# Space-weathering induced changes in hydrated silicates: A multi-scale study combining visible/infrared spectroscopy and electron microscopy

Stefano Rubino, Hugues Leroux, Cateline Lantz, Alice Aléon-Toppani, Donia Baklouti, Zahia Djouadi, Obadias Mivumbi, David Troadec, Ferenc Borondics, Rosario Brunetto

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Stefano Rubino, Hugues Leroux, Cateline Lantz, Alice Aléon-Toppani, Donia Baklouti, et al.

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# Space-weathering induced changes in hydrated silicates: A multi-scale study combining visible/infrared spectroscopy and electron microscopy

Stefano Rubino<sup>a, b, *</sup>, Hugues Leroux<sup>c</sup>, Cateline Lantz<sup>a</sup>, Alice Aléon-Toppani<sup>a</sup>, Donia Baklouti<sup>a</sup>, Zahia Djouadi<sup>a</sup>, Obadias Mivumbi<sup>a</sup>, David Troadec<sup>d</sup>, Ferenc Borondics<sup>e</sup>, Rosario Brunetto<sup>a</sup>

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

### Keywords:

Solar wind

Laboratory astrophysics

Infrared spectroscopy

Asteroids

## ABSTRACT

Investigating the effects of space-weathering on primitive bodies is paramount in assessing their composition and understanding their history. For this purpose, we conducted 40 keV He and Ar ion-implantation experiments on terrestrial phyllosilicates, the main components of primitive C-class asteroids: two serpentines, and one saponite. We emulated the effects of the solar wind component of space weathering on hydrated surfaces over timescales of 1E3–4 years in the inner solar system. Spectral changes (from 0.35 to 30 µm in wavelength), surface topography variations, and elemental and morphological changes were investigated. In the visible range, we saw darkening for all samples, accompanied by reddening for the serpentine samples and blueing for the saponite. Spectroscopic features such as the silicate stretching and bending modes at 10 and 20 µm respectively, as well as the (M)-OH bending mode at 16 µm, exhibited a peak shift towards longer wavelength (red-shift), band area decrease, and band widening. The surface of Ar-implanted samples appeared polished, while no difference was noticeable for He-implanted samples. Morphological changes in the ion-implanted layers of our samples include abundant vesiculation and lattice amorphization. Mg and O appear to be particularly mobile upon ion implantation. In particular, dehydroxylation was observed for all samples. Changes observed for spectroscopic features do not correlate systematically with variations of the Mg/Fe ratio. They do correlate with oxygen loss via dehydroxylation and crystal lattice disordering and amorphization. Spectral changes in the visible range and for 10 and 20 µm features appear to be mainly due to crystal amorphization. Variation of the hydration feature at 16 µm correlates with both dehydroxylation and lattice amorphization. Compositional variations affect the hydration feature at 2.7 µm more consistently. The effects detected upon ion implantation on the samples in this work support the identification of spectral trends from the remote-sensing data of primitive bodies Ryugu and Bennu as changes induced by space-weathering. Findings are also compatible with laboratory analysis on returned weathered grains from Ryugu.

## 1. Introduction

Investigating the youth of our solar system is critical in understanding its evolution. The study of primitive bodies allows us to perform such an investigation. Their composition is tightly linked to the early years of our solar system. However, thermal, aqueous alteration, shocks, and space-weathering effects can alter these bodies' composition (Clark et al., 2001; Marchi et al., 2009; Delbo et al., 2014; R. Brunetto et al., 2015). In particular, space-weathering (SW), which regroups the

ensemble of processes affecting the surface of bodies not protected by an atmosphere - such as micrometeorite impacts and solar-wind ion implantation - can introduce a bias in compositional analysis led by remote sensing (Nittler et al., 2004; Matsuoka et al., 2023), complicating the linking between asteroid families and meteoritical classes (Pieters and Noble, 2016). To study these effects, we can emulate them in a laboratory environment by working on analogous materials (Keller et al., 2015a; Lantz et al., 2017; Thompson et al., 2019, 2020; Brunetto et al., 2020; Laczniak et al., 2021; Rubino et al., 2020; Rubino et al., 2022;

<footnote><sup>a</sup> Institut d'Astrophysique Spatiale, Université Paris-Saclay, CNRS, F-91405 Orsay, France</footnote>

<footnote><sup>b</sup> INAF-Istituto di Astrofisica e Planetologia Spaziali (INAF-IAPS), Rome, Italy</footnote>

<footnote><sup>c</sup> Univ. Lille, CNRS, INRAE, Centrale Lille, UMR 8207—Unité Matériaux et Transformations, F-59000 Lille, France</footnote>

<footnote><sup>d</sup> Institut d'Electronique, de Microélectronique et de Nanotechnologie, Lille, France</footnote>

<footnote><sup>e</sup> SMIS Beamline, SOLEIL Synchrotron, Gif sur Yvette Cedex, France</footnote>

<footnote>* Corresponding author at: Institut d'Astrophysique Spatiale, Université Paris-Saclay, CNRS, F-91405 Orsay, France.
E-mail addresses: stefano.rubino@universite-paris-saclay.fr, stefano.rubino@inaf.it (S. Rubino).</footnote>
Nakauchi et al., 2021; Hallatt et al., 2022) or directly on returned samples. The Hayabusa2 mission (JAXA - Watanabe et al., 2019) has successfully returned 5.4 g of material from the surface of C-type asteroid Ryugu. Ryugu samples appear to be most similar to CI chondrite but are also more chemically pristine, since they have not been altered by terrestrial contamination (Yokoyama et al., 2023; Okazaki et al., 2023; Yabuta et al., 2023). The most common mineralogies contain Mg-rich phyllosilicates, MgCa and MgFe carbonates, hydroxyapatite, Fe sulphides and Fe oxides, suggesting widespread aqueous alteration on Ryugu's parent body (Nakamura et al., 2022), consistent with the samples' organic matter chemical diversity (Naraoka et al., 2023). Weathered grains exhibit surface amorphization and partial melting, Fe reduction and dehydration (Noguchi et al., 2022). The OSIRIS-REx (NASA - Lauretta et al., 2019) craft has recently landed carrying material from B-type asteroid Bennu and investigations are ongoing.

In this work, we emulated in the laboratory the effects of the solar-wind component of space weathering - which is the dominant process for young asteroidal surfaces ($< 10^7$ years in the inner solar system - Vernazza et al., 2009; Takaaki Noguchi et al., 2014; Bonal et al., 2015) - onto primitive hydrated surfaces in a laboratory environment. Anhydrous S- and hydrous C-class asteroids are the major components of the asteroid belt (DeMeo et al., 2009), hence the scientific interest in understanding their history and evolution. On anhydrous surfaces, space weathering is now fairly well constrained, thanks to extensive studies which have been led on the Moon and S-type asteroid Itokawa (e.g., Noguchi et al., 2011). For such bodies, SW is associated with darkening (decrease of albedo) and reddening (increase of the spectral slope) in the visible range, as well as attenuation of spectroscopic features (Pieters et al., 2000; Hapke, 2001). Morphological modification include vesiculation, creation of amorphous layers, emergence of multi-layer structures and appearance of nanop phase Fe-bearing particles (Keller and McKay, 1993; Keller and McKay, 1997; Pieters et al., 2000; Hapke, 2001; Clark et al., 2002; Brunetto et al., 2015). On hydrated surfaces, weathering trends are less straightforward: both darkening/reddening, brightening/bluing (increase of albedo and flattening of the spectral slope) and overlapping of these two regimes have been seen in the visible to near-infrared (near-IR) range (Lantz et al., 2017; Thompson et al., 2019; Matsuoka et al., 2020; Laczniak et al., 2021, 2024), various instances of modifications in spectroscopic bands position and profile from the near- to far-IR (Vernazza et al., 2013; Brunetto et al., 2014; Lantz et al., 2015, 2017; Brunetto et al., 2020) as well as a number of morphological changes, such as surface melts, vapor deposits, vesiculation, amorphization, accompanied by elemental modifications affecting Fe, Mg and/or O (Keller et al., 2015a; Matsuoka et al., 2015; Keller et al., 2015b; Gillis-Davis et al., 2017; Thompson et al., 2020, 2019; Laczniak et al., 2021; Hallatt et al., 2022). Weathered samples from Ryugu also show changes in the position and shape of the 2.7 µm hydration feature in the near-IR (Le Pivert-Jolivet et al., 2023) as well as surface amorphization and partial melting, accompanied by Fe reduction and dehydration (Noguchi et al., 2022).

