Issue |
A&A
Volume 586, February 2016
|
|
---|---|---|
Article Number | A15 | |
Number of page(s) | 9 | |
Section | Planets and planetary systems | |
DOI | https://doi.org/10.1051/0004-6361/201526962 | |
Published online | 20 January 2016 |
Compositional characterisation of the Themis family
1
European Southern Observatory (ESO), Alonso de Córdova 3107, 1900 Casilla Vitacura,
Santiago, Chile
e-mail: mmarsset@eso.org
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
Received:
14
July
2015
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
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