Solar wind ion-bombardment experiment on Lunar meteorites

Introduction. The Moon, as the first planetary body explored beyond Earth, has long been a cornerstone of space exploration. Following early sample return missions by the USA and the USSR, renewed global interest (following recent missions from the USA, China, India, and Japan) has emphasised the need for detailed knowledge of the lunar surface. Mafic minerals like pyroxenes and olivine are widespread on this surface [1, 2], whose spectral features in the near infrared are altered over time by space weathering processes, including solar wind irradiation and micrometeorite impacts [3, 4]. These effects, first observed in Apollo and Luna samples, modify both the chemical composition and optical properties of lunar materials. Therefore, understanding the extent and nature of space weathering is crucial to interpreting remote sensing data. The MoonSWA (Moon Space Weathering Analysis) project, selected in the framework of "Bando Ricerca Fondamentale INAF 2023 (PI: F. Zambon), targets a twofold approach combining remote sensing spectral analysis with laboratory simulations on lunar analogues to investigate space weathering effects on the Moon. The work presented here focuses primarily on the experimental part of this project.

Materials and Methods. We emulate the effects of the solar wind component of SpWe on the Moon’s surface by performing ion-bombardment experiments on several Lunar meteorites, which are not often used for this kind of work due to their scarcity: NWA 8786 (one slab + one pressed-powder pellet), NWA 14188 (one slab + one pellet) and NWA 13859 (slab in epoxy). We used the INGMAR (IrradiatioN de Glaces et Météorites Analysées par Réflectance, Institut d’Astrophysique Spatiale (IAS) - Laboratoire des deux Infinis Irène Joliot Curie (IJCLab), Orsay) vacuum chamber, at room temperature and under vacuum (P ∼ 10⁻⁷ mbar), using 20 keV He⁺ ions provided by the SIDONIE (IJCLab - Orsay) ion-implanter. We then monitored the spectroscopic evolution of our samples in the visible/near-IR range (0.3 to 4 μm) with increasing fluences up to 1x10¹⁷ ions/cm² (10³ - 10⁵ years of exposure at 3AU). Using an infrared microscope, we also acquired additional spectroscopic data in the mid-IR (2 to 16 μm), both before and after the weathering of our samples. We selected multiple ROIs for each sample to investigate ion bombardment's effects with respect to our samples' mineralogical heterogeneity.

Preliminary Results. In the visible to the near-IR range, we focus on the spectral changes affecting several spectral parameters, namely the reflectances at 380, 465 and 550 nm, the absorption band at 1 μm (Band I) and the absorption band centred at ~2 μm (Band II), associated with the spin-allowed crystal field transitions in Fe2+ in olivine and pyroxene, and the spectral slopes associated with Band I and Band II, as well as the Global slope, defined as the slope between the left shoulder of Band I and the right shoulder of Band II. These parameters have been selected since they have been used and deemed the most sensible to ion-bombardment spectral changes on other mafic materials, more specifically HEDs meteorites [5], and are now being tested for applicability for Lunar samples.

Across our samples, we see darkening at all probed wavelengths in the Reflectance space. Band I appears far less affected than the other selected parameters. Finally, in Slope space, we see instances of slope reddening followed by bluing, the bluing phase being associated with the largest achieved ion fluence of 1.0x1017 ions/cm2. An example of this parameter analysis is shown in Figure 1 for the NWA 8786 pellet sample.

Figure 1. The behaviour of NWA 8786 pellet sample upon 20 keV He+ bombardments with increased ion-fluence. The black arrow represents the increase in fluence. The associated fluences are: 0 (pristine), 0.7, 1.5, 3, 5 and 10 (x1016) ions/cm2. We see darkening across all proved fluences, Band I intensity and area virtually unvarying and reddening slope followed by blueing.

In the mid-IR, we focus on the behaviour or the Reststrahlen features associated with the vibrational modes of the meteorite’s mineral crystal lattices. For instance, we see a systematic red-shift (peak position shift towards longer wavelengths) of the Si-O stretching feature around 10 microns (with varying amplitude, from tenths to hundreds of nanometres), coupled with band depth decrease (up to approximately -8% variation in intensity). Band width is not as affected as peak position and band depth. An example of this analysis on the NWA 8687 chip sample can be seen in Figure 2.

Figure 2. Spectral data before (black) and after ion-bombardment (grey) of two spots from the NWA 8687 chip sample, with associated optical images of said 124x124 μm spots. We see a peak position red-shift coupled to a Si-O stretching band decrease upon weathering.

Conclusion and Perspective. This is a work in progress, as the spectral parameters for all samples have not yet been derived. However, preliminary results suggest that the ion-bombardment-induced spectral trends are coherent with what is commonly associated with Lunar-type SpWe. Additional measurements with scanning and transmission electron
microscopy (SEM and STEM) on both the samples’ ion-bombarded surfaces and focused-ion-beam (FIB) sections extracted from them are being considered to probe ion-bombardment-induced morphological and elemental sample modification and possibly associate these changes with the already identified spectroscopic variations.

Acknowledgement. We thank C. Lantz, O. Mivumbi, J. Boderfois, and P. Benoit-Lamaitrie for their 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). This work is part of the INAF MoonSwa mini-grant (PI: F. Zambon) and PRIN INAF MELODY (PI: F. Tosi). SR is supported by the ASI-INAF agreement n.2023-6-HH.0 (Resp: G. Piccioni).

References. [1] McCord, T. B. et al. J. Geophys. Res. 86, 10883–10892 (1981); [2] Pieters, C. M. Reviews of Geophysics 24, 557–578 (1986). [3] Pieters, C. M. et al. Meteorit. Planet. Sci. 35, 1101–1107 (2000). [4] Hapke, B. Journal of Geophysical Research: Planets 106, 10039–10073 (2001). [5] Rubino, S. et al. Icarus, in review.