Open Access
Issue
A&A
Volume 690, October 2024
Article Number A138
Number of page(s) 6
Section Planets and planetary systems
DOI https://doi.org/10.1051/0004-6361/202450888
Published online 04 October 2024

© The Authors 2024

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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1 Introduction

Space weathering refers to the physical and chemical alterations that occur on the surface of airless celestial bodies exposed to solar wind and cosmic rays, as well as the impact of micrometeoroids (Hapke 2001; Pieters & Noble 2016). This process leads to the formation of nanophase iron and other opaque nanophase particles on the surface of regolith, resulting in a reddening and darkening of the surface spectrum with weakened absorption features. This makes it challenging to identify and interpret the acquired spectral data (Chrbolkova et al. 2022; Hapke 2001; Pieters & Noble 2016). Due to the significant spectral alteration effects of space weathering, olivine, as one of the main rock-forming minerals of silicate bodies, has drawn a great deal of attention as the fastest and most broadly responding mineral to space weathering (Pieters & Noble 2016; Sasaki et al. 2002; Yamada et al. 1999).

Spectral analysis, laboratory simulation tests, meteorite and returned sample examination, and numerical simulations have been the main areas of research on the space weathering of olivine. Laboratory studies of olivine space weathering include experimental simulations of solar wind implantation and micrometeoroid bombardment, which are able to identify changes in the microstructure and chemical characteristics of space-weathered samples. Ion irradiation, used to simulate solar wind implantation, results in olivine forming vesicles, partially amorphizes and forms a Si-enriched and Mg-depleted layer on the surface, accompanied by reddening and brightening of its spectral features (Laczniak et al. 2021; Carrez et al. 2002; Demyk et al. 2001). Laser irradiation, used to simulate micrometeorite impacts, creates a dual-layer structure on the olivine surface, consisting of a narrow melt deposition layer and a superimposed vapor deposition layer. The underlying layer is enriched in refractory components, while the superimposed layer is a fluffy, volatile-rich deposit. Within this dual-layer structure, np-Fe0 particles are found, with smaller particles in the upper layer and larger particles in the lower layer (Thompson et al. 2019). The reflectance spectra of olivine subjected to pulsed-laser irradiation exhibit reddening, darkening, and attenuated absorption bands (Thompson et al. 2019, 2020; Loeffler et al. 2016). Other studies include atomic-level mechanisms for the increased susceptibility of olivine to space weathering (Quadery et al. 2015), comparisons among the effects of solar wind injection and micrometeorite on the spectra of olivine (Chrbolkova et al. 2021; Yamada et al. 1999), and comparisons among the influence of the sizes and amounts of np-Fe0 particles on the reflectance spectra of olivine (Kohout et al. 2014; Markley & Kletetschka 2016). These studies have focused on forsterite; although modified Gaussian model (MGM) deconvolution has been applied theoretically to olivine with different iron content levels (Han et al. 2020; Pinet et al. 2022); however, the response of fayalite to space weathering in terms of optical properties remains unclear. Recent studies of Chang’e 5 return samples and formation simulations of Martian moons have revealed that the surfaces of these airless bodies may contain more iron-rich olivine than previously estimated (Li et al. 2022; Citron et al. 2015; Canup & Salmon 2018). Iron-rich olivine has also been detected in asteroids and comets (Sunshine et al. 2007; Dobrica et al. 2022; Frank et al. 2014). Therefore, it is essential to investigate the effects of space weathering on the spectra of iron-rich olivine. To ascertain the impact of variations in space weathering levels and iron content on the spectral properties of olivine, our team irradiated olivine featuring a range of iron content levels (Fa30, Fa50, Fa70, and Fa100) at five energy levels (5 mJ energy× 1 time, 5 mJ energy× 5 times, 5 mJ energy × 15 times, 15 mJ energy× 5 times, and 15 mJ energy × 10 times) and obtained near-infrared (NIR) reflectance spectra; however, the variation among the spectral parameters of NIR spectroscopy has not been studied in depth (Xu et al. 2023). The main goal of this article is to carry out a detailed spectroscopic study using this sample set.

