Thermophysical Modeling of Asteroid (15) Eunomia from Spatially Resolved ALMA and VLA DataOPEN ACCESS 

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Yu Yu Phua, Katherine de Kleer, Bryan Butler, Saverio Cambioni and Michael Shepard

The Planetary Science Journal, Volume 7, Number 2, Published: 23 February 2026

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“Asteroid (15) Eunomia is the largest S-type asteroid in the main belt and the largest member of the Eunomia asteroid family. Previous visible-to-mid-infrared data suggested that Eunomia’s hemispheres have different olivine-to-pyroxene ratios and that ∼2% of asteroids in its family belong to the X-complex, a taxonomic class possibly associated with iron meteorites. This led to the proposal that Eunomia is partially differentiated. In this work, we searched for metallic regions on Eunomia’s surface diagnostic of differentiation by analyzing new brightness temperature data collected with the Atacama Large Millimeter/Submillimeter Array (ALMA) at 1.3 mm and the Karl G. Jansky Very Large Array at 1 and 2 cm, which are particularly sensitive to surface metal content. The ALMA data are resolved at 30 km and allow us to characterize spatial variations in surface properties. We derived best-fit disk-averaged thermal inertia Γ = 75–135 J m−2 s−0.5 K−1 and dielectric constant epsilon = 7–8 (X²r=15.5; high X²r could be due to insufficient data quality or large-scale topography not captured by the shape model). The thermal inertia derived here from 1.3 mm observations may be different from that derived from mid-infrared observations, as the ALMA observations at 1.3 mm probe deeper into the subsurface than mid-infrared observations, where the regolith properties may be different. We do not find hemispherical-scale variations in the thermal inertia and dielectric constant. This suggests that the metal contents of any distinct olivine-rich and pyroxene-rich regions are not significantly different. Overall, our results do not provide conclusive evidence for the state of differentiation of Eunomia.”

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