Compositional characterisation of the Themis family

M. Marsset (1, 2) , P. Vernazza (2) , M. Birlan (3, 4) , F. Demeo (5) , R. P. Binzel (5) , C. Dumas (6) , J. Milli (1) , M. Popescu (3)

1 ESO - European Southern Observatory [Santiago]
2 LAM - Laboratoire d'Astrophysique de Marseille
3 IMCCE - Institut de Mécanique Céleste et de Calcul des Ephémérides
4 Astronomical Institute of Romanian Academy
5 EAPS - Department of Earth, Atmospheric and Planetary Sciences [MIT, Cambridge]
6 CEFREM - Centre de Formation et de Recherche sur les Environnements Méditérranéens

Context. It has recently been proposed that the surface composition of icy main-belt asteroids (B-, C-, Cb-, Cg-, P-, and D-types) may be consistent with that of chondritic porous interplanetary dust particles (CP IDPs). Aims. In the light of this new association, we re-examine the surface composition of a sample of asteroids belonging to the Themis family in order to place new constraints on the formation and evolution of its parent body. Methods. We acquired near-infrared spectral data for 15 members of the Themis family and complemented this dataset with existing spectra in the visible and mid-infrared ranges to perform a thorough analysis of the composition of the family. Assuming end-member minerals and particle sizes (<2 µm) similar to those found in CP IDPs, we used a radiative transfer code adapted for light scattering by small particles to model the spectral properties of these asteroids. Results. Our best-matching models indicate that most objects in our sample (12/15) possess a surface composition that is consistent with the composition of CP IDPs. We find ultra-fine grained (<2 µm) Fe-bearing olivine glasses to be among the dominant constituents. We further detect the presence of minor fractions of Mg-rich crystalline silicates (enstatite and forsterite). The few unsuccessfully matched asteroids may indicate the presence of interlopers in the family or objects sampling a distinct compositional layer of the parent body. Conclusions. The composition inferred for the Themis family members suggests that the parent body accreted from a mixture of ice and anhydrous silicates (mainly amorphous) and subsequently underwent limited heating. By comparison with existing thermal models that assume a 400-km diameter progenitor, the accretion process of the Themis parent body must have occurred relatively late (>4 Myr after CAIs) so that only moderate internal heating occurred in its interior, preventing aqueous alteration of the outer shell.

Mots clés: Comets: general; Interplanetary medium; Meteorites; meteors; meteoroids; Methods: data analysis; Minor planets; asteroids: general; Techniques: spectroscopic

Domaines: Planétologie et astrophysique de la terre [astro-ph.EP]; Géochimie

M. Marsset, P. Vernazza, M. Birlan, F. Demeo, R. P. Binzel, et al.. Compositional characterisation of the Themis family. Astronomy and Astrophysics - A&A, 2016, 586, pp.A15. ⟨10.1051/0004-6361/201526962⟩. ⟨hal-01440143⟩