This thesis concerns the electron and X-ray microanalysis of planetary materials: from Comet 81P/Wild2 to the surface of Mars. Advanced techniques in electron microscopy and X-ray spectroscopy have been developed for the microanalysis of the nakhlite martian meteorites and Comet 81P/Wild2 samples from the Stardust Mission. \ud Electron microprobe analysis and a Focussed Ion Beam - Scanning Electron Microscope (FIB-SEM) technique for Transmission Electron Microscopy (TEM) was used to analyse the secondary mineral assemblages in the nakhlites. Fracture-filling assemblages in the nakhlites are found to be dominated by an amorphous, hydrated Fe-silicate - a ‘gel’. The gel decreases in Mgat/Mgat+Feat ratio going up the expected depth profile of the nakhlites. Other phases, especially 2:1 smectites - 1:1 phyllosilicate and carbonate are associated with the gel. Newly discovered 1:1 phyllosilicate, suggested to be serpentine, is also found in the mesostasis of Lafayette. A model is proposed describing the formation of the nakhlites’ secondary assemblages by an impact-induced hydrothermal system based on the mineralogical and geochemical differences between different samples. \ud A suite of Stardust cometary samples have also been analysed using FIB-TEM and microfocus X-ray spectroscopy that includes: X-ray Fluorescence Spectroscopy (XRF), X-ray Absorption Near-Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) at the Diamond synchrotron. Attempts have been made to distinguish the cometary material from that formed by capture heating in aerogel via the identification of ferric-oxides at track entrances. \ud Finally, the mineralogy and morphology of a terminal particle from Stardust track #154 was studied by analytical TEM. The results show that Comet Wild2 contains a unique Al-diopside-bearing grain, having affinities with the minerals found in refractory objects from the inner Solar System. Upon comparison with different early Solar System materials, the grain’s mineral assemblage most closely resembles Al-rich chondrules. This adds to the refractory inventory identified in Comet 81P/Wild2
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