Preliminary spectroscopic investigation of a potential Mars analog site: Lake Bagno dell’Acqua, Pantelleria, Italy

Abstract

In this study, we characterize the mineralogy and microbiology of a suite of samples from the Bagno dell’Acqua alkaline (pH ≥ 9) lake in Pantelleria, Italy. The aim of the present characterization is the assessment of the lake as a Martian analog site. Nineteen samples were characterized by a combination of VNIR reflectance spectroscopy, micro-Raman (μ-Raman) spectroscopy, and fluorescence microscopy. In addition, DNA extraction was performed on three selected samples to identify the bacteria phyla and to quantify their abundance. The samples were collected as push cores inside the lake and as loose sediments taken along the eastern and southeastern lake's shore. The samples underwent three thermal cycles (between 343 K and 473 K) to remove excess water and organic matter. The VNIR spectra were measured on each sample after each thermal cycle to check the variation of the spectral features as a function of thermal treatment. The mineralogical content of our push core samples is dominated by the presence of Mg-smectite clays always associated with the remanence of K-feldspars and Ca-carbonates while in the sediments we observe also additional minor phases like pyroxene (aegirine NaFe3+Si2O6), hematite (Fe2O3), anatase (TiO2), and volcanic glass. Two organic phases were detected by μ-Raman spectroscopy. One phase was attributable to carotenoids, in line with the high abundance of Bacterial Phyla that can synthesize these molecules. The second phase appeared to be visually indistinguishable from the inorganic matrix and is therefore attributed to EPS (Extracellular Polymeric Substances) forming a biofilm and extensively observed by fluorescent microscopy. The peculiar chemistry of the waters, the presence of Mg-smectite in the sediments of Lake Bagno dell’Acqua associated with Ca-carbonates and sediments derived by biological activity makes Lake Bagno dell’Acqua a very interesting and promising Martian analog site.