The PRIMitive Asteroid Spectroscopic Survey (PRIMASS) was formed with the goal of determining the origins and likely compositions of mission targets (101955) Bennu and (162173) Ryugu. Both Bennu and Ryugu are thought to have originated from primitive asteroid populations in the inner main belt (IMB), specifically from the low inclination (i<8°) dynamical families. Another population that is dynamically able to deliver near-Earth asteroid (NEA) sized fragments to near-Earth space is the background population, i.e., asteroids that do not cluster into families.
A map of the IMB asteroid population with the family members removed leaves a population of background objects that seem to cluster into halos around the location of the families. Dermott et al. (2018) show that the sizes of IMB background asteroids are correlated with eccentricity and anti-correlated with inclination, which suggests that both family and non-family asteroids originated from a small number of large primordial planetesimals. If this is true, then the background objects should be spectrally similar to the family objects in the same region. It is possible that background objects may have belonged to a larger primordial family that has been erased over time due to drifting caused by the Yarkovsky effect. Delbo et al. (2017) identified one such primordial family in the IMB by searching for characteristic “V shaped” groups in (semi-major axis vs 1/diameter) space.
In this study we spectrally characterize the primordial background population by obtaining a representative sample of the IMB background objects identified by Delbo et al. (2017). We compare our results with the population of primitive IMB families already observed and published by PRIMASS. We present visible spectra of 20 objects obtained from the SMASS and S3OS2 surveys and near-infrared spectra of 47 objects that we observed using the NASA InfraRed Telescope Facility and the Telescopio Nazionale Galileo. Our sample shows that the background looks similar to the primordial families in the same region, i.e., the background and families may have originated from the same parent bodies. Within the background population, there is a spectral diversity that is suggestive of a complex origin, i.e., the background did not originate from one single parent body. Based on our spectral comparisons, the primordial background cannot be ruled out as a possible source for near-Earth asteroids Bennu and Ryugu, and we argue that it is the most probable source region for Ryugu.