The observed variability in effects associated with SW on hydrated/carbon-rich materials, in both laboratory studies and in returned samples, prompts us to further investigate this topic in order to better constrain the origins of such a diversity. In this work, we subjected terrestrial serpentine and saponite minerals to ion-implantation using $He^+$ and $Ar^+$ cations. We chose serpentine and saponite since they are the main phases of CI/CM chondrites (Brearley, 2006) and have been detected on primitive bodies (Rivkin et al., 2002). Terrestrial phyllosilicates are more homogenous and coarse-grained compared to meteoritic samples in which the two main phases (serpentine and saponite) are frequently intermixed. This configuration helps us "simplify" the investigation by removing the effect of sample complexity when discussing the effects of ion-implantation on such materials. We then

characterized the samples using micro-spectroscopy (wavelength ranging from 350 nm to 25 µm) and by scanning and transmission electron microscopy. This work is a direct follow-up of Rubino et al., 2020–2022, which focused on the evolution upon weathering of the spectroscopic feature at 2.7 µm, the (M)-OH stretching feature in phyllosilicates. The scientific goal of this work is to link macroscopic spectral effects (over a larger spectral range than what had been done in our previous works) and microscopic elemental/morphological changes in the weathered layers, to better support the interpretation of both remote-sensing and laboratory data from weathered primitive materials.

## 2. Materials and methods

### 2.1. Samples

We worked using three terrestrial rocks, two serpentine polymorphs - serpentine Rawhide (Rawhide, California), serpentine UB-N (Col des Bagenelles, France) - and one saponite (Griffith Park, California). Serpentine Rawhide consists mainly of chrysotile, while serpentine UB-N of lizardite. The saponite sample is from the griffithite variety (particularly Fe-rich). Their chemical formula, derived by SEM-EDS (Scanning Electron Microscopy - Energy Dispersive X-ray Spectroscopy) chemical analysis with 15 kV on loose powder prior to pelletization are $Na_{0.5}Ca_{0.5}Mg_{1.0}Fe_{0.7}Al_{1.1}Si_{3.3}O_{10}(OH)_2$ for saponite, $Mg_{2.8}Fe_{0.2}Al_{0.04}Si_{1.9}O_{5}(OH)_4$ for serpentine Rawhide, and $Ca_{0.05}Mg_{2.6}Fe_{0.4}Al_{0.1}Si_{1.8}O_{5}(OH)_4$ for serpentine UB-N. We prepared three 20 mm diameter pellets for each rock (nine pellets total). The rocky samples were crushed using an agate mortar and pestle. Grain size was estimated to be in the 1–100 µm range using a SEM. The pellets were prepared by pressing 240 mg of powdered sample onto a PEG substrate (powdered polyethylene glycol, average molecular mass of 8000 g/mol, from Fisher Scientific) for a duration of 5 min, under a weight of 7 tons (pressure of approximately 1720 bar, 547 bar/cm²). Expanded sample description, including compositional wt%, can be found in Rubino et al., 2020.

### 2.2. Ion implantation

For each sample, one pellet underwent $He^+$ bombardment, a second underwent $Ar^+$ bombardment, and a third was kept as a control sample. We mounted the samples onto the INGMAR (IrradiationN de Glaces et Météorites Analysées par Réflectance, Institut d'Astrophysique Spatiale (IAS) - Laboratoire des deux Infinis Irène Joliot Curie (IJLab), Orsay) vacuum chamber. Ion bombardment was done at room temperature under vacuum (P ~ $10^{-7}$ mBar). The SIDONIE ion implanter (IJLab, Orsay) provided the $He^+$ and $Ar^+$ beam at 40 keV at ~$10^{13}$ ions/(s.cm²). Subsequent use of the ion-implanter allowed us to acquire data at different ion fluences: $1 \times 10^{16}$, $3 \times 10^{16}$ and $6 \times 10^{16}$ ions/cm² for $He^+$ and $0.5 \times 10^{16}$, $1 \times 10^{16}$ and $2 \times 10^{16}$ ions/cm² for $Ar^+$ on the same pellet, in-situ, without breaking the vacuum seal of the chamber. The entire surface of each pellet was exposed to the ion-beam, except for a small external rim (~500 µm thick) which was masked by the sample holder.

The energy of 40 keV is the standard working condition of the SIDONIE ion-implanter. It is close to the energy of $Ar^+$ in the solar wind (36 keV) but ten times higher than $He^+$ (4 keV). Higher energy $He^+$ can still contribute to SW effects (Matsumoto et al., 2015), and weathering timescales by 40 keV $He^+$ are about ten times longer than 4 keV $He^+$ (Brunetto et al., 2014). Moreover, we used SRIM simulation (Ziegler et al., 2010) to compute ion-implantation parameters (see Table 1). The 40 keV $He^+$ and $Ar^+$ ion-implantation experiments are each dominated by a different energy loss regime, inelastic (electronic) for $He^+$ and elastic (nuclear) for $Ar^+$, which allows us to investigate in a more general way how these two regimes can affect asteroidal surfaces.
<table><caption>**Table 1**
Ion-implantation depth: SRIM simulation results and observations. *multiply by 2.4999 × 10² for units in keV/µm.</caption><thead><tr><th rowspan="2">Samples</th><th rowspan="2"></th><th colspan="2">Serpentine Rawhide</th><th colspan="2">Serpentine UB-N</th><th colspan="2">Saponite</th></tr><tr><th>40 keV He<sup>+</sup></th><th>40 keV Ar<sup>+</sup></th><th>40 keV He<sup>+</sup></th><th>40 keV Ar<sup>+</sup></th><th>40 keV He<sup>+</sup></th><th>40 keV Ar<sup>+</sup></th></tr></thead><tbody><tr><td rowspan="3">Stopping Power*</td><td>Ion implantation depth [nm]</td><td>317.2</td><td>40.6</td><td>322.1</td><td>41.2</td><td>332.1</td><td>42.2</td></tr><tr><td>dE/dx (inelastic)</td><td>5.35 × 10<sup>-1</sup></td><td>8.92 × 10<sup>-1</sup></td><td>5.22 × 10<sup>-1</sup></td><td>8.66 × 10<sup>-1</sup></td><td>5.00 × 10<sup>-1</sup></td><td>8.10 × 10<sup>-1</sup></td></tr><tr><td>dE/dx (elastic)</td><td>2.29 × 10<sup>-2</sup></td><td>2.76</td><td>2.25 × 10<sup>-2</sup></td><td>2.72</td><td>2.19 × 10<sup>-2</sup></td><td>2.64</td></tr></tbody></table>

## 2.3. Visible and IR spectroscopy

The visible and near-IR data were acquired directly inside the INGMAR vacuum chamber with optic fibers. Illumination angles for the visible and near-IR light sources are $i = 15°$ and $20°$. The illumination spot sizes are 20 mm and 12 mm in diameter. A single optical fiber collects reflected signal over a 3–4 mm diameter spot at a fixed collection angle $e = 15°$. The phase angle is $20°$ for visible and $15°$ for near-IR. Data from 350 nm to 1.1 µm is collected using a Maya2000Pro grating spectrometer (Ocean Optics) with a spectral resolution of 0.5 nm using a DTGS (deuterated triglycine sulphate) detector. Data beyond 1 µm and up to 4 µm is acquired with a Tensor 37 FTIR spectrometer (Bruker) at a spectral resolution of $16 \text{ cm}^{-1}$ with an MCT (HgCdTe) detector.

Mid-IR data (2.5 to 12 µm) were acquired with an Agilent Cary 670/620 micro-spectrometer (SMIS beamline, SOLEIL synchrotron) using the internal Globar source. We used a $15 \times$ objective with a numerical aperture of 0.62.

Far-IR data (14 to 50 µm) were acquired using a NicPlan microscope coupled to an iS50 FTIR spectrometer (Thermo Fisher) operating in confocal reflection geometry. This spectral range was measured using a bolometer detector (boron-doped silicon, 4.2 K cooled, Infrared Laboratories) and a solid-state Si beamsplitter.

## 2.4. Electron microscopy

A JEOL JSM-7800F LV operating at 3 kV was used to examine the pellet’s morphology by SEM imaging after being carbon-coated (300 Å). Ultra-thin sections - typically 100 nm thick - were extracted by focused ion beam (FIB) using an FEI Strata dual beam 235 (IEMN, University of Lille). A Titan Themis (Thermo Fisher Scientific™) scanning transmission electron microscope (STEM) operating at 300 kV was used to acquire high-angle annular dark field (HAADF) images and EDS (Energy Dispersive X-Ray Spectroscopy) data on the FIB sections. The He-implanted serpentine UB-N and saponite samples were also studied with selected area electron diffraction (SAED) for structural changes. We used a beam current of 60 pA for the HAADF imaging and 300 pA for hyperspectral EDS maps to minimise beam damage. EDS maps included both the weathered layer and the pristine material below (global compositional profiles are in Appendix E). EDS quantification was achieved using the Bruker software Esprit (version 1.9). The absorption and the k-factor corrections were applied assuming a thickness of 100 nm and using experimentally determined k-factors. We used a diffusion model based on an error function (or erf function, see Appendix D) to fit oxygen curves from the elemental profiles and extract the length of the O-depleted depth and overall oxygen decrease.

## 2.5. Data processing for spectral data

No particular data processing was done in the visible range. We investigate the effects of ion-implantation by following the evolution of the reflectance at 0.55 µm and the visible slope between 0.48 and 0.7 µm (Lantz et al., 2013).

