Optical spectropolarimetry of large C-complex asteroids: polarimetric evidence for heterogeneous surface compositions

TL;DR

This study uses optical spectropolarimetry to analyze large C-complex asteroids, revealing surface heterogeneity and linking polarimetric properties to surface composition and aqueous alteration processes.

Contribution

First spectropolarimetric analysis of large C-complex asteroids, establishing correlations between polarization spectra and surface mineralogy.

Findings

Polarization spectra differ between phyllosilicate-rich and poor asteroids.

Negative polarization branch increases with asteroid subclass type.

Surface composition heterogeneity is influenced by aqueous alteration.

Abstract

This study presents the first optical spectropolarimetric study of large C-complex asteroids. A total of 64 C-complex asteroids of different subclasses are analyzed using archival polarimetric and reflectance data to refine the link between polarimetric parameters and surface properties of the asteroids. We find a consistent difference in the polarization spectra between asteroids containing phyllosilicates and those without, which correlates with the overall morphology of the reflectance spectrum. They exhibit broad similarities in polarization-phase curves; nonetheless, we observe a gradual enhancement of the negative polarization branch in the ascending order of F-B-T-Ch types, along with an increase in reflectance curvature around 500 nm. Our observations suggest at least for large C-complex asteroids a common mechanism underlies the diversity in optical properties. The observed trends would be explained by the surface composition of the asteroids, particularly optical heterogeneity caused by carbon's varying levels of optical influence, primarily regulated by aqueous alteration of the surfaces.