Simulation of Space Weathering on Asteroid Spectra through Hydrogen Ion Irradiation of Meteorites

For decades, remote sensing has been carried out to identify the physical characteristics and chemical composition of planetary bodies (Fernandez et al. 2015). These observations were carried out in visible–near-infrared (VIS-NIR) and mid-infrared spectral regions. The Apollo missions to the Moon delivered material from another planetary body directly to Earth for the first time and were instrumental to our understanding of space weathering. There was a significant mismatch between the optical properties of the interior of the collected samples and the reflectance spectra obtained via ground-based observations (McCord & Johnson 1970; McCord & Adams 1973). Nevertheless, the difference was not only between the interior of the collected samples and remotely sensed data but also between the interior of the collected larger samples and fine-grained (mature) regolith, despite both being collected from the very same area. The reason behind this mismatch was identified as the effects of space weathering (Hapke 2001; Marchi et al. 2005). Space weathering is caused by three main agents: micrometeorite impacts, solar wind, and galactic radiation. The optical properties of a planetary surface are altered through surface amorphization, formation of nano- and microphase opaque particles (npFe⁰), vesicles (Pieters et al. 2000; Hapke 2001), and/or redeposition of thin coatings of sputtered material (McCord & Adams 1973; Pieters et al. 1993; Hapke 2001).

Overall similar (but distinct in detail, for example, through the presence of npFeS) changes to the mineral physical state as described above for lunar samples were recently observed in samples from near-Earth asteroids (162173) Ryugu and (25143) Itokawa (Noguchi et al. 2014; Tsuchiyama 2014; Yada et al. 2022). Such space weathering–induced discrepancies between the remotely sensed spectra and measured fresh material spectra increase the challenge of linking the meteorites with their parent body or quantitatively identifying minerals. Hence, understanding space weathering is important for the correct interpretation of the mineral or chemical composition of asteroids, the Moon, or planets (e.g., Mercury; Hapke 2001; Clark et al. 2002; Moroz et al. 2008; Domingue et al. 2014).

More than 99% of the solar wind consists of hydrogen (96%) and helium (4%) ions with a broad velocity distribution (250–800 km s⁻¹ at 1 au; Schwenn 2000; Gosling 2007). Previous review works on space weathering (Chapman 1996; Keller & McKay 1997; Pieters et al. 2000; Hapke 2001; Bennett et al. 2013; Pieters & Noble 2016) give a quite sufficient general understanding of the mechanisms involved. However, quantitative estimates of the mismatch between asteroid and meteorite reflectance spectra are not clear. For example, when comparing ordinary chondrite (OC) spectra to those of compositionally similar S-type asteroids, space weathering induces albedo decrease, spectral slope reddening, and a decrease in silicate absorption band depths (Clark et al. 2002; Gaffey 2010) at varying relative proportions. However, reflectance spectra matches of howardite–eucrite–diogenite (HED) meteorites with asteroid (4) Vesta show less pronounced alterations than S-type asteroids (McCord et al. 1970; McSween et al. 2013). Therefore, the question is not whether space weathering occurs on asteroids, but why space weathering takes a distinct path on different asteroids (Gaffey 2010).

Many previous studies used high-energy ion irradiation utilizing ions such as H⁺ (Dybwad 1971—silicate minerals, Yamada et al. 1999—olivine, Crandall 2018—nonsilicate minerals, Chrbolková et al. 2021—olivine and pyroxene), Ar⁺ (Fulvio et al. 2012; Chrbolková et al. 2021), and He⁺ (Dybwad 1971; Dukes et al. 1999; Loeffler et al. 2009; Chrbolková et al. 2021) with energy levels of 4–10 keV or higher on silicate minerals, OC meteorites, or carbonaceous meteorites (Laczniak et al. 2021). However, the typical solar wind H⁺ has low energy, around 0.3–3 keV. Only a few studies have been conducted with low-energy fluences with a focus on chemical analysis. Dukes et al. (1999) simulated solar wind irradiation with 1 keV H⁺ on single crystalline San Carlos olivine and carried out a chemical analysis. Kuhlman et al. (2015) and Takigawa et al. (2019) simulated ∼1 keV hydrogen ion irradiation on orthopyroxene polished surfaces and conducted a scanning transmission electron microscope study. It was shown that in certain experimental conditions, hydrogen ions are much more effective in altering olivine spectra than helium and argon ions (Marchi et al. 2005; Chrbolková et al. 2021). Hence, the first objective of this study is to observe how reflectance spectra change under 1 keV H⁺ irradiation mimicking closer solar wind conditions. Furthermore, our second objective is to compare the responses of meteorites with different compositions. The study hypothesizes that different minerals may respond differently at low solar wind energies and low fluence. The initial, shorter-timescale space weathering effect on reflectance spectra may substantially differ on various bodies of distinct composition, while at longer timescales, close to space weathering saturation, these differences in space weathering may smear, and the overall results may become similar.

