Gaia view of primitive inner-belt asteroid families

Abstract

Aims. The near-Earth asteroids Ryugu and Bennu were visited, characterised, and sampled by the Hayabusa2 and OSIRIS-REx missions, where remote sensing data and sample return analysis showed that both asteroids have primitive, hydrated, and organic-rich compositions. The dark families of the inner main belt that belong to the spectroscopic C-complex have been claimed to be the sources of both Ryugu and Bennu, and hence there have been large efforts to spectroscopically characterise them by ground-based observations. Methods. Here we used the Gaia Data Release 3 (Gaia DR3) asteroid reflectance spectra in order to characterise the 11 known inner main belt C-complex families (Chaldaea, Chimaera, Clarissa, Erigone, Eulalia, Klio, Polana, Primordial, Sulamitis, Svea, Tamara), using space-borne visible-light spectroscopic observations. For each family we extracted the family members that have known geometric visible albedo values and Gaia DR3 data, and we created an average reflectance spectrum per family between 370 and 950 nm. These averages were then compared with the ground-based visible spectroscopic surveys of the same families, and to Bennu’s and Ryugu’s space- and ground-based spectra in the same wavelength range. Results. Gaia DR3 reflectance spectra of the dark asteroid families of the inner main belt are in general consistent with previous findings. The only exception is the case of the Svea family: previous surveys classified its members as B-types, whereas the average reflectance spectrum from Gaia DR3 is similar to a C-type. We also showed that the Polana and the Eulalia families can be distinguished in the wavelength region 370–500 nm. Among all the primitive inner main belt families, we found that the average reflectance spectra of the Eulalia and Polana families are the most similar to those of Bennu and Ryugu, respectively. In particular, Eulalia family’s average spectrum is a good match to Bennu’s in the wavelength range 450–800 nm, while beyond 800 nm the spectrum of Bennu is bluer than that of Eulalia. Moreover, the spectrum of the Polana family has the smallest discrepancy (smallest χ2) against the spectrum of Ryugu, although this match is formally unsatisfactory (reduced χ2 ~ 1.9).