Gaia and IRTF abundance of A-type main-belt asteroids

¹ Université Côte d’Azur, CNRS–Lagrange, Observatoire de la Côte d’Azur, CS 34229 – 06304 NICE Cedex 4, France
² University of Leicester, School of Physics and Astronomy, University Road, LE1 7RH Leicester, UK
³ INAF-IAPS, Institute for Space Astrophysics and Planetology, Rome, Italy
⁴ Eureka Scientific, Oakland, CA 94602, USA
⁵ Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone, Italy

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 11 August 2025
Accepted: 24 October 2025

Abstract

Context. The so-called missing-mantle problem is a long-standing issue in planetary science. It states that olivine-rich asteroids should be abundant in the main belt, while this is observationally found not to be the case by dedicated surveys. Conversely, olivine-rich asteroids appear to be more abundant among near-Earth asteroids than those surveys would suggest.

Aims. We aim to provide a revised estimate of the abundance of A-type (olivine-rich) asteroids in the main belt by combining taxonomic classifications from Gaia Data Release 3 reflectance spectra with ground-based near-infrared observations from NASA’s IRTF.

Methods. We performed a principal component analysis on Gaia Data Release 3 visible-light reflectance spectra to identify A-type candidates and confirmed a subset of these using near-infrared spectroscopy from the IRTF. We combined our observations with data from the literature to compute the A-type probability distribution as a function of the principal components of Gaia reflectance spectra. This probability distribution was then used to estimate the abundance of A-type asteroids in the main belt and its sub-populations as a function of heliocentric distance. We also examined the distribution of A-type asteroids among known collisional families.

Results. We found that the abundance of A types in the main belt is (2.00 ± 0.15)%, which is significantly higher than previous estimates for the same region. Our analysis also shows that some collisional families, such as those of Vesta and Flora, have above-average A-type fractions, whereas others, such as Themis and Hygiea, exhibit negligible abundance.

Conclusions. Our results support the idea that olivine-rich material is more widespread than previously thought. In particular, the high A-type abundance in the Flora family is consistent with the hypothesis of a second differentiated parent body in the inner main belt, beyond Vesta. This work provides new observational constraints on the missing-mantle problem and the distribution of differentiated material in the asteroid main belt. In particular, our results deepen the compositional diversity observed in the inner main belt and have important implications for our understanding of early Solar System differentiation processes.