Reflectance-spectroscopic and polarization measurements of meteorite mixtures relevant to E- and M-type asteroids

¹ School of Earth Sciences and Hubei Key Laboratory of Planetary Geology and Space Explorations, China University of Geosciences, Wuhan, China
² Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, China
³ Department of Astronomy and Space Informatics, V. N. Karazin Kharkiv National University, Kharkiv, Ukraine
⁴ State Key Laboratory of Lunar and Planetary Science, Macau University of Science and Technology, Macau
⁵ Department of Physics, University of Central Florida, Orlando, FL, USA
⁶ Purple Mount Observatory, Chinese Academy of Sciences, Nanjing, China

^★ Corresponding authors: zhanghao@cug.edu.cn; zhangpengfei@mail.gyig.ac.cn; liyang@mail.gyig.ac.cn

Received: 21 May 2025
Accepted: 18 September 2025

Abstract

Context. Metal-rich asteroids are of significant interest due to their spectral diversity and their potential to shed light on early planetary differentiation and core formation processes. E- and M-type asteroids are generally thought to consist of iron-poor silicates (e.g., enstatite) and metallic components, yet their precise composition and surface morphology remain poorly constrained. The effects of space weathering on these bodies are also not well understood.

Aims. This study aims to experimentally examine how variations in metal content and simulated micrometeorite bombardment influence the spectral, photometric, and polarimetric properties of silicate-metal mixtures.

Methods. Mixtures of enstatite-rich aubrite and iron meteorite powders were prepared in varying proportions. Pulsed laser irradiation was used to simulate micrometeorite impacts. Scanning electron microscopy (SEM), reflectance spectroscopy, bi-directional reflectance, and linear polarimetry were employed to characterize the samples before and after irradiation.

Results. Increasing metal content enhanced the near-infrared spectral slope and suppressed the 3 μm absorption band while having minimal impact on mid-infrared silicate features. Reflectance increased in the principal plane but decreased outside of it. Polarimetric measurements showed that both the minimum and maximum degrees of polarization (P_min and P_max) were strongly dependent on metal content. Laser irradiation further increased the visible-to-near-infrared spectral slope and altered the scattering behavior, enhancing backscattering and reducing forward scattering, particularly in metal-rich samples. Additionally, irradiation reduced polarization at small phase angles, producing a “squeezing effect” on the polarization curves and confining the results to the E-type asteroid domain in P_min–α_inv space.