Data in the mid- (8–12 µm) and far-IR (15–30 µm) has been continuum-corrected. A linear continuum drawn using the Christiansen feature as the sole anchor point has been subtracted from the mid-IR data. The Christiansen feature occurs where the refractive index of the measured sample undergoes rapid change, approaching the refractive index of the medium surrounding the sample and thus minimising scattering (Salisbury and D’Aria, 1989). For silicate materials, its position indicates the threshold beyond which surface scattering dominates instead of volume scattering. A convex rubber-band continuum considering multiple anchor points was used to correct the far-IR data. We shall look at ion-induced changes in the following spectral features: (a) Si–O stretching, around 10 µm (1000 cm$^{-1}$); (b) Mg–OH bending around 16 µm (625 cm$^{-1}$) (Farmer, 1974; Glotch et al., 2007; Bishop, 2018); (c) Si–O–Si, Si–O–Al, or Si–O–Fe stretching vibrations in the 12 to 16.7 µm (833–600 cm$^{-1}$) (Salisbury et al., 1987); (d–e) Si–O bending, between 16.6 and 25 µm (600 and 400 cm$^{-1}$), Si–O–Si near ~20.6–21.3 µm (470–485 cm$^{-1}$) and Si–O–(M) where (M) is an octahedral cation such as Al, Fe or Mg in the ~22–24 µm (400–450 cm$^{-1}$) (Salisbury et al., 1987; Bishop, 2018). The effects of ion-implantation were followed by extracting the various features’ peak position (associated with their maximum reflectance), band area, and full width at half of the maximum (FWHM). Moreover, for the Far-IR data, we tried using a 5-gaussian fit to discuss the evolution of the Si–O bending feature at 20 µm, but the results were inconsistent when comparing different pellets. Instead, we directly measured two parameters to discuss ion-induced changes: the position of its reflectance maximum and the band area ratio between its left and right shoulders (the Si–O–Si and Si–O–(M) bending contributions, respectively).

# 3. Results

## 3.1. Visible spectroscopy

Changes in reflectance at 0.55 µm and visible slope are shown in Fig. 1. All He-bombarded samples exhibit darkening at the maximum reached fluence of $6 \times 10^{16}$ ions/cm$^2$ (–9% for serpentine Rawhide, –4.9% for serpentine UB-N, and –7.5% for saponite). Serpentines exhibit reddening (+35% slope increase), while the saponite shows slight blueing (–9.3%). No clear trend seems to emerge for the $Ar^+$ bombarded samples.

The complete visible spectral dataset is available in Appendix A. Note that pellets from the same material show different initial reflectances. If there were compositional differences between pellets from the same material, they would not affect the data presented here, due to the large collection spot associated with the measurement (4 mm in diameter). The observed discrepancy is possibly due to differences in crystal orientation and surface morphology of the pellets before ion bombardment. This effect does not hinder our investigation since we are looking at relative changes.
<figure><img src="image_4.png" /><figcaption>Fig. 1. Evolution of the reflectance at 0.55 µm and visible slope between 0.48 and 0.7 µm as a function of ion fluence (colour-bar, yellow to purple for increasing He⁺ fluences and yellow to dark blue for Ar⁺). Slope data have been computed after spectra normalisation at 0.55 µm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

## 3.2. Mid- and far-IR spectroscopy

Mid- and far-IR data are shown in Fig. 2 (8–12 µm and 15–30 µm, respectively, in this study). The gap between 12 and 15 µm could not be covered by either the mid- or the far-IR setup.

### 3.2.1. Mid-IR data

We see evident changes in the shape of the Si—O stretching feature upon both He⁺ and Ar⁺ ion-bombardment (Fig. 2): peak position red-shift, changes in reflectance, and feature widening. Note that the red-shift does not correspond to a band shift “per se” but is the consequence of a difference in the intensities of the 10 µm band various Si—O stretching contributions (Brunetto et al., 2020). For all He-implanted samples, the feature’s position undergoes a significant red-shift (shift >300 nm) and a decrease in band area (up to –25% for serpentine Rawhide). FWHM increases up to +30% for both serpentine samples and >70% for the saponite sample.

Ar-implanted serpentine UB-N and saponite exhibit change analogous to the He-implanted samples but less prominent. The Ar-bombarded serpentine Rawhide sample is an outlier, exhibiting a red-shift close to 200 nm and an increase in band area (+33%).

Pristine and He-implanted spectra are acquired on the same pellet before and after weathering. In contrast, Ar-implanted spectra are acquired on a different pellet. This complicates the comparison of spectral parameters affected by surface state (band area, reflectance, or width).

### 3.2.2. Far-IR data

A 5-Gaussian fit in the far-IR range provides the spectral parameters for the Mg-OH bending mode around 16 µm, such as peak position, band area, and FWHM (Fig. 4). Details and examples for these fits are in Appendix B. He-implanted samples exhibit a significant red-shift (up to 320 nm for serpentine Rawhide), coupled with band area decrease (down to –22%) and widening of the feature (up to +9%). Ar-implanted samples exhibit a very slight blue shift (up to 65 nm), coupled with intense area decrease (down to –64%) and narrowing of the feature (down to –13%).

The band area ratio increases across all samples upon He and Ar implantation. The peak position for the 20 µm feature shows a red-shift for the serpentine Rawhide and saponite sample. For the serpentine UB-N sample, we detect a peak inversion upon weathering: the maximum reflectance changes from being associated with the Si-O-Si contribution to the Si-O-(M) one. Spectra shown in Fig. B2 of Appendix B can help visualise this inversion.

<figure><img src="image_5.png" /><figcaption>Fig. 2. Continuum corrected reflectance spectra from 8 to 30 µm of the pristine, He⁺ and Ar⁺ bombarded phyllosilicate pellets (in black, magenta and blue respectively): (I) serpentine Rawhide, (II) serpentine UB-N, (III) saponite. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
### 3.3. Topography variations probed by SEM

The surface of pressed pellets is topographically heterogeneous, generally with a few fractures. Certain regions exhibited compacted phyllosilicate grains or fibers. We have looked for similar morphological features for each pellet to compare them (Fig. 6). No morphological changes can be detected upon He-implantation while Ar-implanted surfaces show evident changes. Ar-induced changes include softening angles, blurring grain border, and removing every morphological feature smaller than 10 nm. Overall, Ar implantation has a smoothing effect on the weathered surface.

### 3.4. Microstructural and compositional changes probed by STEM

#### 3.4.1. Serpentine Rawhide

3.4.1.1. **He⁺ bombarded.** The FIB section was taken perpendicular to the chrysotile fibers. These fibers typically have a diameter of 30 nm with a 5 nm central hole (Appendix C, Fig. C.1). Below 700 nm in depth, the chrysotile fibers are found in their pristine state (Fig. 7.a). Between 400 and 700 nm in depth, the cohesion among the fibers appears diminished (Fig. 7.b). The region closest to the surface appears highly vesiculated, with bubbles up to several hundreds of nm. The chrysotile fibers do not survive in this region (Fig. 7.c).

Oxygen decreases in concentration in the vesiculated region, from ~64 at.% in the pristine material to ~50 at.% at the utmost surface (~10% total oxygen over 228 nm). There is also a strongly Mg-depleted surface layer, ~20 nm thick (Fig. 8, panels b-e). Mg content decreases by ~20% relative to the pristine material, making this layer rich in SiO₂. Below this depleted region, we see an increase of ~17% in Mg content over a thickness of approximately 18–20 nm (Appendix E, Fig. E.1). The Fe distribution does not change over the entire thickness. No Fe-nanoparticles were detected. Some larger bubbles are partially filled with a MgO-rich material (Fig. 8, panels c-d). We detect Mg depletion over these bubbles’ walls, spotted via the presence of a Si-rich thin rim surrounding the bubbles. Some areas have large surface bubbles/blisters and extensive regions of coalesced bubbles (Appendix C, Fig. C.3).

3.4.1.2. **Ar⁺ bombarded.** Fig. 9 shows that, below 400 nm in depth, the chrysotile fibers show no evidence of ion bombardment. From the surface down to 350–400 nm, the sample is highly vesiculated with large bubbles (20 to 120 nm in diameter). The texture of arranged chrysotile fibers has disappeared. There is a SiO₂-rich rim approximately 30 nm thick, with a smooth texture at the utmost top surface.

Like for the He case, elemental changes highlight an oxygen decrease (better seen in Appendix E Fig. E.2), although less prominent, from ~ 66 at.% in the pristine sample to ~ 57 at.% at the surface, localized in the upper part of the highly-vesiculated area (~6% total oxygen over ~106 nm). A thin Mg-depleted rim (approximately ~30% compared to the pristine material) is located near the surface, followed by an Mg-enriched area (approximately +18% compared to the pristine material). There is also MgO-rich material inside some larger bubbles, coupled with an Mg-depleted thin rim surrounding these bubbles (Fig. 10 panels b-c-d-e). No Ar was detected in the bubbles. There is no notable variation in Fe concentration over the layer. Coalesced bubbles are distributed homogeneously along the ion-bombarded layer. There is no evidence of surface bubbles or blisters.