2 Materials and methods

2.1 Sample synthesis and laser irradiation experiments

Four iron-rich olivine materials (Fa30, Fa50, Fa70 and Fa100) were used in this study. The fayalite (Fa100) end-member was synthesized in the furnace using powders of Fe2O3 and SiO2. The samples of Fa71, Fa50, and Fa29 were then fabricated by mixing the synthetic fayalite with commercial forsterite. To serve as targets for laser irradiation, the synthesized olivine samples were crushed and formed into pellets and placed in the vacuum chamber. For detailed sample preparation and irradiation experimental procedures, we refer to Xu et al. (2023). Compositions of synthetic Fe-rich olivine used in this study is shown in Table A.1. We used computational simulations of the radiant energy on the surface of the Moon and Phobos, and considering the size of the laser spot (0.5 mm diameter), we set up two energy levels, 15 mJ × 5 (M75 mJ) and 15 mJ × 10 (M150 mJ), to represent the space weathering environments of the Moon. Three energy levels, 5 mJ × 1 (P5 mJ), 5 mJ × 5 (P25 mJ), and 5 mJ × 15 (P75 mJ), were determined to represent the space weathering environments of Phobos (Divine 1993; Xu et al. 2023). Furthermore, P75 mJ (5 mJ × 15) and M75 mJ (15 mJ × 5) were used to compare the effects of the single-shot energy level when the total energy level remained the same (Xu et al. 2023). The laser pulse irradiation simulation experiments were carried out at the Institute of Geochemistry, Chinese Academy of Sciences. We refer to Xu et al. (2023) for a detailed description of the nanosecond-pulsed laser irradiation experiments on the Fe-rich olivine samples used in this study.

2.2 Near-infrared spectral acquisition and processing

Xu et al. (2023) obtained a single NIR reflectance spectrum of the laser spot, however, the spatial resolution was too low to identify any finer details. Thus, we conducted measurements of the imaging spectra of these samples to enhance our understanding. The spectra were collected at a spectral resolution of 8 cm−1 over the 1–2.5 µm range with 3112 bands. The spatial resolution is 40 µm/pixel, which is sufficient to identify the locations of weathered samples. As illustrated in Figure 1, we averaged over a 6 × 6 pixel region surrounding each impact pit center to extract the spectra of weathered olivine and pristine olivine from the surrounding unimpacted portions. First, we smoothed these olivine spectra using the Savitzky–Golay algorithm (Savitzky & Golay 1964), with a window of 115 points and a polynomial order of 6. In this study, we employed a segmented approach for envelope removal, using 1.2 µm as the segmentation point to separately remove the envelope for the front and rear segments of the spectrum. To extract the spectral parameters at 1.08 µm and 1.35 µm, we fit a sixth-degree polynomial (Fig. A.1) to the obtained continuum-removed spectra (Yang et al. 2020). For the sixth-degree polynomial fitting, three distinct spectral ranges were chosen, the average values of the band centers and depths were taken as the final results, and their standard deviations represented the errors. To calculate the spectral slope for spectral reddening, we divided the reflectance at 2.4 µm by the reflectance at 1.9 µm, as these wavelengths do not typically exhibit absorption features for olivines.

thumbnail Fig. 1

Grayscale image of radiation crater on olivine at 1500 nm. Impact pits are represented by black areas, unirradiated areas by white patches, and the locations from which we retrieved spectra are shown by boxes.

thumbnail Fig. 2

Unweathered olivine spectra. (a) Spectra of the samples obtained in this study. (b) Spectra from the RELAB spectral library. (c) The band centers and (e) band depths at 1.08 µm. (d) The band centers and (f) band depths at 1.35 µm. The spectral IDs for these spectra are DD-MDD-039 (Fa30), DD-MDD-041 (Fa50), DD-MDD-043 (Fa70), and DD-MDD-046 (Fa100). The particle size of the RELAB sample is 0–45 µm.

