Mem. S.A.It. Vol. 74, 262 °c SAIt 2003 Memorie della Minor bodies of the Solar System: ISO and ESO{VLT infrared spectroscopic data E. Dotto1 and M.A. Barucci2 1 INAF–Osservatorio Astronomico di Torino, Strada Osservatorio 20, 10025 Pino Torinese (TO), Italy e-mail: [email protected] 2 LESIA–Observatoire de Paris, 5 Place J. Jannsen, 92195 Meudon Principal Cedex, France Abstract. Over the last decade our knowledge and understanding of the physical properties of small bodies in the Solar System have been widely improved, due to the results obtained by near– mid– and far–infrared spectroscopy. We observed several main belt asteroids with the Infrared Space Observatory (ISO), obtaining the first asteroid spectrum up to 45 ¹m. Moreover, we started a systematic study of the physical properties of the population of Trans–Neptunians Objects (TNOs). The physical and dynamical characteristics of these bodies, as their origin and evolution, are far to be understood. We carried out near–infrared spectroscopic observations at ESO–VLT of several of these objects, obtaining useful information about their surface composition. Key words. Asteroids – Centaurs – TNOs – infrared – spectroscopy – ISO – VLT 1. Introduction features from minerals, ices, and organic compounds. In spite of their importance It is widely believed that minor bodies of these spectral ranges are not widely ex- the Solar System are remnants of the early plored. For this reason in the last years phases of the planetary formation. Being we devoted a strong effort to the obser- the less thermally evolved bodies in our vation and the analysis of infrared spec- planetary system, they can provide infor- tra of minor bodies in the inner and outer mation about the primordial processes that Solar System, in order to assess their sur- governed the evolution of the protonebula face composition and to infer some infor- and operated at different solar distances. mation about their thermal and dynamical The analysis of near– mid– and far– history. infrared spectra of these bodies is partic- ularly important as these wavelength re- 2. ISO observations of Asteroids gions contain several diagnostic spectral We observed sixteen asteroids with the Send offprint requests to: Elisabetta Dotto Infrared Space Observatory (ISO), using Correspondence to: Strada Osservatorio 20, three different instruments (Barucci et al. 10025 Pino Torinese (TO) 1997; Dotto et al. 1999). We obtained pho- Dotto and Barucci: Minor bodies of the Solar System: infrared data 263 1.6 1.0 Hygiea Pallas 1.4 0.8 0.6 Enstatite 1.2 Warrenton Emissivity 0.4 Emissivity 0.2 Jadeite 1.0 Ornans 6 7 8 9 10 11 12 Wavelength [µm] 0.8 5 10 15 20 25 Wavelength [µm] Fig. 1. Left panel: PHT-S emissivity of 2 Pallas compared with the emissivity of two pyroxenes (enstatite and jadeite) (Dotto et al. 2000). Right panel: comparison between PHT–S and SWS emissivity of 10 Hygiea and the emissivity of the CO3 carbonaceous chondrite meteorites Ornans and Warrenton (Barucci et al. 2002). The spectra are vertically offset for clarity. tometric observations in several filters up to interpret the obtained spectral behav- to 60 ¹m, low resolution spectra up to 11.6 iors and to infer the surface composition ¹m and high resolution spectra between of the objects observed by ISO, we consid- 5.3 and 45 ¹m. For all the observed as- ered all the available data bases of labo- teroids sub–solar temperatures, diameters ratory spectra of minerals and meteorites and albedoes have been computed applying (Salisbury et al. (1991a,b), ASTER spec- the standard thermal model by Lebofsky & tral library on http://speclib.jpl.nasa.gov). Spencer (1989): the obtained results are Moreover, selected samples of mineral and in agreement with the IRAS results (Dotto meteorite particulates at different grain et al. 1999, 2000; Barucci et al. 1997, sizes have been chosen and laboratory ex- 2002). The standard thermal model and an periments have been performed at the advanced thermophysical model (Lagerros Capodimonte Observatory (Dotto et al. 1998), have been applied also to model the 2000; Barucci et al. 2002). As an exam- thermal continuum. The emissivity spectra ple Fig. 1 (left panel) shows the emissiv- of the observed asteroids have been com- ity of 2 Pallas obtained by the ISO spec- puted as the ratio between the ISO spec- trophotomer PHT-S. In the mid–infrared tra and the computed expected fluxes at wavelength range the most important spec- the time of the ISO observations. The in- tral structures are the Christiansen, rest- terpretation of the spectral features above strahlen, and transparency features. The the thermal continuum is still a difficult Christiansen peak is directly related to the task, as asteroid surfaces are composed mineralogy and the grain size, and occurs by mixtures of