DIifferent origins or different evolutions? Decoding the spectral diversity among C-type asteroids

P. Vernazza (1) , J. Castillo-Rogez (2) , P. Beck , J. Emery , R. Brunetto (3) , M. Delbò (4) , M. Marsset (5) , F. Marchis (6) , O. Groussin (1) , B. Zanda (6, 7) , P. Lamy (1) , L. Jorda (1) , O. Mousis (1) , A. Delsanti (8) , Z. Djouadi (3) , Z. Dionnet (3) , F. Borondics (9) , B. Carry (10, 6)

1 LAM - Laboratoire d'Astrophysique de Marseille
2 JPL - Jet Propulsion Laboratory
3 IAS - Institut d'astrophysique spatiale
4 CASSIOPEE - Laboratoire de Cosmologie, Astrophysique Stellaire & Solaire, de Planétologie et de Mécanique des Fluides
5 ARC - Astrophysics Research Centre [Belfast]
6 IMCCE - Institut de Mécanique Céleste et de Calcul des Ephémérides
7 IMPMC - Institut de minéralogie, de physique des matériaux et de cosmochimie
8 LESIA - Laboratoire d'études spatiales et d'instrumentation en astrophysique
9 SSOLEIL - Synchrotron SOLEIL
10 ESAC - European Space Astronomy Centre

Anhydrous pyroxene-rich interplanetary dust particles (IDPs) have been proposed as surface analogs for about two-thirds of all C-complex asteroids. However, this suggestion appears to beinconsistent with the presence of hydrated silicates on the surfaces of some of these asteroids, including Ceres. Here, we report the presence of enstatite (pyroxene) on the surface of two C-type asteroids (Ceres and Eugenia) based on their spectral properties in the mid-infrared range. The presence of this component is particularly unexpected in the case of Ceres, becausemost thermal evolution models predict a surface consisting of hydrated compounds only. The most plausible scenario is that Ceres' surface has been partially contaminated by exogenous enstatite-rich material, possibly coming from the Beagle asteroid family. This scenario questions a similar origin for Ceres and the remaining C-types, and it possibly supports recent results obtained by the Dawn mission (NASA) that Ceres may have formed in the very outer solar system. Concerning the smaller D∼200 km C-types such as Eugenia, both their derived surface composition (enstatite and amorphous silicates) and low density (<1.5 g cm −3) suggest that these bodies accreted from the same building blocks, namely chondritic porous, pyroxene-rich IDPs and volatiles (mostly water ice), and that a significant volume fraction of these bodies has remained unaffected by hydrothermal activity likely implying a late accretion. In addition, their current heliocentric distance may best explain the presence or absence of water ice at their surfaces. Finally, we raise the possibility that CI chondrites, Tagish-Lake-like material, or hydrated IDPs may be representative samples of the cores of these bodies.

Mots clés: asteroids: general – techniques: spectroscopic; meteorites; meteoroids – methods: data analysis – methods: laboratory: solid state – methods: observational – minor planets; meteors

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

P. Vernazza, J. Castillo-Rogez, P. Beck, J. Emery, R. Brunetto, et al.. DIifferent origins or different evolutions? Decoding the spectral diversity among C-type asteroids. The Astronomical Journal, 2017, 153 (2), ⟨10.3847/1538-3881/153/2/72⟩. ⟨hal-01790534⟩