Optical borehole-wall analysis – Useful method for planetary environment reconstruction

A borehole-wall scanner was developed with 0.1 mm spatial resolution and used to analyse borehole surfaces in Earth analogue tests to support the drilling of ExoMars 2020 rover and other future missions. Altogether 13 field sites were analysed, and the results were interpreted together with laboratory measurements of optical, infrared and Raman instruments of the acquired samples. The tests show the instrument and method is useful to identify grains, structural features and their spatial distribution along the strata. A range of borehole-wall features could be identified, including separated grains (with identified size, shape, colour and albedo), aggregates, layering, deformation features, voids left behind after fallen out grains. Two basic lithological types could be identified: the heterogeneous fragile sedimentary, and the homogeneous stronger volcanic/magmatic rocks. The drilling produced artefacts such as smearing of soft material or aggregation of existing small grains could that provide information on the presence of plastic material. The experiences of borehole-wall analysis revealed that walls usually show highly heterogeneous appearance at all spatial scales – but the crushed samples were homogeneous, as structural information were lost during the sample crushing. Thus optical scanning of drilled holes provide important information that we propose should be used during sample acquisition for future lunar drilling and Mars Sample Return.