#### 3.4.2. Serpentine UB-N

3.4.2.1. **He⁺ bombarded.** The weathered layer is about 300 nm thick. Selected area electron diffraction revealed that the layer is almost entirely amorphous with some rare crystalline islands still present (Appendix F, Fig. F.1). There are two populations of bubbles (Fig. 11, panel a-b-c). We see large elongated bubbles (10 to 100 nm in diameter)

parallel to the basal plane of the serpentine down to ~300 nm. A large population of tiny bubbles dotting the material, only a few nanometers in diameter, is also visible from 100 nm down to 300 nm in depth.

The utmost top surface region appears to be significantly depleted in O (Fig. 11.e), from 65 at.% down to 50 at.% (~9% total oxygen over ~135 nm). There is no significant change for the other elements. Bubbles appear to be empty (Fig. 11.d).

3.4.2.2. **Ar⁺ bombarded.** The weathered layer is ~70 nm thick (Fig. 12), appearing darker than the pristine material, suggesting a lower density. The layer exhibits an almost continuous empty structure of coalesced bubbles located at a depth of 40 nm. The bubbles are dotted with bright spots which were identified as Pt and Ga clusters resulting from re-deposition during the FIB section extraction process. A thin, bright layer separates the weathered layer from the pristine material. We have not been able to explain the origin of this contrast.

For this sample, the weathered layer proved particularly sensitive to electron damage. Thus, the elemental quantification analysis must be taken with a grain of salt. The weathered layer is particularly depleted in oxygen and magnesium, from ~58 at.% to ~45 at.% for oxygen (~22% total oxygen over 81 nm) and from ~19 at.% down to ~10 at.% for magnesium (Fig. 13). Iron concentration does not significantly change. Elemental mapping on the thin rim separating the two regions did not give exploitable results.

#### 3.4.3. Saponite

3.4.3.1. **He⁺ bombarded.** The weathered layer extends to ~350 nm deep. It contains elongated bubbles parallel to the basal plane of the serpentine basement, up to approximately 150 nm in diameter (Fig. 14). The fibrous texture of the saponite sample has disappeared. The layer is fully amorphous, as revealed by the absence of Bragg contrast in the TEM bright field image and by the selected area electron diffraction recorded on the layer (Appendix F, Fig. F.2). Oxygen depletion is localized near the sample’s surface (Fig. 15.a), decreasing from ~65 at. % to ~46.5 at.% (~8% total oxygen over ~130 nm). We see no noticeable changes for other elements (Fig. 15.b).

3.4.3.2. **Ar⁺ bombarded.** Effects observed here are similar to those seen for the Ar-implanted serpentine UB-N sample (Fig. 16). The weathered rim, ~80 nm thick, contains in the middle a layer of coalesced bubbles. The fibrous pristine texture, visible below 80 nm, has wholly disappeared upon weathering.

Oxygen depletion is very slight (Fig. 17.a). The concentration of other elements remains the same. Ar has remained captured in some bubbles (Fig. 17.b.II and Appendix E Fig. E.7).

## 4. Discussion

### 4.1. Weathering and spectral trends

Darkening among He-implanted samples may be related to increasing surface porosity with ion implantation (Sultana et al., 2021) which could trap and dissipate incident photons. The relationships between vesiculation and spectral changes have also been suggested by studies on weathered meteorites (Laczniak et al., 2021, Matsuoka et al., 2020, Zhang, 2022). Darkening is also detected in samples showing no significant compositional changes (He-implanted saponite). The visible slope exhibits reddening for both serpentine and blueing for the saponite (Fig. 1). Both our serpentine are Mg-rich, while the saponite sample is particularly Fe-rich. This compositional difference may be the origin of this difference in behavior. Comparing these results with spectral changes of meteorites which have undergone the same weathering procedure further suggests a link between Fe/Mg ratio and the behavior of the visible spectral slope (see Table 2).
<caption>**Table 2**</caption>

Changes in visible slope with respect to Fe/(Fe + Mg) ratio for meteorite and phyllosilicate pellets upon 40 keV He⁺ implantation at 6E16 ions/cm².

<table><thead><tr><th>Sample</th><th>VIS slope*</th><th>Fe/(Fe + Mg)</th></tr></thead><tbody><tr><td>CV Allende</td><td>reddening</td><td>0.48</td></tr><tr><td>CI Alais</td><td>reddening (slight)</td><td>0.51</td></tr><tr><td>CM Mighei</td><td>blueing (slight)</td><td>0.63</td></tr><tr><td>CO Lancé</td><td>reddening</td><td>0.63</td></tr><tr><td>C-ung Tagish Lake</td><td>blueing</td><td>0.64</td></tr><tr><td>Serpentine Rawhide</td><td>reddening</td><td>0.12</td></tr><tr><td>Serpentine UB-N</td><td>reddening</td><td>0.18</td></tr><tr><td>Saponite</td><td>blueing</td><td>0.58</td></tr></tbody></table>

Meteorite data is taken from Lantz et al., 2017, who exposed the samples to the same weathering conditions of the work presented here. The VIS slope for meteorites is computed in the range 0.55–1.05 µm. Elemental ratios are computed from (McSween and Richardson, 1977; Malissa et al., 1972; Brown et al., 2000; S. Rubino et al., 2020).

Alternatively, differences in crystal lattice structures may also play a role. In saponites, each tetrahedral-octahedral-tetrahedral (TOT) crystal unit is separated by an inter-layer of molecular water. Each TOT unit behaves as a separate entity, and lattice stress can dissipate more efficiently than for the more compact TO structure of serpentine. For the Ar-implanted samples, we see no clear trend. Visible spectroscopy may not be sensible to Ar-induced changes since the Ar-implanted layer is much smaller than the visible photon probing depth (~40 µm in serpentine, Brunetto et al., 2020).

The apparent red-shift of the Si—O stretching feature at 10 µm has been seen across all samples (Fig. 2–3). Mg-poor/Fe-rich phyllosilicate often exhibits a Si—O stretching feature located at longer wavelengths (Salisbury et al., 1987). However, Mg-depletion has only been detected for a few samples (He-/Ar-implanted serpentine Rawhide and Ar-implanted serpentine UB-N). Thus, compositional changes alone do not explain the observed band shift. It may be sufficient to break the Si-O-Mg bond to make it spectrally invisible, hence inducing the observed spectral change. Band area decreases and widening (Fig. 3), consistent across almost all samples, suggest that disordering of the mineral crystal lattice dominates spectral modifications of the Si—O stretching feature. The apparent increase in band area for the Ar-implanted serpentine Rawhide is probably related to differences in surface state between the pristine and the weathered pellet: the investigated ROIs from the two pellets probably

<figure><img src="image_6.png" /></figure>

**Fig. 3.** Changes of the SiO-stretching feature at 10 µm (1000 cm⁻¹) upon He⁺ (magenta) and Ar⁺ (blue) implantation. (a) Evolution of the band area as a function of peak position.b) Evolution of full width at half maximum (FWHM) as a function of peak position. The changes are computed with respect to the pristine material (white markers with black outline). Pristine and He-implanted spectra are acquired on the same pellet before and after weathering. In contrast, Ar-implanted spectra are acquired on a different pellet. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

<figure><img src="image_7.png" /></figure>

**Fig. 4.** Changes of the Mg-OH bending feature at 16 µm (625 cm⁻¹) upon He⁺ (magenta) and Ar⁺ (blue) implantation. (a) Evolution of the band area as a function of peak position. (b) Evolution of full width at half maximum (FWHM) as a function of peak position. The changes are computed with respect to the pristine material (white markers with black outline). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

show different crystals with different orientations, which is a common issue in all pressed pellets. The presence of large flat mineral crystals in the ROI of the Ar-implanted pellet - which maximises the reflected signal - could explain the presence of a large band area. In general, different absorption-rates of the incident light along distinct crystallographic axes in monoclinic crystals might explain these differences.

Red-shift, band-area decrease, and band widening for the Mg-OH bending feature in He-implanted samples (Fig. 4) are consistent with the crystal lattice’s disordering and dehydroxylation. Mg-decrease possibly contributes to these changes but does not suffice (since Mg-decrease is seen only on the serpentine Rawhide sample). Here again,

<figure><img src="image_8.png" /></figure>

**Fig. 5.** Changes of the Si—O bending feature around 20 µm (500 cm⁻¹) upon He⁺ (magenta) and Ar⁺ (blue) implantation. Evolution of the band area ratio of its two main contributions, the Si-O-(M) with (M) a metallic cation and Si-O-Si (respectively right and left shoulder of the broad feature) as a function of maximum peak position. Changes for the band area ratio are computed with respect to the pristine material (white markers with black outline) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
the energy input probably suffices to break the bond preferentially, but not to sputter the Mg²⁺ from the sample. The evolution of the Si—O bending feature at 20 µm is consistent with the disordering of the crystal lattice. Upon both He- and Ar-implantation, the feature’s maximum shifts towards longer wavelengths (Fig. 5), indicating changes in the relative contributions of its two spectral constituents: the Si-O-Si bending vibration (left shoulder, ~20.6–21.3 µm) and the Si-O-(M) bending vibration (right shoulder, ~22–24 µm). A shift towards longer wavelengths indicates that weathering is more efficient on the right shoulder, which is the Si-O-Si contribution. This is consistent with the increase of the band area ratio upon weathering. Ion-induced amorphization preferentially affects the tetrahedral layer rather than the octahedral layer.

