Synchrotron radiation as a tool for in situ investigation of extraterrestrial grains in low-density collectors: application to the analyses of the PIE polymid foams targets

The Particle Impact Experiment (PIE) was flown for 11 months outside the MIR station in 1996–97. The grains, both of extraterrestrial and terrestrial origins, captured in the low-density foam collectors, were investigated for using Synchrotron X-ray microfluorescence (SXμF) techniques, developed at LURE (Orsay, France) and ESRF (Grenoble, France). The positions of grains a few microns large are known at better than 10 μm. Chemical identification is arduous; only Z>20 elements are identified and, for the heavy elements, only a rough estimate of their abundances inside the grains can be given. We use the Fe/Ni ratio as the criterion allowing to distinguish between terrestrial orbital debris (OD) and extraterrestrial grains. In the 60 cm2 of foam analyzed by this technique, we identified two or three probable extraterrestrial grains and confirmed the existence of an OD cloud, rich in Fe–Ni alloys, crossed by the MIR station. SXμF is a powerful, non-destructive, technique of in situ identification of absorbing grains trapped in a material otherwise transparent to X-rays. It is the only analytical procedure for foams, opaque to visible light. For aerogels, exposed in many space missions and used as grain collectors in the STARDUST mission, the grains positions can be known after an optical scanning; SXμF appears as the last step for the high resolution in situ identification (size, shape, chemical composition) of the grain before its eventual extraction by microtunneling techniques.