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

TL;DR

This study combines Spitzer observations and OSIRIS shape modeling to analyze asteroid (2867) Steins' thermal properties, composition, and surface inhomogeneities, providing insights into its thermal inertia, roughness, and temperature distribution.

Contribution

It integrates thermal infrared data with shape models to accurately determine Steins' surface properties and highlights the importance of shape accuracy in thermal analysis.

Findings

Thermal inertia estimated at 100±50 JK-1m-2s-1/2.

Infrared emissivity consistent with enstatite composition.

Surface inhomogeneities affect thermal light curve interpretation.

Abstract

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. 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. We obtain a thermal inertia in the range 100\pm50 JK-1m-2s-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.