2:30 PM - 2:45 PM
[PPS03-14] Thermophysical model of main-belt asteroids (15) Eunomia, (16) Psyche, (22) Kalliope from spatially resolved ALMA data
Keywords:ALMA, M-type asteroid, S-type asteroid, thermophysical modeling
Thermally emitted fluxes, which provide information about the regolith properties and composition, can be detected from thermal radio wavelengths but are most often studied in the thermal infrared wavelengths [e.g., 1]. However, we cannot obtain spatially resolved observations of main-belt asteroids at thermal infrared wavelengths from ground-based or spaced-based telescopes existing today. In contrast, radio interferometry from the Atacama Large Millimeter/submillimeter Array (ALMA) can spatially resolve the largest main-belt asteroids at millimeter wavelengths. This allows us to study the spatial variations in lithology across their surfaces. Comparing the spatial variations in thermal and dielectric properties of stony and metal asteroids can provide insights into how compositionally uniform the surfaces of metal-rich and silicate-rich asteroids are to understand planetesimal differentiation and collisions that expose core/mantle material on the surface.
In this work, we have obtained 1.3 mm thermal emission data from ALMA for three asteroids at ~30 km resolution (asteroid diameters are 150-250 km): two M-type asteroids (16) Psyche [2, 3] and (22) Kalliope [4], and S-type asteroid (15) Eunomia [5]. Kalliope is thought to be metal-rich from its high density and lack of spectral feature though its low radar albedo suggests otherwise [6]. Eunomia is thought to be partially differentiated based on its hemispherical compositional heterogeneity [7]. We fit a thermophysical model to the ALMA thermal emission data to determine the thermal inertia and the dielectric constant; the latter is an indicator of surface composition, in particular surface metal content, on asteroids. In general, low thermal inertia suggests regolith, silicate-rich or porous materials while low dielectric constant suggests low metal content. The thermophysical model is based on [8] and adapted to take into account the spatially resolved thermal emission data including emission from the subsurface, which is sensed at these wavelengths [2, 3].
Published results for Psyche [2, 3] and preliminary results for Kalliope [4] and Eunomia [5] reveal spatial variations in both thermal inertia and dielectric constant across their surfaces. The best-fit thermal inertia and dielectric constant of Psyche are 280 ± 100 J m−2 K−1 s−1/2 (hereafter, units are omitted) and 19 ± 2, respectively [2]. A large depression has lower thermal inertia than its surroundings, which could be due to the presence of fine-grained material, impact-induced fractures or higher abundance of silicate materials from impacts [3]. The best-fit thermal inertia and dielectric constant of Kalliope are 210+246-100 and 17 ± 1. At the northern latitudes in the western hemisphere, there is a region of high dielectric constant, suggesting a localized region of high metal content [4]. The best-fit thermal inertia and dielectric constant of Eunomia are 116 ± 42 and 8 ± 1, respectively [5]. The range of dielectric constant is smaller for Eunomia compared to Psyche and Kalliope, so even the most potentially metal-rich portions of Eunomia’s surface are not as metallic as an M-type. These trends are consistent with the expected compositional taxonomy of S-type compared to M-type asteroids, though the dielectric constant of Eunomia here is higher than that of Vesta (~2 [9]). However, the spatial variations of the dielectric constants across the surface on all three asteroids suggest that asteroid surfaces are a mix of metal-rich and silicate-rich regions and are more compositionally complex than suggested by taxonomical classification.
References: [1] Lovell, AJ (2008) Astrophys. Space Sci. 313. [2] de Kleer, K et al. (2021) Planet. Sci. J. 2. [3] Cambioni, S et al. (2022) JGR Planets 127. [4] de Kleer, K et al. in prep. [5] Phua, Y et al. in prep. [6] Magri, C et al. (2007) Icarus 186. [7] Nathues A et al. (2005) Icarus 175. [8] Delbo, M et al. (2015) Asteroids IV 107–128. [9] Palmer, EM et al. (2015) Icarus 262.
In this work, we have obtained 1.3 mm thermal emission data from ALMA for three asteroids at ~30 km resolution (asteroid diameters are 150-250 km): two M-type asteroids (16) Psyche [2, 3] and (22) Kalliope [4], and S-type asteroid (15) Eunomia [5]. Kalliope is thought to be metal-rich from its high density and lack of spectral feature though its low radar albedo suggests otherwise [6]. Eunomia is thought to be partially differentiated based on its hemispherical compositional heterogeneity [7]. We fit a thermophysical model to the ALMA thermal emission data to determine the thermal inertia and the dielectric constant; the latter is an indicator of surface composition, in particular surface metal content, on asteroids. In general, low thermal inertia suggests regolith, silicate-rich or porous materials while low dielectric constant suggests low metal content. The thermophysical model is based on [8] and adapted to take into account the spatially resolved thermal emission data including emission from the subsurface, which is sensed at these wavelengths [2, 3].
Published results for Psyche [2, 3] and preliminary results for Kalliope [4] and Eunomia [5] reveal spatial variations in both thermal inertia and dielectric constant across their surfaces. The best-fit thermal inertia and dielectric constant of Psyche are 280 ± 100 J m−2 K−1 s−1/2 (hereafter, units are omitted) and 19 ± 2, respectively [2]. A large depression has lower thermal inertia than its surroundings, which could be due to the presence of fine-grained material, impact-induced fractures or higher abundance of silicate materials from impacts [3]. The best-fit thermal inertia and dielectric constant of Kalliope are 210+246-100 and 17 ± 1. At the northern latitudes in the western hemisphere, there is a region of high dielectric constant, suggesting a localized region of high metal content [4]. The best-fit thermal inertia and dielectric constant of Eunomia are 116 ± 42 and 8 ± 1, respectively [5]. The range of dielectric constant is smaller for Eunomia compared to Psyche and Kalliope, so even the most potentially metal-rich portions of Eunomia’s surface are not as metallic as an M-type. These trends are consistent with the expected compositional taxonomy of S-type compared to M-type asteroids, though the dielectric constant of Eunomia here is higher than that of Vesta (~2 [9]). However, the spatial variations of the dielectric constants across the surface on all three asteroids suggest that asteroid surfaces are a mix of metal-rich and silicate-rich regions and are more compositionally complex than suggested by taxonomical classification.
References: [1] Lovell, AJ (2008) Astrophys. Space Sci. 313. [2] de Kleer, K et al. (2021) Planet. Sci. J. 2. [3] Cambioni, S et al. (2022) JGR Planets 127. [4] de Kleer, K et al. in prep. [5] Phua, Y et al. in prep. [6] Magri, C et al. (2007) Icarus 186. [7] Nathues A et al. (2005) Icarus 175. [8] Delbo, M et al. (2015) Asteroids IV 107–128. [9] Palmer, EM et al. (2015) Icarus 262.