Dielectric Properties of Asteroid (3) Juno at One Millimeter Wavelength

ALMA observed S-type asteroid (3) Juno at 1.3 mm thermal continuum emission in October 2014 as part of the long-baseline campaign. The observations yielded ten images covering 4.4 hours of the 7.2 hours rotational period of Juno. Here we present our modeling results for the disk-integrated flux of Juno based on those data to constrain the thermal and dielectric properties of the regolith to millimeters depth. The model diurnal temperatures of Juno’s surface and subsurface for the observed season were modeled with the USGS KRC thermal code. The subsurface thermal emission was accounted for with a radiative transfer model characterized by the index of refraction and the dielectric loss tangent. The total flux of Juno is integrated over the visible hemisphere using the triangular plate shape model from the Database of Asteroid Models from Inversion Techniques. Our modeling suggested that the thermal inertia of Juno is about 40 J m^-2 s^-0.5 K^-1, consistent with the previous results derived from the thermal infrared observations. The index of refraction was fitted to be about 1.6. Using the laboratory measurements of the index of refraction of ordinary chondrite meteorites, albeit at centimeter to decimeter wavelengths, we estimated a porosity of 45-65% for the top layer of Juno’s regolith. The best-fit dielectric loss tangent was between 0.1 and 1.0, two orders of magnitude higher than that of lunar material or the values commonly adopted for asteroids with silicate compositions. We noticed that the spectral density distribution of Juno in the sub-millimeter to millimeter wavelengths from the literature shows a high-temperature bump at about one millimeter, possibly indicating a relatively high loss tangent at this wavelength. This behavior is similar to Ceres, but not other S-type asteroids that have been observed. The cause of such a bump is unclear. Our modeling suggests that millimeter to centimeter thermal lightcurve modeling could be used to probe the physical properties of the regolith for irregularly shaped asteroids, including near-Earth asteroids. This work is supported by NASA SSO Grant NNX15AE02G to the Planetary Science Institute and the Solar System Exploration Research Virtual Institute 2016 (SSERVI16) Cooperative Agreement (grant NNH16ZDA001N), SSERVI-TREX to the Planetary Science Institute.