3 Results

3.1 Initial Fe-rich olivine

When the Fe concentration of the unirradiated olivine increased, the reflectance decreased, and both of the absorption peaks at 1 µm shifted toward longer wavelengths (Fig. 2a). The depth of absorption at 1.08 µm decreased with an increasing iron content, whereas at 1.35 µm, it increased (Fig. 2). All the spectral parameters obtained in our unweathered olivine samples were in a high agreement to those from the RELAB spectral library with the same iron content (Fig. 2), confirming the validation of our dataset.

3.2 Space-weathered olivine under Phobos-simulating conditions

Three sets of irradiation energy levels, 5 mJ × 1 (P5 mJ), 5 mJ × 5 (P25 mJ), and 5 mJ × 15 (P75 mJ), were used to simulate the space weathering environment on Phobos. The spectral reflectance of irradiated olivine shows the same decreasing trend with increasing iron content as that of unirradiated olivine (Fig. A.2). The absorption centers at 1.08 µm of iron-rich samples shifted to longer wavelengths with increased irradiation, while the absorption center at 1.35 µm remained relatively stable and did not exhibit significant changes after irradiation. The absorption depths at 1.08 µm and 1.35 µm both decreased (Figs. 3 and A.2), and the slope of the spectrum gradually increased with increasing irradiation energy (Fig. 4). More importantly, the spectral modification effect of high radiant energy is more pronounced with increasing Fe content. The depth of absorption near 1.35 µm increases gradually with increasing iron content, but once the iron content surpasses a certain threshold (Fa# > 70), the absorption depth starts to decrease instead of continuing to increase. The decrease in the absorption depth of the Fa100 sample may be due to the presence of a thick amorphous silica layer on the surface of the pure fayalite after irradiation (Xu et al. 2023).

thumbnail Fig. 3

Band center and band depth under Phobos-like conditions. (a) The band centers and (c) band depths at 1.08 µm. (b) The band centers and (d) band depths at 1.35 µm.

thumbnail Fig. 4

Changes in spectral slope with increasing irradiation energy. The reflectance at 2.4 µm is divided by the reflectance at 1.9 µm to calculate the spectral slope for spectral reddening, the box indicates a comparison between two samples with the same total energy but differing impact frequencies.

3.3 Space-weathered olivine under lunar-simulating conditions

Two sets of irradiation energy levels, 15 mJ × 5 (M75 mJ) and 15 mJ × 10 (M150 mJ), were examined to simulate the space weathering environment on Moon. Although the spectral characteristics of pure fayalite (Fa100) exhibits anomalous brightening (Fig. A.2e,f), the spectral characteristics of iron-rich olivine obtained in the simulated lunar environment are similar to those under Phobos conditions (Fig. 5). Consistent with the findings of Xu et al. (2023), we discovered that the spectral absorption center and absorption depth variations of the Fa50 samples in lunar space weathering settings diverge from the trend of the irradiated samples. To determine whether this aberration is unique to the Fa50 sample, more samples with smaller gradients will be needed.

When we analyzed the reflectance and slope of two control groups with the same total energy, we found that the spectral slopes of all the samples show the same decreasing trend in the transition from P5 mJ×15 to M15 mJ×5 (Fig. 4) and the spectral reflectance of the sample becomes lower (Fig. 6). This indicates that the influence of lower energy and more irradiation times (M15mJ×5) on the spectral change was more noticeable than the impact of greater energy and fewer irradiation times.

thumbnail Fig. 5

Band center and band depth under lunar-like conditions. (a) The band centers and (c) band depths at 1.08 µm. (b) The band centers and (d) band depths at 1.35 µm.

thumbnail Fig. 6

Spectral curves of P75 mJ and M75 mJ.

