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

TL;DR

This study models Juno's thermal and dielectric properties at 1.3 mm wavelength, revealing insights into its regolith's porosity, grain size, and electrical characteristics through combined thermophysical and radiative transfer modeling.

Contribution

It provides the first detailed modeling of Juno's regolith properties at millimeter wavelengths, highlighting the potential of mm observations for asteroid surface characterization.

Findings

Thermal inertia of 13±10 J m$^{-2}$ K$^{-1}$ s$^{-0.5}$ suggests fine-grained regolith.

Modeled porosity of about 45% from refractive index contrasts.

High loss tangent indicates significant electrical skin depth within the regolith.

Abstract

We present the modeling results of the thermal lightcurve of asteroid (3) Juno at the wavelength of $\lambda$ = 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 $\mu$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.