Spectroscopy (VNIR-MIR) of lab-made silicate glasses as analogues for volcaniclastic materials on Mercury: insights on chemical composition and grain size

Introduction: 

Volcanic and magmatic processes shaped the surfaces and contributed to the mineral diversity of Mercury. Beyond the ubiquitous presence of effusive products (smooth plains) explosive volcanism is suggested by the presence of depressions surrounded by high-reflectance halos, interpreted as eruptive centers, as well as calderas and vents linked to impact structures or faults  [1]. BepiColombo hosts SIMBIO-SYS and MERTIS, two instruments that will characterize the surface of Mercury with different methodologies:
SIMBIO-SYS (Spectrometer and Imagers for MPO BepiColombo Integrated Observatory System) is a system for imaging and spectroscopic analysis. Its goals are to study surface geology, volcanism, tectonics, age, composition, and geophysics. One of the systems of SIMBIO-SYS is the Visible Infrared Hyperspectral Imager Channel (VIHI), which performs hyperspectral imaging to map the planet's mineralogical composition in the Visible and Near Infrared (VNIR) range [2].
MERTIS (MErcury Radiometer and Thermal Infrared Spectrometer) aims to provide detailed mineralogical information about Mercury’s surface by globally mapping spectral emittance with high spectral resolution. Operating in the 7-14 µm wavelength range with high spectral resolution, MERTIS can detect key surface features like the Christiansen feature (CF), Reststrahlen bands (RB), and Transparency features (TF) [3].

The interpretation of spectral data from these instruments needs to be based on the spectral response of the reference material. This is complicated by factors such as mineral composition, elemental abundance, temperature, surface roughness, and particle size, which all contribute to spectral response. This study aims to explore the VNIR and MIR spectral response with the goal of building a comprehensive database of Mercury-like silicate glasses to support future spectral interpretations of Mercury's surface, where volcaniclastic materials are thought to be abundant.

	NVP	NVP_Na	NVP_Mg
SiO2	58.09	55.89	53.85
TiO2	0.98	0.90	0.90
Al2O3	15.54	15.11	12.44
FeO	2.93	3.36	3.34
MgO	16.82	13.80	22.76
CaO	5.04	4.36	6.35
Na2O	0.30	6.35	0.16
K2O	0.32	0.22	0.19

Table 1: The compositions of three starting materials prepared for this study, taken from [4].

Methods:

Material preparation: Three compositions resembling that of the Northern Volcanic Plains (NVP) on Mercury were chosen: an iron-poor potassium-rich composition resembling the composition of NVP [4] the same one with an addition of Na (NVP_Na), and another with an addition of Mg (NVP_Mg) (Table 1). From these compositions, glasses were reproduced  following the procedure illustrated in [6]. Glasses were ground and sieved into grain size fractions: 0–25 µm, 25–38 µm, 38–63 µm, 63–106 µm, 106–150 µm. From these, also Gaussian-like distributions of powders were produced, characterized by mean values of 30, 80, and 120 µm, and standard deviations of 20 and 40 µm.

Spectral Analyses: Spectroscopic analysis was performed using a FT-IR apparatus consists of a Bruker Invenio-X spectrometer coupled with Harrick Praying Mantis™ for Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). Samples were measured in atmosphere and corrected to a background measured with a mirror. To focus on SIMBIO-SYS and MERTIS spectral range, we present the spectra in the wavelength ranges 0.4-2µm  and 7 to 14 µm.

Results and perspectives:

VNIR: the spectral shape of all samples is characterized by a generally positive slope and a prominent absorption at ca 1.1 µm, and a second, very weak, absorption at ca. 1.9 µm. These features are the usual ones observed for silicate glasses [5] and are linked to the presence of Fe-O bonds, which are not detectable in iron-free samples.

MIR: shape of spectra in this spectral range is influenced by both chemical and granulometric characteristics. First, it is linked to the connectivity rate of the silicate structure consisting of tetrahedral units T, primarily Si, Al, and Ti, namely by the amount of bridging oxygens (BO). Moreover, the vibrations in T-O bonds occur at different wavelengths depending on the number of BO between two Si-tetrahedra (BO) and therefore on the amount of Si, Al and Ti present in the structure [6]. In our set of spectra, local maxima at 10 µm detectable for all spectra can be correlated to T-O bonds and can be assigned to RB feature. All spectra show a local negative peak at ∼8 μm that can be recognized as the CF, a local minimum which appears on both amorphous and crystalline materials, and that is often used as a diagnostic feature for igneous products.  Moreover, the presence of fine material produces the presence of TF at ca. 12 μm, occurring when a large surface/volume ratio determines a larger ratio of scattered/absorbed light. Only the spectra related to the finest products show such a feature, reasonably, in our set of spectra.

All these spectra will be made available on the SSDC-ASI portal of the PVRG group [7, https://www.ssdc.asi.it/rockspectra/], and the use of these spectra as reference material will be pivotal for the interpretation with data acquired by the instrument onboard the BepiColombo mission such as SIMBIO-SYS and MERTIS. Ongoing parametrization and analyses on these data will also provide models for the identification of putatively unknown igneous materials on Mercury.

References:

[1] https://doi.org/10.1002/jgre.20075

[2] https://doi.org/10.1007/s11214-020-00704-8

[3] https://doi.org/10.1007/s11214-020-00732-4

[4] https://doi.org/10.1002/2015JE004792

[5] https://doi.org/10.1016/j.icarus.2021.114801

[6] https://doi.org/10.1016/j.icarus.2022.115222

[7] https://doi.org/10.5194/epsc2022-539

Additional Information: 

 We acknowledge ASI-UniPG agreement 2019-2-HH.0. Part of this work was supported by the Ministero dell’Istruzione dell’Università e della Ricerca (MUR) through the program “Dipartimenti di Eccellenza 2023-2027” (Grant SUPER-C).