A&A 470, 1157-1164 (2007)
DOI: 10.1051/0004-6361:20066944

Near infra-red spectroscopy of the asteroid 21 Lutetia

II. Rotationally resolved spectroscopy of the surface

D. A. Nedelcu^{1, 2}, M. Birlan¹, P. Vernazza³, P. Descamps¹, R. P. Binzel⁴, F. Colas¹, A. Kryszczynska⁵, and S. J. Bus<sup>6

1</sup>  Institut de Mécanique Céleste et de Calcul des Éphémérides (IMCCE), Observatoire de Paris, 77 avenue Denfert-Rochereau, 75014 Paris Cedex, France
    e-mail: [Mirel.Birlan;Pascal.Descamps;Francois.Colas]@imcce.fr
²  Astronomical Institute of the Romanian Academy, 5 Cu titul de Argint, 75212 Bucharest, Romania
    e-mail: nedelcu@imcce.fr
³  LESIA, Observatoire de Paris-Meudon, 5 place Jules Janssen, 92195 Meudon Cedex, France
    e-mail: Pierre.Vernazza@obspm.fr
⁴  Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge MA 02139, USA
    e-mail: rpb@mit.edu
⁵  Astronomical Observatory, Adam Mickiewicz University, Sloneczna 36, 60-286 Poznan, Poland
    e-mail: agn@amu.edu.pl
⁶  Institute for Astronomy, 640 North A'ohouku Place, Hilo, HI 96720, USA
    e-mail: sjb@ifa.hawaii.edu

(Received 15 December 2006 / Accepted 16 April 2007)

Abstract
Aims.In the framework of the ground-based science campaign dedicated to the encounter with the Rosetta spacecraft, the mineralogy of the asteroid (21) Lutetia was investigated.
Methods.Near-infrared (NIR) spectra of the asteroid in the 0.8-2.5 m spectral range were obtained with SpeX/IRTF in remote observing mode from Meudon, France in March and April 2006. We analysed these data together with previously acquired spectra - March 2003, August 2004. I-band relative photometric data obtained on 20 January 2006 using the 105 cm telescope from Pic du Midi, France has been used to build the ephemeris for physical observations. A  test using meteorite spectra from the RELAB database was performed in order to find the best fit of complete visible + infrared (VNIR) spectra of Lutetia.
Results. The new spectra reveal no absorption features. We find a clear spectral variation (slope), and a good correspondence between spectral variations and rotational phase. Two of the most different spectra correspond to two opposite sides of the asteroid (sub-Earth longitude difference around 180°). For the neutral spectra a carbonaceous chondrite spectrum yields the best fit, while for those with a slightly positive slope the enstatitic chondrite spectra are the best analog. Based on the chosen subset of the meteorite samples, our analysis suggests a primitive, chondritic nature for (21) Lutetia. Differences in spectra are interpreted in terms of the coexistence of several lithologies on the surface where the aqueous alteration played an important role.

Key words: minor planets -- asteroids -- techniques: spectroscopic -- methods: observational

© ESO 2007

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