## 4.2. Morphological/elemental changes

All weathered samples share specific ion-induced changes. The original fibrous/layered phyllosilicate structure does not survive in the weathered layers dominated by amorphous material (Figs. 7, 9, 11, 12 and 16). Vesiculation is always present in various forms (round large bubbles filled with MgO-rich material, small elongated empty bubbles, and layers of coalesced bubbles - Figs. 9, 11 and 16). All samples exhibit oxygen loss (more prominent for He-implanted samples - Appendix E). Samples show evidence of Mg mobility (changes in concentration, significant depletion at the surface, filling of the bubbles with a material

rich in MgO - Figs. 8, 10 and 13), with a characteristic diffusion distance of about 10–20 nm (which corresponds to the thickness of the Mg-depleted surface layers). Some samples also exhibit specific changes. The serpentine Rawhide sample shows the most significant texture (Fig. 7–9) and compositional changes upon both He- and Ar-implantation, possibly due to its tubular microstructure. The Ar-implanted serpentine UB-N sample is the only sample showing a heavy depletion of Mg and O in the weathered layer (Fig. 13). The saponite sample only shows changes relating to O content in the weathered region, indicating a better resilience to ion-weathering processes than the other samples (Figs. 15 and 17).

Observations show that the length of the ion-implantation depth is often larger than the one from SRIM simulations (see Table 3).

For highly vesiculated samples, the ion-implantation depth doubles or more. This behavior is to be expected since SRIM simulations do not account for the initial porosity of the sample nor its increase due to vesiculation upon weathering. The increased porosity in the weathered layer allows incident ions to travel further, hence the deeper implantation depth.

Vesiculation is commonly associated with space weathering. The vesiculated layers presented are compatible with the presence of surface bubbles/vesicles or blisters observed for Itokawa’s samples (Noguchi et al., 2014; Thompson et al., 2014) or highly vesiculated/coalesced regions observed for He-implanted Murchison meteorite (Laczniak et al., 2021) and lunar grains (Noble et al., 2010). Vesiculation is usually

<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Secondary electron images of the central regions of the pellets. The center of the pristine pellet is compared to the center of the He⁺ and Ar⁺ bombarded pellets.: serpentine Rawhide (a), serpentine UB-N (b) and saponite (c).</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Fig. 7.** HAADF-STEM images highlighting three different textures seen in He⁺ bombarded serpentine Rawhide. (a) compacted chrysotile fibers in the pristine region, below 700 nm from the surface. (b) intermediate region between 700 and 400 nm from the surface, with more loose fibers. (c) Surface implanted layer, between 250- and 400 nm from the surface, highly vesiculated, exhibiting no chrysotile fibers.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Fig. 8.** Serpentine Rawhide He⁺ implanted. (a) HAADF image of the sample (b) corresponding RG/SiMg composite X-ray intensity STEM-EDS map. Note the presence of a SiO₂-rich layer at the top surface (in red), (c) HAADF image of multiple bubbles, (d) corresponding RG/SiMg composite image from intensity STEM-EDS. Note the Mg-rich agglomerate inside the bubble (in green), as well as the thin Si-rich surrounding some bubbles (in red), (e) composition profile for O, Mg and Si of the utmost top surface (the dashed lines represent the elemental abundances associated with the pristine material, located deeper in the sample). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>Fig. 9. HAADF image global view of Ar + bombarded serpentine Rawhide. The pristine chrysotile fibers are located below 400 nm from the surface. A highly vesiculated region sits on top of the pristine region, where the texture of the chrysotile fibers has disappeared. The utmost top layer consists of a thin SiO-rich rim.</figcaption></figure>

related to laser heating (Matsuoka et al., 2020), but can also emerge via gas implantation, such as He and Ar gas (Demyk et al., 2001). EDS mapping showed that Ar is trapped in isolated bubbles in the saponite sample. No Ar was detected in the coalesced bubble rim. The rim coalescence may have led captured Ar to escape. Moreover, the diversity in bubble shape highlights that vesiculation is an anisotropic process, with bubble shape following the mineral structure of the ion-implanted material (elongated bubbles follow the mineral basal plane, while round bubbles emerge from a fibrous/tubular structure).

Oxygen loss upon weathering has been observed for carbonaceous

chondrites (Keller et al., 2015b), olivine (Demyk et al., 2001), and terrestrial phyllosilicates (this study). Phyllosilicates are hydrated species characterized by hydroxyls (-OH) bonds and sometimes molecular water. Oxygen loss can happen for these species through dehydroxylation (destruction of the (M)-OH bond). For serpentine, the reaction may happen as follows:

$$ (Mg, Fe)_3Si_2O_5(OH)_4 \rightleftharpoons (Mg, Fe)_3Si_2O_7 + 2H_2O(g) $$

with (Mg,Fe)₃Si₂O₇ being amorphous and H₂O as gaseous species

<figure><img src="image_13.png" /><figcaption>Fig. 10. Serpentine Rawhide Ar⁺ implanted. (a) HAADF image of the sample, (b) RG/SiMg composite image by STEM-EDS, (c) HAADF image of MgO-filled vesicles, (d) compositional variations of selected ROIs (two ROIs inside the bubbles - ROI#1 and ROI#2 - and one ROI in the surrounding material - ROI#3) indicating a Mg and O enrichment in the vesicles, (e) compositional profile (at.%) of the utmost top surface (the dashed lines represent the elemental abundances associated with the pristine material, located deeper in the sample).</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>**Fig. 11.** Serpentine UB-N He⁺ implanted. (a) HAADF image of the sample, (b) Zoomed-in HAADF images of the detected bubbles, (c) Zoomed-in HAADF images of the pristine material (d) RG/SiMg composite image from intensity STEM-EDS, (e) Global composition profile across the entire irradiated layer showing an oxygen depletion extending over about half of the layer (~ 150 nm) (the dashed lines represent the elemental abundances associated with the pristine material, located deeper in the sample).</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>**Fig. 12.** (a) Global HAADF view of Ar⁺ bombarded serpentine UB-N. (b) Zoomed-in HAADF images on the ion-bombarded layer (red selection). (c) Zoomed-in HAADF on another ion-bombarded layer, less contaminated (contamination Pt: small white dots which correspond to small clusters of Pt atoms, redeposited within the bubbles). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>**Fig. 13.** (a) Composition profile across the Ar-bombarded layer, serpentine UB-N sample. Due to noisy data, the elemental profiles have been smoothed using a mobile average (window size of 20 data points, the dashed lines represent the elemental abundances associated with the pristine material) (b) I - HAADF image of the Ar-bombarded layer in serpentine UB-N. II - RG/SiMg composite image from intensity. III - BG/OMg composite image from intensity. The strong Mg depletion is likely due to electron irradiation.</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>**Fig. 14.** He⁺ bombarded saponite (a) HAADF image showing the full irradiated layer and the saponite basement. The basal plane is close to being horizontal. Note the elongation of the bubbles in the irradiated layer along an axis which is parallel to the basal plane (b) Zoomed-in HAADF image on bubbles (red selection). (c) Zoomed-in HAADF image on the pristine material (yellow selection). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

(Gualtieri et al., 2012). Stoichiometry indicates that the atomic concentration of oxygen would shrink from 64 at.% to 58 at.%, with an overall O loss of 6%. This value is compatible with what has been detected for our samples. De-hydroxylation leads to the degassing H₂O which may have contributed to the observed vesiculation in our samples, in addition to the He and Ar gas implanted.

