Space weathering effects and potential spectral alteration on Phobos and the Moon: Clues from the Fe content of olivine

¹ State Key Laboratory of Continental Dynamics and Department of Geology, Northwest University, Xi’an 710069, PR China
e-mail: liushen@nwu.edu.cn
² Center for Lunar and Planetary Sciences, Institute of Geochemistry Chinese Academy of Sciences, Guiyang 550081, PR China
e-mail: zhaoyuyan@mail.gyig.ac.cn
³ State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry Chinese Academy of Sciences, Guiyang 550081, PR China
⁴ College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, PR China
⁵ CAS Center for Excellence in Comparative Planetology, Hefei 230026, PR China
⁶ Analysis and Test Center, Guangdong University of Technology, Guangzhou 510006, PR China
⁷ Research Center for Planetary Science, College of Earth Science, Chengdu University of Technology, Chengdu 610059, PR China
⁸ Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, PR China
⁹ Department of Earth Sciences, University of Hong Kong, Hong Kong 999077, PR China

Received: 13 November 2022
Accepted: 31 January 2023

Abstract

Context. Olivine responds to space weathering in the fastest and most profound way, which results in significant space weathering spectral alteration effects (SWSAEs) on airless silicate bodies. Although Mg-rich olivine (Fa₁₀) has been subjected to extensive studies, SWSAEs of Fe-rich (Fa# > 20) or Fa-dominant (Fa# ⩾ 50) olivine are still poorly understood.

Aims. We aim to systematically characterize the space weathering effects and the associated spectral alterations of Fe-rich olivine on the surface of Phobos and the Moon.

Methods. We conducted nanosecond pulsed laser irradiation experiments on a set of synthetic Fe-rich olivine (Fa₂₉, Fa₅₀, Fa₇₁, and Fa₁₀₀) with energy levels simulated for Phobos and the Moon and analyzed the irradiated olivine for microscopic characteristics and near-infrared (NIR) and Raman spectroscopy.

Results. Micron-level thick alteration layers are found in Fa₁₀₀ compared to those hundreds of nanometers thick in Fa₂₉, Fa₅₀, and Fa₇₁. With increasing irradiation energy levels and Fa# values, nanophase iron (np-Fe⁰) particles increase in size but decrease in quantity. The np-Fe⁰ formed via in situ decomposition are ubiquitously present, while those formed via vapor deposition are primarily found in Fa₂₉ but rarely in Fa# ⩾ 50. The size fraction of intermediate (10–40 nm) and large (40–60 nm) np-Fe⁰ proportionally increases with Fa# values. The NIR spectra of weathered olivine show darkening over reddening in most cases, but Fa100 under the most irradiated condition shows brightening-reddening spectral effects. The Raman spectra of weathered olivine show a reduction in intensity without peak shifts.

Conclusions. The Fa# values of olivine are a more critical factor in controlling the SWSAEs on Phobos than those on the Moon. If Phobos and Deimos contain substantial Fe-rich or Fa-dominant olivine, similar to Mars, thick alteration rims and large np-Fe⁰ formed via space weathering may cause darkening-reddening and potentially brightening-reddening spectral effects on the Martian moons.