Kalliope: Discovery of the first metallic asteroid family

 In the classical theory of planetesimal differentiation, a body would form an iron-rich core, an olivine-dominated mantle, and a pyroxene-rich basaltic crust [1]. The detection of differentiated bodies in the current asteroid main belt will allow us to get insights and study the very initial phases of planetesimal accretion. So far, the only striking proof of a differentiated planetesimal is asteroid (4) Vesta and its family that resulted from the impact formation of two large basins Rheasilvia and Veneneia [2]. 

Asteroid (22) Kalliope is the densest known asteroid with ⍴=4.4±0.46 g.cm^-3 [3] indicating a metal-rich composition. The low radar albedo (0.18±0.05 [4]), however, points towards a lower metal content on the surface but the presence of very high density indicates a differentiated metal-rich interior.  

(22) Kalliope has recently been shown to be the parent body of an asteroid family in the outer main belt consisting of 302 members [5]. Therefore, studying the physical properties of the Kalliope family members we can get insights into the internal structure of the original planetesimal. 

In this work we studied the physical properties of the Kalliope family.

Thirty seven Kalliope family members have visible reflectance spectra from Gaia DR3 and 22 of which were observed at NASA IRTF obtaining their near-infrared spectra. Following the methodology of our previous work on the Athor asteroid family [6], Gaia and IRTF spectra were combined with the available visible SDSS data. The final combined spectra were classified in the Bus-DeMeo taxonomy [7]. Using the reflectance spectra of Kalliope family members as well as their geometric visible albedos we matched them with meteorites that are included in the RELAB and PSF meteorite lab spectra databases. 

We discovered that the Kalliope family is the first family that consists of metallic fragments, confirming the differentiated nature of the original planetesimal.

References: [1] Elkins-Tanton, L. and Weiss, B. (2017), Planetesimals, Cambridge University Press. [2] Marchi S. et al. (2021) Science 336, Issue 6082, 690. [3] Ferraris M. et al. (2022) A&A, 622, A71. [4] Shepard M. K., et al. (2015) Icarus, 245, 38. [5] Brož M. et al. (2022) A&A, 664, A69. [6] Avdellidou C. et al. (2022) A&A, 665, L9. [7] DeMeo F. E. et al. (2009) Icarus, 202, 160.