In Situ Search for Traces of Life in Extraterrestrial Samples Using X-ray Spectromicroscopy at the Sulfur K-edge

Introduction: Extraterrestrial sample return missions have been undertaken (Stardust and Genesis) and are being planned for Mars (Mars Sample Return, 2025). One of the major objectives of the latter is the search for traces of life. It will be important to start searching for traces of life during the preliminary examination stages while returned samples are still stored in their original container in order to determine the quarantine level constraints and to decide on an optimal analytical strategy. In situ X-ray fluorescence using synchrotron microbeams and the derived imaging techniques are among the essential tools to be used during preliminary examination stages [1]. Sulfur is a major element in cells and in sedimentary organic matter. It plays an important role in the formation and preservation of fossil organic matter. Its fluorescence photons are more penetrative than those of the other major biological elements (C, H, O, N, P) and X-ray absorption near edge spectroscopy (XANES) at the S K-edge can distinguish sulfur speciation (8 redox states) in organic [2,3] and inorganic compounds [4]. However, few in situ microfluorescence studies at the S K-edge [5] have been done in comparison to carbon K-edge. Two selected case studies exemplify the relevance of micro-X-ray fluorescence (XRF) at the S K edge performed at the ID21 beamline of the ESRF to search for traces of life in extraterrestrial samples.