Ion irradiation of aubrites: visible to mid-infrared spectroscopic effects of space weathering simulation on Mercury analogues

Introduction

Aubrites are a rare group of highly reduced achondrites. They are characterized by very low amounts of FeO in silicates and the presence of sulphides composed of typically lithophile elements, indicating a formation under extremely reducing conditions [1]. Their highly reduced nature and mineralogy suggest that aubrites represent promising analogues of Mercury’s surface rocks [2] based on MESSENGER mission results [3]. Mercury’s surface is affected by space weathering processes that influence its spectral properties increasing the VIS spectral slope (spectral reddening) and suppressing the absorption features in the VIS-NIR spectral range, due to the presence of iron nanoparticles (npFe⁰) [4]. For a more consistent comparison between the spectral properties of Mercury’s surface and of its potential analogues, the effect of space weathering should be considered. So far, space weathering simulations through ion irradiation on natural highly reduced materials have been conducted on one enstatite chondrite [5]. In this study we investigate ion irradiation spectral effects in the visible-middle infrared region on four aubrites: Norton County, Rantila, Peña Blanca Spring and NWA 14185.

 

Methods

The aubrite samples were crushed into powder (≤100 μm) and pressed to create pellets for the ion irradiation experiments [e.g. 6,7,8]. To simulate the solar wind irradiation affecting Mercury’s surface, we performed two separate ion irradiation experiments using He⁺ at 20 keV and C⁺ at 30 keV. He⁺ was chosen because helium represents an important component of the solar wind ions. C⁺ was chosen to simulate the effects of another solar wind species, whose irradiation effects on meteorites have been investigated only in a few studies [9].

The irradiation experiments were conducted under vacuum (P ~ 10^-7 mbar) using the SIDONIE electromagnetic isotope separator at the IJCLab, Université Paris-Saclay, Orsay, France. The whole sample surface was irradiated at every fluence step (1 x 10¹⁶, 3 x 10¹⁶, 6 x10¹⁶ and 1 x 10¹⁷ ions/cm²).

The visible (VIS) and near-infrared (NIR) reflectance spectroscopy measurements were conducted in-situ, in vacuum and at room temperature, through the INGMAR (IrradiatioN de Glaces et Météorites Analysées par Réflectance VIS-IR) setup, of IAS (Institut d'Astrophysique Spatiale)- IJCLab, Université Paris-Saclay, Orsay, France. VIS bidirectional reflectance spectra were acquired using a halogen visible source and a Maya2000Pro (Ocean Optics) spectrometer. NIR bidirectional reflectance spectra were acquired using a Fourier Transform spectrometer (Tensor37 Bruker). Middle-infrared (MIR) reflectance spectra were taken ex-situ on fresh samples and on samples after the final irradiation dose at SMIS (Spectroscopy and Microscopy in the Infrared using Synchrotron) beamline of the Synchrotron SOLEIL, France, using an Agilent Cary 670/620 micro-spectrometer equipped with MCT detector.

 

Preliminary results and discussion

Spectral effects of ion irradiation show a common general trend for all the investigated samples, and it is possible to observe some differences in the spectral effects produced by the two different ions. The main effects in the VIS-NIR range are a general spectral darkening and reddening, progressive with the increasing of irradiation doses. The extent of darkening and reddening varies among the samples, for both He⁺ and C⁺ irradiations. A possible explanation is that the samples present some differences in their mineralogy and mineral chemistry. For example, Rantila aubrite presents higher FeO contents (up to 0.3 wt%) in silicates than the other samples (<0.02 wt%) which causes a visible absorption band at ~0.9 μm, while the other samples are almost featureless. Rantila spectra also show a small absorption feature around 0.5 µm attributable to a spin-forbidden Fe²⁺ absorption in olivine [10], which is present in larger amount in this sample than in the others. These absorption features flatten with increasing irradiation doses, for both He⁺ and C⁺. Some absorption features are visible in the 1.9-2.1 μm region due to a minor terrestrial aqueous alteration of the samples. These features do not seem to be affected by irradiation.

In the MIR range we observe a shift in the position of the Christiansen feature (CF) and the Reststrahlen bands (RBs) between the spectra of fresh and irradiated samples. Also, the reflectance intensity of the RBs decreases, and they become less sharp.

 

Ongoing activities and perspectives

The data analysis is still ongoing, and additional investigations are planned on rock fragments of these aubrites to compare the effects of He⁺ and C⁺ irradiation on powders (pellets) and rocks. Studying the spectral properties of these highly reduced meteorites could support the future investigation of Mercury with the ESA's BepiColombo mission [11]. In particular, the knowledge of the spectral properties of reduced meteorites, in correlation with their mineralogy, could be useful in the interpretation of the future data from the Visible and near-Infrared Hyperspectral Imager (VIHI)/Spectrometer and Imaging for MPO BepiColombo Integrated Observatory SYStem (SIMBIO- SYS) [12] and the Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) [13] which will map the surface in the 0.4-2 μm and 7–14 μm, respectively.

 

Acknowledgements

This research is supported by the Italian Space Agency (ASI) under ASI-INAF agreement 2017-47-H.0 (Bepicolombo SIMBIO-SYS) and financed by the Space It Up project funded by the Italian Space Agency, ASI, and the Ministry of University and Research, MUR, under contract n. 2024-5-E.0 – CUP n. I53D24000060005. It has been conducted during and with the support of the Italian national inter-university PhD programme in Space Science and Technology, University of Trento.

 

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