The surface composition of Jupiter Trojans: Visible and near-infrared survey of dynamical families☆
Introduction
Beyond the asteroid main belt, in the Jupiter Lagrangian points L4 and L5, there are two clouds of small bodies of the Solar System, called Jupiter Trojans. The origin of these bodies is still far from being completely understood and several hypothesis have been so far proposed (Marzari et al., 2003, Morbidelli et al., 2005). Morbidelli et al. (2005) suggested that Jupiter Trojans, like Kuiper belt objects and the scattered disk, originated in the planetesimal disk which drove the planetary migration. Before being captured in the region where they are still observable, Jupiter Trojans had temporarily large eccentricity that brought them relatively close to the Sun, where cometary activity should have been intense. Although the scenario of their formation has not been definitively assessed, it is widely accepted that Jupiter Trojans formed at large heliocentric distances, in a region rich in frozen volatiles. Moreover, they are widely believed to have now stable orbits and to have suffered an intense collisional evolution. The recent discovery of dynamical families among Jupiter Trojans seems to support this idea.
So far we know more than 1900 Jupiter Trojans. Their physical properties and their surface composition are at present not yet fully understood. Visible and near-infrared spectra are available for a limited sample of these objects (Jones et al., 1990, Jewitt and Luu, 1990, Luu et al., 1994, Fitzsimmons et al., 1994, Lazzarin et al., 1995, Dumas et al., 1998, Emery and Brown, 2001, Emery and Brown, 2003, Bendjoya et al., 2004, Fornasier et al., 2004a). The main characteristics are featureless spectra, low albedo and red colors: the large majority of them belongs to the D taxonomic class but P- and C-types are also present among them.
Although Jupiter Trojans are believed to have formed in a region rich in frozen volatiles, water ice is still undetected in their spectra. A large part of the infrared spectra of Jupiter Trojans available in the literature has been recently published by Emery and Brown (2004) who obtained 0.3–4.0 μm spectra of 17 bodies and presented also models of the surface composition. They did not detect water ice and hydrated silicate features in their V–NIR spectra and they estimated upper limits of a few percents and up to 30%, respectively, for these materials at the surface. To explain this lack of water ice on the surface of the observed objects, space weathering mechanisms can be invoked. Laboratory experiments have shown that solar wind, high-energy particles and microimpacts can alter icy surfaces of atmosphereless bodies, producing an irradiation mantle spectrally red and with low albedo (Moore et al., 1983, Thompson et al., 1987, Strazzulla, 1998, Hudson and Moore, 1999). However, if Jupiter Trojans experienced a phase of cometary activity water ice on their surface could have been devolatized early in their history, when they went through the high eccentricity phase. Alternatively, they could have formed a dust mantle as shown by Tancredi et al. (2006) for large comets (a few km-size comet nuclei). As a consequence, water ice, originally present on the surface of Jupiter Trojans, would be now completely covered and still present only in their interiors. In this scenario ice signatures could be detectable only if recent collisions would expose inner fresh material.
In order to increase the available sample of visible and near-infrared spectra of Jupiter Trojans and to investigate the surface composition of these objects, we started in 2002 an observational program at the European Southern Observatory (ESO, Chile), using both the New Technology Telescope (NTT) and the Very Large Telescope (VLT), and at the 3.6-m Telescopio Nazionale Galileo (TNG, La Palma, Spain). The sample selection done by the other teams which studied the physical properties of Jupiter Trojans was not based on dynamical constraints. On the contrary, we concentrated on members of dynamical families, as defined by Beaugé and Roig (2001), with the aim to look for the presence of water ice on their surfaces. In fact, the collisional disruptions which are supposed to have produced dynamical families might have exposed on the surface of the fragments some of the ices originally present in the interior of the parent body. Therefore, water ice likely present in the interior of larger Trojans, might be observable on the surfaces of the family members if the family formation is somewhat recent.
Section snippets
Observations
We carried out visible and near-infrared spectroscopy and photometry of Jupiter Trojans belonging to different dynamical families. We selected our targets from a list kindly provided by Beaugé and Roig (personal communication) as an update of the list by Beaugé and Roig (2001) and P.E.Tr.A. Project at www.daf.on.br/froig/petra/. In particular our selection was based on a cutoff threshold (Q is the upper limit of a metric so that a cluster be significant), corresponding to relative
Spectral analysis and surface modeling
Fig. 1, Fig. 2 show the whole sample of visible spectra obtained at the TNG telescope and at the ESO-NTT telescope.
Fig. 3, Fig. 4, Fig. 5, Fig. 6 show the visible and near-infrared spectra obtained for each dynamical family. For observations carried out at ESO, where the photometric calibration has been performed, the different spectral ranges have been adjusted using the computed color indices. For TNG observations the photometric calibration was not possible. In any case they are not needed
Discussion
A strong homogeneity of the Trojan population is the main characteristic we derived from our survey. Most of the observed Trojans family members appear to belong to the P–D taxonomic type. No diagnostic spectral signatures have been found to distinguish a family with respect to the others. All the observed spectra have been modeled with a surface composition given by mixtures of amorphous carbon and organic compounds with small percentages of silicates. Different slopes in the region 0.5–1.2
Conclusion
In this paper we present visible and near-infrared data of 24 Jupiter Trojans belonging to seven different families. Absolute magnitudes have been computed for 17 objects and an estimation of the diameter has been presented for 19 bodies. Tentative models of the surface composition of the whole sample of observed targets have been obtained by applying a radiative transfer model based on the Hapke theory. We considered several mixtures of minerals, and organic and icy compounds which are
Acknowledgements
We thank C. Beaugé and F. Roig for kindly providing us with updated Trojan family list. We thank D. Lazzaro and R.P. Binzel for having revised this paper, providing useful comments. E. Dotto, invited scientist at the European Southern Observatory on February–March 2003, thanks all the ESO staff for the hospitality.
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Based on observations carried out at the European Southern Observatory (ESO), La Silla, Chile, ESO proposals 69.C-0524 and 71.C-0650, and at the Telescopio Nazionale Galileo, La Palma, Canary Island, proposals TAC06 (AOT7) and TAC705 (AOT6).