Volume 586, February 2016
|Number of page(s)||9|
|Section||Planets and planetary systems|
|Published online||20 January 2016|
Compositional characterisation of the Themis family
European Southern Observatory (ESO), Alonso de Córdova 3107, 1900 Casilla Vitacura,
2 Aix Marseille University, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
3 IMCCE, Observatoire de Paris, 77 avenue Denfert-Rochereau, 75014 Paris Cedex, France
4 Astronomical Institute of the Romanian Academy, 5 Cuţitul de Argint, 040557 Bucharest, Romania
5 Department of Earth, Atmospheric and Planetary Sciences, MIT, 77 Massachusetts Avenue, Cambridge, MA, 02139, USA
Accepted: 11 November 2015
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.
Key words: interplanetary medium / meteorites, meteors, meteoroids / methods: data analysis / minor planets, asteroids: general / techniques: spectroscopic / comets: general
© ESO, 2016
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.