Near infra-red spectroscopy of the asteroid 21 Lutetia

II. Rotationally resolved spectroscopy of the surface

¹ 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 μm 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.