Mg-mobility has been seen in He-implanted Mg-rich olivine grains embedded in the matrix of the Murchison meteorite (Laczniak et al., 2021) and on terrestrial olivine (Demyk et al., 2001; Carrez et al., 2002). Our samples show a 20 nm surface rim significantly depleted in Mg (serpentine Rawhide and Ar-implanted serpentine UB-N). Ion implantation appears to sputter Mg preferentially during the amorphization process, which is compatible with Mg²⁺ in octahedral sites being more

mobile than Si⁴⁺ in tetrahedral sites (Davoisne et al., 2008). Mobile Mg atoms have left the external free surface and partially filled the bubbles in the volume. However, the slight Mg-enrichment detected just below the Mg-depleted surface layer in both He- and Ar-bombarded serpentine Rawhide indicates that some Mg manages to travel deeper inside the sample, carried by the electrostatic field created during the ion-implantation process. The walls of large bubbles in serpentine Rawhide also show Mg-depleted rims. These bubbles act as an internal sputtering surface, hence why we see a 'replica' of the surface effects on the inferior outline of these bubbles. The sputtered Mg does not escape the sample but remains trapped inside the bubbles. It precipitates together with some sputtered oxygen, creating a MgO-rich agglomerate.
<figure><img src="image_18.png" /><figcaption>Fig. 15. saponite $He^+$ implanted. (a) Global profile, (b) I - HAADF image of the sample, II - RG/SiMg composite image from intensity STEM-EDS.</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>Fig. 16. saponite Ar + bombarded. (a) HAADF image of the sample, (b) Zoomed-in HAADF image of the ion-bombarded layer. Note that the Pt contamination from FIB-sectioning, here located in the pore space between the fibers of the saponite in the non-irradiated basement.</figcaption></figure>

### 4.3. A generalized weathering mechanism for ion-implantation on hydrated silicates

One of the goals of this multi-scale analysis was to probe the link between macroscopic spectral changes and microscopic elemental/morphological changes upon weathering. According to what has been observed in our samples, we suggest the following mechanism, described in Fig. 18.

As hydrated silicates get exposed to an ion flux, three processes emerge. The first is a visible loss of texture due to the amorphization of the silicate crystal lattice, which reflects in the infrared spectrum via the spectral changes affecting the 10 and 20 µm features (band area decrease

coupled with feature widening and feature-position red-shift). A second phenomenon is gas implantation, which leads to vesiculation. We suggest that changes in subsurface porosity affect the overall albedo of the weathered samples, resulting in darkening in the visible spectral range. The third process related to ion-implantation is the preferential destruction of the -Mg-OH bond of the hydrated minerals, which translates into spectral changes in the near- and far-IR (red-shift of the 2.7 µm hydration feature - (M)-OH stretching with (M) a metallic cation - and band area decrease of the 16 µm hydration feature - Mg-OH bending). This preferential destruction of the -Mg-OH bond spurs the emission of gaseous species (possibly O₂, H₂, or H₂O) via dehydroxylation, which may contribute to vesiculation in the sample. Finally, Mg
<figure><img src="image_20.png" /><figcaption>Fig. 17. saponite Ar⁺ implanted. (a) EDS profile across the irradiated layer and HAADF image of the sample, (b) I - HAADF image of the sample, II - RY/SiAr composite image from intensity STEM-EDS, III - RG/SiMg composite image, IV - BG/OMg composite image from a the corresponding STEM-EDS map.</figcaption></figure>

<figure><img src="image_21.png" /><figcaption>Fig. 18. Schematization of weathering processes from ion-implantation on phyllosilicates and their effects on spectral observables. This schematization includes results on the 2.7 µm feature already presented and discussed in Rubino et al., 2020–2022.</figcaption></figure>

<table><caption>**Table 3**
Ion-implantation depth: SRIM simulation results and observations.</caption><thead><tr><th rowspan="2"></th><th rowspan="2">Samples</th><th colspan="2">Serpentine Rawhide</th><th colspan="2">Serpentine UB-N</th><th colspan="2">Saponite</th></tr><tr><th>40 keV He⁺</th><th>40 keV Ar⁺</th><th>40 keV He⁺</th><th>40 keV Ar⁺</th><th>40 keV He⁺</th><th>40 keV Ar⁺</th></tr></thead><tbody><tr><td rowspan="2">Ion implantation depth [nm]</td><td>SRIM simulation</td><td>317.2</td><td>40.6</td><td>322.1</td><td>41.2</td><td>332.1</td><td>42.2</td></tr><tr><td>Observations</td><td>700</td><td>300</td><td>330</td><td>70</td><td>350</td><td>80</td></tr></tbody></table>

mobility can also be observed for samples susceptible to ion damage due to the preferential destruction of the Mg-OH bond.

### 4.4. Implications for Ryugu and Bennu spectra

The spectral and elemental/morphological characterization of the samples presented here has shown several processes associated with the ion-implantation of hydrated surfaces. Some of the effects observed here

are compatible and support the interpretation of data from real-case scenarios. For instance, the spectral changes presented in this work are compatible with observations on primitive bodies Ryugu and Bennu.

In the case of Bennu, an extensive review of SW on the surface of Bennu can be found in (Clark et al., 2023) and references therein. Relating spectral changes of Bennu's surfaces is not trivial due to its surface activity, which contributes to its resurfacing and complicates discussions involving the terrain's age. However, the surrounding of the
Nightingale collect-site appears darker and redder after collection, suggesting that space-weathering on Bennu results in spectral brightening and blueing. Instances of spectral blueing and/or brightening have been observed in simulated space weathering experiments on carbonaceous chondrites for both solar-wind implantation (Lantz et al., 2017; Laczniak et al., 2021; Laczniak et al., 2024) and micrometeoroid bombardment (Thompson et al., 2019; Matsuoka et al., 2020). On the other hand, Bennu's older equatorial region appears darker and redder than the average spectrum of Bennu. Changes in the weathering regime have been observed in the Ar-implanted samples presented in this work, suggesting that different weathering processes dominate above certain fluences. The surface of Bennu is also covered in abundant craters, appearing spectrally redder and brighter than the Bennu's average spectrum (Deshapriya et al., 2021). Bennu's surface seems to be dominated by an "intermediate age" weathered surface, associated with a brighter and bluer spectrum than its subsurface material. This blue continuum's origin has been attributed to insoluble organic matter, large particle size, and/or SW on Fe-rich phyllosilicates via the creation of fine-particulate magnetite (Trang et al., 2021). The work presented here also showed that the Fe-rich phyllosilicate exhibited blueing upon weathering. However, we did not see the creation of any new nanophase in the material. It is also worth addressing that our Fe-rich saponite did not brighten like Bennu's surface but darkened, indicating that we may not be replicating the exact conditions of Bennu's surface in general. The presence of a more reducing environment on Bennu's surface - associated with the presence of insoluble organic matter (IOM) - may explain the difference in behavior between Bennu's surface and our laboratory analog. It is relevant to address that the presence of both organic matter and magnetite has been confirmed in both OVIRS and OTES spectra (Christensen et al., 2018; Hamilton et al., 2019; Simon et al., 2020) as well as by preliminary analysis of returned samples (Tachibana et al., 2023).

The hydration feature in the near-IR on Bennu appears localized at 2.74 µm (± 0.01) (Hamilton et al. 2019). Deshapriya et al. (2021) found that the hydration feature appears slightly blue-shifted for younger surfaces such as craters. These findings are compatible with the results presented by Rubino et al. (2020) regarding the effects of ion implantation on the 2.7 µm hydration feature of phyllosilicates.

Finally, for the case of Bennu, we can also look for weathering-associated changes at longer wavelengths, thanks to the OTES instrument. Previously, Lantz et al. (2020) investigated the distribution of the Si—O stretching feature at 10 µm from the Detailed Survey EQ3 OTES data. The authors highlighted the presence of two end-member peaks, at ~835 cm⁻¹ (11.97 µm) and 965 cm⁻¹ (10.36 µm), plus a third population, peaking at ~915 cm⁻¹ (10.93 µm). Space weathering effects explain this shifted third population (see effects of ion-implantation on carbonaceous chondrites in Lantz et al., 2017). Moreover, Brunetto et al. (2020) have shown that Bennu's OTES spectrum is compatible with an average spectrum of He⁺ bombarded Tagish Lake (C-unclustered type) and Mighei (CM-type) meteorites, which shows sensible position redshift of the 10 and 20 µm features. The detected shifts are also compatible with the work presented here on the 10 and 20 µm of our ion-implanted terrestrial silicates, further supporting this interpretation.

In the case of Ryugu, we do not have access to spectral data at longer wavelengths, but visible and infrared data have sufficed to assess weathering effects. For instance, Kitazato et al. (2019) found that the hydration feature in the surroundings of the artificial SCI crater (Small Carry-on Impactor, Arakawa et al., 2020) also exhibited a blue-shift to the rest of Ryugu's surface, as well as appearing brighter and bluer than the average spectrum of Ryugu. For SW to be the dominant process on Ryugu's surface, it would indicate that weathering on Ryugu induces spectral darkening and reddening, as well as the red-shift of the hydration features (also seen on returned samples, see Hiroi et al., 2023), all processes compatible with what has been observed on our weathered terrestrial phyllosilicates.

