Puzzling asteroid 21 Lutetia: our knowledge prior to the Rosetta fly-by

Abstract

Aims. A wide observational campaign was carried out in [2004][2005][2006][2007][2008][2009] that aimed to complete the ground-based investigation of Lutetia prior to the Rosetta fly-by in July 2010. Methods. We obtained BVRI photometric and V-band polarimetric measurements over a wide range of phase angles, and visible and infrared spectra in the 0.4-2.4 μm range. We analyze them with previously published data to retrieve information about Lutetia's surface properties.Results. Values of lightcurve amplitudes, absolute magnitude, opposition effect, phase coefficient, and BVRI colors of Lutetia surface seen at near pole-on aspect are determined. We define more precisely parameters of polarization phase curve and show their distinct deviation from any other moderate-albedo asteroid. An indication of possible variations in both polarization and spectral data across the asteroid surface are found. To explain features found by different techniques, we propose that (i) Lutetia has a non-convex shape, probably due to a large crater, and heterogeneous surface properties probably related to surface morphology; (ii) at least part of the surface is covered by a fine-grained regolith of particle size smaller than 20 μm; (iii) the closest meteorite analogues of Lutetia's surface composition are particular types of carbonaceous chondrites, or Lutetia has specific surface composition that is not representative among studied meteorites.

Ultraviolet and visible photometry of asteroid (21) Lutetia using the Hubble Space Telescope

“…• 48 on 2008 November 30, which is consistent with the phase law deduced from visible observations (Belskaya et al 2010).…”

“…Lutetia was extensively observed in the 1970s, yielding visible and near-IR reflectance spectra (McCord & Chapman 1975), radiometric albedos and diameter estimates (Morrison 1977), and polarimetric albedos and diameter estimates (Zellner & Gradie 1976), which have been confirmed by similar observations reported during the last decade (see Belskaya et al 2010). Based Fig.…”

“…We obtained these data from the STScI archive, and reanalyzed them using new information on the phase behavior (Belskaya et al 2010) and size of Lutetia (Drummond et al 2010). The IUE spectrum (Fig.…”

“…Numerous researchers in the last few years (Barucci et al 2005(Barucci et al , 2008Lazzarin et al 2009;Perna et al 2010; see summary by Belskaya et al 2010) have argued that Lutetia shows certain spectral characteristics (e.g., in the thermal IR) that resemble several CO and CV meteorites, but not an iron meteorite. However, mineralogical interpretations of thermal IR spectra must be made cautiously because particle size, in addition to composition, can strongly affect the observed spectral features (Vernazza et al 2010).…”

Asteroid (21) Lutetia: Disk-resolved photometric analysis of Baetica region

“…Although we find an expressive phase bluing in the A 0 spectral slope map. Belskaya et al (2010) work on polarimetry of Lutetia was also inconclusive in respect to the existence of coherent-backscattering mechanism. Belskaya et al showed that Lutetia presents one of broadest negative polarization branch, only (234) Barbara and other four asteroids have wider negative polarization than Lutetia.…”