To investigate space weathering effects on different types of meteorites, H⁺ irradiation was carried out at the Helsinki Accelerator Laboratory on powdered samples compressed into pellets as described in detail in Brunetto et al. (2014) and Chrbolková et al. (2021). Three types of meteorites were selected, namely, Avanhandava, Bjurböle, and Luotolax. Basic information on these meteorites is summarized in Table 1. Approximately 600 mg from each meteorite was ground using a mortar and pestle. The ground powder of each sample was homogenized and dry-sieved to a fraction of ≤50 μm to obtain a representative sample. A pressed KBr pellet substrate was used as a base, and an additional 100 mg of meteorite powder was pressed on the substrate using a Specac manual hydraulic press applying 6 ton pressure for 6 minutes. The pellets have a diameter of 13 mm and thickness of approximately 2.2 mm.

The pellets were irradiated with 1 keV H⁺ ions under three fluence steps, 2.17 × 10¹⁷, 5.4 × 10¹⁷, and 1.37 × 10¹⁸ H⁺ cm⁻², to simulate solar wind irradiation at different timescales. A beam current of ∼2 μA cm⁻² with an ion flux of 1.2 × 10¹³ cm² s⁻¹ was used for the irradiation. No sample neutralizer was used. The H⁺ ions with 20 keV energy were decelerated down to 1 keV, keeping the sample at 19 kV potential and using a lens system to gradually decrease the ion energy. The beam was raster scanned with a frequency of about 10 kHz in the horizontal direction and 1 kHz in the vertical direction. The technique is similar to the one used by Brunetto et al. (2014) or Chrbolková et al. (2021) but differs in lower H⁺ energies.

The surface exposure timescale was calculated from the irradiated fluences based on the flux of protons at ∼1 au by Schwenn (2000). The exposure time, t, in years was calculated using

$$\begin{eqnarray}&&t\left(\mathrm{yr}\right)=\displaystyle \frac{F\ ({{\rm{H}}}^{+}\,{\mathrm{cm}}^{-2})\times 4}{2.9\times {10}^{8}\,{\mathrm{cm}}^{-2}\,{{\rm{s}}}^{-1}}\times \displaystyle \frac{1}{31.5792\times {10}^{6}\,{\rm{s}}\,{\mathrm{yr}}^{-1}}\times {d}^{2},\end{eqnarray} \tag{ 1 }$$

where F is the fluence in particles per square centimeter, d is the relative (to 1 au) distance from the Sun, 2.9 × 10⁸ cm⁻² s⁻¹ is the literature-based value for the flux of a photon at 1 au (Schwenn 2000; Moroz et al. 2008), and 31.5792 × 10⁶ is the factor converting the exposure time from seconds to years. This was multiplied by a factor of 4 to approximate the object as a rotating sphere. The correction factor may vary according to the shape of the body. In our calculation, the body was considered as a sphere. As the flux of the solar wind approximately reduced with the square of the distance from the Sun, the solar flux at the asteroid belt is smaller than at 1 au. Since the meteorites in question are from the main asteroid belt, we calculated the exposure time at 2.3 au. The timescale ranges corresponding to our fluences are less than 1000 yr at 1 au and less than 3500 yr at 2.3 au (Table 2).

