Thermal properties of asteroid 21 Lutetia from Spitzer Space Telescope observations
Résumé
Context. Asteroid 21 Lutetia is the second target of the Rosetta space mission with a flyby scheduled in July 2010. To best prepare the observational campaign, Lutetia is being extensively characterized by ground-and space-based astronomical facilities. Aims. We used the Spitzer Space Telescope (SST) to determine the thermal properties of Lutetia and more generally, to contrain its physical properties and nature. Methods. The observations were performed with the infrared spectrograph (IRS) of the SST on 10 and 11 December 2005, when the asteroid was 2.81 AU from the Sun, 2.65 AU from the SST and at a phase angle of 21 degrees. We obtained 14 spectra ranging from 5.2 to 38.0 mu m, and sampling the rotational period of the asteroid. They were interpreted with a standard thermal model incorporating the thermal inertia. Results. We obtained the first thermal light curve of Lutetia. Using the most recent solution for its three-dimensional shape and rotational state, as well as independently determined parameters such as the albedo, we satisfactorily reproduced the 14 spectral energy distributions and the complete thermal light curve of Lutetia. The best thermal model has a thermal inertia I \textless= 30 JK(-1)m(-2)s(-1/2) and a beaming factor in the range similar to 0.70-0.83. This low thermal inertia is typical of main belt asteroids and implies that the surface of Lutetia is likely covered by a thick regolith layer. Since the beaming factor only reflects the effects of surface rugosity, the above range implies a high degree of roughness. In addition, our results show evidence of inhomogeneities in the surface roughness in the equatorial band of Lutetia.