Space Weathering simulation on the Aubrite meteorite NWA 13278, putative analogue of Mercury

Introduction

The surface of Mercury as seen by the MESSENGER spacecraft is mostly featureless in the Visible-to-Near-Infrared range (VIS-to-NIR) [1-4], except for some restricted locations [5,6], making it difficult to constrain its mineralogical composition. However, ground-based measurements of the planet in the Mid-Infrared (MIR) range suggest the occurrence of labradorite (a plagioclase with Anorthite 70-50) and enstatite (a Mg-rich orthopyroxene) on its surface [7], also supported by spectral modelling [8]. Thus, it was suggested that enstatite-rich meteorites (aubrites) could be potential analogues of Mercury [9], generally made of iron-free enstatite, sulfides and metallic iron [10]. 

The spectrometer MASCS [11] and the camera MDIS [12] onboard MESSENGER provided featureless spectra with a red slope on the entire surface of Mercury and with an average reflectance level of 4% in the VIS range [13]. Given the close vicinity to the Sun and the lack of an atmosphere, Mercury is constantly exposed to Space Weathering (SpWe) processes such as solar wind irradiation and micro-meteoritic impacts, which can alter the spectral properties of its surface, producing a reddening and darkening, as well as removing absorption bands [14,15,4].

SpWe processes are usually reproduced in the laboratory with ion irradiation and laser bombardment. In this work, we irradiated the enstatite achondrite meteorite NWA 13278 with He⁺ to simulate solar wind irradiation on Mercury’s surface. Spectral variations caused by the experiment are analysed.

 

NWA 13278

The sample was purchased with dimensions of 8.0×5.0 cm, and a small piece of 1.0×0.9 cm and a thickness of 0.3 cm was obtained for the investigation.  From a petrological analysis, the sample is a breccia with large clasts of enstatite and forsterite (Mg-rich olivine) within a finer-grained matrix composed of enstatite, sodic plagioclase (albite), Cr-troilite, niningerite and Si-bearing kamacite. Diopside (clinopyroxene) is also present through geochemical analysis [16]. The mineralogical composition of Mercury, as spectrally modelled by [17], is quite similar, including orthopyroxene, olivine, Mg-rich clinopyroxene, Na-rich plagioclase, orthoclase, and Fe-Ti agglutinic glasses.

 

Ion irradiation experiment and spectral acquisition

The sample was irradiated in vacuum (pressures of 10^-7 mbar) at the Ion and Energetic Neutral Atoms (I-ENA) facility with a beam of 4.5 keV He⁺ ions. The beam spot size was around 2.5 mm in diameter (Fig.1). Full details of the I-ENA facility can be found in the abstract in the same session (Richards, G., 2025, Planetary space weather experiments at the I-ENA facility, EPSC-DPS Joint Meeting 2025).  After about 40 hours of irradiation, the maximum fluence reached was 4×10¹⁴ ions/cm².

Several reflectance spectra in the VIS-NIR and MIR range of the virgin and processed sample were acquired in the irradiated area to evaluate spectral variations. Average spectra are then obtained. The VIS-NIR spectra were acquired at “SLAB” (Spectroscopy Laboratory), while the MIR data were obtained at “SPARK LAB” (Space Rocks Key Analysis), both at IAPS-INAF.

The VIS-NIR bidirectional reflectance spectra (i=30°, e=0°)  cover the 0.4-2.5 µm spectral range with a spectral resolution of 3 nm in the VIS range and 10 nm in the NIR one. The spot of the acquisition is 6 mm. The MIR spectra (i=0°) acquired in diffused reflectance cover the 3-14 µm spectral range with a spectral resolution of 4 cm^-1 and a spot of 2.5 mm.

 

 

Results

The irradiated sample shows a spectral brightening and a slight bluing in the VIS range with respect to the spectrum of the virgin one, and a spectral reddening in the NIR range (Fig. 2). The reflectance at 0.55 µm increases from 0.124 to 0.128 and the VIS slope in the 0.5-0.8 µm range, estimated as in [13], changes from -0.38 to -0.39 µm^-1. The NIR slope estimated as a linear fit in the 0.8-1.2 µm range reddens from -0.24 to -0.20 µm^-1.

 

The MIR spectrum of the meteorite shows Reststrahlen peaks at ∼8.6 µm, ∼9.0 µm and ∼10.6 µm (due to plagioclase), ∼9.3 µm, ∼10.4 µm and ∼11.6 µm (diopside), ∼9.8 µm and ∼11.1 µm (enstatite), ∼10.2 µm (due to the combination of the peaks of enstatite and plagioclase) (Fig.3). The Christiansen Feature is at 7.60 µm, coherent with plagioclase. After irradiation, the strongest peaks at ∼9.0 µm and ∼9.3 µm shift longwards of about 20 nm, following previous experiments [18,19], while a shift of 30 nm is observed for the peaks at ∼10.4 µm, ∼10.6 µm, and ∼11.1 µm. The other peaks show either a shift toward shorter wavelengths or no variation in their position.

Discussion and future works

The irradiation experiment performed on the NWA 13278 aimed to simulate the solar wind irradiation on Mercury’s surface. The spectral reddening in the NIR range caused by the He⁺ exposition is coherent with the spectral behaviour observed on Mercury, where older and more degraded terrains are spectrally redder than younger ones [1, 20]. Anyway, the spectrum of the irradiated meteorite shows a blue spectral slope, conversely to Mercury’s spectra. It is worth noting that we simulated an exposure to SpWe of about a few hundred years, thus we reproduced the onset of spectral degradation. To better reproduce hermean spectra, we will perform a laser bombardment to simulate the effects caused by micro-meteoritic impacts.

 

References

[1] Robinson et al., Science 321 (2008). [2] Murchie et al., Science 321 (2008). [3] Denevi et al., Science 324 (2009). [4] Blewett et al., EPSL 285 (2009). [5] Vilas et al., Geophys. Res. Lett. 43 (2016). [6] Lucchetti et al., JGR: Planets 123 (2018). [7] Sprague and Rousch, Icarus 133 (1998). [8] Warell and Blewett, Icarus 168 (2004). [9] Burbine et al., LPI 1097 (2001). [10] Watters 10^th LPSC (1979). [11] McClintock and Lankton SSR 131 (2007).  [12] Hawkins et al., SSR 131 (2007).  [13] Galiano et al, Icarus 388 (2022).  [14] Hapke, JGR 106 (2001).[15] Brunetto and Strazzulla, Icarus 179 (2005). [16] Gattacceca et al., MAPS 56 (2021). [17] Warell et al., Icarus 209 (2010) [18] Brunetto et al., PSS 158 (2018). [19] Lantz et al., Icarus 285 (2017).  [20] Domingue et al. SSR 181 (2014).

 

Acknowledgements

The measurements described are part of the “SIMILIS” project, in the outcome of "Mini Grants INAF 2023”.