The hemispherical reflectance VIS-NIR spectra were measured in the shortest possible time (within 2–4 weeks) since the irradiations using an OL-750 automated spectroradiometric measurement system by Gooch & Housego situated at the Department of Physics, University of Helsinki. The OL-750 instrument is equipped with a polytetrafluoroethylene (PTFE) integrating sphere and a specular reflection trap. The spectra from 250 to 2500 nm were measured relative to a PTFE (VIS and NIR) standard and 5 mm slit with identical viewing geometry for all segments of the spectra. The incident angle was 10° to the surface normal, and the spectral resolution was between 5 and 10 nm. The spectrometer measuring parameters for the different segments are summarized in Table 3.

Spectral slopes at the 1 and 2 μm regions were calculated as a linear fit connecting the local maxima of the 1 and 2 μm absorptions and depth as the distance between the linear fit (local continuum) and local spectral minimum. The local maxima positions did not significantly change within the irradiation series; thus, the linear fit was based on similar intervals. The spectral slope was subtracted before reading the band depth. An example is illustrated in Figure 1.

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Individual mineral absorption bands from the spectra were deconvolved using the modified Gaussian model (MGM), which has been identified as an accurate mathematical description of the shape of isolated electronic transition absorption bands (Sunshine et al. 1990; Sunshine & Pieters 1993). Customized code by Sunshine et al. (1990) was used. After the initial user input, a fit of the spectrum better than an rms of 2 × 10⁻⁹ (default MGM code setting) was obtained by the iterative optimization process. The second-order polynomial continuum fit was selected, as it has been proven effective by Clénet et al. (2011) and Han et al. (2020). The key band parameters—band center, band strength, and FWHM—were obtained using the MGM. Figure 2 gives the MGM fit for the fresh samples of the meteorites, and MGM fits of irradiated samples are in Zenodo at DOI:10.5281/zenodo.7053633 in the "Supplementary Figures S1–S12.pdf" file. To determine the error of the spectral parameters derived from MGM fits, we repeated the spectral measurements and MGM fitting three times, and the standard deviation was calculated.

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Possible temporal trends in the spectral parameters were tested between the unaltered material and the material with the largest fluence, corresponding to an exposure time of ∼3100 yr at 2.3 au distance. Gaussian-distributed errors and constant variance were assumed for the parameters, and Student's two-sample t-test for independent samples was conducted. The test results are available in the Zenodo archive as "Supplementary Table S1.xlsx."

Figure 3 gives the reflectance spectra for fresh and irradiated samples of Bjurböle, Avanhandava, and Luotolax. We can observe two major absorption bands at 1 and 2 μm in all meteorite samples. The overall slopes for the spectra above 1500 nm are slightly blue (i.e., decrease in the spectral slope), and those below 1500 nm are red (i.e., increase in the spectral slope). The blue slope at higher wavelength may be explained by the presence of a fine fraction and pressed surface as observed by, e.g., Lazzarin et al. (2006), Brunetto et al. (2014), Serventi & Carli (2017), or Sultana et al. (2021). However, the slope in both regions shows a relative reddening trend with increasing fluences (Figure 8). Figure 4 shows the time evolution of the reflectance at 550 nm with a general decrease as a function of increasing fluences for all three meteorite samples.

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3.1. Absorption Band Parameters

Using the MGM model, for Bjurböle, six was the minimum number of bands to obtain an optimum fit with rms ≤ 0.002 × 10⁻⁶. Bands at 0.8, 0.9, 1, and 1.9 μm were identified as the result of absorption by minerals. Similarly, for Avanhandava, five bands were used to get an optimum fit, and bands at 0.9, 1, and 1.9 μm were identified as the result of mineral absorption. For Luotolax, five bands were used to get an optimum fit, and bands at 0.9, 1, and 1.9 μm were identified as the result of mineral absorption. In all cases, we did not consider the bands in the ultraviolet (UV) range (250–400 nm) in the interpretation because they are not entirely present. We used these bands only to get an optimum fit (reduced rms error) in the UV-dropoff region. Changes in the band center, strength, and FWHM with increasing fluences are plotted against the exposure time in Figures 5–7. Calculated band slopes for the 1 and 2 μm regions are illustrated in Figure 8.

