THERMAL AND DIELECTRIC PROPERTIES OF (15) EUNOMIA DERIVED FROM SPATIALLY-RESOLVED ALMA DATA

Thermal emission informs us about the regolith properties and composition of small bodies. While thermal emission is most often studied at thermal infrared wavelengths, we cannot obtain spatially resolved observations of main-belt asteroids at these wavelengths using ground-based telescopes existing today. Instead, we can study thermally emitted fluxes at millimeter wavelengths using radio interferometry from the Atacama Large Millimeter/submillimeter Array (ALMA), which can spatially resolve the asteroids in the main belt at ~30 km resolution. This allows us to study the spatial variations in lithology and mineralogy across the surfaces of these asteroids.

In this work, we have obtained 1.3 mm thermal emission data from ALMA for (15) Eunomia, the largest S-type asteroid of diameter 270 km. Eunomia has been suggested to be at least partially differentiated, based on the compositional heterogeneity of its surface and of its dynamical family members. Rotationally resolved near-infrared spectroscopy by Reed et al. (1997) showed evidence that one hemisphere of the asteroid has a composition dominated by Fe-rich olivine, similar to stony-iron meteorites while the other hemisphere appears to be more basaltic and dominated by pyroxene. However, the full extent of its differentiation remains unclear.

We fit a model for Eunomia’s 1.3 mm thermal emission to data acquired with ALMA to determine the thermal inertia and the dielectric constant. The thermal inertia is a relatively good indicator of the surface regolith properties while the dielectric constant is an indicator of surface compositions, in particular surface metal content. The thermophysical model we have used is based on Delbo et al. (2015) and adapted by Cambioni et al. (2022) to take into account the spatially resolved thermal emission data including emission from the subsurface, which is sensed at millimeter wavelengths. The thermal and dielectric properties that are mapped across Eunomia’s surface can provide complementary information to existing knowledge on the surface mineralogy. We also compare the thermal and dielectric properties derived for Eunomia to M-type asteroids (16) Psyche and (22) Kalliope (de Kleer et al., 2021; de Kleer et al., submitted) to understand how regolith properties relate with compositional variations arising from the formation and/or differentiation of the parent planetesimal.