Geochemical and physical variability of Icelandic tephra fields and glaciovolcanic sandur to inform spatial sampling in Mars biosignature searches

摘要

In the search for evidence of extant or extinct life on Mars, characterization of the physical and geochemical properties of a sample - as well as the physicochemical conditions under which that sample was collected - may be critical in the interpretation of any biosignatures discovered. To ensure collection of a statistically meaningful set of samples for biosignature assessment, multiple samples should be collected in these same locations, as well as in nearby areas of the same material (i.e., similar in physical and geochemical properties). To determine the optimal spatial sampling for such a sample set, we explored four volcanic regions of Iceland: two recent tephra fields (Holuhraun and Fimmvorouhals) and two older glaciovolcanic sand sheets, or sandur (Dyngjusandur and Maelifellssandur). These regions have a similar mafic rock source but span different time periods and experience geomorphological forces to differing degrees. Such differences can lead to micro-variability in the physical material that make up these Mars analog sites, thus we aimed to characterize the differences in composition and physical aspects of material between and within these four locations. We selected areas that appeared repetitive and homogeneous from visible satellite and uncrewed aerial vehicle (UAV) imagery, and collected samples over a range of spatial scales (10 cm-1 km). We utilized visible to near-infrared and short-wavelength infrared (VNIR/SWIR) reflectance spectroscopy, X-ray fluorescence (XRF), and measurements of sampled sediment moisture content and grain size to characterize the physical and geochemical properties. VNIR/SWIR spectra contained features consistent with iron-bearing minerals such as pyroxene, basaltic glass and ferric oxides/oxyhydroxides. The two tephra fields had a larger contribution of iron phases compared to the two sandurs, and Holuhraun spectra in particular showed evidence of Fe-bearing glass. Average spectra depicted trends that appear to correlate with location age, including an increase in Fe oxide absorption (0.54 mu m), a shift of the broad absorption at 1.0 mu m to shorter wavelengths, an increase in structural and molecular water at 1.4 and 1.9 mu m, and an increase in the hydroxylated mineral (likely Si-OH) absorption near 2.2 mu m. The bulk geochemical compositions of the four sites were largely undifferentiable within SiO2, Al2O3, MgO, MnO, Na2O, and TiO2, whereas other elements (K2O, CaO, FeO, and P2O5) showed trends that grouped the two northern sites (Dyngjusandur and Holuhraun) and the two southern sites (Maelifellssandur and Fimmvorouhals) together. While there were some differences between the four regions studied, statistical analysis of moisture content, grain size, and summary products derived from VNIR/SWIR data indicate comparable variability in sample geochemical and physical properties up to the 10-m scale, and substantially increased variability at the 100-m and 1-km scales, suggesting that current and future missions in search of biosignatures should target separate sampling areas no more than 10 m apart for repeat measurements of the same material.