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

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 Millimeter/submillimeter Array at a resolution of 30 km over two-thirds of its rotation. The diurnal temperature variations are at the similar to 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, 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.

Automated summary

Asteroid (16) Psyche, known as the largest M-type asteroid, has been theorized to be the core of disrupted planetesimals and parent bodies of iron meteorites. Recent observations suggest that its composition may not be pure metal, but still exhibit anomalous spectral and radar properties. The thermal emission observations indicate a possible surface composition of porous mixture of silicates and metals, with a metal content of at least 20%, and suggest a highly scattering surface due to the absence of polarized emission.