4 Discussion

We conducted nanosecond pulsed laser irradiation experiments on a set of synthetic Fe-rich olivine samples (Fa29, Fa50, Fa71, and Fa100) to simulate space weathering and analyze the spectral changes before and after irradiation. Our findings revealed that the absorption centers of Fe-rich samples shifted to longer wavelengths with increased irradiation energy, while the absorption depths decreased, and the spectral slope gradually increased. In comparing the spectral slopes and reflectances of the P5 mJ × 15 and M15 mJ × 5 samples, we found that spectral changes are more noticeable at low energy and high impact frequency. We observed anomalous changes in the spectral absorption center and absorption depth of Fa50 samples in the lunar space weathering environment. We attributed this phenomenon to the combined effects of the laser ablation layer, np-Fe0 particles, and local redistribution of elements. Xu et al. (2023) previously observed the microstructure and chemical features of these bombarded sample, finding that under P75 mJ (5 mJ × 15) and M75 mJ (15 mJ × 5) conditions, higher single-shot energy consistently produces thicker laser alteration layers and larger np-Fe0 particles in all olivine samples, with Fa50 produces thinner laser alteration layers than Fa29 and Fa71. Additionally, these authors observed that the formation of np-Fe0 particles is accompanied by a local redistribution of elements; Fe ends up concentrated in the np-Fe0 particles and migrates into the amorphous layer, leaving mg-rich features in the subsolid layer.

By comparing the absorption centers at 1.35 µm of olivine in the Phobos environment with those in the lunar environment, we find that the absorption center near 1.35 µm remains stable, serving as a reliable indicator for determining the Fo# values of olivine on space-weathered bodies from remote sensing observations. Previous studies have identified numerous exposures of olivine on the Moon, including a small crater northwest of the rim of the Moscoviense basin (Yamamoto et al. 2010; Isaacson et al. 2011), the Copernicus central peak (Pieters 1982; Pinet et al. 1993; Yamamoto et al. 2010; Isaacson et al. 2011), Aristarchus (Lucey et al. 1986; Le Mouélic et al. 1999; Chevrel et al. 2009; Isaacson et al. 2011), and Marius crater in the eastern Marius Hills (Besse et al. 2011; Isaacson et al. 2011). Fe-rich olivine is expected to be abundant in Phobos (Koeppen & Hamilton 2008; Citron et al. 2015; Canup & Salmon 2018), and future missions are expected to find similar exposures of pure olivine on its surface. Additionally, asteroid 2016 HO3, which is set to be explored by the Tianwen-2 mission, is believed to have possibly originated from the Earth-Moon system (Sharkey et al. 2021). It may have been ejected from the lunar surface as impact debris (Gladman et al. 1995) or could be a fragment resulting from a near-Earth object’s tidal or rotational breakup during a close encounter with the Earth-Moon system (Holsapple & Michel 2008). This asteroid might also exhibit olivine exposures similar to those found on the Moon. Our results provide a reference for determining the Fo# of these olivines and understanding the spectral changes of olivine in such airless celestial bodies.

5 Conclusions

In this study, we analyzed the spectra of a set of synthetic iron-rich olivine samples (Fa30, Fa50, Fa70, and Fa100) after irradiation with nanosecond pulsed lasers. Our findings are summarized below:

  1. When receiving the same amount of radiation energy, changes in the spectra are more noticeable with low single-shot energy and high irradiation times;

  2. Following irradiation, the absorption depth at 1.08 µm weakens and the absorption center shifts toward longer wavelengths. The spectral modification impact of high radiant energy becomes more noticeable as the iron concentration increases;

  3. The band center at 1.35 µm of olivine is steady after irradiation. This can be a good indicator for determining Mg# of olvines from remote sensing observations under weathering conditions.

Understanding the spectral response of iron-rich olivine to space weathering processes is critical for interpreting the data in the exploration of airless bodies that may contain such materials, such as the Moon, Phobos and Deimos, and asteroids. In addition, studying the magmatic evolution of the Moon also benefits from the ability to invert olivine magnesium values based on the spectra of olivine. In the next decades, many missions such as Chang’e-7 and Lunar Trailblazer etc. for the Moon and Martian Moons eXploration (MMX) for Phobos are planned, our findings provide a reference for their spectral analysis in the near future.