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4.1. Exposure Time

4.2. Silicate Absorptions

4.3. Significance of Absorption Band Changes and Observed Trends

As explained in Section 2, temporal trends in the spectral parameters were tested for independent samples using Student's two-sample t-test. Parameter changes with p < 0.05 have been considered as significant. These are highlighted in green in the MS/Excel file in the Zenodo repository, "Supplementary Table S1.xlsx." Looking at changes in the absorption band centers, no significant trends as a function of fluence are observed in the fitted bands in all meteorites except the 1.9 μm bands of Bjurböle and Avanhandava, where a slight but still significant change is detected. The magnitude of this change is ∼30 nm toward higher wavelengths, indicating a possible Fs change of 10% (Moriarty & Pieters 2016). Alternatively, this can also be attributed to the fitting error due to the poorly constrained minimum position of the broadband feature, rather than a change in the mineralogy.

Concerning band strength, the Bjurböle absorption bands at 1 μm and Avahandava band at 0.9 μm all fulfill the statistical test condition as a significant change. Other absorption bands of Bjurböle in the 1 μm region show a monotonous decrease (Figure 6), but they do not pass the test due to higher standard deviations. Even though we cannot separate all of the existing absorption bands in this region, it can be assumed that all of the overlapping bands behave in a similar manner. The initial change in the band depth is relatively large and then gradually decreases with further irradiation, e.g., being nonlinear. For Avanhandava, the changes are less prominent compared to Bjurböle. The band strength change with increasing fluence shows mixed trends, or rather, fluctuations. The amplitude of these fluctuations is well within the error bars. When it comes to Luotolax, the band strengths also do not show a clear correlation with increasing fluence with variations and p > 0.05.

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The FWHM behavior (Figure 7) is similar to that of the band depth, but the observed trends are less pronounced. Initially, Bjurböle shows a slight decrease in FWHM (e.g., the 0.8 μm band with p < 0.05). Subsequently, further changes are negligible. Avanhandava and Luotolax do not show any significant change in FWHM.

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In all three meteorites, the spectral slope in the 1 μm region (Figure 8) tends to monotonously increase with fluence (reddening), passing the significant change test with p < 0.05. In the 2 μm region, Avanhandava also shows significant reddening. For Bjurböle and Luotolax, the change of slope in the 2 μm region is less significant.

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The reflectance at 550 nm shows a monotonous decreasing trend with increasing fluences for all three meteorite samples that pass the temporal change test, indicating a valid change in all three meteorites.

To summarize, the most prominent changes in all three meteorites are (1) a decrease of the 550 nm reflectance, (2) reddening in the 1 μm region, and (3) a monotonous decrease in absorption band strengths in Bjurböle, while in the other meteorites, these are rather insignificant or random. A possible explanation for the difference between Bjurböle and Avanhandava is that Avanhandava, being an H chondrite, is more reduced than Bjurböle (being LL) and has less Fe in silicates (lower Fa and Fs numbers). Subsequently, the availability of Fe²⁺ in silicates to potentially create reduced npFe⁰ is less, and this is why the band depth and width changes are less pronounced compared to Bjurböle. The reason behind the increased reddening of Avanhandava in the 2 μm region is not clear.

Comparable studies observed that H⁺ ion irradiation resulted in similar spectral changes (Chrbolková et al. 2021) as observed in our samples, but these were caused predominantly by a thin, partially amorphous layer (up to 200 nm) and small vesicles with no npFe⁰ (Takigawa et al. 2019; Chrbolková et al. 2022). According to Chrbolková et al. (2022), thickness of the amorphous layer plays a major role in absorption spectra, as the amorphous layer absorbs the wavelengths that are equivalent to its thickness. We carried out H⁺ ion penetration depth and associated damage simulations using SRIM software (Ziegler & Biersack 1985) for energies of 1–5 keV, and we obtained a penetration depth and thickness of the damaged area of <40 nm for our case of 1 keV (see the "Supplementary Figure S13.pdf" file in the Zenodo repository). The simulation only imitates individual events and does not take into account the cumulative damage. The damaged zone in our case will be slightly thinner than that observed by Chrbolková et al. (2022) for 5 keV H⁺ experiments, but the observed features (partial amorphization, vesicles) may be similar. Hence, we attribute the significant reddening and albedo reduction in the 1 μm region to be predominantly caused by a thin, partially amorphous layer and the presence of vesicles, rather than the presence of npFe⁰. Low-energy ion irradiation of pyroxene by Kuhlman et al. (2015) and olivine crystal by Dukes et al. (1999) observed sparse npFe⁰ particles on the topmost surfaces of their sample. We cannot rule out the presence of npFe⁰ in our samples, but its abundance is most likely lower compared to those generated during laser irradiation experiments. While Chrbolková et al. (2022) did not observe npFe⁰ on He⁺-irradiated samples, they did observe it on identical samples irradiated by a femtosecond laser, pointing to a more likely npFe⁰ appearance during spot heating compared to ion irradiation associated with milder thermal conditions. Loeffer et al. (2009) also observed during 4 keV He ion irradiation that, due to partial oxidation of the redeposited sputter, npFe⁰ formed more slowly on a powder (as in our case) than on a flat surface of olivine.

