The Planetary Terrestrial Analogues Library (PTAL) project had two main tasks, collecting and characterising the analogue rock samples and building an online platform and library allowing public access.
The PTAL collection now consists of 102 rock samples from 17 localities on Earth. The rocks represent the broadly the diversity of volcanic rocks of Mars (known from meteorites, rover and satellite measurements), so that they resemble the composition of martian crust and a range of geological processes on Mars. Individual rocks are made of several minerals and represent different stages of weathering that occurred in contrasting chemical alteration environments, such as due to rain, in rivers, lakes, glaciers, or semi-arid deserts. Furthermore, physical mineral alteration occurs during impact crater formation (impact-related shock-induced metamorphism), therefore, the collection includes a number of so-called impactites.
We characterised all samples with X-ray diffraction (XRD) and optical microscopy to identify the main rock-forming minerals as well as alteration products. We recorded the structural field context, providing sample mineralogy in the framework of geological processes. Each sample was characterized by Near-Infrared (NIR), Raman and Laser-Induced Breakdown (LIBS) spectroscopy using of commercial systems and dedicated instrumentation derived for current (NASA-Curiosity and Perseverance) and forthcoming (ESA-Roscosmos/Rosalind Franklin) rover planetary missions. The PTAL online platform contains rock-sample and sampling-site images, coordinates, mineral identification separated by method and the respective spectral data set. The database is searchable and all images and spectra can be displayed or are downloadable for further usage.
Our unprecedented, coordinated characterization of rocks with multiple techniques provided detailed and complementary information. We identified interfering spectral features showing the advantage of studying natural rocks instead of isolated minerals or synthetic materials, and the importance of the geological context for the interpretation of mineral assemblages. Scientifically, we explored martian environmental and climate conditions on weathering using laboratory experiments. Chemical conditions differ, such as pH-value due to the different atmosphere composition, or physical conditions such as atmospheric pressure and temperature. In experiments, we artificially weathered typical martian rocks and identified the presence of specific mineral groups that are difficult to detect by remote sensing, but important indicators of environmental conditions during aqueous alteration and which correlate with clay mineral structure. Our experiments also suggest a typical response of basalt weathering under dense CO2-rich atmosphere, found for example in Mawrth Vallis (a candidate landing site for future robotic and human exploration). Materials at the ExoMars2022 landing site resemble clay minerals alike vermiculite. Our experiments reproduced this mineral from chlorite in anoxic conditions with significant water amount and time, supporting wet conditions in the martian past, and we have vermiculite in natural rock assemblages as part of the collection. Several of our samples are already used during the calibration campaign of SuperCam on board NASA-Mars2020/Perseverance, for testing operational procedures and additional instruments of the ExoMars2022-Rosalind Franklin rover, and a set of six samples may become witness samples for the same mission.
Overall, the public PTAL rock and spectral library already supported several studies on Mars climatic, environmental and geological evolution. Making the library accessible to the public provides comprehensive and multifaceted information that will enhance the scientific outcome of past, current and future planetary missions. All data are public and accessible in the PTAL database and described in several open access publications.
