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

Abstract

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 ∼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.

Detection of Radio Thermal Emission from the Kuiper Belt Object (486958) Arrokoth during the New Horizons Encounter

“…The thermal inertia for Ganymede at λ = 1.3 mm was found to lie in the range 200 J m −2 K −1 s −1/2 < I therm < 700 J m −2 K −1 s −1/2 (de Kleer et al 2021a). At the same wavelength the best fits to the diurnal temperature variations at Psyche were obtained for a smooth surface and I therm = 280 ± 100 J m −2 K −1 s −1/2 (de Kleer et al 2021b). Recent direct measurements of the heat capacity and thermal conductivity were utilized to derive the thermal inertia of five CM carbonaceous chondrite meteorites (Opeil et al 2020) over a wide range of temperatures 5-300 K. All five samples yielded values I therm ≈ 100 J m −2 K −1 s −1/2 at T ≈ 40 K. The thermal inertia was found to be very sensitive to temperature, increasing monotonically by an order of magnitude at T = 300 K.…”

“…A value of I therm = 295 ± 18 J m −2 K −1 s −1/2 was determined for a single boulder on the surface of the asteroid (Hamm et al 2020). Recent ALMA observations have yielded values of thermal inertia for Jupiterʼs moon Ganymede (de Kleer et al 2021a) and the asteroid (16) Psyche (de Kleer et al 2021b). The thermal inertia for Ganymede at λ = 1.3 mm was found to lie in the range 200 J m −2 K −1 s −1/2 < I therm < 700 J m −2 K −1 s −1/2 (de Kleer et al 2021a).…”

Porosity Evolution in Metallic Asteroids: Implications for the Origin and Thermal History of Asteroid 16 Psyche

“…The bulk composition of Lutetia is still generally thought to be metal‐rich with CB, CH, CR, and enstatite chondrites as potential meteorite analogs of the surface (Coradini et al., 2011). Although Psyche is also thought have a metal‐rich composition, the thermal inertia and radar albedo of Psyche (de Kleer et al., 2021; Shepard et al., 2021) are greater than those of Lutetia (Coradini et al., 2011; Shepard et al., 2010), which could indicate a higher surface metal content for Psyche. If Psyche and Lutetia are observed to have similar structures and compositions, Psyche may therefore have a greater fraction of metal in its metal‐silicate mixture than Lutetia.…”

Asteroid Measurements at Millimeter Wavelengths with the South Pole Telescope

“…In order to expand on this work, astronomers need more high-sensitivity measurements of asteroids at submm to cm wavelengths, where observations exist for only a handful of large asteroids. Recently, the Atacama Large Millimeter/submillimeter Array (ALMA) has carried out dedicated studies of asteroids and other small Solar System bodies, including asteroids (1) Ceres (Li et al 2020), (3) Juno (The ALMA Partnership et al 2015), and ( 16) Psyche (de Kleer et al 2021), as well as Centaurs and trans-Neptunian objects (Lellouch et al 2017). Measurements like these at submm to cm wavelengths serve an important role in asteroid studies because they lie on the ill-understood boundary between two observable regimes-the highly emissive radiation in IR and the supposedly less emissive radiation in cm-and ultimately can improve modeling of surface properties, including thermal inertia and regolith roughness.…”