4.4. Comparison with Previous Studies and Application in Solar System Research

4.4.1. S- and Q-type Asteroids

The observed albedo reduction and rapid slope change in the 1 μm region in both OCs are consistent with reflectance spectra of S-type asteroids, where more reddening is often observed in the 1 μm region than at higher wavelengths (Binzel et al. 2010). Our results indicate that this can already occur on short timescales (10²–10³ yr) and that the dominant agent in this change is low-energy solar wind. Only on longer timescales, where cumulative damage specific to slower space weathering processes, such as heavier, higher-energy ion irradiation or micrometeorite impacts, starts to prevail, do the spectral changes in the 2 μm region also become apparent. This is supported by, for example, Brunetto et al. (2005) simulating timescales of 10⁷ yr with 60 keV Ar⁺⁺ on Epinal H5 chondrite and observing reddening in the whole NIR region. Laser irradiation experiments simulating microimpacts, typically on timescales larger than 10⁴ yr, also show spectral changes in the whole NIR region (e.g., Moroz et al. 1996; Yamada et al. 1999; Sasaki et al. 2002; Chrbolková et al. 2021, 2022). The dominant spectrally significant agent in these experiments is identified as npFe⁰ embedded in surficial amorphous rims, which was also observed in lunar soil samples (e.g., Keller & McKay 1997; Hapke 2001).

An important factor is the distinct response of olivine and pyroxene to space weathering. Yamada et al. (1999), Sasaki et al. (2002), and Chrbolková et al. (2021, 2022) conducted space weathering by pulse laser irradiation or photon implantation (1 MeV) on olivine and pyroxene. Their results indicate that simulated space weathering changes are more prominent in olivine than in pyroxene and that pyroxene shows less alteration in the 2 μm region relative to the 1 μm region. Similar differences in the progress of space weathering between olivine and pyroxene were revealed through systematic changes in the 2 μm/1 μm band depth ratio as observed in space weathering simulations by Kohout et al. (2020). The observational manifestation of the above-described different mineral responses to space weathering is the higher spectral slope in olivine-rich asteroids compared to those with higher pyroxene content (Vernazza et al. 2009).

Interesting insight into the different responses of olivine and pyroxene to space weathering is provided through atomic-scale simulations by Quadery et al. (2015). The simulations predict pyroxene's lower resistance to ion irradiation due to one-dimensional anion mobility, which prevents healing of the lattice. On the contrary, pyroxene is more resistant to alterations caused by localized rapid surficial heating (e.g., laser pulses, microimpacts), as rapid anion diffusion inhibits surface reduction and extensive npFe⁰ production. Thus, pyroxene may be more susceptible to rapid space weathering caused by low-energy solar ions but more resistant to the long-term effects of heavier ions, microimpacts, and associated npFe⁰ formation. This can also explain the apparent lack of spectral changes in the pyroxene 2 μm region relative to olivine even on long timescales, as the production of npFe⁰ is lower than in olivine.

Figure 9 synthesizes our findings (yellow rectangle) with the previously published results relevant to space weathering of OC asteroids. On short timescales (<10⁴ yr), space weathering is dominated by low-energy solar wind and the associated amorphization and vesicle formation of the thin, ∼100 nm topmost layer of silicate mineral grains. Spectral changes are most apparent in the 1 μm region and manifested through a rapid decrease of albedo, moderate reddening, and a slow reduction of 1 μm silicate absorption. Only on longer timescales (>10⁴ yr) the effects of higher-energy, heavier ions and microimpacts start to accumulate with the associated appearance of npFe⁰ (being more abundant in olivine compared to pyroxenes). The associated spectral changes progress slowly into the 2 μm region but are rather related to slope change (predominantly related to olivine) with only a small reduction of the 2 μm band depth (as this band is attributed to pyroxene).

