Compositional study of the Themis family
Résumé
Themis is an outer main-belt family comprising more than 4,000 dynamically well-established members (Nesvorny 2012), mainly B- and C-type asteroids (Florczak et al. 1999; Mothé-Diniz et al. 2005; Ziffer et al. 2011). This family is rather unique for a number of reasons:- It is believed to be the only main-belt family formed from the catastrophic disruption of a large (D>200 km) B-/C-type body (Brož et al. 2013). As such, it offers a unique view on the internal composition of a primitive asteroid.- Elst-Pizarro, one of the family member, was one of the first main belt comets to be discovered (Hsieh & Jewitt 2006).- 24 Themis is the first main belt asteroid for which water ice was detected at its surface (Campins et al. 2010; Rivkin & Emery 2010).- The low density values recorded for two family members (<1.3 g/cm3 , Descamps et al. 2007; Marchis et al. 2008) likely imply high fractions of ice(s) in the interior of these bodies.The last three features all point towards an ice-rich composition for the Themis parent body while showing little compatibility with a thermally metamorphosed body that has been heated throughout at temperatures exceeding 300 K. The latter interpretation was previously suggested on the basis of a similarity between the near-infrared spectral properties of heated CI/CM chondrites and those of the family members’ surfaces (Clark et al. 2010; Ziffer et al. 2011). In brief, the Themis family members appear unsampled by our meteorite collections. Recently, Vernazza et al. (2015) instead proposed that Interplanetary Dust Particles (IDPs) may be more appropriate extraterrestrial analogs for these objects’ surfaces.In the light of Vernazza et al. (2015)’s recent work, we investigated the surface mineralogy of a sample of Themis family members using a combined dataset of spectra covering the visible (Bus & Binzel 2002; Lazzaro et al. 2004), near-infrared (this work), and mid-infrared (Licandro et al. 2012; Hargrove et al. 2015) spectral ranges.Assuming particle sizes (typically sub- to micrometer sizes) and end-members composition similar to those found in chondritic porous IDPs, we modeled the spectral properties of the Themis family members. The results of this study will be presented in details.