Thermal and Dielectric Properties of Juno's Regolith at One Millimeter Wavelength

We present the modeling results of the thermal lightcurve of asteroid (3) Juno at the wavelength of $λ$ = 1.3 mm measured by the Atacama Large Millimeter-submillimeter Array. A thermophysical model together with a radiative transfer model suggest a thermal inertia of 13$\pm$10 [J m$^{-2}$ K$^{-1}$ s$^{-0.5}$], an equivalent emissivity of 0.8$\pm$0.1, a loss tangent of 0.4$\pm$0.3, and an index of refraction 1.8$\pm$0.3. Based on previous laboratory measurements, the modeled index of refraction suggests a regolith porosity of about 45%. However, thermal inertia models using the material parameters of ordinary chondrite indicate a grain size of 10s $μ$m and require a high porosity of $\sim$90% to explain the low thermal inertia. In order to explain such a contradiction, we postulate that some repulsive mechanism might be in effect to reduce the contact of grains and therefore the thermal inertia. The loss tangent of Juno’s regolith corrected for the modeled thermal skin depth is in the order of 0.5, much higher than that of the lunar regolith and indicating an electrical skin depth of L = 0.1 - 1.4 mm that is within the thermal skin depth. The shape of the rotational lightcurve of Juno in the mm wavelengths is dominated by its irregular shape, but rotational variations in the thermal and/or dielectric properties cannot be ruled out. Our results demonstrate that mm-wavelength observations of asteroids provide an extra dimension of constraints to the porosity and grain size of asteroid regolith compared to the thermal infrared observations.

💡 Research Summary

This paper presents a comprehensive analysis of the thermal lightcurve of asteroid (3) Juno at a wavelength of 1.3 mm, using data obtained with the Atacama Large Millimeter‑submillimeter Array (ALMA) during its first long‑baseline campaign in October 2014. Ten disk‑integrated flux measurements covering ~60 % of Juno’s 7.2‑hour rotation were modeled with a coupled thermophysical‑radiative‑transfer framework. The thermophysical component employs the KRC code to compute surface and subsurface temperatures based on a shape model (triangular‑plate representation from DAMIT) and assumes uniform albedo (A = 0.09), bulk density (ρ = 1300 kg m⁻³), and specific heat (c = 630 J kg⁻¹ K⁻¹). The radiative‑transfer module incorporates the complex dielectric constant of the regolith, allowing the extraction of the index of refraction (n), loss tangent (tan Δ), and consequently the electric skin depth (L).

A grid search over thermal inertia (Γ = 5–640 tiu), equivalent emissivity (ε_eq = 0.7–1.0), loss tangent (10⁻⁴–1.0), and index of refraction (1.001–2.5) produced 19 statistically indistinguishable solutions (Δχ² ≤ 1). The weighted averages of these solutions are:

• Thermal inertia Γ = 13 ± 10 J m⁻² K⁻¹ s⁻⁰·⁵ (tiu),
• Equivalent emissivity ε_eq = 0.8 ± 0.1,
• Index of refraction n = 1.8 ± 0.3,
• Loss tangent tan Δ = 0.4 ± 0.3.

Using the Lichtenecker mixing rule, the derived n translates to a bulk regolith density of ~1.8 g cm⁻³, implying a porosity of ~45 % when compared with the grain density of ordinary chondrite material (3.3 g cm⁻³). The loss tangent yields an electric skin depth L ≈ 0.1–1.4 mm, which overlaps the thermal skin depth (δ ≈ 0.4–3.5 mm) for the diurnal wave period. Thus, the 1 mm observations probe temperatures a few millimetres beneath the surface, providing a unique window into subsurface thermophysical and dielectric properties.

The modeled lightcurve shows that the amplitude (~10 % of the mean flux) is dominated by Juno’s irregular shape; variations in thermal inertia or dielectric parameters affect the absolute flux level and the longitudinal position of extrema by only a few millijanskys. Nonetheless, thermal inertia influences the phase of the lightcurve, shifting extrema by up to ~30° in longitude, indicating that rotationally resolved observations could further constrain surface heterogeneities.

A key tension emerges between the low thermal inertia (≈ 13 tiu) and the moderate porosity inferred from the index of refraction. Thermal‑inertia theory, combined with ordinary‑chondrite material properties, would require a porosity near 90 % (or extremely fine grains of tens of micrometres) to reproduce such a low Γ. The authors propose that a repulsive mechanism—potentially electrostatic or van der Waals forces—reduces grain‑to‑grain contact, thereby suppressing effective thermal conductivity without necessitating extreme porosity. This hypothesis challenges the conventional view that grain contact dominates heat transport in asteroid regolith.

The paper also situates its findings within the broader context of mm‑wave asteroid studies. Previous ALMA observations of large asteroids (e.g., Ceres, Vesta, Lutetia) have shown that mm‑wavelength data can reveal subsurface dielectric constants and thermal skin depths, complementing infrared thermal‑inertia measurements. By jointly fitting thermal and dielectric parameters, the authors demonstrate that mm‑wave observations add an “extra dimension” to regolith characterization, enabling simultaneous constraints on grain size, porosity, and electrical properties.

In summary, the study delivers the first detailed mm‑wavelength thermophysical‑dielectric model of Juno, finding a low thermal inertia, moderate porosity (~45 %), a relatively high loss tangent, and an electric skin depth comparable to the thermal skin depth. The apparent discrepancy between thermal‑inertia‑derived and dielectric‑derived porosities suggests that non‑contact forces may play a significant role in regulating heat transport on asteroid surfaces. These results highlight the diagnostic power of millimetre observations for probing the shallow subsurface of small bodies and provide valuable inputs for future mission planning, surface‑process modeling, and laboratory investigations of regolith physics.