Synchrotron infrared microscopy of micron-sized extraterrestrial grains

In the study of the Solar System and its environment, a specific class of samples analysed in the laboratory consists of particles in the micron and sub-micron range. The collection of such valuable samples is to be completed in the future with the samples to be delivered by the coming sample return missions (STARDUST, Mars Sample Return). Among the various microanalysis tools available, Synchrotron Infrared Microspectrometry (SIRM) is of particular interest, as it is a relatively “light” and non-destructive technique, well suited to provide mineralogical and chemical information on micrometer scales. Such an instrument was recently developed on the SA5 infrared beamline at LURE (Orsay, France), and has since shown quite good performances. However, the experimental conditions lead to scattering-induced spectral artifacts, as evidenced by experiments on mineral and organic micro-spheres and test mineral grains of various sizes. It appears that the particles should be crushed prior to analysis, in order to minimize the influence of scattering phenomena. A series of experiments on meteorite samples and five IDPs (L2021D7, -C8, -C18, -B4 and L2036G15) confirms the performances of SIRM for the characterization of mineral phases, of the state of hydratation, and for the detection of the organic content.