The properties of asteroid (2867) Steins from Spitzer Space Telescope observations and OSIRIS shape reconstruction

¹ Laboratoire d’Astrophysique de Marseille, UMR 6110 CNRS/Université de Provence, 38 rue Frédéric Joliot Curie, 13388 Marseille Cedex 13, France
e-mail: olivier.groussin@oamp
² LESIA, Observatoire de Paris, 92195 Meudon Principal Cedex, France

Received: 2 November 2010
Accepted: 14 February 2011

Abstract

Aims. We report on the thermal properties and composition of asteroid (2867) Steins derived from an analysis of new Spitzer Space Telescope (SST) observations performed in March 2008, in addition to previously published SST observations performed in November 2005.

Methods. We consider the three-dimensional shape model and photometric properties derived from OSIRIS images obtained during the flyby of the Rosetta spacecraft in September 2008, which we combine with a thermal model to properly interpret the observed SST thermal light curve and spectral energy distributions.

Results. We obtain a thermal inertia of 100  ±  50 J K^-1 m^-2 s^−1/2 and a beaming factor (roughness) in the range 0.7–1.0. We confirm that the infrared emissivity of Steins is consistent with an enstatite composition. The November 2005 SST thermal light curve is most reliably interpreted by assuming inhomogeneities in the thermal properties of the surface, with two different regions of slightly different roughness, as observed on other small bodies, such as the nucleus of comet 9P/Tempel 1. Our results emphasize that the shape model is important to an accurate determination of the thermal inertia and roughness. Finally, we present temperature maps of Steins, as seen by Rosetta during its flyby, and discuss the interpretation of the observations performed by the VIRTIS and MIRO instruments.