different minerals whose at wavelength that for silicates is just be- spectral properties are combined following low the Si–O stretching vibration bands. non-linear paths. Moreover, asteroid spec- The reststrahlen bands are due to vibra- tra are affected by several unknown phys- tional modes of molecular complexes. The ical parameters, such as density, mineral- transparency features usually occur at 11– ogy, particle size and packing. In order 13 ¹m between main reststrahlen bands. In 264 Dotto and Barucci: Minor bodies of the Solar System: infrared data the spectrum of Pallas the feature at about 3. ESO–VLT observations of 8 ¹m is the Christiansen peak which occurs Centaurs and Trans–Neptunian at a shorter wavelength than in most min- Objects eral and meteorite spectra. The only me- teorite with similar position of this emis- Observational evidences have shown, be- sion peak is the aubrite which is essentially yond the orbit of Neptune, a region densely composed of enstatite. The behavior of the populated by small bodies, named Trans– obtained spectrum of Pallas beyond 7.6 ¹m Neptunians Objects (TNOs). This region, suggests the presence on the surface of this named Edgeworth–Kuiper Belt (EKB), is object of a mixture of pyroxenes. The nar- supposed to be the source of short-period row feature at about 10.5 ¹m may be re- comets and Centaurs (whose orbits are be- lated to the presence of water ice on the tween Jupiter and Neptune). EKB objects surface. If confirmed, this is surprising and are believed to be remnants of the early suggests the existence of renewal mecha- phases of the planetary formation at these nisms. solar distances and to contain the most pri- Our observations with ISO provided also mordial material that we can observe to- the spectrum of 10 Hygiea obtained by the day. Up to now more than 600 TNOs and Short Wavelength Spectrometer (SWS) up about 40 Centaurs are known: their physi- to 45 ¹m, the first high resolution spec- cal and dynamical properties show a wide trum of an asteroid at these wavelengths. diversity. This implies that, although they To interpret this spectrum several miner- all have the same origin, they have experi- als have been selected as possible candi- enced different dynamical and physical evo- dates to be present on the surface of this lution over the age of the Solar System. We C-type asteroid. None of the analysed min- carried out systematic near–infrared spec- eral spectra matched the Hygiea spectrum. troscopic observations at ESO–VLT to look Many attempts have been made by com- for spectral features which are diagnostic of bining different components while main- ices and other mineral compounds as sili- taining consistency with the hypothesized cates and/or organics, in order to retrieve composition, and the quality of the numer- the surface composition of these objects ous matches has been always very poor and to investigate their history and ther- (Barucci et al. 2002). We obtained the mal evolution. The stony ingredients of the most consistent analogy with spectra of original EKB objects should be primarily the carbonaceous chondrite meteorites. As of silicate nature. Even if their detection is shown in Fig. 1 (right panel), CO3–type extremely difficult, since they may be cov- carbonaceous chondrite meteorites Ornans ered by ice and/or organic mantles, some and Warrenton represent the best similar- signature seen in the Centaur Pholus be- ity with the spectral behavior of Hygiea up tween 0.6 and 1.6 micron is attributed to to about 27 ¹m: the analogy is supported silicates (Cruikshank et al. 1998). Water by the comparison of the Christiansen ice absorption bands at 1.5 and 2 ¹m have peak at about 9.2 ¹m and by the trans- been identified in the reflectance TNO 1996 parency features around 13 ¹m and at 26 TO66 (Brown et al. 1999). Fig. 2 shows ¹m (Barucci et al. 2002). The similar- the result obtained for the Centaur 8405 ity of 10 Hygiea with CO3 carbonaceous Asbolus. Barucci et al. (2000) found a chondrite meteorites would imply that this rather featureless spectral behavior with no is a “primitive” asteroid which may have detection of water ice. On the basis of this started some metamorphism. observation and using a radiative transfer model they argued that, to reproduce the observed spectral features, no more than few percents of the surface of this Centaur can be covered by water ice. This is sur- Dotto and Barucci: Minor bodies of the Solar System: infrared data 265 Fig. 2. Visible and near–IR spectrum of 8405 Asbolus (Barucci et al. 2000). The surface has been modeled as a mixture of amorphous carbon and kerogen (dotted line) and as a mixture of amorphous carbon, tholin and a few percent of water ice (dashed line). prising since objects at that solar distance sample of small bodies of the Solar System.