Thermal properties of the asteroid (2867) Steins as observed by VIRTIS/Rosetta

¹ LESIA, Observatoire de Paris, CNRS, UPMC, Université Paris-Diderot, 5 place Jules Janssen, 92195 Meudon, France
e-mail: cedric.leyrat@obspm.fr
² INAF-IFSI Istituto di Fisica dello Spazio Interplanetario, via del Fosso del Cavaliere 100, 00133 Roma, Italy
³ INAF-IASF Istituto di Astrofisica Spaziale e Fisica Cosmica, via del Fosso del Cavaliere 100, 00133 Roma, Italy

Received: 17 January 2011
Accepted: 1 May 2011

Abstract

Context. To investigate thermal properties of the small asteroid (2867) Steins, we analyzed its surface thermal emission measured by the VIRTIS instrument during the Rosetta spacecraft (ESA) flyby that occurred in September 2008. VIRTIS performed the first thermal mapping of an asteroid surface.

Aims. We determine the thermal properties of different locations at the surface of Steins and the thermal inertia is constrained by measuring the variations of the surface temperature for different local times.

Methods. The surface temperature on the morning side of Steins is measured by inverting IR spectra between 4 and 5 microns acquired by the VIRTIS-M instrument onboard the Rosetta spacecraft. It is then compared to the theoretical temperatures obtained with a thermophysical model assuming either a smooth or rough surface. The heat conduction is computed radially into the first millimeters below the surface assuming a non-zero thermal inertia. The shape of the asteroid is reproduced by a 3D shape model composed of thousands of independent facets. We then compare both observed and computed temperatures within the footprint at different local times.

Results. The observed temperatures are reproduced by a thermal inertia Γ = 110  ±  13   J   m^-2   K^-1   s^−1/2 for a smooth surface but some discrepancies remain between data and the thermal model at high incidence angles on the morning side near the terminator. This suggests that roughness that creates self-heating especially at low incidence angles is needed to reproduce all data. Surface roughness is modeled statistically. We assumed an infrared emissivity of 0.9 which corresponds to an effective emissivity ϵ_IReff = 0.73  ±  0.02 when combined with a small scale self heating parameter ξ = 0.2 derived from OSIRIS Hapke parameters. This is consistent with the emissivity of 0.6−0.7 measured by the microwave instrument MIRO at 0.53 mm. Surface temperature variations are then well reproduced with a thermal inertia of Γ = 210  ±  30   J   m^-2   K^-1   s ^− 1/2. This value is higher than the one found for main-belt asteroids, but is consistent with kilometer-sized objects such as the near-Earth objects. It suggests a thin regolith layer and a low porosity.