Returned material from Ryugu includes several space-weathered

grains (Noguchi et al., 2022–2023; Thompson et al., 2022; Matsumoto et al., 2022). The surface of 5% of grains from chamber A and 3% from chamber C exhibits a smooth amorphous layer with exposed blisters (<50 nm in diameter). This smooth layer is similar to that observed in this work, supporting its origin from solar wind ion implantation. These smooth layers are more reduced than the matrix (richer in Fe²⁺ than Fe³⁺). Additionally, 1% of grains from chamber A and 2% of chamber C showed frothy layers and melt splashes (Noguchi et al., 2022). These frothy layers show abundant large vesicles (0.1–1 µm in diameter) and numerous nanophases (Fe—Ni sulfide beads) localized in a thin top surface layer. They are also more reduced than the internal matrix and show similarities to laser irradiation experiments on carbonaceous chondrites (Thompson et al., 2019, 2020). This texture may originate from micrometeorite impact. Overall, Ryugu weathered grains showcase an exceptionally low abundance of nppFe⁰, contrasting with weathered lunar and Itokawa samples. Finally, dehydration and dehydroxylation have also been detected, similar to that identified on our terrestrial analogs, suggesting that solar-wind ion implantation is partially responsible for this effect.

## 5. Conclusion

This work has presented He⁺ and Ar⁺ ion-bombardment experiments focused the effects solar wind on hydrated primitive bodies by working on pellets made from terrestrial hydrated silicates. Spectral characterization highlighted darkening and reddening for the He-implanted serpentine and darkening and blueing for the He-implanted saponite sample in the visible range. No clear trend was detected for the Ar-implanted samples. Hydration spectroscopic features at 2.7 µm and 16 µm exhibit a noticeable red-shift towards longer wavelengths (except for the 2.7 µm feature for the saponite sample), coupled with the 16 µm feature band area decrease. Spectroscopic features related to the silicate structure, the 10 and 20 µm features (Si—O stretching and bending, respectively), also exhibit such a red-shift, coupled with band area decrease and width increase. These changes are consistent with lattice induced amorphization via ion-bombardment. SEM images showed the polishing effect of Ar⁺ bombardment on the surface of our pellet, while no noticeable topographical changes could be seen for the He⁺ case. STEM analysis on FIB sections of He⁺ and Ar⁺-implanted serpentine and saponite samples showed vesiculation, amorphization, dehydroxylation with oxygen loss and Mg transfer.

The multi-technique and multi-scale approach presented in this work has allowed us to probe the link between spectral and elemental changes of ion-implanted samples. This work highlights that the variability in the 2.7 µm hydration feature's position between fresh and older surfaces on hydrated bodies is clearly related to dehydroxylation via solar-wind ion implantation. On future missions where features at longer wavelengths are observable, the hydration feature at 16 µm could also be used to probe for solar-wind dehydroxylation. Dehydroxylation also contributes to the emergence and abundance of vesicles in solar-wind ion-implanted hydrated materials by creating gaseous species upon dehydroxylation. We have seen that this vesiculation affects the local porosity of the ion-implanted layers, which affects the overall albedo of the samples. Finally, we hope this work may have proved the relevance of working with terrestrial analogs and creating "ion-bombarded one mineral phase standards" to support the interpretation of space weathering hydrated surfaces for current and future studies.

## CRediT authorship contribution statement

**Stefano Rubino:** Writing – review & editing, Writing – original draft, Visualization, Validation, Software, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Hugues Leroux:** Writing – review & editing, Supervision, Funding acquisition. **Cateline Lantz:** Writing – review & editing, Validation. **Alice Aléon-Toppani:** Writing – review & editing, Validation. **Donia Baklouti:** Writing –
review & editing, Validation. **Zahia Djouadi:** Writing – review & editing, Validation. **Obadias Mivumbi:** Investigation. **David Troadec:** Investigation. **Ferenc Borondics:** Writing – review & editing, Validation, Supervision, Methodology. **Rosario Brunetto:** Writing – review & editing, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Data availability

The data that support the findings of this study are available from the corresponding author, Stefano Rubino (stefano.rubino@inaf.it) upon reasonable request. Spectroscopic data is directly available on the SSHADE-DAYSY repository (Brunetto and Baklouti, 2017):●

visible and near-IR: https://www.sshade.eu/data/experiment/EXPERIMENT_SR_20200204_00, https://www.sshade.eu/data/experiment/EXPERIMENT_SR_20200204_01, https://www.sshade.eu/data/experiment/EXPERIMENT_SR_20200204_02, https://www.sshade.eu/data/EXPERIMENT_SR_20200204_03●

Mid-IR: https://www.sshade.eu/data/experiment/EXPERIMENT_ZeD_20230103_01●

far-IR: https://www.sshade.eu/data/experiment/EXPERIMENT_ZeD_20230103_02

## Acknowledgements

We thank D. Ledu, C.O. Bacri, and P. Benoit-Lamaitrie for help and technical support with SIDONIE and INGMAR. INGMAR is a joint IAS-IJCLab (Orsay, France) facility funded by the P2IO LabEx (ANR-10-LABX-0038) in the framework Investissements d'Avenir (ANR-11-IDEX-0003-01). We would also like to thank A. Fadel for help and technical support with the SEM acquisitions. We also thank Dr. Moe Matsuoka, Dr. David Trang and the two other anonymous reviewers that went through the previous iterations of this work for their crucial comments and suggestions. This work has been funded by the ANR project CLASSY (Grant ANR-17-CE31-0004-02) of the French Agence Nationale de la Recherche. The micro-spectros-copy measurements were supported by grants from Region Ile-de-France (DIM-ACAV) and SOLEIL. HL thanks I-SITE ULNE and the MEL for funding. The electron microscopy works at the University of Lille were performed with the support of the Chevreul Institute and funding from the CPER projects ARCHI-CM and CHEMACT.

## Appendix A. Full visible and near-IR dataset

In this section, we show the complete dataset from our in-situ visible and near-IR spectroscopy measurements, acquired after different ion fluences under vacuum. Results from the data in the near-IR range have already been presented and discussed in Rubino et al., 2020–2022.

<figure><img src="image_22.png" /><figcaption>Fig. A.1. Full visible dataset acquired in-situ, under vacuum, with the INGMAR setup of our sample in both pristine condition and upon ion bombardment.</figcaption></figure>
<figure><img src="image_23.png" /><figcaption>Fig. A.2. near-IR dataset - In-situ evolution of the 2.7 µm feature before (non-bombarded - black dots) and after maximum ion fluence (6*1016 ions/cm2 for He⁺ (left panel) and (2*1016 ions/cm2 for Ar⁺ (right panel) - grey dots). Spectra were shifted for clarity. A linear continuum correction from 2.64 µm to 4 µm was applied. Solid lines represent the Savitzky-Golay filter used for denoising the spectra. For plotting, the linear scale is used with respect to wavenumbers. The right sup-panel of each panel is a zoom on the region of the 2.7 µm bands.</figcaption></figure>

## Appendix B. 5-Gaussian fitting of far-IR dataset

In this section, we showcase the result of the 5 Gaussian fitting procedure which was applied on our far-IR dataset to investigate the evolution of spectral parameters describing the (M)-OH bending mode around 16 µm and the broad 20 µm silicate feature upon 40 keV He⁺ (6E16 ions/cm²) and 40 keV Ar⁺ (2E16 ions/cm²) bombardment. The fitting procedure was applied on data which had been cut between 680 and 200 cm⁻¹ (14.7 to 50 µm) and continuum-corrected using a rubber-band type continuum. In the following table, the reduced χ² value associated with each fit is disclosed.

<caption>**Table B.1**</caption>

Reduced χ² value associated to each instance of 5 Gaussian fitting of our far-IR dataset.

<table><thead><tr><th>Sample</th><th></th><th>Reduced χ²</th></tr></thead><tbody><tr><td rowspan="3">serpentine Rawhide</td><td>pristine</td><td>0.35</td></tr><tr><td>He⁺</td><td>0.20</td></tr><tr><td>Ar⁺</td><td>0.40</td></tr><tr><td rowspan="3">serpentine UB-N</td><td>pristine</td><td>0.20</td></tr><tr><td>He⁺</td><td>0.09</td></tr><tr><td>Ar⁺</td><td>0.06</td></tr><tr><td rowspan="3">saponite</td><td>pristine</td><td>0.04</td></tr><tr><td>He⁺</td><td>0.05</td></tr><tr><td>Ar⁺</td><td>0.12</td></tr></tbody></table>

Fig. B.1-3 shows the results of the 5 Gaussian fitting procedure on our far-IR dataset.
<figure><img src="image_24.png" /><figcaption>Fig. B.1. 5-Gaussian fitting of pristine and ion-bombarded spectroscopic data from serpentine Rawhide.</figcaption></figure>

<figure><img src="image_25.png" /><figcaption>Fig. B.2. 5-Gaussian fitting of pristine and ion-bombarded spectroscopic data from serpentine UB-N.</figcaption></figure>
<figure><img src="image_26.png" /><figcaption>Fig. B.3. 5-Gaussian fitting of pristine and ion-bombarded spectroscopic data from saponite.</figcaption></figure>

## Appendix C. Supplementary STEM Fig.s

In this section, we show additional STEM figures supporting claims and observations done in the main text of this work.

