Gaia and IRTF abundance of A-type main belt asteroids

Abstract

The Missing Mantle Problem refers to the apparent scarcity of olivine rich (A-type) asteroids in the main belt, despite expectations that such bodies should be common if differentiated parent bodies were widely disrupted. Conversely, A-types appear relatively more abundant among near-Earth asteroids. We present a revised estimate of the A-type abundance in the main belt by combining Gaia Data Release 3 (DR3) reflectance spectra with near-infrared observations from NASA IRTF. We applied principal component analysis (PCA) to Gaia DR3 visible spectra to identify A-type candidates and confirmed a subset through IRTF spectroscopy. Using these data and literature results, we derived the A-type probability distribution as a function of DR3 principal components, and from this, estimated their abundance across heliocentric distances and collisional families. We find that A-types constitute (2.00 +/- 0.15)% of main-belt asteroids significantly higher than previous estimates. Some families, such as Vesta and Flora, show enhanced A-type fractions, while others (e.g. Themis, Hygiea) contain few or none. The elevated abundance in the Flora family supports the existence of an additional differentiated parent body in the inner belt, besides Vesta. These findings indicate that olivine-rich material is more widespread than previously thought, offering new constraints on the Missing Mantle Problem and on early Solar System differentiation processes.