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

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.

Alteration in the Raman spectra of characteristic rock‐forming silicate mixtures due to micrometeorite bombardment

“…As a result of our study and also as it is known from previous investigations, 39,40 a distinct surface darkening can be confirmed for the olivine-and pyroxenecontaining pellets in the craters left in the irradiated areas. In contrast to this, the plagioclase-rich pellets show only laser-induced in parts amorphized impact craters.…”

MoonIndex, an Open-Source Tool to Generate Spectral Indexes for the Moon from M3 Data

“…But the bigger factor is that the lunar spectra show an overall positive and steep slope (Figure 2a), this effect is known as spectral reddening and is the result of space weathering. This alteration is produced by the combined action of solar wind, cosmic radiation, and micrometeoroids bombardment; which produces nanophase iron particles, responsible for the increased spectral slope, a reduction of the reflectance, and the weakening of some absorption bands (Hapke, 2001;Xu et al, 2023). Spectral reddening hinders the absorption band analysis; therefore, it is necessary to remove the spectral slope.…”

MoonIndex, an Open‐Source Tool to Generate Spectral Indexes for the Moon From M3 Data

“…This alteration is produced by the combined action of solar wind, cosmic radiation, and micrometeoroid bombardment. This produces nanophase iron particles, responsible for the increased spectral slope, a reduction of the reflectance, and the weakening of some absorption bands (Hapke, 2001; Xu et al., 2023). Spectral reddening hinders the absorption band analysis; therefore, it is necessary to remove the spectral slope.…”