<figure><img src="image_27.png" /><figcaption>Fig. C.1. shows the serpentine Rawhide fibrous morphology as seen in their pristine state.</figcaption></figure>

Fig. C.1. serpentine Rawhide fibrous morphology, as seen in their pristine state. (a) SEM image of chrysotile fibers on the surface of the pristine serpentine Rawhide pellet, (b) HAADF image of chrysotile fibers sections, as seen in the FIB sections, (c) circular shape of the chrysotile fibers with various sizes, highlighted with dashed-line yellow circles to help the reader.

Fig. C.2 showcases the three distinct regions observed upon He⁺ bombardment of the serpentine Rawhide sample, as well as a study on the variation of local porosity as a function of depth. The porosity pores' size variations make the distinction among these three regions clearer.
<figure><img src="image_28.png" /><figcaption>**Fig. C.2.** (a) HAADF global image of $He^+$ bombarded serpentine Rawhide. (b) Threshold-ed image highlighting the porosity inside the sample (bubbles, fractures and voids). (c) Evolution of the porosity as a function of depth (binned along the x-axis for a bin-size of 5 nm). (d) distribution of the Feret's diameter of 881 voids (identified by thresholding) as a function of depth (x-axis bin-size of 5 nm, y-axis bin-size of 10 nm).</figcaption></figure>

<figure><img src="image_29.png" /><figcaption>**Fig. C.3.** shows regions where we saw surface bubbles and coalesced bubbles cracking the structure of serpentine Rawhide in the $He^+$ bombarded sample.</figcaption></figure>

**Fig. C.3.:** STEM-HAADF images. For serpentine Rawhide $He^+$ (and only for this sample), we see a few *surface bubble* (a), slightly larger than those seen deeper in the sample (b). Localization and size could hint at another origin for these surface bubbles. In the right panel, we see the aftermath of coalesced bubbles, ripping up the sample.

**Fig. C.4** showcases the two distinct regions observed upon $Ar^+$ bombardment of the serpentine Rawhide sample, as well as a study on the variation of local porosity as a function of depth. The porosity pores' size variations make the distinction among these two regions clearer.
<figure><img src="image_30.png" /><figcaption>Fig. C.4. (a) HAADF global image of Ar⁺ bombarded serpentine Rawhide, seen parallel to the a crystallographic axis. (b) threshold-ed image highlighting the porosity inside the sample (bubbles, fractures and voids). (c) Evolution of the porosity as a function of depth (binned along the x-axis for a bin-size of 5 nm). (d) Distribution of the Feret's diameter of 653 voids (identified by thresholding) as a function of depth (x-axis bin-size of 5 nm, y-axis bin-size of 10 nm).</figcaption></figure>

Fig. C.5 shows regions where we saw surface bubbles and coalesced bubbles cracking the structure of serpentine Rawhide in the Ar⁺ bombarded sample.

<figure><img src="image_31.png" /><figcaption>Fig. C.5. STEM-HAADF image. Coalesced bubbles in Ar⁺ bombarded serpentine Rawhide.</figcaption></figure>

## Appendix D. Oxygen curve fitting via diffusion model

To better investigate the evolution of oxygen as a function of depth in our FIB sections, the oxygen profiles were fitted as a function of depth x using an diffusion model based on the error function (erf function):

$$ \frac{C_x - C_s}{C_0 - C_s} = \erf\left(\frac{x}{2\sqrt{Dt}}\right) $$

with:
* Cₓ the measured concentration of oxygen through the irradiated layer, the interface is positioned at x = 0;

* C₀: the starting concentration of oxygen in the phyllosilicate prior to the experiment;

* Cₛ: the concentration of oxygen at the surface;

* 2√Dt is the characteristic diffusion distance of oxygen.

One instance of this fitting is shown in the following Fig. D.1, applied to the oxygen profile in the He⁺ bombarded serpentine Rawhide. Here the measured diffusion distance is ~230 nm.

<figure><img src="image_32.png" /><figcaption>Fig. D.1. Oxygen concentration profile in He⁺ bombarded serpentine Rawhide in blue, as measured by STEM-EDS. Fitted profile using a diffusion model in red. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

In the following Table D.1, all fitting parameters for all our samples are summarised.

**Table D.1**

Fitted parameters for oxygen profiles across all our samples.

<table><thead><tr><th rowspan="2">Samples</th><th rowspan="2"></th><th colspan="3">Fitted parameters</th><th rowspan="2">Reduced Chi-squared</th></tr><tr><th>C₀ [%]</th><th>Cₛ [%]</th><th>2√Dt [nm]</th></tr></thead><tbody><tr><td rowspan="2">serpentine Rawhide</td><td>He⁺</td><td>64.7</td><td>50.6</td><td>228</td><td>1.14</td></tr><tr><td>Ar⁺</td><td>66.8</td><td>50.3</td><td>106</td><td>2.81</td></tr><tr><td rowspan="2">serpentine UB-N</td><td>He⁺</td><td>65.1</td><td>39.4</td><td>135</td><td>2.20</td></tr><tr><td>Ar⁺</td><td>59.8</td><td>32</td><td>81</td><td>30.16</td></tr><tr><td rowspan="2">saponite</td><td>He⁺</td><td>64.6</td><td>46.2</td><td>130</td><td>1.54</td></tr><tr><td>Ar⁺</td><td>64</td><td>42</td><td>110</td><td>4.26</td></tr></tbody></table>

It is relevant to address that the model works less efficiently for Ar⁺ serpentine UB-N and saponite, so the values derived for the fitting parameters need to be taken grain of salt. It is possible that for these two samples, the oxygen removal upon Ar⁺ bombarded is not dominated by “gentle” diffusion and escape, but by other processes more “ballistic” in nature, such as sputtering, due to the larger effective cross section of Ar⁺ pushing forward amorphization and atomic displacement. The presence of large voids, which hinders the compositional mapping of these samples, may also lead to difficulties in fitting the obtained compositional profiles.

**Appendix E. Compositional profiles full dataset**

In this section, we show the complete range of large compositional profiles from our FIB section, to better appreciate the evolution of the composition at the surface with respect to deeper, non-bombarded matter.
<figure><img src="image_33.png" /><figcaption>Fig. E.1. STEM-EDS compositional profile in He⁺ bombarded serpentine Rawhide.</figcaption></figure>

<figure><img src="image_34.png" /><figcaption>Fig. E.2. STEM-EDS Compositional profile in Ar⁺ bombarded serpentine Rawhide.</figcaption></figure>

<figure><img src="image_35.png" /><figcaption>Fig. E.3. STEM-EDS Compositional profile in He⁺ bombarded serpentine UB-N.</figcaption></figure>
<figure><img src="image_36.png" /><figcaption>**Fig. E.4.** STEM-EDS profile across the Ar-bombarded serpentine UB-N sample. Due to noisy data, elemental profiles have been smoothed using a mobile average (window size of 20 data points). Note that there is a normalisation issue with the global profile here, possibly to data processing malfunctions. The origin of this discrepancy is still being investigated. However, this does not hinder us from discussing relative elemental variations across the profile.</figcaption></figure>

<figure><img src="image_37.png" /><figcaption>**Fig. E.5.** STEM-EDS Compositional profile in He⁺ bombarded saponite.</figcaption></figure>
<figure><img src="image_38.png" /><figcaption>Fig. E.6. STEM-EDS Compositional profile in Ar⁺ bombarded saponite.</figcaption></figure>

In the following figure, the evolution of Ar at.% abundance at the surface of the Ar⁺ bombarded saponite sample is reported, to further showcase the effect of Ar "trapping" inside the bombarded sample.

<figure><img src="image_39.png" /><figcaption>Fig. E.7. STEM-EDS Composition profile highlighting the evolution of Ar at.% along surface profiles extracted from the Ar⁺ bombarded saponite FIB section. The blue curve is associated to an Ar⁺ bombarded feldspar grain, identified in the saponite FIB section, which also underwent Ar⁺ bombardment. The effects of Ar⁺ bombardment on this isolated feldspar grain are discussed in the following section. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

## Appendix F. Electron diffraction analysis

Electron diffraction images were acquired on FIB section from the He⁺ bombarded Serpentine UB-N and Saponite samples. No data was acquired from the Serpentine Rawhide sample. Moreover, we were not able to acquire data from the Ar⁺ bombarded sample due to the short implantation depth of Ar⁺ on our samples. Fig. F.1 highlights that the weathered layer is dominated by amorphous material, with crystalline islands still present locally. On the other hand, Fig. F.2 shows that the weathered layer in the Saponite sample is rendered completely amorphous.
<figure><img src="image_40.png" /><figcaption>Fig. F.1. Serpentine SARM - Selected area electron diffraction (a) unirradiated serpentine basement (b) He⁺ bombarded layer. The layer is dominated by amorphous material, as evidenced by the diffuse ring, but crystalline islands are still present locally.</figcaption></figure>

<figure><img src="image_41.png" /><figcaption>Fig. F.2. Saponite - Selected area electron diffraction (a) unirradiated saponite basement (b) He⁺ bombarded layer. The diffuse ring attests to a complete amorphization of the initial crystal structure.</figcaption></figure>

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