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4.4.2. V-type Asteroids

Compared to the OCs in our experiments, Luotolax shows a significant initial change in the albedo and 1 μm slope, but any subsequent changes to increasing fluence are minimal. The rapid onset of space weathering in pyroxene-dominated Luotolax on timescales of <10⁴ yr is consistent with the atomic-scale simulations by Quadery et al. (2015) described in the previous section. The observed spectral changes are mostly due to the thin, partially amorphous layer with vesicles (Takigawa et al. 2019; Chrbolková et al. 2022) caused by low-energy solar wind.

Only heavier ions such as Ar⁺ and C⁺ with high energies of 60–200 keV applied to eucrites by Fulvio et al. (2012) reveal the reduction of the 1 μm band strength with reddening and darkening to be more intense in the 1 μm than the 2 μm region. Fulvio et al. (2012) also observed different reddening behavior among various HEDs, indicating the importance of individual mineral and chemical compositions or surface textures. These findings are consistent with the reflectance spectra of asteroid (4) Vesta, as observed by the Dawn mission, indicating a relatively fresh surface with deep absorption bands and a lack of reddening compared to S-type asteroids (e.g., Pieters et al. 2012). Laser irradiation experiments with pyroxene (Yamada et al. 1999; Sasaki et al. 2002; Chrbolková et al. 2021) show the onset of reddening and darkening in the 2 μm region, but these changes are still of lower magnitude compared to those in 1 μm region.

Thus, based on our and previous results, we summarize space weathering on pyroxene-rich planetary surfaces in Figure 10 and predict that, initially, only a change in the 1 μm region albedo and slope (reddening) will be observed as a consequence of low-energy solar wind irradiation. Little band depth change will occur, and the 2 μm region remains largely unaffected. These changes are predominantly due to the thin, partially amorphous layer with vesicles (Chrbolková et al. 2022). On longer timescales (>10⁴ yr), the 1 μm absorption band will start to slowly decrease, and subtle spectral changes will also start to appear in the 2 μm region but at lower magnitude compared to the 1 μm region.

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Low-energy H⁺ irradiation with 1 keV energy was carried out under three different fluences with two olivine/pyroxene-dominated OCs (Avanhandava H4 and Bjurböle L/LL4) and pyroxene-dominated howardite Luotolax to simulate the typical solar wind irradiation effect. The most prominent changes in all three meteorites are (1) a decrease of the 550 nm reflectance, (2) reddening in the 1 μm region, and (3) a monotonous decrease in absorption band strengths in Bjurböle. The band strength changes in the other meteorites are rather insignificant or random. No significant changes were observed in the 2 μm region. The results imply that at short timescales (10²–10³ yr), radiation damage as amorphization and vesicle formation caused by low-energy solar wind is the dominant space weathering factor causing spectral changes mostly in the 1 μm region. Only on longer timescales do the spectral changes proceed into the 2 μm region due to the increasing cumulative effect of slower processes such as heavy-ion and micrometeorite bombardment and the associated occurrence of npFe⁰. This will be more prominent in olivine-rich materials (A- or S-type asteroids) compared to pyroxene-rich basaltic (V-type) asteroids.

This work was supported by the Academy of Finland, project No. 335595, the NASA Solar System Exploration Research Virtual Institute Center for Lunar and Asteroid Surface Science and conducted with the institutional support of RVO67985831 of the Institute of Geology of the Czech Academy of Sciences. The open-access publication cost was provided by the APC fund of the Library University of Helsinki. We would like to thank the anonymous referees for the valuable comments and suggestions, which greatly improved the quality of the article.

A Zenodo repository contains additional supplementary materials. This includes two figure files. The first, "Supplementary Figure S1–S12.pdf," contains additional MGM fits figures like Figure 2, while the "Supplementary Figure S13.pdf" file shows the penetration depth and thickness obtained from SRIM simulation. The "Supplementary Table S1.xlsx" file provides the temporal change test results. Finally, the raw data of the spectral measurements are available in .csv files. All of these supplementary items can be found at doi:10.5281/zenodo.7053633.