Illuminating the dark side of the asteroid population: Visible near-infrared (0.7-2.45 μm) surface mineralogy modeling of D-type asteroids using Shkuratov theory

D-type asteroids are a prime example of the many dark, low-albedo asteroids which do not reflect sufficient light to reveal detectable mineral absorptions. While D-type asteroids are relatively rare in the inner solar system and the main asteroid belt, they are dominant among the Jovian Trojans. In this study, we have applied Shkuratov radiative transfer modeling to laboratory spectra of meteorites for which mineral abundances have been measured using X-ray diffraction (XRD) and Rietveld refinement. The general agreement of radiative transfer and XRD estimates of mineral abundances demonstrates the applicability of the radiative transfer approach to featureless, low-albedo spectra. Shkuratov modeling was then applied to new spectral observations of D-type asteroids, along with numerous previously published spectra. The surface mineral abundances of 81 D-type objects, including NASA's Lucy Mission target (21900) Orus, were modeled using assemblages that are plausible based on meteorite analogues. Modeling results reveal D-types are composed of: low-iron olivine; magnesium saponite-dominant phyllosilicates; opaques such as pyrrhotite and tholin; as well as traces of water-ice and other constituents. Subtle compositional differences in model mineralogies exist between Trojan and non-Trojan D-types as well as between L4 and L5 Trojans suggesting differing formational as well as evolutional conditions have affected these bodies.

Automated summary

D-type asteroids are dark, low-albedo objects found predominantly among the Jovian Trojans, challenging to reveal mineral absorption due to their low reflectivity. By applying Shkuratov radiative transfer modeling and XRD measurements, the mineral abundances and compositions of D-type asteroids have been effectively estimated, showing they are mainly composed of low-iron olivine, magnesium saponite-dominant phyllosilicates, pyrrhotite, tholin, as well as traces of water-ice. Subtle compositional differences exist between Trojan and non-Trojan D-types, as well as between L4 and L5 Trojans, indicating different formational and evolutional conditions have influenced these bodies.