The Surface of (16) Psyche from Thermal Emission and Polarization MappingOPEN ACCESS 

Katherine de Kleer, Saverio Cambioni, Michael Shepard

To be published in Planetary Science Journal


“The asteroid (16) Psyche is the largest of the M-type asteroids, which have been hypothesized to be the cores of disrupted planetesimals and the parent bodies of the iron meteorites. While recent evidence has collected against a pure metal composition for Psyche, its spectrum and radar properties remain anomalous. We observed (16) Psyche in thermal emission with the Atacama Large (sub-)Millimeter Array (ALMA) at a resolution of 30 km over 2/3 of its rotation. The diurnal temperature variations are at the ∼10 K level over most of the surface and are best fit by a smooth surface with a thermal inertia of 280±100 J m−2 K−1 s−1/2. We measure a millimeter emissivity of 0.61±0.02, which we interpret via a model that treats the surface as a porous mixture of silicates and metals, where the latter may take the form of iron sulfides/oxides or alternatively as conducting metallic inclusions. The emissivity indicates a metal content of no less than 20\% and potentially much higher, but the polarized emission that should be present for a surface with ≥20\% metal content is almost completely absent. This requires a highly scattering surface, which may be due to the presence of reflective metallic inclusions. If such is the case, a consequence is that metal-rich asteroids may produce less polarized emission than metal-poor asteroids, exactly the opposite prediction from standard theory, arising from the dominance of scattering over the bulk material properties.”