Experimental petrology and spectroscopy: building analogue samples in laboratory for planetary exploration

Interpretation of spectral data acquired remotely and/or in situ from other planets requires an exhaustive database taking into account well-characterized spectra. Silicate glasses are one of the main constituents of volcanic rocks and a deep knowledge of their spectral response is fundamental to characterize volcanic terrains that we can observe on other planets.

We will show a study concerning spectroscopy properties of silicate glasses, which were synthesized at the Petro-Volcanology Laboratory of the University of Perugia. This study has the main objective to simulate and characterize putative compositions of lavas present on the Northern Volcanic Province of Mercury.

Glasses were synthesized mixing pure oxides and melting them at high temperatures. Once produced, glasses were partly crushed to powders in order to obtain different grain size classes and distributions. Furthermore, they were partly embedded as bulk fragments in epoxy and irradiated by laser ablation at different powers to simulate space weathering effects on Mercury.

The spectroscopic characterization of the samples was performed at the INAF-Astrophysical Observatory of Arcetri, Firenze, where mid-IR biconical diffuse reflectance FTIR spectra in the range 7-15 µm range were acquired on samples characterized by different granulometry. Spectroscopic measurements were performed first on 7 different homogeneous granulometric classes (ranging from 25 to 500 µm), then on six heterogeneous granulometric classes presenting gaussian distributions with varying values of average granulometry and standard deviation. Finally, spectra were acquired on slabs of glasses, which were previously irradiated by laser ablation simulating both weathered surface and re-deposited fine material after meteoritic impacts.

The results showed that spectroscopic features depend on the grain sizes, and in particular they are strongly influenced by presence of fine materials in the heterogeneous samples. Such information was used to retrieve detailed granulometrical data of the bulk samples which were covered by ablated and redeposited particles.

The study shows that experimental petrology is indeed a powerful tool to obtain planetary analogues of any terrestrial and planet product. The spectral characterization and space weathering simulation in the laboratory represent useful techniques to develop instrumental and analytical knowledge for space and planetary exploration. This study was performed on a specific Mercurian product, but in general this kind of approach can be preparatory to design future exploration missions of any planetary/asteroidal site in particular.