Gaia view of primitive inner-belt asteroid families: Searching for the origins of asteroids Bennu and Ryugu

Abstract

Near-Earth asteroids Ryugu and Bennu, were visited, characterised, and sampled by the Hayabusa2 and OSIRIS-REx missions: 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 (IMB) that belong to the spectroscopic C-complex have been claimed to be the sources of both Ryugu and Bennu. Hence, there has been large effort to characterise them. Here we used the Gaia Data Release 3 (DR3) asteroid reflectance spectra to investigate the 11 known IMB C-complex families (Chaldaea, Chimaera, Clarissa, Erigone, Eulalia, Klio, Polana, Primordial, Sulamitis, Svea, Tamara). 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. The average DR3 reflectance spectra of each family were compared with the previous literature data and to Bennu's and Ryugu's spectra. We found that DR3 reflectance spectra of the IMB C-complex families are in general consistent with previous findings with the only exception of the Svea family. We also showed that the Polana and the Eulalia families can be distinguished in the wavelength region 370 - 500 nm. Among all the IMB C-complex families, we determined 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 against the spectrum of Ryugu, although this match is formally unsatisfactory (reduced chi^2 ~ 1.9).