Analysis of surface morphology of basaltic grains as environmental indicators for Mars

摘要

Analysing the microtextures of grains from different sediments helps to determine their transport medium. This method is well developed in the case of quartz grains, because quartz is usually dominant in sediments on Earth. In this study, basaltic sands from aeolian and fluvial environments from Iceland and from the Azores Islands were analysed, since basaltic rocks have been found on Mars, and paleo-environment reconstruction is an important goal of future missions. Fixed grains were imaged and analysed by a high-resolution electron microscope. Several different surface identification marks were found corresponding to known examples from quartz micro morphologies: conchoidal fractures, steps, and percussion marks, which are connected to fluvial environments; while precipitation features and bulbous edges were identified and connected to the aeolian transport. In these generally very diverse samples some new micromorphologies, which have not previously been found on quartz grains, were also identified. The identification of such features which are not present on quartz is expected, as the sample has a different composition than quartz. These newly identified features were named as: fan-shaped percussion marks; isometric shaped grains with a rough surface (fluvial); "large voids" with abraded surface and cracked surface (aeolian). Based on our results, a further improved version of the method evaluating grain surface micro-morphology could be useful not only for quartz grains, but in the case of the basaltic grains too. Such information could help to identify the settling environments of grains on Mars, we therefore also discuss the spatial resolution and observational conditions required to do so by in-situ Mars missions there. We conclude that a one order of magnitude increase in the spatial resolution (down to about 1 mu m) would greatly support the identification of grain surface features and interpretation of transport methods. This level of resolution is not far beyond that which is currently available.