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

I. BELSKAYA, S. FORNASIER, YU. KRUGLY, V. SHEVCHENKO, N. GAFTONYUK, M. A. BARUCCI, M. FULCHIGNONI, AND R. GIL-HUTTON

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Año: 2010 vol. 515 p. 29 - 36

0004-6361

Aims. A wide observational campaign was carried out in 2004–2009 that aimed to complete the ground-based investigation of Lutetiaprior 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 andinfrared spectra in the 0.4–2.4 μm range. We analyze them with previously published data to retrieve information about Lutetia’ssurface properties.Results. Values of lightcurve amplitudes, absolute magnitude, opposition effect, phase coefficient, and BVRI colors of Lutetia surfaceseen at near pole-on aspect are determined. We define more precisely parameters of polarization phase curve and show their distinctdeviation from any other moderate-albedo asteroid. An indication of possible variations in both polarization and spectral data acrossthe 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 thesurface is covered by a fine-grained regolith of particle size smaller than 20 μm; (iii) the closest meteorite analogues of Lutetia’s sur-face composition are particular types of carbonaceous chondrites, or Lutetia has specific surface composition that is not representativeamong studied meteorites.