Acknowledgements

This work was supported by the National Natural Science Foundation of China(42241106),the Youth Innovation Promotion Association of the Chinese Academy of Sciences (Grant No. 2023071), the Young Elite Scientists Sponsorship Program by CAST (Grant No. 2021QNRC001), and the Key Research Program of the Institute of Geology and Geophysics, Chinese Academy of Sciences (Grant No. IGGCAS-201905). The authors declare no conflicts of interest.

Appendix A Supplementary information

Table A.1

Elemental abundance of synthetic olivine data from Xu et al. (2023)

thumbnail Fig. A.1

Olivine reflectance spectra and processing methods.(a) Final obtained olivine spectra(b) Smoothed spectrum of Fa70 P5×1mJ (c) continuum-removed spectrum of Fa70 P5×1mJ near 1.05 µm(d) continuum-removed spectrum of Fa70 P5×1mJ near 1.35 µm. The band center is defined as the wavelength with the minimum reflectance/continuum value calculated using the best-fitting polynomial function, and the band depth is defined as 1-Rminimum, where Rminimum is the reflectance/continuum value at the band center.

thumbnail Fig. A.2

Spectral curves of olivine with different iron content levels at the same energy level.

thumbnail Fig. A.3

Spectral curves of olivine with different energy levels with the same iron content.

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All Tables

Table A.1

Elemental abundance of synthetic olivine data from Xu et al. (2023)

All Figures

thumbnail Fig. 1

Grayscale image of radiation crater on olivine at 1500 nm. Impact pits are represented by black areas, unirradiated areas by white patches, and the locations from which we retrieved spectra are shown by boxes.

In the text
thumbnail Fig. 2

Unweathered olivine spectra. (a) Spectra of the samples obtained in this study. (b) Spectra from the RELAB spectral library. (c) The band centers and (e) band depths at 1.08 µm. (d) The band centers and (f) band depths at 1.35 µm. The spectral IDs for these spectra are DD-MDD-039 (Fa30), DD-MDD-041 (Fa50), DD-MDD-043 (Fa70), and DD-MDD-046 (Fa100). The particle size of the RELAB sample is 0–45 µm.

In the text
thumbnail Fig. 3

Band center and band depth under Phobos-like conditions. (a) The band centers and (c) band depths at 1.08 µm. (b) The band centers and (d) band depths at 1.35 µm.

In the text
thumbnail Fig. 4

Changes in spectral slope with increasing irradiation energy. The reflectance at 2.4 µm is divided by the reflectance at 1.9 µm to calculate the spectral slope for spectral reddening, the box indicates a comparison between two samples with the same total energy but differing impact frequencies.

In the text
thumbnail Fig. 5

Band center and band depth under lunar-like conditions. (a) The band centers and (c) band depths at 1.08 µm. (b) The band centers and (d) band depths at 1.35 µm.

In the text
thumbnail Fig. 6

Spectral curves of P75 mJ and M75 mJ.

In the text
thumbnail Fig. A.1

Olivine reflectance spectra and processing methods.(a) Final obtained olivine spectra(b) Smoothed spectrum of Fa70 P5×1mJ (c) continuum-removed spectrum of Fa70 P5×1mJ near 1.05 µm(d) continuum-removed spectrum of Fa70 P5×1mJ near 1.35 µm. The band center is defined as the wavelength with the minimum reflectance/continuum value calculated using the best-fitting polynomial function, and the band depth is defined as 1-Rminimum, where Rminimum is the reflectance/continuum value at the band center.

In the text
thumbnail Fig. A.2

Spectral curves of olivine with different iron content levels at the same energy level.

In the text
thumbnail Fig. A.3

Spectral curves of olivine with different energy levels with the same iron content.

In the text

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