In-situ analysis of 1.9 Ga chert with a miniature mass spectrometer for space

In-situ Mars exploration requires new promising instrumentation that will be capable of delivering highly accurate chemical information about soils and rocks present at the Martian surface. Specific attention is drawn to the instruments that are capable of identifying extinct or extant microbes within the bulk of various solid samples (Tulej et al., 2015; Westall et al., 2015; Wiesendanger et al., 2018). A miniature Laser Ablation/Ionization Mass Spectrometer (LIMS) developed at the University of Bern is among the valid candidates (Wurz et al., 2012). The size of the mass analyzer is only Ø 60 mm × 160 mm and thus capable of being deployed on a rover or lander platform. In this contribution, we will present data collected from a 1.88 Ga Gunflint sample using a deep UV fs laser system as ablation ion source. The gunflint chert sample contains a population of microfossils entombed in the silica matrix and was chosen as a Martian analogue. Using the high stability of the UV laser and consequent uniform ablation, we performed large-scale spectra collection (90’000) in two modes - chemical imaging and depth profiling. With the current setup, we achieved a diameter of the analytical spot of ~10 µm for the depth profiling and ~5 µm for the imaging. Our results reveal that our LIMS instrument can identify the location of the microfossil lamination area as well as single microfossils by chemical means. We show how single mass unit spectral decomposition and subsequent kernel clustering reveal masses and intensity regions unique to the microfossils and inorganic host, thus providing the opportunity for automated identification of the spectra that are collected from the microfossils. We also show how transforming spectral intensities into spectral proximities can help to navigate the rich multidimensional datasets. We also address common interpretation problems in LIMS, when multiple mineralogical inclusions contribute to the output spectra acquired within the single analytical spot using ρ-networks and Principal Component Analysis (PCA). In combination with analysis of spectral proximities, this approach is particularly useful in attempts to assess the biogenicity of the putative terrestrial microfossils as well as potential Martian microfossils. Additionally, we discuss the data analysis pipeline and chemical composition of the microfossils and surrounding inorganic host in detail. 

Tulej M., Neubeck A., Ivarsson M., Riedo A., Neuland M. B., Meyer S., and Wurz P. (2015) Chemical Composition of Micrometer-Sized Filaments in an Aragonite Host by a Miniature Laser Ablation/Ionization Mass Spectrometer. Astrobiology, 15: 669-682.

Westall F., Foucher F., Bost N., Bertrand M., Loizeau D., Vago J. L., Kminek G., Gaboyer F., Campbell K. A., Bréhéret J.-G. and others. (2015) Biosignatures on Mars: What, Where, and How? Implications for the Search for Martian Life. Astrobiology, 15: 998-1029.

Wiesendanger R., Wacey D., Tulej M., Neubeck A., Ivarsson M., Grimaudo V., Moreno-García P., Cedeño-López A., Riedo A., Wurz P. and others. (2018) Chemical and Optical Identification of Micrometer-Sized 1.9 Billion-Year-Old Fossils by Combining a Miniature Laser Ablation Ionization Mass Spectrometry System with an Optical Microscope. Astrobiology, 18: 1071-1080.

Wurz P., Abplanalp D., Tulej M., Iakovleva M., Fernandes V. A., Chumikov A., and Managadze G. G. (2012) Mass spectrometric analysis in planetary science: Investigation of the surface and the atmosphere. Solar System Research, 46: 408-422.