Asteroids 2867 Steins and 21 Lutetia: surface composition from far infrared observations with the Spitzer space telescope

¹ LESIA, Observatoire de Paris, 92195 Meudon Principal Cedex, France e-mail: antonella.barucci@obspm.fr
² Université Paris Diderot, Paris VII, France
³ INAF, Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Roma, Italy
⁴ Laboratoire d'Astrophysique de Marseille, BP 8, 13376 Marseille Cedex 12, France
⁵ INAF, Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy
⁶ Observatorio National (COAA), rua Gal. José Cristino 77, CEP20921–400 Rio de Janeiro, Brazil
⁷ NASA Ames Research Center, MS 245-6, Moffett Field, CA 94035-1000, USA

Received: 14 June 2007
Accepted: 3 October 2007

Abstract

Aims.The aim of this paper is to investigate the surface composition of the two asteroids 21 Lutetia and 2867 Steins, targets of the Rosetta space mission.

Methods.We observed the two asteroids through their full rotational periods with the Infrared Spectrograph of the Spitzer Space Telescope to investigate the surface properties. The analysis of their thermal emission spectra was carried out to detect emissivity features that diagnose the surface composition.

Results.For both asteroids, the Christiansen peak, the Reststrahlen, and the Transparency features were detected. The thermal emissivity shows a clear analogy to carbonaceous chondrite meteorites, in particular to the CO–CV types for 21 Lutetia, while for 2867 Steins, already suggested as belonging to the E-type asteroids, the similarity to the enstatite achondrite meteorite is confirmed.