Gaia and IRTF abundance of A-type main belt asteroids

TL;DR

The paper addresses the Missing Mantle Problem by integrating Gaia DR3 visible reflectance spectra with IRTF near-infrared data to produce a PCA-based target selection for A-type asteroids. By calibrating A-type probability in PC1–PC2 space with literature and new NIR classifications, the authors estimate the main-belt A-type abundance as $2.00 \\pm 0.15$% and reveal spatial and family-based variations, including notable enrichments in Vesta and Flora. This work strengthens the case that olivine-rich material is more widespread in the asteroid belt than previously thought, with implications for early Solar System differentiation and multiple differentiated parent bodies in the inner belt. The methodology showcases the value of multi-band spectral datasets and points toward SPHEREx-induced advances in taxonomic classification of small bodies.

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.

Figures (11)

• Figure 1: PC1 vs. PC2 plot of main belt asteroid reflectance spectra from the Gaia DR3 having S/N$>$50 (blue circles) gaiacollaborationGaiaDataRelease2023. The red stars indicate the A-types that we selected from the literature following the method explained in the text and have also a DR3 reflectance spectrum. The green triangles show the asteroids that we observed with the IRTF.
• Figure 2: (A) is as Fig. \ref{['F:PCA50']} but now showing A-type and non A-type asteroids with VIS+NIR spectroscopy after our survey with the IRTF. (B) shows a contour plot of the value of $K_a(x,y) / K_c(x,y)$, which approximates the probability $P$ to find an A-type at $(x,y)$ given there is an asteroid at $(x,y)$, i.e., $P(\text{A-type}~|~(x,y))$. The KDEs were calculated here for display with $h$=0.2.
• Figure 3: As in Fig. \ref{['F:PCA50']} but showing the PC1, PC2 positions of the A-type candidates selected by demeoOlivinedominatedAtypeAsteroids2019 from the SDSS that have values within the Gaia DR3.
• Figure 4: Size frequency distribution of A-type asteroids from the Gaia DR3 that have a known diameter, uncorrected (blue) and corrected (orange) for observational biases (see text).
• Figure 5: A-type abundance (with values) in collisional families with more than 2,000 members according to the definition of nesvornyIdentificationFamiliesAstIV2015, compared to the average value of the main belt and the host region of the family in the belt (inner, central, outer). Values per family, given within the plot, are in %. The values after the $\pm$ symbol represent the standard deviation of the distribution of the A-type abundance.