- E = ( e2 + sin2 ( i ) )1/2 with e and i as lengths a certain reddening range tra taken about one year later (see eccentricity and inclination of the orbit, seems to be continuously populated, a Fig. 7); this would argue for a non-uni-

27 Figure 7: Two “puzzling” Plutinos: reflectivity spectra of 2000 EB173 and 2000 GN171. The reflectivity spectra of both objects show shallow and wide absorption features during one observing epoch (left panel), but a straight featureless slope during a second observing run about one year later (right panel). The dots and triangles in the panels indicate the corresponding ‘broadband’ reflectivity from quasi-simultaneous BVRI filter photometry of the objects. form distribution of the absorbing mate- have a great impact on our under- these distant bodies; however, on the rial on the surface of the objects. (2) standing of the physical nature of these other hand, the identification of further Features resembling the absorptions in bodies at the edge of the planetary surface materials as well as quantita- these two Plutinos and identified in system. tive estimations are difficult because of other solar system bodies and in lab Near-IR spectra: We have found the quality of the data fits and the exis- experiments are produced by silicate water ice absorption in H and K band tence of multiple model solutions for material on asteroids that was exposed spectra of 1 TNO and 3 Centaurs (see each spectrum. to long-term aqueous alteration on the for instance the 2001 PT13 spectrum in surface. A similar scenario in the ex- Figure 8). No features are seen in the tremely cold and icy environment of the spectra of other objects in our sample. Conclusion and Outlook outer solar system obviously poses For 2001 PT13, a non-uniform water ice some problems. The answer to this distribution on a large scale is conclud- The ESO Large Programme on the puzzle calls for a systematic study of ed from the existence and absence of physical studies of TNOs and Centaurs the objects by rotation-phase resolved the absorption features in the spectra is now approaching the end of its as- spectroscopy, and, if confirmed, it will measured at two different epochs. The signed period for the observations, and model fits to the it is a little more than half way through combined near-IR the data reduction. The analysis of the and visible spec- results and model interpretation will tra use a radiative certainly continue for another year be- transfer code with yond the official end of the observing simple geographi- programme. cal mixtures of or- With the data from this programme it ganics, minerals has been possible to identify the first and ices. The steep taxonomic group in the Edgeworth- slopes in the visi- Kuiper Belt, the cluster of red Cube- ble require the ad- wanos beyond 41 AU, and to get clear mixture of Tholins indications of resurfacing effects on or kerogene in the TNOs and Centaurs. At the moment, model fits. Never- the observed results appear to be theless, despite somewhat contradictory to each other the decent match and a synoptic picture for their interpre- that is achieved tations has not yet evolved. Water ice between the ob- seems to be abundant in these objects; served spectra and however, the identification of other sur- the radiative trans- face ice features appears to be a diffi- Figure 8: Near-IR spectrum of the Centaur 2001 PT13. The plot fer model results, cult task. Spectra of two Plutinos may shows the near-IR spectrum of the object on 10/9/2001 (upper spec- at present our con- have provided the first glance of stony trum) and on 8/10/2001 (lower spectrum). The continuous and material, and if so, they contribute yet broken lines show model fits to the observed spectra using the ma- clusions are limit- terial mixture as indicated in the figure insert. The dots indicate the ed. On one hand, another puzzle with the potential to reflectivity from quasi-simultaneous broadband JHK photometry of water ice seems drastically change our view of these icy the object. to be present in worlds in the outer solar system.