Searching for a concentration of olivine-rich bodies in asteroid collisional families in the main belt

It is understood that large asteroids that were (partially) melted by the heat produced by the decay of radioactive elements at the beginning of our Solar System, differentiated into layers of distinct compositions: an iron core, an olivine-rich mantle and a basaltic crust (Šrámek et al. 2012; Kruijer et al. 2014; Elkins-Tanton & Weiss 2017). Traces of material corresponding to each of these layers have been identified by studying the composition of asteroids in the current main asteroid belt. In addition, the collisional break-up of a differentiated asteroid is expected to produce fragments of different compositions representative of each layer. However, no family with a clear abundance of olivine-rich mantle-like asteroids has been found to date (DeMeo et al. 2019). There is a scarcity of olivine-rich asteroids in the main belt compared to other compositions, known as the ’missing mantle problem’. DeMeo et al. (2019) states that, up to now, there is no statistical concentration of olivine-rich objects in any asteroid family, and that these objects are evenly distributed throughout the main belt.

Using the Gaia DR3 dataset, which contains more than 60 000 Solar System small bodies with reflectance spectra in the visible wavelength range (Gaia Collaboration et al. 2023), we analysed the collisional families of Nesvorny et al. (2015) to search for a potential concentration of olivine-rich asteroids in any family. This composition corresponds to the A-type spectroscopic class in several taxonomic schemes (Bus & Binzel 2002; DeMeo et al. 2009; Mahlke et al. 2022). We found from the study of literature data that the family (36256) 1999 XT17 (FIN 629 in Nesvorny et al. 2015) was the most probable to show a concentration of potential olivine-rich objects. This family is located in the ’pristine zone’ of the main belt (Brož et al. 2013), and it contains 58 members in Nesvorny et al. (2015), 15 of which show a spectrum in the Gaia DR3 dataset.

We classified these 15 members with a χ2 procedure, using a combination of their Gaia DR3 spectra and their literature data, when available. We used the Bus-DeMeo (DeMeo et al. 2009) taxonomic templates to perform this classification, following the methods of Avdellidou et al. (2022). We obtained 12 objects classified as A-types out of the 15. We analysed the spectra of these objects and their position in the proper orbital elements space, and we concluded that a cluster of objects within the collisional (36256) 1999 XT17 family might show homogeneous olivine-rich compositions. This cluster could have once been part of a completely or partially-differentiated body, or could have been formed from nebular processes.

We will present the implications of our findings, including the possibility that despite being rare, A-type asteroids might be better revealed by large-scale spectroscopic surveys, such as ESA Gaia DR3/DR4 and the future NASA’s SPHEREx mission.