Identification of Alteration Features in Basalts by Atomic Force Microscopy: New Approach for Understanding the Climate History of Mars.

Abstract:

Studies on Martian meteorites, as well as the analysis of data obtained during space missions, indicate the presence of basalts on the Martian surface. Analysis of their alteration features may provide insights into the climate evolution and its local variations. In this work we consider Atomic Force Microscopy (AFM) as a tool to identify diagnostic alteration features on a basaltic surface.

AFM is one of the most widely used tools for surface science and has already been introduced in space missions. Its advantages include, among others, low instrument weight and high resolution capability. It allows to observe objects in the nanoscale and analyze their topography and roughness. AFM can operate in ambient atmosphere and can be used for different, also non-conductive, materials. However, it has some significant limitations, e.g. it is not possible to determine the chemical composition directly. In order to assess the suitability of this method for analysis of basalt alteration it is necessary to complement AFM with other methods, in this case, Scanning Electron Microscopy (SEM).

Basalt samples from arid cold (Udokan, Siberia) and arid hot (Ogaden, Ethiopia) environments, two of the possible Martian end-member paleo-environments, have been studied. Interesting places in the samples were marked with a diamond stylus for easy identification with AFM and SEM. Mineralogical analysis based on backscattered electron spectra (BSE) was carried out in selected areas of the samples. Structures examined by AFM were assigned to specific mineral phases; it is possible to perceive details of structures that may result from weathering. The study of rocks exposed to contrasted climatic conditions (arid cold and arid hot) aims at determining effect of temperature on the formation of these features. With the used experimental approach, the same area in a given sample can be studied both by SEM and AFM. The structure of minerals identified in the BSE can be located and observed at higher resolution on AFM images, leading to interpretations at a scale that was not attainable before.