Letter to the Editor

Inhomogeneities on the surface of 21 Lutetia, the asteroid target of the Rosetta mission^*

Ground-based results before the Rosetta fly-by

¹ Dipartimento di Fisica, Università di Roma Tor Vergata, via della Ricerca Scientifica 1, 00133 Roma, Italy e-mail: dperna@oa-roma.inaf.it
² LESIA, Observatoire de Paris, 5 Place J. Janssen, 92195 Meudon Principal Cedex, France
³ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone (Roma), Italy
⁴ Dipartimento di Astronomia, Università di Padova, Vicolo dell'Osservatorio 3, 35122 Padova, Italy
⁵ Université Paris Diderot – Paris 7, 4 rue Elsa Morante, 75013 Paris, France

Received: 12 January 2010
Accepted: 7 March 2010

Abstract

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.