5 Conclusion
The Planetary Analogues and Exobiology Lava Tube Expedition (PELE) was formed to investigate biosignatures in terrestrial lava tubes and inform the search for extant and extinct life in Martian lava tubes. OurStudy in Blue , a case study of blue, copper-rich speleothems collected in Icelandic lava tubes, gave insights into how we might approach sampling these analogue environments, and what we might expect to find. A portable XRF gave us a good indication of elemental composition inside the caves and helped us distinguish regions of interest, i.e., secondary minerals with obvious variations in composition to that of the background lava rock. 16S rRNA sequencing of microbial mats revealed Proteobacteria as the dominant bacterial phylum, several carotenoid-containing genera, chemolithoautotrophic genera, and genera known for their high metal resistance, all characteristics which make them good candidates for surviving on Mars, but also potential contaminants in Mars missions. Raman analysis produced a prominent carotenoid signal, which is a promising biosignature feasibly detectable for robotic lava tube explorers carrying Raman instruments. The morphology and elemental composition of the blue secondary mineral precipitates were elucidated by SEM/EDS, identifying chrysocolla crusts, and a complex, layered precipitate consisting of chrysocolla, pseudomalachite, manganese (hydr)oxide, and a carbonate-bearing species. It appears that copper may be leached from hyaloclastite and ash deposits, enriching the rainwater and groundwater that filters into the caves. Copper-rich speleothems are then precipitated out and serve as biotopes for metal-resistant organisms. As chrysocolla and other oxidized copper mineral phases have been detected on the surface of Mars, they may also be present in Martian lava tubes, and if so, would be worthy targets for astrobiological investigations by future missions.