Effects of ion irradiation on Mercury terrestrial analogues in the visible to mid-infrared

The surface of Mercury is subject to space weathering that complicates remote sensing data analysis [1]. Constraining the effect of space weathering is necessary to reconstruct the geological history and surface evolution of the planet using spectral data. Previous studies highlighted the necessity to deeper investigate spectral changes induced by ion irradiation simulating solar wind reaching Mercury [1, 2].

We present an experimental study performed on Mercury’s volcanic surface analogues. We used 20 keV He⁺with fluences up to 5.10¹⁷ ions/cm² to simulate ion irradiation reaching the surface. Terrestrial ultramafic lava already identified as good analogues for Mercury were used [3, 4, 5]: a boninite, a basaltic komatiite and a komatiite. Spectra were acquired in the VMIR wavelengths range between 0.4 and 16 μm. Ion irradiations were performed with the SIDONIE electromagnetic isotope separator (CSNSM, Orsay, France) [6] interfaced with the INGMAR (IrradiatioN de Glaces et Météorites Analysées par Réflectance VIS-IR, IAS-CSNSM, Orsay, France) setup [7]. Using INGMAR, we performed VNIR in-situ reflectance spectroscopy measurements. MIR analysis were conducted at the Synchrotron SOLEIL (France) using the SMIS beamline (Spectroscopy and Microscopy in the Infrared using Synchrotron) [8]. 

Several spectral modifications induced by irradiation are observed. In the VNIR samples show an exponential darkening, a reddening and a flattening of spectra. Above a certain irradiation dose, the darkening reaches a plateau while the reddening and flattening do not show any definable trend. After a certain irradiation dose/time exposure spectral differences among different units on Mercury will be limited. In the MIR we observe a red-shift of Reststrahlen bands. The Christiansen feature is red or blue shifted according to the irradiation dose. Spectral modifications in the VMIR are closely influenced by the composition and will likely participate in the origin of spectral heterogeneities on Mercury.

This work provides ground-truth data for the future ESA/JAXA/BepiColombo observations [9]. VMIR spectral modifications induced by ion irradiation simulated in laboratory will be used for future SIMBIO-SYS (Spectrometer and Imaging for MPO BepiColombo Integrated Observatory SYStem) [10] and MERTIS (Mercury Radiometer and Thermal Infrared Spectrometer) [11] data analysis.

 

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