Inhomogeneities on the surface of 21 Lutetia, the asteroid target of the Rosetta mission. Ground-based results before the Rosetta fly-by

Context. In July 2010 the ESA spacecraft Rosetta will fly by the main belt asteroid 21 Lutetia. Several observations of this asteroid have been performed so far, but its surface composition and nature are still a matter of debate. For a long time Lutetia was supposed to have a metallic nature due to its high IRAS albedo. Later on it has been suggested that the asteroid has a surface composition similar to primitive carbonaceous chondrite meteorites, while further observations proposed a possible genetic link with more evolved enstatite chondrite meteorites.
Aims: We performed visible spectroscopic observations of 21 Lutetia in November 2008 at the Telescopio Nazionale Galileo (TNG, La Palma, Spain) to make a decisive contribution to solving the conundrum of its nature.
Methods: Thirteen visible spectra were acquired at different rotational phases and subsequently analyzed.
Results: We confirm a narrow spectral feature at about 0.47-0.48 μm which was already found by Lazzarin et al. (2004, A&A, 425, L25) in the spectra of Lutetia. We also confirm an earlier find of Lazzarin et al. (2004), who detected a spectral feature at about 0.6 μm in one of their Lutetia's spectra. More remarkable is the difference of our spectra though, which exhibit different spectral slopes between 0.6 and 0.75 μm and, in particular, we found that up to 20% of the Lutetia surface could have flatter spectra.
Conclusions: We detected a variation of the spectral slopes at different rotational phases that could be interpreted as possibly due to differences in the chemical/mineralogical composition as well as to inhomogeneities of the structure of the Lutetia's surface (e.g., to craters or albedo spots) in the southern hemisphere.

Based on observations made with the Italian Telescopio Nazionale Galileo (TNG) operated on the island of La Palma by the Fundacion Galileo Galilei of the INAF (Istituto Nazionale di Astrofisica) at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias (DDT program partially performed during program AOT18/TAC21).