Study of iron nanophases in ordinary chondrites by means of nearfield microscopy

The aim of this work is to study the role, the characteristics and the formation process of iron nanoparticles (npFe) in asteroids. These npFe are considered the main responsible for the reddening (i.e. red-IR reflectance increase at increasing wavelength) observed in the asteroids' spectra and it is believed that they are formed as consequence of the Space Weathering (i.e. the ensemble of processes acting on a body exposed to space environment). Moretti et al. (2005) discusses a scenario regarding npFe formation, according to which they originate from shock-induced phase transformations of Fe-Ni alloys caused by collisions. We looked for npFe in samples of Ordinary Chondrites (OCs), whose parent bodies are S-type asteroids, and are trying to link the amount of metal and the mechanical shock degree (which would confirm the scenario above mentioned). For this purpose, we have choosen to use SNOM (Scanning Near-field Optical Microscopy). This technique permits to collect at the same time high resolution topography images and optical images of the analyzed sample. For the first time, a multi-colour SNOM experiment (i.e. every sample has been analyzed at dierent wavelengths) has been performed on extraterrestrial samples: because npFe are more reflective at longer wavelengths, comparison of reflectance images obtained at dierent wavelength gives a strong aid in npFe detection and identification. Finally, laboratory analysis has been supported by simulation methods.