New visible spectra and mineralogical assessment of (21) Lutetia, a target of the Rosetta mission

The Rosetta spacecraft, launched on March 2nd 2004, in the course of its journey to the comet 67P/Churyumov-Gerasimenko (encounter foreseen in 2014), will fly past two asteroids: (2867) Steins and (21) Lutetia. On September 5th 2008 (2867) Steins was encountered. In this paper, we present two visible spectra of (21) Lutetia of different spectral resolutions covering the spectral ranges where possible absorption bands were previously revealed by Lazzarin and collaborators. We confirm detection of a broad complex feature between 0.45 and 0.55 mu m and two narrower features around 0.47 and 0.52 mu m. We discuss possible assignments of these bands and suggest that they might originate from electronic transitions in pyroxenes, although unambiguous identification is difficult and the published thermal infrared (TIR) spectrum of (21) Lutetia suggests that pyroxene cannot be the dominant silicate component at its surface. Furthermore, we discuss the published spectra of (21) Lutetia in the range from near-UV to thermal infrared. We conclude that carbonaceous meteorites (chondrites and achondrites) appear to be the closest meteorite analogues of (21) Lutetia, based on the observed spectral features. Among these meteorites, metal-rich carbonaceous chondrites seem to be the most plausible analogue materials.